<%BANNER%>

Metallicity, Distance and Distribution of Populous Clusters in the Large Magellanic Cloud


PAGE 1

METALLICITY,DISTANCE,ANDDISTRIBUTIONOFPOPULOUS CLUSTERSINTHELARGEMAGELLANICCLOUD By AARONJ.GROCHOLSKI ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT OFTHEREQUIREMENTSFORTHEDEGREEOF DOCTOROFPHILOSOPHY UNIVERSITYOFFLORIDA 2006

PAGE 2

Copyright2006 by AaronJ.Grocholski

PAGE 3

“Space...itseemstogoonandonforever.Butthenyougettotheendandagorillastartsthrowingbarrelsatyou.”-PhilipJ.Fry

PAGE 4

ACKNOWLEDGMENTS Firstandforemost,Iwouldliketothankmyadviser,Dr.AtaS arajedini.Through incrediblepatiencewithmyquestions(andmyforgettingto writedowntheanswers) andgivingmetheopportunitytotraveltomanymeetingsandt elescopes,hehasmade metheastronomerthatIamtoday.Ifeelthatthisdissertati onisasmuchareection ofhisabilitiesasitisofmine(so,hopefullyitisgood). Indoingtheresearchthatwentintothisdissertation,Ihav ehadtheopportunity toworkwithanumberofastronomers.Dr.AndrewColeserveda sabitofasecond adviser,teachingmehowtoprocessstellarspectraandbett erunderstandwhytheylook thewaytheydo.Drs.DougGeislerandVerneSmithalsocontri butedgreatlytoChapter3,providingtherawdataaswellasmanyusefulsuggestio nonthenumerousdrafts ofthatpaper.ForChapter4,Drs.KnutOlsenandGlennTiedeo fferedconsiderable inputonthetargetlist,dataacquisition,andinterpretat ion.ConorMancone,withhis hardworkontheclusterages,helpedtomaketheresultsofth atchapterconsiderably moreaccuratethantheywouldhavebeenotherwise. Iwouldalsoliketothankmydissertationcommittee(Drs.St eveDetweiler,Fred Hamann,ElizabethLada,andVickiSarajedini)forputtingu pwithmymanye-mails andchangesinschedulingmydefenseaswellasreadingthrou gh(apparentlyquite thoroughly)mydissertation. JoannaLevine...orrather,Dr.JoannaLevine,wasanexcell entdissertationbuddy. Beingabletosharethefunandexcitementofwriting,submit ting,anddefending(only twodaysapart)withhermadethewholeprocessalittlemoreb earable.Shealso talkedmyadviserintosendingmewithhertoChileonmyrstr ealobservingrun, whereIgottoseetheLMCandSMCinpersonforthersttime.On thattrip,Ialso iv

PAGE 5

learnedfromaightattendantthatJoannaandIwerelivingi nsinandthattheyhave specialcustomsformsforthat. ThefutureDr.MikeBarkerhasbeenagreatofcemate,neverc omplaining aboutmyaskingbasicastronomyquestionsorrandomtapping ,humming,mumbling, banging,etc,whenI'mlostinthoughtorannoyedatmydata,c omputer,orthestupid L A T E Xtemplate.Itsdoubtfulthatwewilleverbeatthesameinsti tutionagain,but IknowthatifIeverhaveanyquestionsaboutM33orsynthetic color-magnitude diagrams,heistherstpersonI'llask. IwouldliketonotthankJeffJulianandMaj.Dr.SteveNovotn y,USAF.Without themdraggingmetolunchatSonny'sforallyoucaneatribsso often,Iprobably wouldhavenishedmydissertationalongtimeago.But,ther ibsweretasty... And,nally,Iwanttothankmyparents.In28years(andIexpe ctformany more),Ihavehadnothingbutsupportfromthem,whateverpat hIchosetotake.Even withallthebigwordsIhavelearnedwhilewritingupmydisse rtation,Istillcannot ndthewordstoexpresswhatthishasmeanttome.Thankyou. Myresearch(specically,mysalary,travel,andpublicati onmoney)wassupported byNSFCAREERgrantAST-0094048toAtaSarajedini.Thisdiss ertationisbroughtto youbyThompson'sTeeth.Theonlyteethstrongenoughtoeato therteeth. v

PAGE 6

TABLEOFCONTENTS page ACKNOWLEDGMENTS ................................iv LISTOFTABLES ...................................viii LISTOFFIGURES ...................................ix ABSTRACT .......................................xiii CHAPTER1INTRODUCTION ................................1 2 K -BANDREDCLUMPMAGNITUDEASADISTANCEINDICATOR ...7 2.1Introduction .................................7 2.2TheData ..................................9 2.2.1OpenClusters ............................9 2.2.2GlobularClusters ..........................15 2.3ResultsandDiscussion ...........................17 2.3.1ClusterData .............................17 2.3.2FieldStarData ...........................17 2.3.3ComparisonWithTheoreticalModels ...............19 2.4ApplicationAsaDistanceIndicator ....................23 2.5Conclusions .................................25 2.6BeyondGrocholski&Sarajedini(2002) ..................26 3ABUNDANCESANDVELOCITIESOFASAMPLEOFLMCCLUSTERS 31 3.1Introduction .................................31 3.2Data .....................................36 3.2.1TargetSelection ...........................36 3.2.2Acquisition .............................39 3.2.3Processing ..............................41 3.2.4RadialVelocities ..........................42 3.2.5EquivalentWidthsandAbundances ................45 3.3Analysis ...................................49 3.3.1ClusterMembership .........................50 3.3.2ClusterProperties ..........................56 3.3.2.1Metallicities .......................57 3.3.2.2Kinematics ........................62 vi

PAGE 7

3.4ComparisonwithPreviousWork ......................63 3.5Summary ..................................71 3.6NotesonIndividualClusters ........................73 3.6.1NGC1718 ..............................73 3.6.2NGC1846 ..............................75 3.6.3NGC1861 ..............................75 4DISTANCESANDDISTRIBUTIONOFPOPULOUSLMCCLUSTERS ..149 4.1Introduction .................................149 4.2Data .....................................152 4.2.1Observations .............................152 4.2.2Reduction ..............................153 4.2.3Photometry .............................155 4.3ClusterAgesandAbundances .......................158 4.4ApparentandAbsolute K -bandRCMagnitudes ..............161 4.5ClusterDistancesandtheDistancetotheLMC ..............167 4.5.1AbsoluteDistanceModuli .....................167 4.5.2LMCClusterDistribution ......................167 4.5.3TheDistancetotheLMCCenter ..................173 4.5.4SystematicErrors ..........................175 4.6ComparisontoPreviousDistances .....................176 4.7Summary ..................................177 5SUMMARY ....................................180 REFERENCES .....................................186 BIOGRAPHICALSKETCH ..............................193 vii

PAGE 8

LISTOFTABLES Table page 2–1OpenandGlobularClusterInformation .....................17 2–2LMCClusterInformation ............................28 3–1LMCTargetClusterInformation .........................38 3–2CaTLineandContinuumBandpasses ......................45 3–3DerivedLMCClusterProperties .........................57 3–4PublishedLMCClusterMetallicities .......................66 3–5MetallicitiesofYoungandIntermediate-AgeStellarPo pulations ........71 3–6PositionsandMeasuredValuesforClusterMembers .............131 3–7PositionsandMeasuredValuesforFieldStars .................136 4–1ExposureTimesatEachDitherPoint ......................153 4–2LMCClusterSampleInformation ........................154 4–3LMCClusterAgesandMetallicities .......................158 4–4CalculatedRedClumpValuesandClusterDistances ..............163 4–5LMCGlobularClusterInformation .......................173 4–6LMCCenterDistances ..............................175 4–7EffectofLMCGeometry .............................175 viii

PAGE 9

LISTOFFIGURES Figure page 2–1Comparisonofopenclusterages .........................11 2–2Near-IRopenclusterCMDs ...........................13 2–3Near-IRglobularclusterCMDs .........................15 2–4Comparisonof M RC K and M RC I ..........................18 2–5Effectsofageon M RC K ..............................20 2–6Effectsof[Fe/H]on M RC K .............................21 2–7AgeeffectsonsolarneighborhoodRCstars ...................22 2–8Intrinsicredclumpcolor .............................25 2–9Near-IRCMDsforHodge4andNGC1651 ..................30 3–1SchematicdiagramoftheLMC .........................37 3–2SampleofspectrafromRGBstarsinourtargetclusters ............43 3–3The xy positionsofourtargetstarsintheHodge11eld ...........51 3–4RadialvelocitiesforourspectroscopictargetsinHodg e11 ..........52 3–5Hodge11targetstarmetallicities ........................53 3–6 S W vs. V V HB forHodge11 ..........................54 3–7CMDfortheentireHodge11eld .......................55 3–8Positionsontheskyandderivedmetallicitiesforourta rgetclusters ......59 3–9Clustermetallcityvs.positionangle .......................60 3–10Clustermetallicityvsradialdistance .......................61 3–11Clusterradialvelocityvs.positionangle ....................64 3–12MetallicitycomparisonwithOSSH .......................67 3–13MetallicitydistributionofLMCclusters .....................70 ix

PAGE 10

3–14ClusterCMDforNGC1718 ...........................74 3–15IC2146clustermemberselection ........................77 3–16IC2146clusterandeldCMD .........................78 3–17NGC1651clustermemberselection .......................79 3–18NGC1651clusterandeldCMD ........................80 3–19NGC1652clustermemberselection .......................81 3–20NGC1652clusterandeldCMD ........................82 3–21NGC1718clustermemberselection .......................83 3–22NGC1718clusterandeldCMD ........................84 3–23NGC1751clustermemberselection .......................85 3–24NGC1751clusterandeldCMD ........................86 3–25NGC1841clustermemberselection .......................87 3–26NGC1841clusterandeldCMD ........................88 3–27NGC1846clustermemberselection .......................89 3–28NGC1846clusterandeldCMD ........................90 3–29NGC1942clustermemberselection .......................91 3–30NGC1942clusterandeldCMD ........................92 3–31NGC2019clustermemberselection .......................93 3–32NGC2019clusterandeldCMD ........................94 3–33NGC2121clustermemberselection .......................95 3–34NGC2121clusterandeldCMD ........................96 3–35NGC2155clustermemberselection .......................97 3–36NGC2155clusterandeldCMD ........................98 3–37NGC2162clustermemberselection .......................99 3–38NGC2162clusterandeldCMD ........................100 3–39NGC2173clustermemberselection .......................101 3–40NGC2173clusterandeldCMD ........................102 x

PAGE 11

3–41NGC2193clustermemberselection .......................103 3–42NGC2193clusterandeldCMD ........................104 3–43NGC2203clustermemberselection .......................105 3–44NGC2203clusterandeldCMD ........................106 3–45NGC2213clustermemberselection .......................107 3–46NGC2213clusterandeldCMD ........................108 3–47NGC2231clustermemberselection .......................109 3–48NGC2231clusterandeldCMD ........................110 3–49NGC2257clustermemberselection .......................111 3–50NGC2257clusterandeldCMD ........................112 3–51Reticulumclustermemberselection .......................113 3–52ReticulumclusterandeldCMD ........................114 3–53SL396clustermemberselection .........................115 3–54SL396clusterandeldCMD ..........................116 3–55SL41clustermemberselection .........................117 3–56SL41clusterandeldCMD ...........................118 3–57SL4clustermemberselection ..........................119 3–58SL4clusterandeldCMD ...........................120 3–59Hodge4clustermemberselection ........................121 3–60Hodge4clusterandeldCMD .........................122 3–61Hodge3clustermemberselection ........................123 3–62Hodge3clusterandeldCMD .........................124 3–63SL61clustermemberselection .........................125 3–64SL61clusterandeldCMD ...........................126 3–65SL663clustermemberselection .........................127 3–66SL663clusterandeldCMD ..........................128 3–67SL869clustermemberselection .........................129 xi

PAGE 12

3–68SL869clusterandeldCMD ..........................130 4–1 K 0 -bandimagesforalltargetclusters ......................155 4–2OpticalphotometryforNGC1651andNGC2173 ...............161 4–3Near-infraredCMDs ...............................164 4–4Schematicdiagram ................................169 4–5Clusterdistancesasafunctionoftheirposition .................170 xii

PAGE 13

AbstractofDissertationPresentedtotheGraduateSchool oftheUniversityofFloridainPartialFulllmentofthe RequirementsfortheDegreeofDoctorofPhilosophy METALLICITY,DISTANCE,ANDDISTRIBUTIONOFPOPULOUS CLUSTERSINTHELARGEMAGELLANICCLOUD By AaronJ.Grocholski December2006 Chair:AtaSarajediniMajorDepartment:Astronomy TheLargeMagellanicCloud(LMC),withitsproximitytotheM ilkyWayand locationwellawayfromtheGalacticplane,offersusanexce llentopportunitytostudy theeffectsoftidalinteractionsontheformationandevolu tionofstellarpopulationsin asatellitegalaxy.Inthisworkwepresentresultsfromapro gramaimedatdetermining theages,kinematics,metallicities,distancesandspatia ldistributionofpopulous clustersintheLMC. TofurtherourunderstandingoftheLMCclustersystem,weha veacquirednearinfrared(NIR)photometryandspectroscopyandcombinedth esedatawithoptical andNIRphotometryfromtheliterature.WithFORS2ontheVLT ,weobtainedNIR spectraformorethan800starsinandaround29LMCclusterst hatspanalarge rangeofages( 1-13Gyr)andmetallicities( 0 : 3 > [Fe/H] > 2 : 0).Weuse thesespectratocalculatestellarvelocitiesandmetallic ities,andidentifymorethan 200membersin28clusters.Usingpublishedphotometry,VLT FORS2images,and archivalHSTWFPC2images,wecompileddeepopticalphotome tryfor15clusters. Thesedataextendbeloweachcluster'smainsequenceturnof fand,combinedwith ourabundances,allowustobreakthewellknownage-metalli citydegeneracyand xiii

PAGE 14

determineunequivocalagesviamainsequencetting(MSF). Astherststepinour distancecalculations,weuse JK s photometryof14Galacticopenclustersfromthe 2MASStocalibratethe K -bandluminosityofcoreheliumburningredclumpstars ( M RC K )asafunctionofageandmetallicity.Next,withISPIontheC TIO4mtelescope, weimaged17LMCclustersintheNIR( JK 0 )downto K 0 18 : 5,orabout1.5mag belowtheRC,allowingustomeasuretheapparent K -bandRCmagnitudeofeach cluster.WecombinetheLMCclusteragesandabundanceswith ourRCcalibrationto predict M RC K foreachclusterandtherebycalculateaccuratedistancesa ndexplorethe geometryoftheclustersystem. Clustervelocitiesareingoodargeementwithpreviousresu ltsandshowthat theLMCclustersrotatewithdisk-likekinematics.Ourabun dancesindicatethatthe metallicitydistributionofthemoremetal-richclustersi smuchtighterthanpreviously believed,withnotailtowardsolarmetallicities.Thepeak ofthisdistributionissimilar tothatofthebar,whichisingoodagreementwithdynamicalm odelsthatsuggestthat boththerestartofclusterformationandtheformationofth ebaroccurredasaresultof therstcloseencounterwiththeSMC( 4Gyrago).ClusteragesderivedfromMSF rangefrom 1-3GyrforallclustersinoursampleexceptESO121-SC03( 9Gyr), theonlyknownclusterintheLMCwithanagebetween 3-13Gyr.Theintermediate agerange,aswellastheageofESO121-SC03,issimilartopre viousresults. Finally,wendthatthespatialdistributionoftheLMCclus tersystemisin goodageementwiththick,inclineddiskgeometryfoundfrom LMCeldstars.In addition,usingRRLyraebaseddistances,wendthattheold globularclustersare alsoconsistentwiththisgeometry.Giventhedisk-likekin ematicsoftheentirecluster system,thisimpliesthattheLMC'sdiskformedataboutthes ametimeastheold clusters, 13Gyrago.CombiningtheLMCgeometrywithclusterdistance s,we calculateadistancetothecenteroftheLMCof ( m M ) 0 = 18 : 40 0 : 04 0 : 08,which is0.1magshorterthanthecommonlyacceptedLMCdistance. xiv

PAGE 15

CHAPTER1 INTRODUCTION Thecurrentscenarioofhierarchicalformationsuggeststh atlargegalaxieslikethe MilkyWay(MW),andinparticulartheirspheroidcomponents (bulgeandhalo),are builtupthroughtheaccretionofdwarfsatellitegalaxiesa ndprotogalacticfragments (e.g., Cˆoteetal.2000 ; Zentner&Bullock2003 ).Itisclearfromanobservational standpointthatthechemicalenrichmentandstarformation historiesofgalaxiesin general,whetherlargeorsmall,athighorlowredshift,are dominatedbyinteractions andmergerevents( Abraham1999 ; Schweizer1999 ).Thus,understandingtheeffects ofgravitationalinteractionsontheevolutionofsatellit egalaxiesisanimportantpiece inthepuzzleoflargegalaxyformation. TheMWanditssatellitegalaxiesareaprimeexampleofinter actionsandmergers inaction,withsignaturesofthemanystagesofhierarchica lformationvisibleinand aroundtheGalaxy.Themoststrikingevidenceofthisisthec annibalizationofthe Sagittariusdwarfgalaxy(Sgr, Ibataetal.1994 ).Lying 25kpcfromusandalmost directlybehindtheGalacticcenter,Sgriselongatedalong itsorbitaroundtheMW asaresultoftidalstripping.AlsovisiblethroughouttheM Waremanytidalstreams (e.g., Majewskietal.2006 ),allofwhichareremnantsofdwarfgalaxiesthatwere longagodisruptedbyandaccretedintoourGalaxy.Evenhalo globularclustersthat havelargeGalactocentricradii( > 8kpc)likelyformedinsatellitegalaxiesbefore beingabsorbedbytheMW( Searle&Zinn1978 ).Whilesignaturesofpastaccretions aboundintheGalaxy,thefactthatthesesystemsweredisrup tedwellinthepast makesitdifcultifnotimpossibletouncovertheformation historyoftheirparent galaxies.Similarly,thelocationofSgralmostdirectlybe hindtheGalacticbulge causeswidespreadcontaminationbyforegroundstars,rend eringobservationsofthis 1

PAGE 16

2 galaxydifculttointerpret.Incontrast,theLargeMagell anicCloud(LMC)isa nearbygalaxythatshowssignaturesofinteractions,butre mainsmostlyintact,andis relativelyfreefromforegroundcontamination.Sinceitsp roximityallowsustoresolve individualstarsanddeterminetheirphysicalproperties, theLMCoffersusanexcellent environmentinwhichtostudytheeffectsoftidalforcesont heformationhistoryofa satellitegalaxy. UsinggasdynamicalN-bodysimulations, Bekkietal. ( 2004 )modeltheorbitsof theLMCandSmallMagellanicCloud(SMC)intheirpathsaroun dtheMWandpay specialattentiontotheeffectsoftidalinteractionsbetw eentheClouds.Priorto5Gyr ago,theLMCorbitedtheMWevery 2Gyrwithahighlyeccentricorbitthatranged inGalactocentricradiusfrom 50 150kpc.Originally,theSMCorbitedtheMWon asimilarpathto,butindependentfrom,theLMC.Itssmaller Galactocentricradius( 50 100kpc)resultedinafasterorbit,withtheSMCcompletingo nerevolutionaround theMWevery1.5Gyr.Approximately5Gyrago,theLMCandSMCp assedwithin 25kpcofeachother,anencounterthatcausedasmalldecayin theLMC'sorbitand anincreaseintheradiusoftheSMC'sorbit,whichultimatel yledtotheMagellanic Cloudsbecomingbound 1Gyrlater.Sincebecomingbound,theLMCandSMC havehadanumberofcloseencounters( < 10kpc,seeFig.1in Bekkietal.2004 ), withtheirrstcloseencounter(6.4kpc,3.6Gyrago)having asignicanteffectonthe starformationhistoryoftheLMC(seebelow). Quitepossibly,themostremarkablesignatureofinteracti onsintheLMC-SMCMWsystemisseenatradiowavelengths;columndensitymapso fH I showacomplex envelopeofgasinandaroundtheMagellanicClouds.TheLMCa ndSMCare connectedbyasubstantialbridgeofmaterialthatwaslikel ystrippedfromtheSMCin apreviousencounter.Inaddition,boththeshortLeadingAr mandlongerMagellanic Stream,whichtrailstheMagellanicCloudsintheirorbitar oundtheMWandstretches

PAGE 17

3 100 acrossthesky,aretheresultoftidalstrippingofmaterial fromtheMagellanic CloudsbytheMW( Putmanetal.2003 ). Moresubtle,butjustasrevealing,arethemarkersofintera ctionsfoundinthe LMC'sstellarpopulations.Studyingcarbonstarkinematic s, Alves&Nelson ( 2000 ) showedthatthediskoftheLMCisared,withthediskscalehe ightincreasingfrom 0.3kpcata R = 0 : 5kpcto1.6kpcat R = 5 : 6kpc.Usinganexpandedsample ofcarbonstars, vanderMareletal. ( 2002 )ndsimilarresultsandalsoshowthat theLMC'sdisk,with V = s 2 : 9 0 : 9,ismuchthickerthantheMWthindisk ( V = s 9 : 8)andslightlythickerthantheMWthickdisk( V = s 3 : 9).Inaddition, both Olsen&Salyk ( 2002 )and Nikolaevetal. ( 2004 )useeldstarsasrelative distanceindicatorstoshowthattheLMCdiskmayalsobewarp ed.Theseresultsare inagreementwiththeN-bodysimulationsby Weinberg ( 2000 )whichpredictthatthe GalaxyisasignicantdriveroftheLMC'sevolutionandthat tidalforcesfromthe MWwillheat(thicken)andpossiblywarpthediskoftheLMC. However,themostimpressivefeatureoftheLMCthatislikel ylinkedtogravitationalforcesisitsclusterformationhistory.TheLMCiskn owntohaveapopulation ofold,metal-poorglobularclustersthatformed 13Gyragoandamorerecentepoch ofintermediatemetallicityclusterformationthatbegan 3Gyragoandhascontinued tothepresent.Inbetweenthesetwoepochsisthewellknown“ agegap”inwhich onlyonecluster,ESO121-SC03(ESO121;9Gyr),isknowntore side(e.g., DaCosta 1991 ; Geisleretal.1997 ; DaCosta2002 ).Similartotheintermediate-ageclusters,starsintheLMCbarformedonlywithinthepast 5Gyr( Coleetal.2005 ), whereasstarformationhistoriesforthediskoftheLMCshow thateldstarshada constant,althoughlow,starformationrateduringtheclus teragegap( Holtzmanetal. 1999 ; Smecker-Haneetal.2002 ).Theaforementionedmodelof Bekkietal. ( 2004 ) showsthat,whilethecauseoftheapparentendofclusterfor mation 13Gyrago isunknown,therstverycloseencounterbetweentheLMCand SMC 4Gyrago

PAGE 18

4 wouldhavecausedtheformationoftheLMCbaraswellas“dram aticgascloud collisions”thatresultedintherestartofclusterformati onintheLMC;continued stronginteractionshavesustainedtheLMC'sclusterforma tion.Duringtheagegap, weakinteractionsbetweenLMC,SMC,andMWwouldhaveonlybe ensufcientto supportstarformationintheeld.Thus,thestarformation historyoftheLMC,and, inparticular,thestarformationthathasoccurredinwithi nthelast 4Gyr,isadirect resultoftidalforcesactingontheLMC. Asmentionedabove,tracersoftheLMC'seldpopulationssh owthatthespatial distributionoftheeldstarshasbeenaffectedbyinteract ions;incontrast,the3dimensionaldistributionofclustershasnotbeenfullyexp lored.Typically,theLMC istreatedasaplanargalaxythatcanbeassumedtolieatasin gledistancefromus. However,theproximityoftheLMC( 48kpc)combinedwiththefactthatitis inclinedappreciablywithrespecttotheplaneofthesky(e. g., Caldwell&Coulson 1986 )leadstoasignicantdistancegradientacrossthefaceoft heLMC.Recently, both vanderMarel&Cioni ( 2001 )and Olsen&Salyk ( 2002 )haveusedeldstars totracethegeometryoftheLMC,wheretheyhaveassumedthat variationsinthe brightnessoftheirobservedeldswereduetodifferencesi ndistance(seeChapter 4 formoredetail).Usingthismethod,bothauthorsndthatth eLMCeldpopulations lieinadiskthatisinclined 35 (0 isfaceon)withthenortheastportionofthe LMCclosertotheMWthanthesouthwest.Forstarslying5kpcf romthecenterof theLMC,thisinclinationcanleadtodifferenceindistance fromtheMWof 8kpc. WhilerelativedistanceshavebeenusedtoshowthattheLMC eldstarslieinathick, inclineddisk,thedistributionoftheclustershasonlybee ninferredfromkinematics. Schommeretal. ( 1992 )calculatedvelocitiesfor 80populousclustersandfoundthat theentireclustersystemrotateswithdisk-likekinematic s,whilenoclustersappearto resideinapressuresupportedhalo.Wenotethat,althoughb oththeclustersystemand

PAGE 19

5 themajorityofeldstarsresideinthediskoftheLMC,there doesseemtoexista tenuoushaloofmetal-poorRRLyraestars( Borissovaetal.2004 ). Starclustersareanimportanttoolforstudyingthestructu reandformation historyofagalaxybecausetheyhaveonemainadvantageover eldstars.Whereas asampleofeldstarsmaycoverawiderangeofages,allstars inagivencluster canbeconsideredtobecoeval,allowingthecluster'sageto bereadilydetermined fromdeepcolor-magnitudediagrams.Thus,clustersplacea muchneededtime-stamp onavarietyofeventsinthehistoryofagalaxy.Forexample, sinceclusterscontain arecordoftheirhostgalaxy'schemicalabundancesattheti meoftheirformation, theypermitustoplacetightconstraintsontheage-metalli cityrelationofthegalaxy. Thisisparticularlyimportantwhenweconsiderthattidalf orcescanresultina galaxyexperiencinginfalloroutowofmaterial,whichmay leavemarkersofthese interactionsonthegalaxy'sage-metallicityrelation.In addition,givenknowledgeof thekinematicsanddistributionoftheclustersystem,itma ybepossibletodetermine thetimescaleofformationoffeatures,suchasthediskofth egalaxy.Finally,for manystandardcandles,theirabsolutebrightnessvariesas afunctionofbothageand metallicity.Duetothefactthattheagesofeldstarsaredi fculttodetermine,itis usuallyonlypossibletocalculate relative distancesfromeldpopulations.Clusters,on theotherhand,withtheiravailableagesandabundances,en ableustoproperlyapplya standardcandleandtherebycalculateaccurateabsolutecl usterdistances.Therefore,we canuseasampleofclustersnotonlytoexploretheirspatial distribution,butwecan combinethisdistributionwiththeirabsolutedistancesto determineanaccuratedistance totheirhostgalaxy.FortheLMC,anaccuratedistanceisofp articularimportance duetoitsuseasthezeropointintheextragalacticdistance scale(e.g., Freedmanetal. 2001 ). Inthisdissertation,wepresenttheresultsfromaprogramd esignedtobetter understandtheages,kinematics,abundances,distances,a ndspatialdistribution

PAGE 20

6 ofpopulousclustersintheLMC.First,inChapter 2 ,wehavedevelopedthecore heliumburningredclump(RC)starsasastandardcandle.Spe cically,wecalibrated theabsolute K -bandmagnitudeoftheredclumpasafunctionofclusteragea nd metallicityforasampleofGalacticopenclusters.Next,in Chapter 3 ,weobtained moderate-resolutionnear-infraredspectraforalargenum berofstarsinandarounda sampleofLMCclusters.Withthesedatawewereabletoidenti fyclustermembers andsubsequentlydetermineclusterabundancesandvelocit ies.InChapter 4 ,wehave acquirednear-infrared( JK )imagesforanumberofintermediate-ageLMCclusters andusedtheresultingphotometrytomeasuretheapparent K -bandRCmagnitude fortheseclusters.Wecombinenewlycalculatedagesfromde epopticalphotometry (see x 4.3 )withtheabundancesfromChapter 3 andtheRCcalibrationpresentedin Chapter 2 topredicttheabsolute K -bandmagnitudeoftheRCforourLMCclusters. Absoluteclusterdistancesarethenreadilycalculatedand thespatialdistributionofthe clustersystemisexplored,inadditiontodeterminingthea bsolutedistancetotheLMC. Finally,inChapter 5 ,wesummarizeourresults.

PAGE 21

CHAPTER2 K -BANDREDCLUMPMAGNITUDEASADISTANCEINDICATOR 2.1Introduction Duringthepastfewyears,theheliumburningredclump(RC)h asgained considerableattentionforitspotentialasastandardcand le.Theprimaryadvantage oftheRCistheeasewithwhichitcanberecognizedinthecolo r-magnitudediagram (CMD).However,thereiscurrentlyagreatdealofcontrover syintheliterature regardingtheappropriatetreatmentofpossiblemetallici tyandageeffectsontheI-band absolutemagnitudeoftheRC( M RC I ).Therearetwoschoolsofthoughtonthisissue. Therstassumesaconstantvaluefor M RC I whichisthenusedtofacilitateasingle-step distancedeterminationviaknowledgeoftheapparentRCmag nitudeandtheextinction (e.g., Paczynski&Stanek1998 ; Stanek&Garnavich1998 ).Thesecondapproachis foundedontheclaimthatbothageandmetalabundancehaveas ignicantinuenceon theluminosityofRCstars(e.g., Cole1998 ; Sarajedini1999 ,hereafter,S99)andmust beaccountedforindetermining M RC I andthereforethedistance. Both Paczynski&Stanek ( 1998 )and Stanek&Garnavich ( 1998 )use Hipparcos RCstarswithparallaxerrorsoflessthan10%tocalculateth e I -bandabsolutemagnitudeofthesolarneighborhoodredclump.Intheiranalysis, Paczynski&Stanek ( 1998 ) ndthat M RC I showsnovariationwithcolorovertherange0 : 8 < ( V I ) 0 < 1 : 4and, fromaGaussianttotheRCluminosityfunction,nd M RC I = 0 : 28 0 : 09.Followingthesamemethodologyandbuildingupontheearlierwork, Stanek&Garnavich ( 1998 )ndasimilarresultwith M RC I = 0 : 23 0 : 03.Withthiscalibration,asinglestepcalculationisthenusedtodeterminethedistancet otheGalacticcenter 7

PAGE 22

8 ( Paczynski&Stanek1998 )andM31( Stanek&Garnavich1998 ).Bothoftheseinvestigationsfoundlittleornovariationin M I oftheRCstarswithcolor;thiswastakento implythat M RC I doesnotvarysignicantlywithmetallicity. Incontrast,theoreticalmodelsfrom Girardi&Salaris ( 2001 )andtheearlier modelsof Seidel,Demarque,&Weinberg ( 1987 ,seealso Cole1998 )showthat M RC I isdependentonbothageandmetallicity,becomingfaintera sbothincrease.These modelsareingoodagreementwiththeobservationspresente dby S99 .Usingpublished photometryforeightopenclusters, S99 'smostimportantresultisthatwhile M RC I is lesssensitivetometalabundancethan M RC V ,bothstillretainaconsiderabledependence ontheageandmetallicityofthestellarpopulation.Asares ult,thesingle-stepmethod ofapplyingthesolar-neighborhood M RC I topopulationswithadifferentage-metallicity mixcouldbeproblematic. Alves ( 2000 )alsousesthe Hipparcos RCforhiscalibration;however,herelies uponthe K -bandluminosity( M K )ofRCstarsinthehopethat,sincethe K -bandisless sensitivetoextinction(andpossiblymetallicityaswell) thanthe I -band,itmightmake abetterchoiceasastandardcandle. Alves ( 2000 )restrictshisRCstarstothosethat havemetallicitiesfromhighresolutionspectroscopicdat a.Forthisgroupof238RC stars,hendsapeakvalueof M RC K = 1 : 61 0 : 03withnocorrelationbetween[Fe/H] and M K .However,heisnotabletoexploretheeffectofageon M RC K duetothelack ofsuchinformationfortheindividualstarsinhissample. Thesepreviousworkspromptedustocombinetheapproacheso f S99 and Alves ( 2000 )toinvestigatetheinuenceofageandmetalabundanceon M RC K foranumberof openclusterswithwell-knowndistancesandmetallicities .Ourndingswereoriginally publishedin Grocholski&Sarajedini ( 2002 ,hereafter,GS02),however,theresults reliedonphotometryfromtheSecondIncrementalDataRelea seoftheTwoMicron AllSkySurvey(2MASS).Sincetheoriginalpublicationofou rpaper,theAllSkyData Releaseofthe2MASScataloghasbeenmadepubliclyavailabl eand,inthischapter,

PAGE 23

9 wehaveupdatedtheworkpresented GS02 ,basedonthenewest2MASSphotometry. Additionally,in§ 2.6 ,wehaveupdatedourinitialapplicationoftheRCcalibrati onto theLMC( Sarajedinietal.2002 ).WenotethattheimprovedphotometryfromtheAll SkyReleaseof2MASShashadverylittleeffectonthecalcula tionsineither GS02 or Sarajedinietal. ( 2002 )andhasnotchangedanyoftheconclusionsinthesepapers. In§ 2.2 wediscusstheobservationaldatausedincalibratingthe K -bandluminosityoftheredclump.Section 2.3 comparesourdatawiththeresultsoftheoretical modelsandpresentsadiscussionoftheresults.Wetesttheu tilityofourresultsfor calculatingthedistancetoaGalacticopenclusterin§ 2.4 andourconclusionsare summarizedin§ 2.5 .Finally,in§ 2.6 ,wediscusstheworkof Sarajedinietal. ( 2002 ), whichsoughttoapplyourRCcalibrationtodeterminingthed istancetoapairofLMC clusters. 2.2TheData 2.2.1OpenClusters Inthepresentstudy,themostimportantcriterionthattheo bservationaldata mustfulllisthatofinternalconsistency.Forexample,we mustensurethatthe distancemoduli,reddenings,ages,andmetallicitiesofal loftheclustersinoursample havebeendeterminedusingthesametechniques.Inaddition ,itisimperativethat theinfraredphotometrywerelyuponbemeasuredandcalibra tedinaconsistent manner.Fortheformer,weusethedatabaseofopenclusterpr opertiesasmeasuredby Twarog,Ashman,&Anthony-Twarog ( 1997 ),supplementedbyclusteragesfromthe WEBDAdatabase,andforthelatter,weutilizetheAllSkyRel easeoftheTwoMicron AllSkySurvey(2MASS)PointSourceCatalog.Wenowdiscusse achoftheseinmore detail. Twarogetal. ( 1997 ),havecompiledalistof76openclustersforwhichthey providereddenings,distancemoduli,andmetallicities.F orthepurposesofthe presentpaper,welimitourselvestodistancemoduliderive dviathetechniqueof

PAGE 24

10 mainsequencetting(MSF)soastoremainindependentofmet hodsthatrelyonthe luminosityoftheRC.Theirmetallicitieshaveallbeenmeas uredonthesamesystem andthereddeningshavebeendeterminedusinganinternally consistentmethod.The vastmajorityofthesevaluesareconsistentwiththosefoun dintheliterature,exceptfor thereddeningofNGC6819forwhichthe Twarogetal. ( 1997 )valueismuchhigher thanotherpublishedvalues.Asaresult,wehavedecidedtoa doptthe S99 reddening forNGC6819insteadoftheapparentlydiscrepantvaluetabu latedby Twarogetal. ( 1997 ).Inaddition,becausethedeterminationofthereddeninga nddistancemodulus iscoupled,wealsoadoptthe S99 distancemodulusforNGC6819. TheagesoftheopenclustershavebeenobtainedfromWEBDA,w hichisa compilationofopenclusterdatafromvarioussources.Toch eckthereliabilityofthe WEBDAages,wecomparethemwiththeclusteragesdetermined by S99 inFig. 2–1 S99 presentsisochrone-ttingagesforeightopenclusterstha thavebeendetermined inaconsistentmannerusingthe Bertellietal. ( 1994 )theoreticalisochrones.Theleft panelofFig. 2–1 plotstheagesfrom S99 versustheagesinWEBDA,whereboth axesareinlogspaceandthedashedlinerepresentsazeroage differencebetween thesystems.Fromthisplot,itisevidentthatthereisasyst ematicoffsetbetweenthe twosystemswiththeWEBDAagesbeingyoungerthanthoseof S99 .Theaverage difference, D log(Age)=0.191,isusedtoshifttheagesgiveninWEBDAonto the S99 system.TherightpanelofFig. 2–1 showstheagesfrom S99 plottedagainst theshiftedWEBDAages;itisclearfromFig. 2–1 thattheshiftedagesareinbetter agreementwiththoseof S99 ;asaresult,wewillapplythisshifttonineoftheclusters inourstudyandusethe S99 agesfortheveclustersthatarecommontobothstudies. Fortheopenclustersinthe Twarogetal. ( 1997 )studythatpossessMSFdistances,weextracted JHK S photometryfromtheAllSkyReleaseofthe2MASS PointSourceCatalog.Asnotedabove,thesedatahavebeenob tainedusingsimilar instrumentsandreducedwiththesamepipelinetechniques. Foreachcluster,wehave

PAGE 25

11 Figure2–1:Comparisonofopenclusterages.AgesfromWEBDA ( leftpanel )plotted againstthosefrom S99 .Duetothesystematicdifferencebetweenthesystems,weshifttheWEBDAagesolderby D log(Age)=0.191( rightpanel ) toplacethemonthesamesystemas S99 utilizedthesamecriteriaforthe2MASSdataretrieval.The eldsizeisoriginally setto30 0 inradiusandthenreducedtoeldsassmallas5 0 inradiusinanattemptto isolatetheclusterstars.Thesourcesarelimitedtoabrigh tnessof6thmagnitudeor fainterduetosaturationeffectsatthebrightend( Carpenter2000 ).Lastly,wehave extractedonlythehighestqualityphotometryfromthe2MAS Scatalog,whichprovides areadag(rd g)indicatinghowthephotometryofeachstarwasmeasured. Wehave chosentoexcludeanysourcethathasareadagofzeroinanyb andsincethisimplies thatthesourcewasnotdetectedinthatbandandthemagnitud egivenisanupperlimit. Wenotethatthevastmajorityofthestellarmagnitudesused inthisstudyarebasedon point-spread-functiontting(i.e.,rd g=2);however,inordertoincludethebrighter redclumpsofmorenearbyclusters,wehavehadtousetheaper turephotometryina minorityofcases. The2MASSprogramusesa K -short( K S )lterfortheirobservations.Wehave chosentoconvertthesemagnitudestothe K -bandadoptingthe Bessell&Brett ( 1988

PAGE 26

12 hereafter,BB)system,whichisalsousedinthetheoretical modelsof Girardietal. ( 2000 )and Girardi&Salaris ( 2001 ).Thetransformationequationsarederivedby Carpenter ( 2001 )andareadoptedasfollows: ( J K BB )=[( J K S ) ( 0 : 011 0 : 005 )] = ( 0 : 972 0 : 006 ) (2–1) and K BB =[ K S ( 0 : 044 0 : 003 )] ( 0 : 000 0 : 005 )( J K BB ) : (2–2) Wenote,however,thattransformationtothe Koornneef ( 1983 )K-band(used by Alves2000 ;§ 2.3.2 )wouldhaveanegligibleeffectonourresults.Tocorrectfortheinterstellarreddening,weadopttheextinctio nlawdeterminedby Cardelli,Clayton,&Mathis ( 1989 ),which,usingtheirvalueof R V = 3 : 1,gives A K = 0 : 11 A V and A J = 0 : 28 A V .Fromthis,itisasimplemattertocalculatethe absolute K -bandmagnitudeanddereddened J K coloroftheopenclusterstars. WehavedeterminedtheRCluminosityforourclustersbytaki ngthemedian valueof M K forallstarswithinastandardsizedboxplacedaroundtheRC .Weuse themedianvalueof M K alongwithaconstantboxsizeinanattempttoeliminateany selectioneffectsthatmayoccurinchoosingthelocationof theRCandtolimitthe effectofoutlierson M RC K .Fig. 2–2 showstheCMDsforall14clusters,focusedonthe RCandmainsequenceturnoff.TheboxusedtoselecttheRCsta rsforeachcluster isalsoshown.Wenotethat,whereavailable,wehaveusedpub lishedopticalCMDs forourclusterstohelpisolatetheapproximateRClocation .Theuncertaintyin M RC K is calculatedbycombiningthestandarderroraboutthemean K -magnitudeforallstars insidetheRCboxesalongwiththeerrorsin E ( B V ) and ( m M ) V ,alladdedin quadrature.Exceptwhereotherwisenoted,weadopt20%ofth evalueastheerrorin E ( B V ) and10%ofthevalueastheerrorin ( m M ) V

PAGE 27

13 Figure2–2:Near-IRopenclusterCMDs.InfraredCMDsforthe 14openclustersin oursampleareshown,withaboxindicatingthelocationofth ecluster's redclump.Allstarswithintheboxareusedincalculatingth emedian K magnitudeoftheredclump.

PAGE 28

14 Figure2–2:Near-IRopenclusterCMDs-Continued

PAGE 29

15 Figure2–2:Near-IRopenclusterCMDs-Continued. Figure2–3:Near-IRglobularclusterCMDs.SameasFig. 2–2 ,butfortheglobular clustersinoursample. 2.2.2GlobularClusters Itisdifculttoensurethatthedistances,ages,andmetall icitiesofglobularclusterswithRCsareonthesamesystemasthoseoftheopencluste rs.Fortunately,the

PAGE 30

16 agesandmetallicitiesofthetwoglobularsinoursample–47 TucandNGC362–are sufcientlydifferentfromthebulkoftheopenclusterstha tsmallsystematicdiscrepanciesinthesequantitiesshouldnotbeasignicanthindra ncetotheinterpretationof theresults.Inanycase,wehavedecidedtoadoptliterature valuesforthebasiccluster parametersandusetheglobularclusterRCsasaconsistency check. Inthecaseof47Tuc,weadoptthemetallicityquotedby Carretta&Gratton ( 1997 )of [ Fe = H ]= 0 : 70 0 : 07,whichhappenstobeveryclosetothe Zinn&West ( 1984 )value.Forthedistancemodulusandreddening,weaveraget hepublished valueslistedinTable2of Zoccalietal. ( 2001 )toobtain ( m M ) V = 13 : 45 0 : 21and E ( B V )= 0 : 044 0 : 008,wheretheerrorsrepresenthalfoftherangeoftabulate d values.Lastly,fortheageof47Tuc,weadopttheoldestagef orwhichthemodels predictthepresenceofaRCatitsmetallicity–12Gyr. ForNGC362,weadoptasimilarapproach.Themetalabundance of [ Fe = H ]= 1 : 15 0 : 06istakenfrom Carretta&Gratton ( 1997 ),whichisapproximately0.1 dexmoremetal-richthanthe Zinn&West ( 1984 )value.Oursearchoftheliterature hasrevealeddistancemodulithatrangefrom ( m M ) V =14.49( Zinn1985 )to14.95 ( Burki&Meylan1986 ,seealso Bolte1987 )andreddeningsintherange E ( B V ) =0.032( VandenBerg2000 )to0.08( Alcaino1976 )leadingtoadoptedvaluesof 14 : 70 0 : 23and0 : 048 0 : 024fortheapparentdistancemodulusandreddeningof NGC362,respectively.Onceagain,weadoptanageof12Gyr. TheRCsoftheseglobularshavebeenisolatedinthe2MASSpoi ntsourcecatalog inthesamewayasfortheopenclusters.The[ M K ; ( J K ) 0 ]CMDsfor47Tucand NGC362areshowninFigure 2–3 alongwiththeboxusedtodenetheirRCs.All oftherelevantobservationalparametersfortheopenandgl obularclustersarelistedin Table 2–1

PAGE 31

17 Table2–1.OpenandGlobularClusterInformation NameLogAge ( m M ) V a E ( B V ) a [ Fe = H ] a s ([ Fe = H ]) a M K s ( M K )( J K ) o s ( J K ) o NGC7529.248.350.04 0.0880.018 1.5380.1180.6030.011 NGC18178.8012.150.26 0.2680.023 1.8750.1810.5650.028 NGC20998.7311.550.270.0890.073 2.1110.1850.5550.029 NGC22049.28 b 13.300.08 0.3380.120 1.6070.1140.6120.010 Be399.88 b 13.500.11 0.1770.032 1.5950.1220.6770.013 NGC23608.9410.350.09 0.1500.026 1.1770.1200.6040.012 NGC24209.2412.100.05 0.2660.017 1.6900.1150.6170.009 NGC24779.0411.550.230.0190.047 1.4360.1630.5970.024 NGC25069.2412.600.05 0.3760.029 1.5730.1070.6190.007 NGC25278.849.300.09 0.0800.090 1.7000.1250.5540.014 NGC25398.7610.750.090.1370.028 1.5640.1250.5400.016 M679.60 b 9.800.040.0000.092 1.6870.1060.6680.011 NGC67919.98 b 13.400.150.1500.041 1.4220.1330.6870.016 NGC68199.42 b 12.44 b 0.16 b 0.0740.035 1.6580.1360.6480.017 47Tuc10.0813.45 c 0.044 c 0.70 d 0.07 d 1 : 3400.2110.5380.016 NGC36210.0814.70 c 0.048 c 1.15 d 0.06 d 0 : 8310.2400.4400.029 a From Twarogetal. ( 1997 )unlessotherwisenoted b From Sarajedini ( 1999 ) c See§ 2.2.2 d From Carretta&Gratton ( 1997 ) 2.3ResultsandDiscussion 2.3.1ClusterData Asdescribedin§ 2.1 ,weareinterestedinexploringthedependenceof M RC K on [Fe/H]andage.PlottedinFigure 2–4 arethe M RC K valuesforthe14openclusters ( opencircles )andthetwoglobulars( lledcircles )inoursampleversusthelogarithm oftheage( toprightpanel )andthemetallicity( topleftpanel ).Theredclumpsstart outverybrightatyoungagesanddecreaseinbrightnessbyal most1magasthecluster agesapproach10 9 yrafterwhichtheybrightenby 0.5mag.Then,theRCsbecome slightlyfainterastheclusteragesincreaseupto10 10 yr.ThetoptwopanelsofFigure 2–4 alsoinclude M RC I ( opensquares )from S99 .Keepinginmindthatthenumbersof clustersissmall,overtheageandmetallicityrangecommon tobothstudies,the K bandabsolutemagnitudeoftheRCexhibitslesssensitivity toageandmetalabundance thandoes M RC I 2.3.2FieldStarData Alves ( 2000 )reports K -bandabsolutemagnitudesofsolar-neighborhoodstars inthe Hipparcos catalogalongwithparallaxesandpropermotions.Followin gthe

PAGE 32

18 Figure2–4:Comparisonof M RC K and M RC I .Upperpanelsshowthevariationofthe RCabsolutemagnitudeasafunctionof[Fe/H]( topleftpanel )andage ( toprightpanel ).Theopencirclesrepresent K -bandabsolutemagnitudes ( M RC K )forthe14openclusterswhilethelledcirclessignify M RC K values forthetwoglobularclustersinthepresentsample.Theopen squaresdesignate M RC I valuesfrom S99 .Inthebottompanel, M K for Hipparcos Solar neighborhoodredclumpstarsfrom Alves ( 2000 )( opencircles )arecomparedwith M RC K forclustersinthepresentpaper( lledcircles ).Thesetwo datasetsshowremarkableagreementintheirmean K -bandmagnitudes.

PAGE 33

19 analysisof Alves ( 2000 ),wehavelimitedourselvestostarswith2 : 2 < ( V K ) 0 < 2 : 5 and 2 : 5 < M K < 0 : 8inTable1of Alves 'paperinordertoisolateasampleof nearbyRCstars.Wehaveplotted M K versus[Fe/H]forthesestarsinthebottompanel ofFigure 2–4 ( opencircles ).Forcomparison,theopenandglobularclusterdatafrom thepresentwork( lledcircles )arealsoshown.Keepinginmindthattherearefar feweropenclustersthaneldstarsinFigure 2–4 ,wendgoodconsistencyinthe locationsofthetwosamples. Alves ( 2000 )nds h M K ( RC ) i = 1 : 61 0 : 03,whilethe openclustersinoursamplegive h M K ( RC ) i = 1 : 61 0 : 06.Thisisremarkablegiven thefactthattheopenclusterdistancesarebasedonthemain sequencettingresults of Twarogetal. ( 1997 )andtheeldstarsareonthe Hipparcos distancescale.Both showmean K -bandmagnitudesof –1.6andverylittleifanydependenceonmetal abundanceoverthesamerange.2.3.3ComparisonWithTheoreticalModels LeoGirardihaskindlyprovideduswiththeoreticalmodelst hatrepresentthe medianmagnitudeoftheredclumpasafunctionofageandmeta labundance ( Girardi&Salaris2001 ; Crowletal.2001 ).Figures 2–5 and 2–6 showthesetheoreticalmodelsinthe K -bandcomparedwithouropenandglobularclusterdata.The ninepanelsofFigure 2–5 displaythe K -bandluminosityoftheredclumpasafunction ofmetallicityforarangeofages.ThevepanelsinFigure 2–6 illustratethevariation ofthe K -bandluminositywithageforarangeofmetalabundances. Inbothofthesegures,clustersthataresimilarinmetalli cityoragetothemodel plottedineachpanelarerepresentedbylledcircleswhile theremainingclustersare denotedbyopencircles.Figures 2–5 and 2–6 suggestthatatagesyoungerthan 2 Gyr,theredclumpluminosityisgreatlydependentontheage oftheclusterandshows littleeffectfromthemetallicitywhereasclustersoldert han 2Gyrshowtheexact opposite,havinglittleagedependencewhilestillshowing theeffectsofmetallicity.

PAGE 34

20 Figure2–5:Effectsofageon M RC K .Plottedistheobservedvariationof M RC K withthe logarithmoftheageascomparedwiththepredictionsoftheo reticalmodels( Girardietal.2000 )fortheindicatedmetallicities.Thelledcircles representclusterswithagesthatarewithin 0.1dexofthemodelage ineachpanel.Fortheupperleftandlowerrightpanels,the lledcircles representclusterswith log ( Age ) 8 : 7and log ( Age ) 10 : 05,respectively. Theremainingclustersineachpanelaremarkedbyopencircl es. Tofacilitateamoredetailedcomparisonbetweenthemodels andtheobservations, weutilizeaninterpolationroutinebasedonloworderpolyn omialstocomparethe theoretical M K valueswiththeobservedones.Asatestoftheinterpolation ,wehave appliedittotheobservationaldataalonecomparingtheint erpolated M RC K valuesto

PAGE 35

21 Figure2–6:Effectsof[Fe/H]on M RC K .Plottedistheobservedvariationof M RC K with metalabundanceascomparedwiththepredictionsoftheoret icalmodels ( Girardietal.2000 )forspecicages.Thelledcirclesdenotetheclusters with[Fe/H] min [Fe/H] [Fe/H] max ,where[Fe/H] min and[Fe/H] max are halfwaybetweenthemodelshownandthenextlowerandnexthi gher metallicitymodels,respectively.Forthemodelsof [ Fe = H ]= 1 : 3and [ Fe = H ]= 0 : 2,clustershaving [ Fe = H ] 1 : 0and [ Fe = H ] 0 : 1,respectively,aremarkedwithlledcircles.Theremainingcluste rsineachpanel areshownbyopencircles. theactualvaluesattheageandabundanceofeachcluster.We ndthatthermsof theresidualsisnegligiblein M RC K withnosystematictrendsasafunctionofageor abundance.Wehavealsotestedtheinterpolationroutineon thetheoreticalmodelswith similarencouragingresults.Theaccuracyoftheinterpola tionallowsustocomparethe M RC K valuespredictedbythemodelsforagivenageandmetallicit ytotheobserved M RC K foreachcluster.Fromthiscomparison,wendthatthermsde viationofthe theoreticalmodelsfromtheobservationsis0.16mag,withn osystematicvariation intheresidualsasafunctionofageormetallicity.Thisdev iationisslightlylarger

PAGE 36

22 Figure2–7:AgeeffectsonsolarneighborhoodRCstars.Wepl ottheK-bandabsolute magnitudeofsolar-neighborhoodRCstarswith Hipparcos parallaxesvs. theirmetallicities.Thesolidlinesrepresentthepredict ionsoftheoretical modelsconstructedby Girardietal. ( 2000 ).Themodelssuggestthatthe verticalspreadin M K canbeexplainedbyvariationsintheagesofthe stars. thanthemeanerrorinthe M RC K valuesofthe16clusters,whichwendtobe0.13 mag.Giventhatthemeandeviationofthemodelsfromtheobse rvationsisroughly consistentwiththeerrorsinherentinthelatter,itisreas onabletoconcludethatthe modelsaregenerallyconsistentwiththeobservationaldat a.

PAGE 37

23 Figure 2–7 showstheGirardimodelsplottedalongwiththe Alves ( 2000 )eld redclumpstardata.Themodelsreinforcetheconclusiondra wnby Alves ( 2000 )that M K isinsensitivetometallicityfornearbystarsinthisabund ancerange.Furthermore, giventhatatypical M K errorin Alves ( 2000 )datais0.11mag,thisguresuggests thattheverticalspreadinthe M K valuesismainlytheresultofageeffectsamongthe eldstars.Boththe10 8 : 8 and10 9 : 2 9 : 4 yearisochronesagreewiththemajorityofthe data.However,giventheexpectationthatstarsintheSolar neighborhoodarelikelyto benearSolar-age,mostofthe Hipparcos starsinFigure 2–7 probablyhaveLogages between9.2and9.6(1.6to4.0Gyr). Itisinterestingtonotethattheagesofthesolarneighborh oodstarsaspredicted bythemodelsshowalackofstarsaround10 9 yr(Figure 2–7 ).Incontrast,using amodelofthesolarneighborhoodRCthatassumesaconstants tarformationrate, Girardi&Salaris ( 2001 )expectanagedistributionfortheRCstarsthatpeaksat1Gy r withapproximately60%ofthestarshavingthisage.Thedisc repancyinthisresult withtheapparentagesofthe Hipparcos RCstarslikelyindicatesanonconstantstar formationrateinthesolarneighborhood;thisisnotsurpri singiftheformationofstars istriggeredbydensitywavestravelingthroughthesolarne ighborhood,whichisan intrinsicallyepisodicprocess. 2.4ApplicationAsaDistanceIndicator Animportantaspectofthisstudyistheapplicationofthe K -bandRCabsolute magnitudeasadistanceindicator.Tooptimizethisapplica tioninthepresentwork, weseekarangeofageandabundanceoverwhichvariationsin M RC K areminimized. InspectingFigure 2–4 ,weseethatiftheageofthestellarpopulationisintherang e 2 < Age < 6Gyrandthemetalabundanceisbetween–0.5 < [Fe/H] < 0.0,thenthe intrinsicvariationin M RC K isminimizedsuggestingthatuncertaintiesinourknowledg e ofthesepropertiesareinconsequentialinthedeterminati onofthedistance.Onthe basisoftheseconsiderations,wehaveselectedtheopenclu sterNGC2158.This

PAGE 38

24 clusterpossesses2MASSphotometry,anditisincludedinth estudyof Twarogetal. ( 1997 ),sowehaveametallicityvalue( [ Fe = H ]= 0 : 24 0 : 06)thatisonthesame systemastheotherclustersinthisstudy.Theageshiftdesc ribedin§ 2.2.1 isalso appliedtoNGC2158givingusanageof1 : 6 0 : 2Gyr.WenoteinpassingthatNGC 2158wasnotincludedaspartofour M K ( RC ) calibrationbecausethedistancegiven in Twarogetal. ( 1997 )wasdeterminedusingthemagnitudeoftheRCandnotmain sequencetting. ForthereddeningtowardNGC2158wecanutilizethedatainTa ble 2–1 to parameterizetheintrinsiccoloroftheRC[ ( J K ) 0 ]intermsofthemetalabundance andage.Figure 2–8 shows ( J K ) 0 versus [ Fe = H ] ( leftpanel )andage( rightpanel ) fortheclustersinoursample.Usingtheinterpolationdisc ussedin§ 2.3.3 ,we candeterminetheintrinsiccolorofNGC2158givenitsmetal licityandage,for whichwend ( J K ) 0 = 0 : 618 0 : 003.Wecalculatetheerrorin ( J K ) 0 by determiningtheuncertaintyresultingfrom s age and s [ Fe = H ] andaddingthesein quadrature.ComparingtheimpliedintrinsiccoloroftheRC withtheapparentcolor, ( J K )= 0 : 837 0 : 005,wend E ( J K )= 0 : 219 0 : 006.Convertingthistoa colorexcessintheopticalregime,wend E ( B V )= 0 : 42 0 : 012,whichisingood agreementwithpublishedvalues(e.g., Christianetal.1985 ; Twarogetal.1997 ).The precedingmethodrepresentsaninternallyconsistentform alismwhichcanbeutilizedto estimatethereddeningofacluster. Theinterpolationon M RC K usingonlytheopenclusterdatapredicts M K = 1 : 67 0 : 09forNGC2158.Alongwith E ( B V )= 0 : 42andtheapparentRC K -bandmagnitude, K ( RC )= 11 : 53 0 : 02,wend ( m M ) V = 14 : 35 0 : 09.Our distancemodulusforNGC2158agreeswithintheerrorswitht hemainsequencetting modulusof ( m M ) V = 14 : 4 0 : 2foundby Christianetal. ( 1985 ),butisslightly lowerthanthatdeterminedby Twarogetal. ( 1997 )of ( m M ) V = 14 : 5.

PAGE 39

25 Figure2–8:Intrinsicredclumpcolor.Theintrinsiccoloro ftheRCisplottedasa functionof [ Fe = H ] ( left )andage( right ),wheretheopencirclesrepresent theopenclusterswhilethelledcirclesaretheglobulars. 2.5Conclusions Inthiswork,wehavesoughttoestablishthe K -bandabsolutemagnitudeofthe heliumburningredclumpstars( M RC K )asadistanceindicator.Tofacilitatethis,we haveutilizedinfraredphotometryfromthe2MASScatalogal ongwithdistances, metallicities,andagesfor14openclustersandtwoglobula rclusters.Oursample encompassesanagerangefrom0.63Gyrto12Gyrandmetallici tiesfrom–1.15to 0.15dex.Basedonananalysisofthesedata,wedrawthefollo wingconclusions. 1.Thereisastatisticallysignicantrangeof M RC K valuesamongthestarclusters inoursample.Inparticular,forthe14openclusters,wecal culate h M RC K i = 1 : 61with astandarddeviationof0.22mag.Incontrast,themeanerror inthese M RC K valuesis 0.13mag. 2.UponinspectionofFigures 2–5 and 2–6 ,wendthatforclustersyoungerthan 2Gyr, M RC K isinsensitivetometallicitybutshowsadependenceonage. Incontrast,

PAGE 40

26 forclustersolderthan 2Gyr, M RC K isinuencedprimarilybythemetallicityofthe populationandshowslittleornodependenceontheage. 3.Ingeneral, M RC K islesssensitivetoageandmetallicitythan M RC I overthe parameterrangecommontoboththispaperand Sarajedini ( 1999 )fromwhichthe M RC I valuesaretaken. 4.Overcomparablemetallicityandageranges,ouraverage M RC K valueof–1.61 magisconsistentwiththatof Alves ( 2000 )whichisbasedonsolar-neighborhood RCstarswith Hipparcos parallaxes.Wealsosuggestthatthesignicantscatterin the Alves ( 2000 ) M K dataislikelyduetoarangeofagesbetween 1.6and 4Gyr amongthesestars. 5.ThetheoreticalRCmodelsbasedontheformalismof Girardietal. ( 2000 ) agreereasonablywellwithourobservationaldata,indicat ingthatageplaysanimportantroleindetermining M RC K foryoungerpopulationswhilemetallicitymainlyaffects olderpopulations. 6.Usingthe K -bandabsolutemagnitudeoftheRC,weareabletocompute thedistancetotheopenclusterNGC2158.Adoptinganageof1 : 6 0 : 2Gyrand [ Fe = H ]= 0 : 24 0 : 06,ourcalibrationyieldsadistanceof ( m M ) V = 14 : 35 0 : 09. 7.Whendeterminingdistancesforstarclustershaving 0 : 5 [ Fe = H ] 0 : 0and 10 9 : 2 age 10 9 : 9 ,onecanignoretheinterpolationdiscussedin§ 2.3.3 andsimply use h M K ( RC ) i = 1 : 61 0 : 04. 2.6BeyondGrocholski&Sarajedini(2002) Asdiscussedintheintroduction,weultimatelywanttoappl yourRCcalibration todeterminingthedistancetotheLMC.In Sarajedinietal. ( 2002 )wetestedthe feasibilityofapplyingtheRCcalibrationpresentedin§ 2.3.3 tothestellarpopulations oftheLMC.OverthreenightsinDecember2001,usingOSIRISo ntheCTIO4m, weobtained JK s photometryoftheLMCclustersHodge4andNGC1651downto aphotometricdepthof K s 19,orabout2magnitudesbelowtheRC.Imageswere

PAGE 41

27 processedusinganumberofIRAFscripts,whichperformedth efollowingtasks.The nineditheredimagesofeachclusterweredarksubtractedan dmediancombined, withoutshifting,soastoremoveanyobjectsfromthecombin edframe,leavingonlyan exposureofthesky.Wecreatedaat-eldimagebynormalizi ngthisskyframe.The originalimageswerethenskysubtractedandat-eldedusi ngthesecalibrationframes. Afterprocessing,spatialoffsetsintheditherpatternwer ecalculatedandtheimages wereshiftedandaveragecombinedresultinginapairofimag esforeachclusterwith signal-to-noiseratiosequivalenttoasingle198sexposur ein J andasingle270s exposurein K Standardstarimageswerecombinedinthesamefashionasthe scienceframes andtheywereanalyzedwiththeQPHOTsoftwarepackageinIRA F,whichwasusedto measureinstrumentalmagnitudesusinga15pixelradiusape rture.Standardstarswere observedoverthecourseofthreenightsandwehavecombined theseobservationsby offsettingthemeasuredinstrumentalmagnitudestothesam ezeropointusingstars incommonbetweenthethreenights.Thecombineddatasetofs evenstandardstar observationswasttedusingaleast-squaresalgorithmtoc alculatetheextinction coefcientsandzeropoints.Thisprocedureyieldedthefol lowingequations: j J =( 2 : 03 0 : 03 )+( 0 : 06 0 : 02 ) X J ; (2–3) k S K S =( 2 : 23 0 : 03 )+( 0 : 09 0 : 02 ) X K ; (2–4) wherethelowercaselettersrepresenttheinstrumentalmag nitudes, X istheairmass, andthecapitallettersaretheapparentmagnitudes. Thecombined JK S clusterimagesweremeasuredusingtheaperturephotometry routinesinDAOPHOT( Stetson1994 )andcalibratedusingequations 2–3 and 2–4 K S magnitudesareconvertedtothe K -bandusingequations 2–1 and 2–2 andFigure 2–9 showstheresulting[ K ( J K ) ]CMDs.Bothoftheseclustersshowawellpopulated redgiantbranch(RGB)andaneasilyidentiableheliumburn ingRC.Theboxineach

PAGE 42

28 Table2–2.LMCClusterInformation Cluster [ Fe = H ] Age(Gyr) K ( RC ) M K ( RC ) E ( J K )( m M ) 0 Hodge4 0 : 17 0 : 041 : 7 0 : 316 : 90 0 : 02 1 : 64 0 : 170 : 03 0 : 0118 : 52 0 : 17 NGC1651 0 : 07 0 : 101 : 8 0 : 317 : 03 0 : 02 1 : 54 0 : 120 : 06 0 : 0118 : 53 0 : 12 panelindicatesthestarsthatwereusedincalculatingthem edianapparent K -band magnitudeofeachclusterRC,whichwendtobe16 : 90 0 : 02magand17 : 03 0 : 02 magforHodge4andNGC1651,respectively.Asmentionedinth eprevioussection, knowledgeofacluster'sageandmetallicityarenecessaryt odeterminingitsdistance using M RC K .Workby Tiede,Martini,&Frogel ( 1997 )showedthattheslopeofa cluster'sRGBinthenearinfraredisrelatedtoit's [ Fe = H ] .InFigure 2–9 wemark theclusterRGBstarswithlledcirclesandthesolidlinede notesthelineartto thesestars.Utilizingthesetswiththecalibrationof Tiedeetal. ( 1997 ),wend [ Fe = H ]= 0 : 17 0 : 04forHodge4and [ Fe = H ]= 0 : 07 0 : 10forNGC1651. Additionally,weusedpublishedopticalphotometryoftheM STOfortheseclusters ( Sarajedini1998 ,Hodge4and Mouldetal.1997 ,NGC1651)tocalculateages viaMSFwiththeoreticalisochrones( Girardietal.2000 ).Theresultingagesare 1 : 7 0 : 3GyrforHodge4and1 : 8 0 : 3GyrforNGC1651.Combiningagesand metallicitieswithourRCcalibration,wendforHodge4, M RC K = 1 : 64 0 : 17and forNGC1651, M RC K = 1 : 54 0 : 12.Sincetheyaresufcientlyclosetoeachother, weaveragedthereddeningvaluesderivedfromboththe Burstein&Heiles ( 1982 )and Schlegel,Finkbeiner,&Davis ( 1998 )dustmaps,foreachcluster.Usingthereddening relationsquotedby Schlegeletal. ( 1998 ), A V = 3 : 1 E ( B V ) A K = 0 : 11 A V ,and A J = 0 : 28 A V ,weconvert E ( B V ) to E ( J K ) .Thesevalues,alongwithotherderived clusterparameters,aregiveninTable 2–2 Finally,combiningourapparentandabsolute K -bandRCmagnitudeswiththe derivedreddenings,wecalculatetheabsoluteclusterdist ancestobe ( m M ) 0 = 18 : 52 0 : 17and ( m M ) 0 = 18 : 53 0 : 12forHodge4andNGC1651,respectively. ThesenumbersareingoodagreementwiththeLMCdistanceof1 8 : 515 0 : 085mag

PAGE 43

29 publishedby Clementinietal. ( 2003 ).Thisvalueisderivedbyaveragingpublished LMCdistancesdeterminedthroughavarietyofdistanceindi cators.Additionally, theseclusterdistancesareingoodagreementwiththeLMCge ometrydeterminedby vanderMarel&Cioni ( 2001 )and Olsen&Salyk ( 2002 ),whichindicatesthatHodge 4,intheNortheastportionoftheLMC,shouldbeclosertoust hanNGC1651,found intheSouthwest.

PAGE 44

30 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Hodge 4 18 16 14 K 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 = = = = = = = == = = = = = = = = = = = = = = = = = = = = = NGC 1651 0.0 0.5 1.0 18 16 14 12 J K K Figure2–9:Near-IR JK CMDsfortheLMCclustersHodge4andNGC1651.The rectanglesareusedtooutlinetheclusterRCs.Allstarswit hintheseboxes areusedincalculatingtheapparent K -bandRCmagnitude.ClusterRGB starsaredenotedbythelledcircleswiththesolidlinessh owingthe least-squarests.

PAGE 45

CHAPTER3 ABUNDANCESANDVELOCITIESOFASAMPLEOFLMCCLUSTERS 3.1Introduction Inthecurrentparadigmofgalaxyformation,itisbelievedt hattheformation historyofspiralgalaxyspheroids,suchastheMilkyWay(MW )haloandbulge,may bedominatedbytheaccretion/mergerofsmaller,satellite galaxies(e.g., Searle&Zinn 1978 ; Zentner&Bullock2003 ).Thistypeofgalacticinteractioniscurrentlyseen intheSagittariusdwarfgalaxy(Sgr),whichisinthemidsto fbeingcannibalized bytheMW.However,duetoitslocationontheoppositesideof theGalaxyfrom us( Ibataetal.1994 ),contaminationbyMWforegroundstarsmakesitdifcultto studystellarpopulationsinSgr.Incontrast,boththeLarg eMagellanicCloud(LMC) andSmallMagellanicCloud(SMC),twosatellitegalaxiesth atmayeventuallybe consumedintotheMWhalo,sufferlittlefromforegroundcon taminationdueto theirdirectiononthesky,whichplacesthemwelloutofthep laneoftheMW.In addition,therelativeproximityofthesegalaxiesallowsu stoeasilyresolvestellar populationsintheMagellanicCloudsdownbelowtheiroldes tmainsequenceturnoffs (MSTOs).Thus,theLMCandSMCofferusagoldenopportunityt ostudytheeffects ofdynamicalinteractionsontheformationandevolutionof satellitegalaxies;this informationplaysanintegralpartindiscoveringthesecre tsofspiralgalaxyformation. Oneofthemostdirectwaystodeterminethechemicalevoluti onhistory(CEH) andstarformationhistory(SFH)ofagalaxyisthroughthest udyofitsstarclusters, whichpreservearecordoftheirhostgalaxy'schemicalabun dancesatthetimeof theirformation.TheLMCstarclusterscontinuetoplayacri ticalroleinshaping ourunderstandingoftheage-metallicityrelationofirreg ulargalaxies.Therichstar clustersystemoftheLMCisalsoauniqueresourceformanyex perimentsinstellar 31

PAGE 46

32 andgalacticastronomy,largelyduetothefactthattheLMCh arborswellpopulated clustersthatoccupyregionsoftheage-metallicityplanet hataredevoidofMW clusters.Thus,LMCclustershavebeenwidelystudiedasate stofstellarevolution modelsatintermediatemetallicityandage(e.g., Brocatoetal.1994 ; Ferraroetal. 1995 ; Bertellietal.2003 )andasempiricaltemplatesofsimplestellarpopulationsf or applicationstopopulationsynthesismodelsofunresolved galaxies(e.g., Beasleyetal. 2002 ; Leonardi&Rose2003 ; Maraston2005 ). TheLMCclustersystem,however,iswellknowntoshowapuzzl ingagedistribution,withahandfulofold( 13Gyr),metal-poorglobularclusters;anumberof intermediateage(1 3Gyr),relativelymetal-richpopulousclusters;and,appa rently, onlyonecluster,ESO121-SC03( 9Gyr,hereafterESO121),thatfallsintothe LMC'sso-called“agegap”(e.g., DaCosta1991 ; Geisleretal.1997 ; DaCosta2002 ). WenotethattheLMCbarseemstoshowaformationhistoryvery similartothatof theclusters( Coleetal.2005 ),whileeldSFHsderivedfromdeepcolor-magnitude diagrams(CMDs)suggestthatstarsintheLMCdiskhadaconst ant,albeitlow,star formationrateduringtheclusteragegap( Holtzmanetal.1999 ; Smecker-Haneetal. 2002 ).Whilethecauseofthecessationofclusterformation(the beginningofthe agegap)isnotknown,dynamicalsimulationsby Bekkietal. ( 2004 )suggestthatthe recentburstofclusterformationislinkedtotherstveryc loseencounterbetweenthe Cloudsabout4Gyrago,whichwouldhaveinduced“dramaticga scloudcollisions,” allowingtheLMCtobeginanewepochofclusterandstarforma tion;strongtidal interactionsbetweentheCloudshavelikelysustainedthee nhancedclusterformation. Bekkietal. ( 2004 )alsondthatthecloseencounterbetweentheCloudswouldh ave beensufcienttocausetheformationoftheLMCbararoundth etimeofthenew epochofclusterformation,givingrisetothesimilarSFHss eenintheclustersystem andthebar.Inadditiontoenhancingstarformation,tidalf orcescanresultintheinfall oroutowofmaterial,therebyaffectingtheCEHoftheLMCan d,atthesametime,

PAGE 47

33 leavingbehindasignatureoftheinteraction.Thus,accura teknowledgeoftheages andmetallicitiesofLMCclustersisnecessarytofullyunde rstandtheformationand dynamicalhistoryofthisgalaxy. Whileageandmetallicityestimatesfromisochronettingt oCMDsexistfor alargenumberofclusters,thedegeneracybetweenageandme tallicitymakesthese estimatesinherentlyuncertainintheabsenceofsolidmeta llicitymeasurementsbased onspectroscopicdata.Integratedlighthasbeenusedtomea sure[Fe/H]formanyof theseclusters;however,thesevaluesareoftenproblemati c,sincetheclusterlightcan bedominatedbyafewluminousstars,andtheresultsaresusc eptibletosmall-number statisticaleffects.Inrecentyears,highspectralresolu tionstudiesofafewprominent clustershavebeenundertaken,yielding,forthersttime, detailedabundanceestimates ofawidevarietyofelements,includingiron,for individualstars withintheseclusters ( Hill2004 ; Johnsonetal.2006 ).Thisworkishighlyvaluable,butbecauseofthelarge investmentintelescopetimenecessarytoobtaindataofsuf cientlyhighsignal-to-noise ratio(S/N),ithasnecessarilybeenlimitedtoonlyafewsta rsinafewclusters;most ofthesetargetsareveryold,leavingthevastmajorityofyo ungandintermediateage clustersunmeasured. Moderate-resolutionstudiesareanexcellentcomplementt ohigh-resolutionwork foracoupleofreasons.First,themulti-objectcapability availableformanymoderateresolutionspectrographsmakesitpossibletoobservemany potentialclustermembers inagiveneld.Thisincreasestheprobabilityofobserving trueclustermembersand facilitatestheiridentication,eveninsparseclusters. Second,lessintegrationtimeis neededtoachievethedesiredS/Natmoderateresolution,al lowingtheobservationof manymoretargetsinagivenamountoftime.Thus,withmodera te-resolutionspectra wecanobservealargenumberoftargetsinashortperiodofti meandtherebycreate anoverviewofagalaxy'sglobalmetallicitydistribution, bothspatialandtemporal.

PAGE 48

34 ThisapproachisparticularlyimportantfortheLMC,sincei tsmetallicitydistributionis verybroadandtheintrinsicshapeisnotverywellknown. Todate,theonlylarge-scalespectroscopicmetallicityde terminationforLMCclustersbasedonindividualclusterstarshasbeenthelandmark studyby( Olszewskietal. 1991 ,hereafter,OSSH; Suntzeffetal.1992 ).Theyobtainedmedium-resolutionspectra ofredgiantbranch(RGB)starsin 80clustersatawavelengthof 8600 A,centeredontheveryprominenttripletofCa II (CaT)lines.Theirworkwasmotivatedby therecognitionthattheCaTlineswereprovingtobeareliab lemetallicityindicator forGalacticglobularclusters(e.g., Armandroff&Zinn1988 ; Armandroff&DaCosta 1991 )Additionally,thisspectralfeatureiseasilymeasuredin distanttargetsandat mediumresolutionsincetheCaTlinesareextremelystronga ndRGBstarsarenear theirbrightestinthenear-infrared.UsingtheCaT, OSSH calculatedmetallicitiesand radialvelocitiesfor72oftheirtargetclusters.Analysis ofthemetallicitydistribution showedthatthemean[Fe/H]valuesforallclustersintheinn er(radius < 5 )andouter (radius > 5 )LMCarealmostidentical( 0 : 29 0 : 2and 0 : 42 0 : 2,respectively), suggestingthepresenceoflittle,ifany,radialmetallici tygradient,insharpcontrast towhatisseenintheMW(e.g., Frieletal.2002 )andM33( Tiedeetal.2004 ).Usingradialvelocitiesfromthe OSSH sample, Schommeretal. ( 1992 )foundthatthe LMCclustersystemrotatesasadisk,withnoindicationthat anyoftheclustershave kinematicsconsistentwiththatofapressure-supportedha lo. However,theresultsof OSSH presentsomedifcultiesowingtothelimitations oftechnologyatthetime.Theuseofasingle-slitspectrogr aphseverelylimitedthe numberoftargetsobservedtowardeachcluster.Additional ly,thedistanceoftheLMC pairedwitha4mtelescoperequiredthattheyobservethebri ghteststarsintheclusters. ManyofthesestarsareMgiants,whichhavespectracontamin atedbyTiO(although itmaynotbesignicantuntilspectraltypeM5orlater),orc arbonstars,neitherof whicharesuitableforusingtheCaTtodetermine[Fe/H].Thu s,thecombinationofa

PAGE 49

35 single-slitspectrographwithamidsizedtelescopemadeit difcultfor OSSH tobuild upthenumberoftargetstarsnecessarytodifferentiatebet weenclustermembersand eldstars.Mostoftheresultingclustervaluesarebasedon onlyoneortwostars;in somecases,therearemetallicityorradialvelocitydiscre panciesbetweenthefewstars measured,anditisunclearwhichofthevaluestorelyon. Theinterpretationofthe OSSH resultsisfurthercomplicatedbysubsequent advancesbothinknowledgeoftheglobularclustermetallic ityscaletowhichthe CaTstrengthsarereferred( Rutledgeetal.1997a )andinthestandardprocedureused toremovegravityandtemperaturedependenciesfromtheCaT equivalentwidths ( Rutledgeetal.1997b ).Itisnotasimplemattertorederiveabundancesfromthe equivalentwidthsof OSSH becauseofthelackofhomogeneousphotometryformany oftheclusters;mappingthe OSSH abundancestoamodernabundancescale(e.g.,that denedatthemetal-poorendby Carretta&Gratton1997 andnear-solarmetallicityby Frieletal.2002 )isinsufcient,becausethetransformationisnonlineara ndrandom metallicityerrorstendtobegreatlymagnied(see, Coleetal.2005 ). InanefforttoproduceamodernandreliablecatalogofLMCcl ustermetallicities, wehaveobtainednear-infraredspectraofanaverageofeigh tstarsineachof28LMC clusters.Wehavetakenadvantageofthemultiplexcapabili tyandextraordinaryimage qualityandlight-gatheringpoweroftheEuropeanSouthern Observatory's(ESO) 8.2mVeryLargeTelescope(VLT)andofthegreatstridesinth einterpretationand calibrationofCaTspectroscopymadeinthepast15yearstop rovideaccuratecluster abundanceswithmeanrandomerrorsof0.04dex.Hereweprese ntourderivedcluster metallicitiesandradialvelocitiesandcomparetheseresu ltstopreviouslypublished spectroscopicmetallicities.Themetallicitydistributi onofseveralhundrednon-cluster LMCeldstarswillbepresentedinaforthcomingpaper(A.A. Coleetal.,inpreparation).Thecurrentchapterislaidoutasfollows:Section 3.2 discussestheobservations anddataprocessing.In x 3.3 wepresentthederivedclusterproperties,andcomparisons

PAGE 50

36 topreviousworksaredetailedin x 3.4 .Finally,in x 3.5 wesummarizeourresults.We notethatthesedataandresultswereoriginallypresentedi n Grocholskietal.2006 3.2Data 3.2.1TargetSelection Weobserved28prominentstarclustersscatteredacrossthe faceoftheLMC, inenvironmentsrangingfromthedensecentralbartothelow -densityregionsnear thetidalradius(a29thclusterwasobserved,butitappears tobetooyoungtoapply theCaTmethod;see x 3.6.2 ).Ourobservationswereaimedatclustersrichenough andsufcientlylargeanddiffusetogiveuscondenceinhar vestingatleastfour deniteclustermembersfromwhichtoderivetheclustermet allicity.Inordertoobtain leverageontheLMCage-metallicityrelation,weincludedc lustersfromSWBclass IVB VII,spanningtheagerangeofclusterscontainingbright,w ell-populatedRGBs ( Perssonetal.1983 ; Ferraroetal.1995 ).Oursamplewasintentionallybiasedtowards thoseclusterswithconictingoruncertainpreviousabund ancemeasurements,those thoughttolieneartheedgeoftheagegap,andthosewhoserad ialvelocitiesmight providenewinsightintothedynamicalhistoryoftheLMC-SM Csystem,basedon theirlocation.Ourtargetsandtheirpositions,sizes,int egrated V magnitudes,andSWB typesarelistedinTable 3–1 .AschematicoftheLMCispresentedinFig. 4–4 .Shown arenear-infraredisoplethsfromvanderMarel(2001;solid ellipses),atsemi-major axisvaluesof1 ,1 : 5,2 ,3 ,4 ,6 ,and8 .ProminentH I features(dashedlines; Staveley-Smithetal.2003 )andthetwolargestcentersofLMCstarformation(30Dor andN11;opencircles)arealsoplotted.Finally,therotati oncenterofintermediate-age starsisdenotedbytheopensquare( vanderMareletal.2002 ),andtheH I rotation centerfrom Kimetal. ( 1998 )isplottedastheopentriangle.Ourtargetclustersare plottedwithsolidsymbols,withtheexceptionofNGC1841,w hichliesfarthersouth thantheareacoveredbythisdiagram.

PAGE 51

37 Figure3–1:SchematicdiagramoftheLMCshowingthelocatio nofourtargetclusters alongwithprominentfeatures.Filledsymbolsrepresentth etargetclusters,withsymbolsizedirectlyrelatedto V magnitudeandshapedenoting SWBtype,wherethetriangles,squares,andpentagonsarety peV,VI, andVII,respectively.NotethatNGC1841(declination 84 )isoutsideoftherangeofthisplotandNGC1861(SWBtypeIVB)ismar ked byalledtriangle.Near-infraredisoplethsfrom vanderMarel ( 2001 ) aremarkedbysolidlineswhilethedashedlinesoutlinemajo rH I features(see Staveley-Smithetal.2003 .TheH I rotationcenter( Kimetal. 1998 )ismarkedwiththeopentriangle,andtherotationcenterof the intermediate-agestars( vanderMareletal.2002 )isshownbytheopen square.Finally,thetwolargestH II regionsaremarkedbyopencircles.

PAGE 52

38 Table3–1.LMCTargetClusterInformation ClusterAlternateR.A. a Decl a Diameter a V mag b SWBExposure Name(J2000.0)(J2000.0)(arcmin)Type b Time(s) SL4LW404 h 32 m 38 s -72 20 0 27 00 1.7 c 14.2VI c 2 300 ReticulumESO118-SC31043611-5851404.714.25VII2 600 NGC1651SL7,LW12043733-7035082.712.28V2 300 NGC1652SL10,LW14043823-6840221.513.13VI300NGC1841ESO4-SC15044523-8359490.911.43VII500SL41LW64044730-7235181.414.14V2 600 SL61LW79045045-7532002.313.99VI2 300 NGC1718SL65045225-6703061.812.25VI500NGC1751SL89045412-6948231.511.73VI2 300 NGC1846SL243050735-6727313.811.31VI600NGC1861SL286051021-7046381.513.16IVB600SL396LW187051936-7306401.313.56VI2 300 NGC1942SL445,LW203052443-6356241.913.46VI2 300 NGC2019SL554053157-7009341.510.86VII600Hodge4SL556,LW237053225-6444122.513.33V500Hodge3SL569053320-6808081.813.42VI500IC2146SL632,LW258053746-7447003.312.41V500SL663LW273054229-6521480.8 c 13.8V c 600 NGC2121SL725,LW303054812-7128522.412.37VI600NGC2173SL807,LW348055758-7258412.611.88VI600NGC2155SL803,LW347055833-6528352.412.60VI600NGC2162SL814,LW351060030-6343193.012.70V2 300 NGC2203SL836,LW380060443-7526183.211.29VI500NGC2193SL839,LW387060618-6505571.713.42V600NGC2213SL857,LW419061042-7131442.112.38V2 300 Hodge11SL868,LW437061422-6950542.711.93VII600SL869LW441061441-6948071.6 c 15.0VI c 600 NGC2231SL884,LW466062043-6731072.113.20V500NGC2257SL895,LW481063013-6419294.012.62VII600 a FromBicaetal.(1999). b FromBicaetal.(1996)unlessnoted. c Estimatedviacomparisonswithtargetclusters.

PAGE 53

39 Pre-imagesofourtargeteldsinthe V and I bandsweretakenbyESOParanal staffinthefallof2004,severalmonthspriortoourobservi ngrun.Thepre-images wereprocessedwithinIRAF,andstarswereidentiedandpho tometeredusingthe aperturephotometryroutinesinDAOPHOT( Stetson1987 ).Starswerecatalogedusing theFINDroutineinDAOPHOTandphotometeredwithanapertur esizeof3pixels. The V -and I -banddatawerematchedtoformcolors.Redgianttargetswer echosen basedontheinstrumentalCMD,andeachcandidatewasvisual lyinspectedtoensure locationwithintheclusterradius(judgedbyeye)andfreed omfromcontaminationby verynearbybrightneighbors.Ineachclusterwelookedform aximumpackingofthe 8 00 longslitsintotheclusterareaandforthebestpossiblecov erageofthemagnitude rangefromthehorizontalbranch/redclump( V 19.2)tothetipoftheRGB( V 16.4).Thepositionsofeachtargetweredenedontheastrom etricsystemofthe FORS2pre-imagessothattheslitscouldbecenteredasaccur atelyaspossible,andthe slitidenticationsweredenedusingtheFORSInstrumentM askSimulatorsoftware providedbyESO;theslitmaskswerecutonParanalbytheFORS 2team. 3.2.2Acquisition ThespectroscopicobservationswerecarriedoutwithFORS2 invisitormodeat theAntu(VLT-UT1)8.2mtelescopeatESO'sParanalObservat oryduringthersthalf ofthenightsof21-24December2004;weatherconditionswer everyclearandstable duringallfournights,withseeingtypically0 : 00 5-1 : 00 0.WeusedtheFORS2spectrograph inmaskexchangeunit(MXU)mode,withthe1028z+29grismand OG590+32order blockinglter.TheMXUslitmaskcongurationallowsthepl acementofmoreslits ontheskythanthe19movableslitsprovidedinMulti-Object Spectrographmode.We usedslitsthatwere1 00 wideand8 00 long(7 00 inafewcases),and,asmentionedabove, targetswereselectedsoastomaximizethenumberoflikelyc lustermembersobserved; typically10starsinsideourestimatedclusterradiuswere observed,withanadditional

PAGE 54

40 20starsoutsideofthisradiusthatappearedtobeLMCeldre dgiantsbasedonour preimagingCMDs. FORS2usesapairof2k 4kMITLincolnLaboratoryCCDs,andthetarget clusterswerecenteredontheupper(master)CCD,whichhasa readoutnoiseof2.9 electrons,whilethelower(secondary)CCD,withareadoutn oiseof3.15electrons, wasusedtoobserveeldstars.Theonlyexceptiontothiswas theHodge11 SL869 eld,where,witharotationoftheinstrument,wewereablet ocenterHodge11in themasterCCDandSL869inthesecondaryCCD.BothCCDshavea ninversegain of0.7e ADU 1 .Pixelswerebinned2 2,yieldingaplatescaleof0 : 00 25pixel 1 andtheresultingspectracover1750 A,withacentralwavelengthof8440 Aanda dispersionof 0.85 Apixel 1 (resolutionof2-3 A).WhiletheFORS2eldofview is6 : 0 8across,itislimitedto4 : 0 8ofusablewidthinthedispersiondirectioninorderto keepimportantspectralfeaturesfromfallingofftheendso ftheCCD. Eacheldwasobservedtwice,withoffsetsof2 00 betweenexposures,toameliorate theeffectsofcosmicrays,badpixels,andskyfringing.The totalexposuretimein eachsetupwaseither2 300,2 500or2 600s.Boththereadouttime(26 s)andsetuptimepereld(some6-10minutes)wereveryquick andallowedusto obtainlongerexposuresthanoriginallyplannedinmanycas es.Formostofourtargets withshortexposuretimes(300s)wecombinedthespectrasoa stoimprovetheS/N. However,withthelongerexposures(500and600s)wefoundth attheS/Ninasingle exposurewasadequate,andcosmicraysandbadpixelswereno taproblem,sowe haveusedonlyoneofthepairofexposuresinouranalysis.Co lumn8ofTable 3–1 givesthetotalexposuretimethatwehaveusedinouranalysi sofeachcluster. CalibrationexposuresweretakenindaytimeundertheFORS2 InstrumentTeam's standardcalibrationplan.Thesecompriselampat-eldex posureswithtwodifferent illuminationcongurationsandHe-Ne-Arlampexposuresfo reachmask.Twolamp

PAGE 55

41 settingsarerequiredfortheateldsbecauseofparasitic lightintheinternalFORS2 calibrationassembly. InadditiontotheLMCclusters,weobservedfourGalacticst arclusters(47 Tuc,M67,NGC2298,andNGC288),threeofwhichareasubsampl eoftheCaT calibrationclustersin Coleetal. ( 2004 ,hereafter,C04).Sinceweusedthesame instrumentsetupas C04 ,weexpectedtousetheirCaTcalibration,andthesethree clusterswereobservedtoserveasacheckonthevalidityoft hatapproach.Processing ofthesethreeclustersshowsthatourresultsareidentical towithintheerrors;thus, weusetheCaTcalibrationof C04 ratherthanderivingourownCaTcalibration coefcients.3.2.3Processing ImageprocessingwasperformedwithavarietyoftasksinIRA F.TheIRAFtask ccdproc wasusedtotandsubtracttheoverscanregion,trimtheimag es,xbadpixels,andateldeachimagewiththeappropriatedomeats.T heat-eldedimages werethencorrectedfordistortionsinordertofacilitatee xtractionanddispersioncorrectionofthespectra.Thedistortioncorrectionisatwo-s tepprocess,wherebyrstthe imageofeachslitletisrectiedtoaconstantrangeof y -pixel(spatialdirection)values ontheCCD,andthenthebrightskylinesaretracedalongeach slitletandbrought perpendiculartothedispersiondirection.Theamountofth edistortionisminimalnear thecenteroftheeldofviewandincreasestowardtheedges; inallcasesitistwith apolynomialthatisatmostquadraticin y andlinearin x .Althoughthedistortion correctionsaresmall,theygreatlyreducetheresidualsle ftafterskysubtractionand improvetheprecisionandaccuracyofthedispersionsoluti on(seebelow). Oncedistortioncorrectionswerecompleted,thetask apall (intheHYDRA package)wasusedtodenetheskybackgroundandextractthe stellarspectraintoone dimension.Theskylevelwasdenedbyperformingalineart acrossthedispersion directiontosky“windows”oneachsideofthestar.Thisproc edurepresentedfew

PAGE 56

42 difculties,sincethetargetstarswereusuallybrightcom paredtotheskyandthe seeingdisksweresmallcomparedtothelengthoftheslitlet s.Theonlyproblems arosewhenthestarfellnearthetoporbottomoftheslitlet; inthesecasesthesky regionswerechoseninteractively,andwefoundforallofth esespectrathatthe resultingskysubtractionwasindistinguishablefromthat ofmorecentrallylocated stars.Whiledailyarclampexposuresareavailablefordisp ersion-correctingthe spectra,telescopeexureduringthenight,alongwithsmal lslitcenteringerrors,makes thisalessdesirablemethodforcorrectingthespectra.Ass uch,morethan30OH night-skyemissionlines( Osterbrock&Martel1992 )wereusedbytheIRAFtasks identify refspectra ,and dispcor tocalculateandapplythedispersionsolutionforeach spectrum,whichwasfoundtobe 0.85 Apixel 1 withacharacteristicrmsscatter of 0.06 A.Fortheshort(300s)exposuredata,weprocessedbothsets ofimages foreachpointingandcombinedthedispersion-correctedsp ectrausing scombine to improvetheS/Nsforthesestars.Inafewcaseswefoundthata veragingthestellar spectraactuallydecreasedtheS/N;forthesestarswechose tousethehigherquality ofthetwoindividualspectrainplaceoftheaveragedspectr um.Allspectrawerethen continuum-normalizedbyttingapolynomialtothestellar continuum,excluding strongabsorptionfeatures(bothtelluricandstellar).Fo rthenalspectra,S/Nsare typically25 50pixel 1 withsomestarsashighas 90pixel 1 and,inonlyafew cases,aslowas 15pixel 1 .SamplespectrashowingtheCaTregionarepresentedin Fig. 3–2 3.2.4RadialVelocities Accurateradialvelocitiesforourtargetstarsareimporta ntfortworeasons.First andforemost,sinceacluster'svelocitydispersionisexpe ctedtoberelativelysmall comparedtothesurroundingeldanditsmeanvelocityquite possiblydistinctfrom theeld,radialvelocitiesareanexcellenttoolfordeterm iningclustermembership.In

PAGE 57

43 Figure3–2:SampleofspectrafromRGBstarsinourtargetclu sterscoveringarange inmetallicities.ThethreeCaTlines,alongwithsomenearb yFe I lines, aremarkedforreference;thechangeinCaTlinestrengthwit h[Fe/H]is readilyvisible.Calculatedsummedequivalentwidthsandm etallicitiesfor eachstararegiven. addition,ourequivalent-widthmeasuringprogramusesrad ialvelocitiestoderivethe expectedCaTlinecenters. Radialvelocitiesforalltargetstarsweredeterminedthro ughcross-correlation with30templatestarsusingtheIRAFtask fxcor ( Tonry&Davis1979 ),andwehave chosentousetemplatespectrafrom C04 .Thetemplatestarswereobservedasapart

PAGE 58

44 oftheirCaTcalibrationprogram;thus,theirsampleoffers agoodmatchtothespectral typesofourtargetstars.Inaddition,theirobservationsw eremadewithatelescopeand instrumentsetupthatisalmostidenticaltoours. C04 chosetemplatestarsforwhich reliablepublishedradialvelocitymeasurementswereavai lable.Templatevelocities camefromthefollowingsources:11starsfromNGC2298,NGC1 904,andNGC 4590( Geisleretal.1995 );8starsfromBerkeley20andBerkeley39( Frieletal. 2002 );2starsfromMelotte66( Friel&Janes1993 );6starsfromM67( Mathieuetal. 1986 );and3starsfrom47Tuc( Mayoretal.1983 ).Inadditiontocalculatingrelative radialvelocities, fxcor usesinformationabouttheobservatorylocationandthedat e andtimeoftheobservations(oncetheESOheaderhasbeenapp ropriatelyreformatted) tocorrectthederivedvelocitiestotheheliocentricrefer enceframe.Forastar'snal heliocentricradialvelocity,weadopttheaveragevalueof eachcross-correlationresult. Wendgoodagreementamongthetemplate-derivedvelocitie s,withatypicalstandard deviationof 6kms 1 foreachstar. Whenthestellarimageissignicantlysmallerthantheslit width,systematicerrorsduetoimprecisealignmentoftheslitcenterandtheste llarcentroidcandominate theerrorbudgetintheradialvelocitymeasurements.Witht hegrismandCCDused here,anoffsetof1pixelacrossthe4pixelwideslitwouldin troduceanerrorinthe measuredvelocityof 30kms 1 .Wefollowtheapproachof Tolstoyetal. ( 2001 ) inapplyingacorrectiontoeachmeasuredradialvelocityba sedontheindividualslit offsets;following C04 ,wemeasuretheslitoffsetsusingacquisition(so-calledt hroughslit)imagestakenimmediatelypriortothespectroscopicm easurementandestimate aprecisionof 0.14pixelsonthemeasuredoffsetvalue.Thisintroducesan error of 4.2kms 1 and,addedinquadraturewiththeerrorresultingfromtheve locity cross-correlations,givesanerrorofroughly7.5kms 1 .Weadoptthisastheerrorin measuringtheradialvelocityofanindividualstar.

PAGE 59

45 Table3–2.CaTLineandContinuumBandpasses FeatureLineBandpass( A)BlueContinuum( A)RedContinuum( A) Ca II l 84988490 85068474 84898521 8531 Ca II l 85428532 85528521 85318555 8595 Ca II l 86628653 86718626 86508695 8725 3.2.5EquivalentWidthsandAbundances TomeasuretheequivalentwidthsoftheCaTlines,wehaveuse dapreviously writtenFORTRANprogram(see C04 fordetails).However,sincethisregionofa star'sspectrumcanbecontaminatedbyweakmetallinesand, insomecases,weak molecularbands,measuringthetrueequivalentwidthofthe CaTlinesatallbut thehighestspectralresolutionsisimpossible.Instead,w efollowthemethodof Armandroff&Zinn ( 1988 )anddenecontinuumbandpassesoneithersideofeach CaTfeature.Inthiswavelengthrange,thecontinuumslopeo faredgiantstaris virtuallyat;thus,the“pseudo-continuum”foreachCaTli neiseasilydenedby alinearttothemeanvalueineachpairofcontinuumwindows .The“pseudoequivalentwidth”isthencalculatedbyttingthesumofaGa ussianandaLorentzian, requiredtohaveacommonlinecenter,toeachCaTlinewithre specttothe“pseudocontinuum.”Forreference,therestwavelengthsoftheline andcontinuumbandpasses, asdenedby Armandroff&Zinn ( 1988 ),arelistedinTable 3–2 .Formanyyearsit hasbeenknownthatevenatthemoderatespectralresolution susedhere,aGaussian ttotheCaTlinesissusceptibletolossofsensitivityathi ghmetallicitybecause theGaussianfailstoaccuratelymeasuretheextremelybroa dwingsofthelines(see discussionin Rutledgeetal.1997b ).Wefollowtheprocedureestablishedin C04 andaddaLorentzianproletotheGaussianinordertorecove rsensitivitytothefull rangeofmetallicities.Errorsintheequivalentwidthmeas urementswereestimatedby measuringthermsscatterofthedataaboutthets.

PAGE 60

46 Anumberofpreviousauthorshavecalibratedtherelationsh ipbetweenthe strengthsofthethreeCaTlinesandstellarabundanceusing avarietyofmethods(see Table3in Rutledgeetal.1997a ).Inallcases,alinearcombinationoftheindividual linestrengthswasusedtoproducethesummedequivalentwid th, S W ,withweighting andinclusionoflines(someauthorsdroppedtheweakestlin e,8498 A)varyingbased onthequalityoftheirdata.Sincethequalityofourdataiss uchthatallthreelinesare wellmeasured,weadoptthesamedenitionfor S W as C04 S W EW 8498 + EW 8542 + EW 8662 : (3–1) ItiswellknownthatTiO,whichhasastrongabsorptionbandb eginningnear8440 A(e.g., Cenarroetal.2001 ),canaffectthespectraofcool( M5orlater),metal-rich stars.Thisabsorptionfeature,whichdepressesthe“pseud o-continuum”aroundthe CaTlinesandresultsinanunderestimationofthemeasurede quivalentwidths,was notedby OSSH insomeoftheirLMCspectra.Duringprocessing,wecheckede ach spectrumfortheappearanceofthisTiOabsorptionbandandf oundnoevidencethat TiOhadaffectedanyofourobservations. Boththeoretical( Jrgensenetal.1992 )andempirical( Cenarroetal.2002 ) studieshaveshownthateffectivetemperature,surfacegra vity,andmetallicityall playsignicantrolesindeterminingtheCaTlinestrengths .However,itiswell establishedthatforredgiantsofagivenmetallicity,ther eisalinearrelationship betweenastar'sabsolutemagnitudeand S W ( Armandroff&DaCosta1991 ),where starsfartheruptheRGBhavelarger S W values.Thisisprimarilyduetothechange insurfacegravityasastarmovesalongtheRGB;starsnearth ebottomoftheRGB havesmallerradii,thuslargersurfacegravities,whichin creasestheH opacity.Since H isthedominantopacitysourceinredgiants,increasingthe H opacitydepresses the“pseudo-continuum”,whichinturndrivesdownthemeasu redvaluefor S W .To removetheeffectsofluminosityon S W ,similartopreviousauthors,wedenea

PAGE 61

47 reducedequivalentwidth, W 0 ,as W 0 S W + b ( V V HB ) ; (3–2) wheretheintroductionofthebrightnessofacluster'shori zontalbranch(HB), V HB removesanydependenceonclusterdistanceorreddening(se ethethoroughdiscussion in Rutledgeetal.1997b ).Duetothefactthatamajorityofourclustersaretoo youngandmetal-richtohaveafullyformedHB,weinsteadado ptthemedianvalue ofthecorehelium-burningredclump(RC)starsfortheseclu sters(see x 3formore information).Valuesfor b havebeenderivedempiricallybypreviousauthors,withthe mostrobustdeterminationbeingthatof Rutledgeetal. ( 1997b ).Utilizingstarsfrom52 Galacticglobularclusters,theyfoundametallicity-inde pendentvalueof b = 0 : 64 0 : 02 Amag 1 ,coveringclustersintherange 2 : 1 < [Fe/H] < 0 : 6.Similarly, C04 found b = 0 : 66 0 : 03fortheglobularclustersintheirsample.However,whent heiropen clusterswereincluded,theslopesteepenedto b = 0 : 73 0 : 04.Thissteepeningofthe relationshipbetween W 0 and V V HB with[Fe/H]isinqualitativeagreementwiththe theoreticalresultsof Jrgensenetal. ( 1992 ).Sinceourtargetclustersspananageand metallicityrangesimilartotheentirecalibrationcluste rsampleobservedby C04 ,for b wehavechosentoadopttheirvalueof0.73,whichisbasedonb oththeiropenand globularcalibrationclusters.Tovalidatethisapproach, asmentionedin x 3.2.2 ,during ourscienceobservationsweobservedasubsampleofthecali brationclustersusedby C04 andfoundthat,towithintheerrors,ourmeasurementsareid enticaltotheirs,asis expected,giventhatessentiallythesameinstrumentsetup wasusedinbothprograms. BeforeproceedingtothelaststepoftheCaTcalibration,we needtoaddress theissueofpossibleageeffectsonthesecalculations.Asn otedbypreviousauthors (e.g., DaCosta&Hatzidimitriou1998 ; C04 ; Kochetal.2006 ),theageofastellar populationaffectstheluminosityofcoreheliumburningst arsandmayintroduce systematicerrorsindetermining V V HB and,therefore,metallicitiesderivedvia

PAGE 62

48 theCaTmethod.Experimentsby C04 and Kochetal. ( 2006 )haveshownthatage effectsbroughtaboutbyusinganinappropriate V HB foranygivenRGBstarwill typicallycauseerrorsin[Fe/H]ontheorderof 0.05dex,buttheseerrorscan, inextremecases,beaslargeas 0.1dex.Onecanavoidthistypeofuncertainty byobservingpopulousclusters,sincethisallowsthecorre lationofagivenRGB startoaspecicHB/RC,whichiscomposedofstarsoftheappr opriateageand, therefore,hasawell-denedmeanmagnitude.However, DaCosta&Hatzidimitriou ( 1998 )stillhadtoaddresstheissueofageeffectsfortheirsampl eofSMCclusters duetothefactthatmanyoftheirtargetclusterswereconsid erablyyoungerthanthe GalacticglobularclustersusedintheCaTcalibrationof DaCosta&Armandroff ( 1995 );thus,theysoughttocorrectforthedifferenceinagebetw eenthetargetand calibrationclusters.Usingadoptedclusterages,alongwi ththeoreticalisochrones, DaCosta&Hatzidimitriou ( 1998 )estimatedthechangein V HB fromtheoldtothe youngpopulations,therebycreatingage-correctedmetall icitiesfortheirtargets.Their correctionswereoftheorderof0.05dex,whichissmallerth antheprecisionofthe abundances.Incontrastto DaCosta&Hatzidimitriou ( 1998 ),wehavemadeno attempttocalculateanyagecorrectionsforthefollowingr eason.WeusetheCaT calibrationof C04 ,whichisbasedonasampleofbothglobularandopenclusters coveringawiderangeofagesandmetallicities.Withtheinc lusionofyoungerclusters, thevariationof V HB withageisbuiltintotheCaTcalibration,specicallyinEq 3–2 andthesteepervaluefor b thanwhathasbeenfoundbyauthorsonlyconsidering globularclusters.Thus,agecorrectionsarenotrequiredf orourabundancedata. Finally, Rutledgeetal. ( 1997a )showedthatforMWglobularclustersthereisa linearrelationshipbetweenacluster'sreducedequivalen twidthanditsmetallicityon the Carretta&Gratton ( 1997 )abundancescale. C04 extendedthiscalibrationtocover alargerrangeofages(2.5Gyr < age < 13Gyr)andmetallicities( 2 < [ Fe = H ] < 0.2)thanpreviousauthors,and,becausetheircalibration iscloserinparameterspace

PAGE 63

49 toourclustersample,weadopttheirrelationship,where [ Fe = H ]=( 2 : 966 0 : 032 )+( 0 : 362 0 : 014 ) W 0 : (3–3) Wenotethat,whilethiscalibrationactuallycombinestwom etallicityscales ( Carretta&Gratton1997 fortheglobularclustersand Frieletal.2002 fortheopen clusters), C04 ndnoevidenceofageeffectsonthecalibrationoranysigni cant deviationfromalinearttosuggestthatthesetwopopulati onsarenotultimatelyon thesame[Fe/H]scale(seetheirFigure4).Althoughsomeofo urclustersarelikely youngerthanthe2.5Gyragelimitestablishedinthecalibra tionof C04 ,theCaTline strengthsforredgiantsof 1Gyrarenotexpectedtodeviatestronglyfromasimple extrapolationofthettingformula(basedontheempirical ttingfunctionsfrom Cenarroetal.2002 appliedtoisochronespublishedin Girardietal.2000 ),soweuse theabovecalibrationforallofourclusters. 3.3Analysis Asmentionedin x 3.2.5 ,knowledgeoftherelativebrightnessofeachtargetstar andtheclusterHBisimperativetotheaccuratecalculation of W 0 andthus[Fe/H]for eachstar.Todetermine V V HB weutilizedthepreimagesnecessaryforcreatingthe slitmasksusedbyFORS2.Small-aperturephotometrywasper formedonthese V -and I -bandimagessoastoallowustocreateclusterCMDsbelowthe corehelium-burning RCstars.Fortheyoungerclustersinoursample, V HB wasmeasuredasthemedian magnitudeofclusterRCstars.Clusterstarswereisolatedf romtheeldbyselecting starswithintheinnerhalfoftheapparentclusterradius.W ethenplacedastandardsizedbox(0.8magin V and0.2magin V I )aroundeachclusterRCandused onlythestarswithinthisboxinourcalculationof V HB .Regardingclusterswithbona deHBs,i.e.oldclusters,wecomparedourinstrumentalpho tometrytopublished photometryandcalculatedaroughzeropointforourdata,al lowingtheconversion ofpublished V HB valuesontoourinstrumentalsystem.Literaturesourcesfo rtheve

PAGE 64

50 oldclustersareasfollows:NGC1841, Alcainoetal. ( 1996 );NGC2019, Olsenetal. ( 1998 );NGC2257andHodge11, Johnsonetal. ( 1999 );andReticulum, Walker ( 1992a ).Errorsin V HB aretakenasthestandarderrorofthemedianforclustersin whichwemeasuredtheRCdirectly;fortheHBintheoldcluste rsweadopt0.1mag. Wenotethatalthoughwehavenotcalibratedourphotometryo ntoastandardsystem, the V I colortermfortheFORS2ltersystemisexpectedtobesmall( < 0.02mag), thushavinglittleeffectontherelativebrightnessesofou rtargetstarsoverthesmall rangeofcolorscoveredbytheRGB.3.3.1ClusterMembership Weuseacombinationofthreecriteriatoisolateclustermem bersfromeld stars.Thisprocessisidenticalforallclusters,soweillu stratetheprocessusing Hodge11.First,theclustercentersandradiiarechosenbye ye,basedprimarilyon thephotometriccatalog.Asanexample,Fig. 3–3 shows xy -positionsforallstars photometeredintheHodge11eld,withlargelledsymbolsd enotingourtargetstars andthelargeopencirclerepresentingtheadoptedclusterr adius;targetstarsmarked inblue(seegurelegendsforadiscussionofthecolorcodin gusedinFigs. 3–3 through 3–5 andFig. 3–7 )areconsiderednon-membersduetotheirdistancefromthe clustercenter.Wenotethatstarsoutsideoftheclusterrad iuswereobservedsoasto deneparametersfortheLMCeld,whichaidsinisolatingcl ustermembers.Next, radialvelocityversusdistanceisplottedinFig. 3–4 .Starsmovingatthevelocityof Hodge11areeasilyidentiedduetotheirsmallervelocityd ispersionandlowermean velocitythanthatoftheeldstars.Ourvelocitycut,denot edbythehorizontallines, hasbeenchosentorepresenttheexpectedobservedvelocity dispersionineachcluster. Todeterminethis,wehaveadoptedanintrinsicclustervelo citydispersionof5km s 1 andaddedthisinquadraturewithouradoptedradialvelocit yerror,7.5kms 1 whichresultsinanexpecteddispersionof 9kms 1 .Thus,wehaveroundedthis upandadoptedawidthof 10kms 1 forourradialvelocitycut.Theclusterradius

PAGE 65

51 Figure3–3:The xy positionsofourtargetstars(largelledsymbols)intheHo dge11 eld.Theadoptedclusterradiusismarkedbythelargeopenc ircle,and starsoutsideofthisradiusareconsiderednonmembers.The colorcoding ofsymbolsinFigs. 3–3 through 3–5 andFig. 3–7 isasfollows:blue pointsrepresentnonmembersthatareoutsidetheclusterra dius;tealand greensymbolsrepresentnonmembersthatwerecutbecauseof discrepant radialvelocitiesandmetallicities,respectively;and, nally,redsymbols denoteclustermembers.

PAGE 66

52 Figure3–4:Radialvelocitiesforourspectroscopictarget sinHodge11,plottedasa functionofdistancefromtheclustercenter.Thehorizonta llinesrepresent ourvelocitycutandhaveawidthof 10kms 1 .Theclusterradiusis shownbytheverticalline,andthecolorcodingofsymbolsis discussed inFig. 3–3 .Theerrorbarsrepresenttherandomerrorindeterminingth e radialvelocityforeachstar,wherewehaveaddedinquadrat uretheslit centeringandcross-correlationerrors. (Fig. 3–4 ,verticalline)ismarkedforreference.Finally,Fig. 3–5 showsmetallicityas afunctionofdistanceforthestarsinHodge11,withhorizon tallinesrepresentingthe [Fe/H]cutthathasbeenappliedtothesedata.Forthestarsi nsixofourclusterswe haveprocessedbothsetsofspectraandcomparedthetwo[Fe/ H]measurementssoas

PAGE 67

53 Figure3–5:Hodge11targetstarmetallicities.Metallicit iesareplottedasafunctionofdistanceforalltargetstarsinHodge11.The[Fe/H]c utof 0.20 dexisdenotedbythehorizontallines.Forthisold,metal-p oorcluster, theeld([Fe/H] 0 : 5)iseasilydistinguishedfromthecluster(red symbols).WenotethatthecolorcodingisthesameasinFig. 3–3 .The plottederrorbarsrepresenttherandomerrorincalculatin g[Fe/H],where wehavepropagatedtheerrorinmeasuringtheequivalentwid thsthrough ourcalculations. todirectlydeterminethemetallicityerrorforeachstar.B asedonthesedatawend s [ Fe = H ] 0.15dex,whichweadoptastherandomerrorin[Fe/H]foreach star.We haveroundedthisupto 0.20dexforuseasthemetallicitycutshowninFig. 3–5

PAGE 68

54 Figure3–6: S W vs. V V HB forHodge11;onlystarsconsideredtobeclustermembersareplotted.Thedashedlineisanisoabundancelineatt hemean metallicityofthecluster,[Fe/H]= 1.84,andhasaslope b =0.73. Redsymbolsdenotestarsthathavemadeallthreecutsandare thereforeconsidered tobeclustermembers.Sincewehadnoapriorimembershipinf ormation,uptothis pointwehaveusedavaluefor V HB thatwasderivedfromtheentireeld,ratherthan justthecluster.Thus,wehaverecalculated W 0 (and[Fe/H])usingtheappropriate cluster V HB value.InFig. 3–6 wepresentthetraditional S W versus V V HB plotfor clustermembers,withthedashedlinerepresentingthemean metallicityofHodge

PAGE 69

55 Figure3–7:CMDfortheentireHodge11eld,withtargetstar smarkedasdescribed inFig. 3–3 ;clustermembersliealongtheRGBandAGB. 11.TheCMDinFig. 3–7 showsallstarsphotometeredintheHodge11eld;cluster members(redsymbols)lieontheRGBandasymptoticgiantbra nch(AGB).Figures 3–15 through 3–68 presenttheclustermemberselectionplotsoftheremaining clusters, inthefollowingformat:eachclusterissplitovertwogure s,withtherstgurein eachpair(oddnumberedgures)showingthe xy -positionsofthetargetstars(upperleft panel),heliocentricradialvelocityand[Fe/H]versusdis tancefromtheclustercenter (upperrightandlowerleft,respectively),andnally, S W versus V V HB forallcluster

PAGE 70

56 members(lowerright).Thesecondgureforeachcluster(ev ennumbers)showsthe CMDforallstarsineachpointing(clusterandeldstars),w iththespectroscopic targetsmarked,usingthecolorcodingasdiscussedinFig. 3–3 .Wenotethatthese guresarelocatedattheendofthischapter. InTable 3–6 ,alsolocatedattheendofthischapter,forallstarsdeterm ined tobemembersoftheobservedLMCclusters,welistthefollow inginformation: stellaridenticationnumber,rightascensionanddeclina tion(asdeterminedfromthe preimages),heliocentricradialvelocityanditsassociat ederror, V V HB ,and S W alongwiththeerrorinmeasuring S W .Althoughwedonotdiscusstheeldstars, forcompleteness,inTable 3–7 wepresentourmeasuredvaluesforalleldstars.In thistable,“primary”referstoallstarsthatfellonthesam eFORS2chipasthetarget cluster,butwerefoundtobenon-membersoftheclusterand“ secondary”denotesthe starsthatwereobservedonthenon-clusterarray.Wenoteth atHodge11andSL869 wereobservedinthesamepointing,withHodge11ontheprima ryarrayandSL869 onthesecondary,hencethelackofsecondaryeldslistedfo reithercluster. 3.3.2ClusterProperties ClusterpropertiesderivedfromourdataarepresentedinTa ble 3–3 ,withthe numberofclusterstarsgivenincolumn2,themeanheliocent ricradialvelocities andmeanmetallicitiesincolumns3and5,andtheirrespecti vestandarderrorofthe meanvaluesincolumns4and6.FortheclustersSL4,SL41,SL3 96,Hodge3,SL 663,andSL869,wereporttherstspectroscopicallyderive dmetallicityandradial velocityvaluesbasedonindividualstarswithintheseclus ters.Inaddition,NGC1718 andNGC2193havenopreviouslyreportedspectroscopic[Fe/ H]values;however, OSSH derivedvelocitiesforthesetwoclusters.Oftheseeightcl usters,NGC1718 occupiesaparticularlyinterestingareaofparameterspac e,asitisthemostmetal-poor ofourintermediate-ageclusters,withametallicitycompa rabletothatofESO121(see

PAGE 71

57 Table3–3.DerivedLMCClusterProperties Cluster n StarsRV s RV [Fe/H] s [ Fe = H ] Name(kms 1 )(kms 1 )(dex)(dex) SL45227.13.6-0.510.06Reticulum13247.51.5-1.570.03NGC16519228.22.3-0.530.03NGC16527275.71.3-0.460.04NGC184116210.30.9-2.020.02SL416229.31.3-0.440.03SL618221.92.0-0.350.04NGC17183278.42.2-0.800.03NGC17516245.42.1-0.440.05NGC184617235.20.9-0.490.03NGC1861...............SL3965225.21.1-0.390.05NGC19428293.72.3-0.500.04NGC20195280.62.3-1.310.05Hodge47310.81.9-0.550.06Hodge37277.40.8-0.320.05IC214618226.30.6-0.410.02SL6638301.41.5-0.540.05NGC212112232.51.2-0.500.03NGC21736237.40.7-0.420.03NGC21557309.11.6-0.500.05NGC21625322.63.5-0.460.07NGC22039245.51.4-0.410.03NGC21935291.22.0-0.490.05NGC22136242.71.2-0.520.04Hodge1112245.11.0-1.840.04SL8693258.42.1-0.400.04NGC22319277.61.4-0.520.03NGC225716301.60.8-1.590.02 discussionin x 3.6.1 ).Asmentionedpreviously,wehavenotderivedvaluesforNG C 1861,sinceitappearstobeyoungerthan1Gyr(see x 3.6.3 ). 3.3.2.1Metallicities PositionsontheskyforeachclusterareshowninFig. 3–8 ,alongwiththe metallicitybinintowhicheachclusterfalls,represented bythecoloroftheplotting symbol.Fortwoofthehighermetallicitybins(orangeandgr eensymbols),thebin sizeisroughlytwicethestandarderrorin[Fe/H],soitispo ssiblethatclustererrors could“move”clustersbetweentheseandadjacentbins.Thea doptedcenterofthe LMC( a = 5 h 27 m 36 s d = 69 52 0 12 00 ; vanderMareletal.2002 )ismarkedby thelledsquare,andthedashedovalrepresentsthe2 near-infraredisoplethfrom vanderMarel ( 2001 ),whichroughlyoutlinesthelocationoftheLMCbar.Conver sion fromrightascensionanddeclinationtoCartesiancoordina teswasperformedusing azenithalequidistantprojection(e.g., vanderMarel&Cioni2001 ,theirequations

PAGE 72

58 1 4);forreference,linesofrightascensionanddeclination aremarkedwithdotted lines.InFigs. 3–9 and 3–10 wefurtherexplorethemetallicity-positionrelationship forLMCclustersbyplottingmetallicityasafunctionofdep rojectedpositionangle andradialdistance(inkiloparsecs),respectively.Wehav ecorrectedforprojection effectsbyadopting34 : 7astheinclinationand122 : 5forthepositionangleofthe lineofnodesoftheLMC( vanderMarel&Cioni2001 ).Inthisrotatedcoordinate system,aclusterwithapositionangleofzeroliesalongthe lineofnodes,andangles increasecounterclockwise;forreference,NGC2019hasapo sitionangleof 8 Radialdistanceswereconvertedfromangularseparationto kiloparsecsbyassuming anLMCdistanceof ( m M ) 0 = 18 : 5( 50kpc);atthisdistance,1 is 870pc. Combined,thesethreeguresillustratethat,similartowh atwasfoundby OSSH (and Geisleretal.2003 ),thereisno[Fe/H]gradientintermsofeitherpositionang leor radialdistanceforthehighermetallicityclustersinours ample.Whilewecannotmake strongcommentsonthemetal-poorclustersduetooursmalls amplesize,itiswell knownthatanumberofmetal-poorclusters([Fe/H] < 1 : 5)existintheinnerportions oftheLMC(e.g., OSSH ),suggestingthatneitherthePopulationInorthePopulati onII clustersexhibitametallicitygradient. InFig. 3–10 wehaveoverplottedboththeMWopenclustermetallicitygra dient from Frieletal. ( 2002 ,dashedline)andtheM33gradientfrom Tiedeetal. ( 2004 solidline).Neitherofthesediskabundancegradientsrese mbleswhatweseeamong theLMCclusters.Thequestionofhowtointerpretthisdiffe rencetakesustothe workof Zaritskyetal. ( 1994 ).TheystudiedtheH II regionoxygenabundancesin 39diskgalaxies.Theirdatasuggestthatdiskabundancegra dientsareubiquitousin spiralgalaxies.However,thepresenceofaclassicalbarin thegalaxy onethat extendsoverasignicantfractionofthedisklength tendstoweakenthegradient. ThisobservationseemstondsupportintheappearanceofFi g. 3–10 .Inthecaseof theLMC,thepresenceofastrongbarcomponentmayhavedilut edthemetallicity

PAGE 73

59 Figure3–8:Positionsontheskyandderivedmetallicitiesf orourtargetclusters. Metallicitybinsaregiveninthelowerleftcorneroftheplo t.Theadopted LMCcenterismarkedwiththelledsquare,andthedashedlin eroughly outlinesthebar.See x 3.3.2 foradetaileddiscussion.

PAGE 74

60 Figure3–9:Clustermetallcityvs.positionangle.Weplott hemetallicitiesofour targetclustersasafunctionofdeprojectedpositionangle ,wherewehave usedtheLMCgeometryof vanderMarel&Cioni ( 2001 )tocorrectfor projectioneffects.Thisplotillustratesthatthereisnoa pparentrelation betweenpositionangleandmetallicityintheLMC.Theerror barshown inthelowerleftcorneroftheplotillustratestheaverager andomerrorin [Fe/H].

PAGE 75

61 Figure3–10:Clustermetallicityvs.radialdistance.Clus termetallicitiesareplottedas afunctionofdeprojecteddistance(inkpc)fromthecentero ftheLMC. Wehaveassumedadistanceof ( m M ) 0 = 18 : 5.Overplottedarethe metallicitygradientsobservedintheMWopenclusters(das hedline; Frieletal.2002 )andM33(solidline; Tiedeetal.2004 ),whichhelp tofurtherillustratethattheLMC'sclustersystemlacksth emetallicity gradienttypicallyseeninspiralgalaxies.Thisattenedg radientislikely causedbythepresenceofthecentralbar( Zaritskyetal.1994 ).Asin Fig. 3–9 ,theaveragerandomerrorisillustratedbytheerrorbaront he lowerleft.

PAGE 76

62 gradientoriginallypresentinthestarclusters,leadingt oaclusterpopulationthat iswellmixed.Wenotethatthisresultisalsoconsistentwit htheconclusionof Pageletal. ( 1978 ),whofoundlittleevidenceforagradientinoxygenabundan cebased onasurveyofH II regionswithin4kpcoftheLMCcenter.ThePagelresult,that d log(O/H)/ d R = 0 : 03 0 : 02dexkpc 1 ,parallelsournon-detectionofagradientin clustermetallicities.3.3.2.2Kinematics Tocharacterizetherotationoftheirclusters, Schommeretal. ( 1992 )tan equationoftheform V ( q )= V m f [ tan ( q q 0 ) sec i ] 2 + 1 g 0 : 5 + V sys (3–4) totheirradialvelocitydatausingaleast-squarestechniq uetoderivethesystemic velocity( V sys ),theamplitudeoftherotationvelocity( V m ),andtheorientationofthe lineofnodes( q 0 );theyadoptedaninclinationof27 .Theirbest-tparametersgive arotationamplitudeanddispersionconsistentwiththeLMC clustershavingdisk-like kinematics,withnoindicationsoftheexistenceofapressu resupportedhalo.Wenote that,duetothenon-circularityoftheLMC, q 0 inEq. 3–4 isnotthetrueorientationof thelineofnodes(theintersectionoftheplaneoftheskyand theplaneoftheLMC), butratheritmarksthelineofmaximumvelocitygradient( vanderMarel&Cioni 2001 ).Morerecently, vanderMareletal. ( 2002 )usedvelocitiesof1041carbon starstostudykinematicsintheLMC.Similarly,theyfoundt hatthesestarsexhibita disk-likerotationwith V = s =2.9 0.9,suggestingthatthesestarsresideinadiskthat isslightlythickerthantheMWthickdisk( V = s 3.9). InFig. 3–11 wehaveplottedgalactocentricradialvelocityversusposi tion angleontheskyforoursample,alongwithvelocitydatafora llclusterslistedin Schommeretal. ( 1992 ).Tobeconsistentwiththeapproachof Schommeretal. ( 1992 ),wehaveadoptedthegalactocentricvelocitycorrections calculatedby

PAGE 77

63 Feitzinger&Weiss ( 1979 ).Additionally,forthisgureonly,wehaveadopted theirLMCcenter( a = 5 h 20 m 40 s d = 69 14 0 10 00 ;J2000.0)foruseincalculatingthe positionanglesofourclusters.Wehaveusedthestandardas tronomicalconventionin whichnorthhasapositionangleofzeroandanglesincreaset otheeast;NGC1942has apositionangleof 4 inthiscoordinatesystem.Datafrom Schommeretal. ( 1992 ) areplottedasopencircles,andourdataareplottedaslled starsfortheclusterswith previouslyunpublishedvelocitiesandlledcirclesforth eremainderofourclusters; overplottedonthisgure(dashedline)istherotationcurv esolutionnumber3from Schommeretal. ( 1992 ).Fortheclustersincommonbetweenthesetwodatasets, wendexcellentagreement,withameanoffsetof0.15kms 1 ,whereourvelocities arefasterthanthoseof Schommeretal. ( 1992 ).Additionally,thederivedvelocities forthesix“new”clustersshowthattheirmotionsareconsis tentwiththendingsof Schommeretal. ( 1992 )inthattheLMCclustersystemexhibitsdisk-likekinemati cs thatareverysimilartotheH I diskandhasnoobvioussignatureofastellarhalo. 3.4ComparisonwithPreviousWork Asmentionedin x 3.1 OSSH and Suntzeffetal. ( 1992 )haveprovidedtheonly previouslarge-scale,spectroscopic[Fe/H]calculations forclustersintheLMC.Similar toourwork,theyutilizedtheCaTlinesasaproxyformeasuri ngFeabundance directly,butwithtwoimportantdifferences:theyusedthe absolutemagnitudeoftheir stars,basedonthespectralintensityat8600 A,asasurfacegravityestimatorinstead of V V HB ,andtheir[Fe/H]calibrationwasbasedlargelyonthe Zinn&West ( 1984 ) metallicitysystem,withtheadditionoftwoopenclusterst hathavemetallicitiesderived fromvariousspectrophotometricindices(seetheirTable7 ).Thisintroducedtwo systematicoffsetsthatmakeitinappropriatetodirectlyc omparethe OSSH valuesto ourworkandotherrecentlymeasuredCaTabundances:rst,t heuseof M 8600 creates adependenceontherelativedistancesofthecalibratingcl ustersandtheLMC,andthe globularclusterdistancescalehasbeenmuchrevisedinthe postHipparcos era( Reid

PAGE 78

64 Figure3–11:Clusterradialvelocityvs.positionangle.Ga lactocentricradialvelocities asafunctionofpositionangleontheskyareplottedforthec lustersin oursample(lledsymbols)aswellasthosefrom Schommeretal. ( 1992 opencircles).Thesixclustersinoursamplewithnopreviou svelocity determinationsareplottedaslledstarsandallothersino ursampleare lledcircles.Rotationcurvesolutionnumber3from Schommeretal. ( 1992 )isoverplottedasthedashedline,showingthatbothdatase tsare consistentwithcircularrotation.Wenotethatwehavenotp lottedarepresentativeerrorbarsinceourplottingsymbolsareroughl ythesamesize astheaveragerandomvelocityerror.

PAGE 79

65 1999 ).Second,ithasbeenshown(e.g., Rutledgeetal.1997a )thattheZinn&West scaleisnon-linearcomparedtothemorerecent Carretta&Gratton ( 1997 )scalebased onhigh-resolutionspectraofglobularclusterredgiants. Toputtheworkof OSSH on theCarretta&Grattonsystem, Coleetal. ( 2005 )performanon-linearleast-squarest tocalibrationclustersincommonwiththeirworkandthatof OSSH .Theyndthat onecanestimatetheabundanceof OSSH clustersonthemetallicitysystemwehave usedviathefollowingconversion: [ Fe = H ] 0 : 212 + 0 : 498 [ Fe = H ] OSSH 0 : 128 [ Fe = H ] 2OSSH : (3–5) Thisequationapproximatesthemetallicitythat OSSH would havederivedfromtheir spectroscopicdataandcalibrationprocedurebutwithupda tedmetallicitiesfortheir calibrationclusters;itdoesnotattempttoaccountforany otherdifferencesinthe treatmentofthedata. Incolumns3and4ofTable 3–4 welist[Fe/H]forclustersin OSSH and Suntzeffetal. ( 1992 )incommonwithourtargetclusters,wherecolumn3gives theirpublishedvaluesandincolumn4wehaveconvertedthei rnumbersontoour metallicitysystemusingEq. 3–5 .Thenumberofstarsusedby OSSH incalculating nalclustermetallicitiesisgiveninparenthesesincolum n4,andourderivedmetallicitiesaregivenincolumn2forreference.InFig. 3–12 weplotthedifferencebetween ourmetallicitiesandtheirconverted[Fe/H]valuesasafun ctionofourmetallicities. OSSH givetheir[Fe/H]errorsforanindividualstaras0.2dex;th erefore,deviations betweenthesedatasetsaslargeas 0.2arenotunexpected,suggestingthatthese resultsareinrelativeagreement,withnooffset.Wenote,h owever,thatevenwiththe useofEq. 3–5 ,itisverydifculttodirectlycomparethederivedcluster abundances becauseofthedifferencesintargetselectionandcalibrat ionstrategy. Whileadirectcomparisonof[Fe/H]valuesisdifcult,weca nreadilycompare themetallicitydistributionsofthesetwodatasets.Assuc h,inFig. 3–13 wehave

PAGE 80

66 Table3–4.PublishedLMCClusterMetallicities Cluster[Fe/H][Fe/H] a [Fe/H] b [Fe/H] Name(ThisWork)CaTCaTHigh-Res. SL4 0 : 51......... Reticulum 1 : 57 1 : 71 c 1 : 44 d (9)... NGC1651 0 : 53 0 : 37 0 : 41(0.5)... NGC1652 0 : 46 0 : 45 0 : 46(2)... NGC1841 2 : 02 2 : 11 c 1 : 83 d (8) 2 : 07 e SL41 0 : 44......... SL61 0 : 35 0 : 50 0 : 49(1)... NGC1718 0 : 80......... NGC1751 0 : 44 0 : 18 0 : 31(0.5)... NGC1846 0 : 49 0 : 70 0 : 62(1)... SL396 0 : 39......... NGC1942 0 : 50 + 0 : 16 0 : 14(1)... NGC2019 1 : 31 1 : 81 1 : 53(1) 1 : 24 f (3) Hodge4 0 : 55 0 : 15 0 : 29(1)... Hodge3 0 : 32......... IC2146 0 : 41 0 : 40 0 : 43(2)... SL663 0 : 54......... NGC2121 0 : 50 0 : 61 0 : 56(1.5)... NGC2173 0 : 42 0 : 24 0 : 34(1)... NGC2155 0 : 50 0 : 55 0 : 52(2.5)... NGC2162 0 : 46 0 : 23 0 : 33(2)... NGC2203 0 : 41 0 : 52 0 : 51(2)... NGC2193 0 : 49......... NGC2213 0 : 52 0 : 01 0 : 22(1)... Hodge11 1 : 84 2 : 06 1 : 78(2) 2 : 13 f (2) SL869 0 : 40......... NGC2231 0 : 52 0 : 67 0 : 60(1.5)... NGC2257 1 : 59...... 1 : 86 e a FromOSSH,unlessotherwisenoted. b FromOSSH,unlessotherwisenoted,convertedontooursyste m usingEq. 3–5 c FromSuntzeffetal.(1992). d FromSuntzeffetal.(1992),convertedontooursystemusing Eq. 3–5 e FromHill(2004) f FromJohnsonetal.(2006)

PAGE 81

67 Figure3–12:Metallicitycomparisonwith OSSH .Wecomparederivedmetallicitiesfor clustersincommonbetweenourstudyandthatof OSSH .Wenotethat [Fe/H]valuesfrom OSSH wereconvertedontothemetallicityscalewe haveusedviaEq. 3–5 .Thiscomparisonshowsthat,towithintheerrors, thereisrelativelygoodagreementbetweenourresultsandt hoseof OSSH (see x 3.4 formoredetails). plottedthemetallicitydistributionof OSSH 'srawdata(toppanel),converted[Fe/H] values(middlepanel),andourresults(bottompanel).Thed arkshadedhistogram showsonlythe20clustersincommonbetweenthethreepanels ,whilethelighter histogramplotsalltheclustersineachsample.Fromthisg ureitisclearthatboth

PAGE 82

68 therawandconverted OSSH samplesshowanextendeddistributionofintermediatemetallicityclusters,whereasourclustersampleexhibits averytightdistribution.For the20clustersincommon,wendamean[Fe/H]= 0 : 47with s = 0 : 06,while theconverted OSSH metallicitiesgive[Fe/H]= 0 : 42 0 : 14.Ourtightmetallicity distribution,withalackofhighermetallicityclusters([ Fe/H] > 0 : 30),isanimportant featureofourdataforthefollowingreason.Chemicalevolu tionmodelssuggestthat metallicityisaroughestimatorofage,inthatyoungerstel larpopulationsshouldbe moremetal-richthanolderpopulations,sincetherehasbee nmoretimetoprocess materialandenrichtheinterstellarmedium.Thus,interme diate-ageclustersshould bemoremetal-poorthanyoungerstellarpopulationsintheL MC.However,some intermediate-ageclustersinthesampleof OSSH appearedtobemoremetal-richthan muchyoungerstellarpopulationsintheLMC,whichwouldind icatethepresence ofalargespreadofmetallicitiesatanygivenage.InTable 3–5 wegivethemean metallicityandspreadofourentiresampleofintermediate -ageclustersandallclusters in OSSH withconvertedmetallicitiesabove 1 : 0dex,alongwithpublishedresultsfor asampleofyoungerstellarpopulations(e.g.,Bdwarfs, Rollestonetal.2002 ;Cepheid variables, Lucketal.1998 ;youngredgiants, Smithetal.2002 )andintermediateage RGBeldstarsintheLMCbar( Coleetal.2005 ).Thistableshowsthat,aswewould expectfromchemicalenrichmentmodels,theintermediateageclustersareslightly moremetal-poorthantheyoungerpopulationsintheLMC.Thu s,themuchtighter metallicitydistributionseeninourclustersisinexcelle ntagreementwiththeexpected chemicalenrichmentpatternintheLMCandalleviatesthepr oblemcreatedbythehigh metallicitytailofintermediate-ageclustersinthe OSSH results.Inaddition,Table 3–5 showsthatourintermediate-ageclustershaveameanmetall icityanddistribution similartothatofthemetal-richcomponentofthebareldst udiedby Coleetal. ( 2005 ).Thesimilaritybetweenthesetwopopulationsisingoodag reementwiththe modelsof Bekkietal. ( 2004 ),inwhichtheformationoftheLMCbarandtherestart

PAGE 83

69 ofclusterformation(theendoftheagegap)arebotharesult ofthesameveryclose encounterwiththeSMC. Finally,inTable 3–4 wehavealsoincluded[Fe/H]valuesderivedfromhighresolutionspectraforNGC1841andNGC2257from Hill ( 2004 )andNGC2019 andHodge11from Johnsonetal. ( 2006 ).Forthetwoclustersfrom Johnsonetal. ( 2006 ),welist[Fe/H]valuesthataretheaverageoftheirmetalli citiesdeterminedfrom Fe I andFe II lines,andthenumberofstarsobservedineachclusterisgiv en.Two clusters,NGC1841andNGC2019,showgoodagreementbetween ourmetallicities, calculatedfromtheCaTlines,andmetallicitiesderivedfr omttingtohigh-resolution spectra.Incontrast,Hodge11andNGC2257showaroughly0.3 dexoffsetbetween thesemethodsinthesensethatourvaluesaremoremetal-ric hthantheresultsfrom high-resolutionspectra.Similarly,apreliminaryresult forESO121,whichismore metal-richthantheaforementionedclusters,suggestsano ffsetinthesamedirection, wheretheCaTmethodgivesa[Fe/H]valuehigherthanwhatism easuredwithhighresolutionspectra(A.A.Cole,privatecommunication).It hasbeensuggestedthat variationsin[Ca/Fe]betweencalibratingclustersintheM Wandtargetclustersin theLMCmaycauseabreakdownintheutilityofCaTlinesasame tallicityindicator. However,abundancesbasedonhigh-resolutionspectrashow that[Ca/Fe]istypically lowerforLMCclustergiantsthanforMWgiantsofthesame[Fe /H],whichisinthe oppositedirectionofwhatisneededtoexplainthedifferen cebetweenCaTandhighresolutionresults.Wealsonotethat,forlow-metallicity stars,previousauthorshave shownthatmetallicitiesderivedfromhigh-resolutionspe ctracanvaryconsiderably(0.3 dexisnotuncommon),dependingonwhichionizationstages, whattemperaturescale, andwhatmodelatmospheresarebeingused(e.g., Johnsonetal.2006 ; Kraft&Ivans 2003 ).

PAGE 84

70 Figure3–13:MetallicitydistributionofLMCclustersasde terminedby OSSH and thispaper.Publishedvaluesfrom OSSH aregiveninthetoppanel,while themiddlepanelshowstheirvaluesconvertedontoourmetal licityscale usingEq. 3–5 ;inthebottompanelwehaveplottedourresults.Inall threepanels,thedarkshadedregionshowsthedistribution forthe20 clustersincommonbetween OSSH andthispaper,whilethelightshaded regionshowstheentireclustersamplefromeachstudy.Ourr esultsindicatethattheLMC'sintermediate-ageclustermetallicit ydistributionis actuallymuchtighterthansuggestedbytheresultsof OSSH

PAGE 85

71 Table3–5.MetallicitiesofYoungandIntermediate-AgeSte llarPopulations PopulationAgeEstimate[Fe/H] s [ Fe = H ] Reference (Myr) Bdwarfs < 20 0 : 310.04Rollestonetal.(2002) Cepheidvariables10 60 0 : 340.15Lucketal.(1998) YoungRGBstars200 1000 0 : 450.10Smithetal.(2002) Intermediate-ageclusters1000 3000 0 : 480.09Thispaper Intermediate-ageclusters1000 3000 0 : 480.17OSSH BarRGBstars,metal-rich1000 5000 0 : 370.15Coleetal.(2005) BarRGBstars,metal-poor > 5000 1 : 080.47Coleetal.(2005) 3.5Summary Asdiscussedin x 3.1 ,determiningabundancesforpopulousclusterswithin theLMCisanimportantstepinunderstandingthehistoryoft hissatellitegalaxy. Accurate[Fe/H]valueshelptobreaktheage-metallicityde generacythatariseswhen tryingtottheoreticalisochronestoclusterCMDs,whicha llowstheunequivocal determinationofclusterages,therebyprovidingaclearpi ctureoftheLMC'scluster age-metallicityrelation.Theseclustersalsoservetoll aregionoftheage-metallicity planethatisvoidofMWclusters;thismakestheLMCclusters ystemanimportant testbedforavarietyofstellarpopulationmodels.Additio nally,inapreviouspaper Grocholski&Sarajedini2002 ,weshowedthatknowledgeofacluster'sageand metallicityisessentialtopredictingthe K -bandluminosityoftheRCforuseasa standardcandle.Inafutureworkwewillusethemetalliciti esderivedhereinto determinedistancestoindividualpopulousLMCclusters,w hichwillallowusto comparetheclusterdistributiontotheLMCgeometrycalcul atedfromeldstars(e.g., vanderMarel&Cioni2001 ). Inthischapterwehavepresentedtheresultsofourspectros copicstudyofthe near-infraredCa II tripletlinesinindividualRGBstarsin28populousLMCclus ters. Utilizingthemulti-objectspectrograph,FORS2,ontheVLT ,wehavebeenableto determinemembershipandcalculatemetallicitiesandradi alvelocitiesfor,onaverage, eightstarspercluster,withsmallrandomerrors(1.6kms 1 invelocityand0.04dex

PAGE 86

72 in[Fe/H]).Thenumberofclustermembersobserved,combine dwiththeupdated CaTcalibrationof C04 (theyextendedthecalibrationtoyoungerandmoremetalric h clustersthanpreviouswork),hasallowedustoimproveonth eworkof OSSH ,which istheonlypreviouslargescalespectroscopicstudyofindi vidualclusterstarswithinthe LMC.Themainresultsofourpaperareasfollows: 1.Wereporttherstspectroscopicallyderivedmetallicit iesandradialvelocities forthefollowingclusters:SL4,SL41,SL396,SL663,SL869, andHodge3.In addition,NGC1718andNGC2193havenopreviouslyreporteds pectroscopic[Fe/H] values. 2.NGC1718istheonlyclusterinoursamplethatfallsintoth erange 1 : 3 [Fe/H] 0 : 6.Thismetallicityregioncorrespondstothewellknown3 13Gyr“age gap,”withinwhichthereisonlyonecluster,ESO121.Howeve r,unlikeESO121,the CMDofNGC1718suggestsanage( 2Gyr)muchyoungerthantheagegap;we usearchival HubbleSpaceTelescope (HST)WideFieldPlanetaryCamera(WFPC2) photometrytoinvestigatethispointinthe x 3.6.1 .ThisagemakesNGC1718oneof themostmetal-poorintermediate-ageclustersintheLMC. 3.Theintermediate-ageclustersinoursampleshowaveryti ghtdistribution, withameanmetallicityof 0 : 48dex( s =0.09)andnoclusterswithmetallicities approachingsolar.Whilethisisincontrasttopreviousclu sterresults,itsuggeststhat theformationhistoryofthebar(mean[Fe/H]= 0 : 37, s = 0 : 15; Coleetal.2005 ) isverysimilartothatoftheclusters.Thisagreeswellwith thetheoreticalworkof Bekkietal. ( 2004 ),whichindicatesthatacloseencounterbetweentheLMCand SMC causednotonlytherestartofclusterformationintheLMCbu tthegenerationofthe centralbaraswell. 4.Similartopreviouswork,wendnoevidencefortheexiste nceofametallicity gradientintheLMCclustersystem.Thisisinstarkcontrast tothestellarpopulations ofboththeMWandM33,whichshowthatmetallicitydecreases asgalactocentric

PAGE 87

73 distanceincreases;theLMC'sstellarbarislikelyrespons ibleforthewell-mixedcluster system. 5.Wendthatourderivedclustervelocities,includingthe six“new”clusters,are ingoodagreementwiththeresultsof Schommeretal. ( 1992 )inthattheLMCcluster systemexhibitsdisk-likerotationwithnoclustersappear ingtohavehalokinematics. 6.Comparingourresultsforfourclustersto[Fe/H]valuesr ecentlyderived throughhigh-resolutionspectra,wendthattwoofthefour clustersareingood agreement,whiletheothertwohave[Fe/H]valuesderivedvi atheCaTmethodthatare 0.3dexmoremetal-richthanwhatisfoundfromhigh-resolut ionspectra;asimilar effectisseeninpreliminaryresultsforanadditionaltwoL MCclusters.Thesourceof thisdifferenceisunclear,anditisnotimmediatelyexplai nedbyvariationsin[Ca/Fe] betweentheCaTcalibrationclustersintheMWandtheLMCtar getclusters.Further high-resolutionstudies,especiallycoveringtheLMC'sin termediate-ageclusters,are neededtofullyaddressthisissue. 3.6NotesonIndividualClusters 3.6.1NGC1718 WhileonlythreeofthestarsobservedinNGC1718appeartobe clustermembers,thesestarsare,onaverage,0.3dexmoremetal-poorth anallbutoneoftheother starsobservedinthiseld.Asmentionedin x 3.3.2 ,thiscausesNGC1718tooccupy aninterestingpositionintheLMC'sage-metallicityrelat ion;itsmetallicityiscomparabletothatofESO121,whichseemstobetheonlyclusterr esidingintheLMC havinganagebetween 3and13Gyr( DaCosta2002 ).TheclusterCMDresulting fromouraperturephotometryisnotwellpopulatedaroundth eMSTO,sowehave usedarchival HST /WFPC2data(GO-5475)tocreateaclusterCMDreachingbelow the MSTO.Theimageswerereducedusingtheprocedureoutlinedb y Sarajedini ( 1998 ). Insummary,alldetectedstarsonthePlanetaryCameraCCDwe rephotometeredinthe F450WandF555Wltersusingasmallaperture.Thesewerethe ncorrectedtoa0 : 00 5

PAGE 88

74 = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = == = = = = = = = = = = = = = = = = = === = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = == = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = == = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = == = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = == = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = NGC 1718Z = 0.0041.3, 2.0, 2.5 Gyr 0.0 0.5 1.0 1.5 22 20 18 B V V Figure3–14:ClusterCMDforNGC1718,basedonaperturephot ometryofarchival HST/WFPC2images.Weoverplotisochronesof1.3,2.0,and2. 5Gyr (toptobottom)from Girardietal. ( 2002 )thathaveametallicity( 0.7 dex)similartothevaluewehavederivedforthiscluster( 0.8dex). AlthoughthisclusterhasametallicitysimilartothatofES O121,the isochronessuggestanageof 2.0Gyrforthiscluster,leavingESO121 astheonlyknownLMCclusterwithanagebetweenapproximate ly3 and13Gyr.

PAGE 89

75 radius,adjustedfortheexposuretime,andtransformedtot hestandardsystemusing theequationsfrom Holtzmanetal. ( 1995 ).InFig. 3–14 wepresenttheCMDofNGC 1718withisochronesfrom Girardietal. ( 2002 )overplotted;theisochroneshave[Fe/H] 0 : 7,closetoourmeasuredclustervalueof 0.8dex,andagesrangingfrom1.3to 2.5Gyr.ThisguresuggeststhatNGC1718hasanageofroughl y2.0Gyr,making itanintermediate-ageclusterandleavingESO121asstillt heonlyclusterknownto occupytheLMC'sclusteragegap.However,theexistenceofa nintermediate-age clusteratthislowmetallicityisintriguing,asitindicat esthatsomepocketsofunenrichedmaterialmusthaveremainedintacteventhoughmosto fthegasthatformedthe intermediate-ageclusterswaswellmixed.3.6.2NGC1846 GiventheslopedappearanceoftheRCandthewidthoftheRGB( seeFig. 3–28 ), NGC1846issufferingfromdifferentialreddening,makingi tdifculttoaccurately measurethetruelocationoftheclusterRC,aswellas V V HB fortargetstars. Toaddressthisproblem,wemakenoadjustmentstotheinstru mentalmagnitudes, butwemeasurethemedianmagnitudeoftheentiredifferenti allyreddenedRC, effectivelymeasuringtheRCatthemeanreddeningoftheclu ster.Sincetheamountof extinctionsufferedbytheRGBstarsshouldbescatteredabo utthemeanreddening,this approachsmoothsoverthedifferentialreddening,allowin gustoaccuratelymeasure theclustermetallicity.Wenotethatthismethodincreases thescatterin[Fe/H]for clustermembers;assuch,wehaverelaxedthemetallicitycu tinourmemberselection methodtoincludeallstarsmovingattheradialvelocityoft hecluster.Forreference, if V V HB foranygivenstarisoffby 0.2mag(weestimatethatthedifferential reddeningis0.4magin V ),theeffecton[Fe/H]forthatstarisroughly 0.05. 3.6.3NGC1861 ThisclusterislistedasSWBtypeIVB,suggestinganagerang eof0.4 0.8Gyr ( Bicaetal.1996 ),whichisroughlytheageatwhichtheRCrstforms( 0.5Gyr;

PAGE 90

76 Girardi&Salaris2001 ).PlottingaCMDofstarswithintheapparentclusterradius revealswhatappearstobeafairlyyoungMSTOinadditionton oobviousclusterRC orRGB.Therefore,weassumethatNGC1861isayoungclustera ndallobserved RGBstarsareactuallypartofanoldereldpopulation.

PAGE 91

77 Figure3–15:IC2146clustermemberselection.Inthisgure weillustrateourclustermemberselectionprocessforIC2146,usingacombinatio nofa star'sdistancefromtheclustercenter( upperleft ),radialvelocity( upper right ),andmetallicity( lowerleft )toseparateeldstars(blue,teal,and greenpoints)fromclustermembers(redpoints;seetextfor acomplete discussionofthecolorcode).Thelowerrightpanelplotsth esummed equivalentwidthasafunctionof V V HB forallstarsconsideredtobe clustermembers;thedashedlineisanisoabundancelineatt hemean metallicityofthecluster.

PAGE 92

78 Figure3–16:IC2146clusterandeldCMD.Shownistheinstru mentalCMDforthe entireIC2146eld(clusterandsurroundingeldstars),wi ththetarget starsmarkedasdescribedinthetext;clustermembers(redp oints)lie alongtheRGB,AGBorintheRC.

PAGE 93

79 Figure3–17:NGC1651clustermemberselection.SameasFig. 3–15 excepttheplots shownareforNGC1651.

PAGE 94

80 Figure3–18:NGC1651clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireNGC1651eld.

PAGE 95

81 Figure3–19:NGC1652clustermemberselection.SameasFig. 3–15 excepttheplots shownareforNGC1652.

PAGE 96

82 Figure3–20:NGC1652clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireNGC1652eld.

PAGE 97

83 Figure3–21:NGC1718clustermemberselection.SameasFig. 3–15 excepttheplots shownareforNGC1718.

PAGE 98

84 Figure3–22:NGC1718clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireNGC1718eld.

PAGE 99

85 Figure3–23:NGC1751clustermemberselection.SameasFig. 3–15 excepttheplots shownareforNGC1751.

PAGE 100

86 Figure3–24:NGC1751clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireNGC1751eld.

PAGE 101

87 Figure3–25:NGC1841clustermemberselection.SameasFig. 3–15 excepttheplots shownareforNGC1841.

PAGE 102

88 Figure3–26:NGC1841clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireNGC1841eld.

PAGE 103

89 Figure3–27:NGC1846clustermemberselection.SameasFig. 3–15 excepttheplots shownareforNGC1846.

PAGE 104

90 Figure3–28:NGC1846clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireNGC1846eld.

PAGE 105

91 Figure3–29:NGC1942clustermemberselection.SameasFig. 3–15 excepttheplots shownareforNGC1942.

PAGE 106

92 Figure3–30:NGC1942clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireNGC1942eld.

PAGE 107

93 Figure3–31:NGC2019clustermemberselection.SameasFig. 3–15 excepttheplots shownareforNGC2019.

PAGE 108

94 Figure3–32:NGC2019clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireNGC2019eld.

PAGE 109

95 Figure3–33:NGC2121clustermemberselection.SameasFig. 3–15 excepttheplots shownareforNGC2121.

PAGE 110

96 Figure3–34:NGC2121clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireNGC2121eld.

PAGE 111

97 Figure3–35:NGC2155clustermemberselection.SameasFig. 3–15 excepttheplots shownareforNGC2155.

PAGE 112

98 Figure3–36:NGC2155clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireNGC2155eld.

PAGE 113

99 Figure3–37:NGC2162clustermemberselection.SameasFig. 3–15 excepttheplots shownareforNGC2162.

PAGE 114

100 Figure3–38:NGC2162clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireNGC2162eld.

PAGE 115

101 Figure3–39:NGC2173clustermemberselection.SameasFig. 3–15 excepttheplots shownareforNGC2173.

PAGE 116

102 Figure3–40:NGC2173clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireNGC2173eld.

PAGE 117

103 Figure3–41:NGC2193clustermemberselection.SameasFig. 3–15 excepttheplots shownareforNGC2193.

PAGE 118

104 Figure3–42:NGC2193clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireNGC2193eld.

PAGE 119

105 Figure3–43:NGC2203clustermemberselection.SameasFig. 3–15 excepttheplots shownareforNGC2203.

PAGE 120

106 Figure3–44:NGC2203clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireNGC2203eld.

PAGE 121

107 Figure3–45:NGC2213clustermemberselection.SameasFig. 3–15 excepttheplots shownareforNGC2213.

PAGE 122

108 Figure3–46:NGC2213clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireNGC2213eld.

PAGE 123

109 Figure3–47:NGC2231clustermemberselection.SameasFig. 3–15 excepttheplots shownareforNGC2231.

PAGE 124

110 Figure3–48:NGC2231clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireNGC2231eld.

PAGE 125

111 Figure3–49:NGC2257clustermemberselection.SameasFig. 3–15 excepttheplots shownareforNGC2257.

PAGE 126

112 Figure3–50:NGC2257clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireNGC2257eld.

PAGE 127

113 Figure3–51:Reticulumclustermemberselection.SameasFi g. 3–15 excepttheplots shownareforReticulum.

PAGE 128

114 Figure3–52:ReticulumclusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireReticulumeld.

PAGE 129

115 Figure3–53:SL396clustermemberselection.SameasFig. 3–15 excepttheplots shownareforSL396.

PAGE 130

116 Figure3–54:SL396clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireSL396eld.

PAGE 131

117 Figure3–55:SL41clustermemberselection.SameasFig. 3–15 excepttheplots shownareforSL41.

PAGE 132

118 Figure3–56:SL41clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireSL41eld.

PAGE 133

119 Figure3–57:SL4clustermemberselection.SameasFig. 3–15 excepttheplots shownareforSL4.

PAGE 134

120 Figure3–58:SL4clusterandeldCMD.SameasFig. 3–16 excepttheCMDshown isfortheentireSL4eld.

PAGE 135

121 Figure3–59:Hodge4clustermemberselection.SameasFig. 3–15 excepttheplots shownareforHodge4.

PAGE 136

122 Figure3–60:Hodge4clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireHodge4eld.

PAGE 137

123 Figure3–61:Hodge3clustermemberselection.SameasFig. 3–15 excepttheplots shownareforHodge3.

PAGE 138

124 Figure3–62:Hodge3clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireHodge3eld.

PAGE 139

125 Figure3–63:SL61clustermemberselection.SameasFig. 3–15 excepttheplots shownareforSL61.

PAGE 140

126 Figure3–64:SL61clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireSL61eld.

PAGE 141

127 Figure3–65:SL663clustermemberselection.SameasFig. 3–15 excepttheplots shownareforSL663.

PAGE 142

128 Figure3–66:SL663clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireSL633eld.

PAGE 143

129 Figure3–67:SL869clustermemberselection.SameasFig. 3–15 excepttheplots shownareforSL869.

PAGE 144

130 Figure3–68:SL869clusterandeldCMD.SameasFig. 3–16 excepttheCMD shownisfortheentireSL869eld.

PAGE 145

131 Table3–6.PositionsandMeasuredValuesforClusterMember s IDR.A.DeclRV s RV V V HB S W s S W (J2000.0)(J2000.0)(kms 1 )(kms 1 )(mag)( A)( A) SL4 6.........4 h 32 m 41 : s 65-72 20 0 59 : 00 1237.46.7-0.897.650.23 7.........43240.46-722050.3221.66.8-0.647.090.229.........43236.78-722032.2234.56.8-0.706.760.3111........43241.01-722012.3221.06.9-1.077.590.1612........43239.18-722000.4221.36.9-0.037.270.36Reticulum 2.........43613.17-585249.6250.96.8-1.445.370.123.........43614.49-585240.1254.36.8-0.814.340.195.........43618.63-585219.6244.17.30.783.600.486.........43617.19-585209.6249.67.1-0.063.530.338.........43610.73-585146.6245.87.10.363.190.469.........43606.80-585130.3251.36.9-1.034.330.1711........43607.52-585111.5252.87.1-0.534.000.1612........43608.07-585100.6250.07.0-0.524.060.2413........43604.51-585050.6237.06.7-0.554.470.2214........43601.47-585033.3243.16.80.233.880.4715........43609.60-585023.9239.97.1-0.333.660.3116........43602.14-585012.6252.67.30.074.320.3917........43614.10-584957.0245.86.8-1.164.980.12NGC1651 6.........43734.49-703550.2235.46.9-0.396.820.258.........43733.25-703531.7235.56.70.446.730.7910........43737.35-703513.9223.26.80.276.180.4611........43734.63-703505.5233.36.8-1.147.880.4112........43734.76-703502.6225.46.8-0.877.290.1713........43729.94-703453.8223.16.8-0.016.430.3515........43731.06-703435.2237.26.8-1.077.740.1416........43731.29-703425.0219.26.8-2.268.430.1017........43729.58-703416.0221.77.1-0.236.870.26NGC1652 5.........43821.50-684055.4271.36.80.026.850.346.........43823.75-684047.1281.36.9-0.637.680.267.........43823.43-684039.6276.76.80.086.500.528.........43819.31-684030.6272.76.8-0.557.340.199.........43819.84-684020.9276.36.8-1.598.500.1410........43822.03-684011.1273.96.9-0.707.410.2212........43820.33-683946.5277.96.90.176.480.48NGC1841 1.........44405.58-840124.0212.06.8-1.603.510.162.........44452.68-840114.3205.26.9-1.793.880.124.........44527.22-840053.5211.87.0-1.183.720.166.........44541.54-840033.7212.36.9-2.114.250.108.........44442.94-840009.1212.86.9-0.883.410.209.........44519.87-835959.0210.46.8-1.493.550.1810........44533.89-835950.4204.36.8-2.254.240.1211........44510.36-835940.7204.66.8-1.563.800.1512........44515.53-835929.8214.37.1-1.273.620.2313........44543.84-835917.7209.86.9-0.933.460.2214........44512.82-835902.5207.46.9-0.703.300.3015........44528.38-835848.4211.37.4-1.523.710.1216........44520.51-835838.4215.86.8-2.604.640.1017........44622.26-835828.7212.16.9-1.043.180.2019........44555.01-835800.9208.97.0-1.293.600.1920........44522.59-835745.5211.66.8-2.464.000.11

PAGE 146

132 Table3–6—Continued IDR.A.DeclRV s RV V V HB S W s S W (J2000.0)(J2000.0)(kms 1 )(kms 1 )(mag)( A)( A) SL41 4.........44730.78-723555.6223.87.1-0.427.290.315.........44729.32-723542.7229.06.9-0.137.390.206.........44728.06-723532.0228.56.8-0.857.400.157.........44731.86-723522.1230.56.7-1.898.200.118.........44732.39-723514.1230.56.8-2.338.750.109.........44730.92-723506.0233.47.0-0.217.060.22SL61 4.........45049.97-753245.0224.06.8-0.587.580.278.........45044.33-753207.1226.16.8-1.077.500.229.........45048.31-753159.2222.36.6-1.848.550.1810........45034.50-753148.6215.87.00.207.320.6211........45044.07-753141.2227.86.8-1.718.320.1212........45050.74-753131.6225.56.7-0.157.070.4313........45042.63-753122.9211.37.30.147.690.6114........45044.32-753114.1222.86.8-2.419.300.11NGC1718 8.........45227.20-670325.7277.56.7-1.307.030.2510........45227.67-670308.6275.36.7-1.827.340.1113........45220.40-670238.5282.66.8-0.596.260.25NGC1751 5.........45419.46-694918.9242.86.7-2.218.550.116.........45401.54-694909.0237.76.9-1.267.250.218.........45407.81-694848.1249.76.7-2.999.480.1410........45408.30-694820.1252.16.7-1.638.300.1511........45413.67-694807.2244.86.7-2.068.700.1114........45415.30-694739.1245.46.8-1.568.090.17NGC1846 2.........50739.69-672859.4233.26.7-1.818.370.114.........50732.76-672831.0236.26.7-1.398.200.145.........50739.03-672822.3231.06.8-1.528.030.126.........50735.07-672813.2240.16.7-0.056.920.298.........50733.80-672756.4241.17.0-0.126.640.429.........50733.87-672754.6229.57.0-1.457.940.1110........50736.86-672745.0234.36.8-2.499.280.1511........50739.26-672735.1234.86.7-1.988.140.1212........50730.12-672726.5238.06.8-1.778.430.1013........50740.15-672714.6235.28.4-0.637.080.4614........50735.98-672706.2235.36.7-1.518.000.1415........50738.81-672657.7230.56.7-2.108.490.1116........50732.46-672648.5232.96.7-1.557.700.1217........50733.62-672640.4236.86.8-1.988.510.1218........50734.04-672631.8231.46.8-0.376.630.2220........50743.43-672612.6235.96.8-0.357.160.2021........50733.58-672602.6242.46.9-0.336.340.29SL396 8.........51938.32-730657.8228.46.9-0.657.170.189.........51935.96-730643.7224.36.7-1.488.140.1110........51938.67-730637.3221.76.8-0.917.930.1711........51938.97-730625.7226.16.8-1.838.890.1313........51941.07-730603.8225.66.8-0.006.920.35

PAGE 147

133 Table3–6—Continued IDR.A.DeclRV s RV V V HB S W s S W (J2000.0)(J2000.0)(kms 1 )(kms 1 )(mag)( A)( A) NGC1942 6.........52444.67-635647.2286.86.8-1.658.370.137.........52445.95-635638.8298.46.8-0.006.750.248.........52443.59-635630.0286.66.8-1.687.630.439.........52446.05-635620.1289.66.8-2.439.040.1210........52446.56-635611.8301.06.8-0.427.210.2011........52446.80-635601.5299.56.6-0.026.720.3312........52441.90-635551.6287.86.7-0.837.540.1513........52441.84-635537.9300.27.0-0.066.330.28NGC2019 8.........53200.40-701019.1281.36.7-2.596.320.0910........53202.96-701001.4287.26.8-2.416.830.1311........53159.36-700951.9283.66.7-2.606.590.1712........53203.01-700943.9274.16.8-1.735.700.1413........53155.94-700935.0276.86.8-1.615.390.14Hodge4 6.........53224.81-644456.5317.06.80.176.020.297.........53225.96-644447.5301.86.80.076.670.308.........53228.14-644438.0309.66.8-0.807.750.149.........53224.60-644430.0310.96.8-0.827.080.1610........53227.95-644418.4312.46.8-0.357.110.2112........53226.11-644359.7315.17.0-0.166.370.2213........53225.97-644347.3308.76.8-1.738.410.12Hodge3 5.........53314.78-680950.9277.06.8-2.168.300.107.........53311.87-680929.3280.16.8-0.247.750.338.........53319.42-680920.3279.76.8-0.977.680.2010........53322.35-680910.0275.87.2-1.548.670.1511........53316.19-680859.8277.16.8-1.368.320.1512........53319.61-680850.1277.96.8-2.129.250.1213........53320.49-680835.5274.46.8-1.608.540.15IC2146 2.........53742.00-744831.0224.16.7-0.357.040.503.........53737.67-744820.0225.46.8-1.578.370.154.........53748.70-744811.0225.36.8-1.888.670.155.........53743.94-744800.2224.16.8-2.449.240.127.........53738.53-744740.3227.66.7-1.558.170.158.........53753.30-744731.6227.26.9-0.157.250.499.........53742.15-744723.4223.56.8-1.137.600.1710........53743.16-744713.3224.46.8-0.657.710.5411........53751.92-744703.4229.56.7-1.468.360.1312........53744.66-744654.6227.36.8-0.617.520.3013........53745.72-744646.5222.66.8-1.317.850.1614........53747.30-744636.7226.46.8-1.157.440.2115........53740.09-744626.6231.56.8-0.107.340.4016........53740.38-744615.2232.36.8-2.499.220.1217........53750.48-744606.6226.06.6-0.146.700.5618........53803.62-744554.4225.16.8-2.228.800.1119........53803.11-744545.1225.76.9-1.057.600.1721........53741.58-744516.8224.96.8-1.998.580.11

PAGE 148

134 Table3–6—Continued IDR.A.DeclRV s RV V V HB S W s S W (J2000.0)(J2000.0)(kms 1 )(kms 1 )(mag)( A)( A) SL663 5.........54229.19-652235.1299.16.7-1.898.570.108.........54230.17-652159.6297.36.8-1.177.440.139.........54229.59-652147.3301.66.8-1.698.430.1510........54226.31-652137.7303.37.0-0.066.280.3011........54229.92-652129.5299.86.7-0.076.730.2712........54229.07-652119.4300.26.8-1.087.240.1413........54228.41-652105.4299.26.80.036.900.3614........54229.73-652053.8311.16.8-0.176.590.32NGC2121 2.........54810.29-713016.8228.46.7-2.048.600.117.........54807.09-712924.4236.56.7-1.307.250.198.........54811.85-712910.9234.76.8-1.988.360.109.........54806.03-712901.3233.96.8-1.908.510.1110........54822.29-712846.2235.46.7-1.457.960.1311........54812.54-712838.0231.46.8-0.747.560.1812........54809.70-712829.8239.76.8-1.657.830.1214........54815.15-712812.3236.26.8-1.708.120.1115........54815.71-712801.6229.76.8-2.148.510.1216........54757.44-712753.2227.66.90.036.370.2317........54825.44-712745.2227.96.8-1.627.810.1120........54813.58-712711.7229.06.8-1.878.150.12NGC2173 6.........55754.09-725911.1238.56.8-1.377.760.168.........55755.77-725852.4237.46.8-2.338.910.129.........55752.62-725842.2235.66.8-2.218.670.1310........55757.94-725829.8238.66.8-1.878.240.2511........55759.18-725820.8235.46.8-1.047.790.1914........55758.02-725753.9239.36.8-2.889.410.11NGC2155 3.........55836.33-652939.0310.86.8-1.237.940.145.........55837.83-652913.2311.76.7-0.837.110.166.........55832.85-652903.4314.26.8-2.198.840.127.........55832.27-652854.3308.86.8-0.147.040.229.........55828.86-652838.4305.96.8-1.727.980.1010........55835.94-652829.3311.06.8-1.808.320.1214........55835.67-652741.6301.56.7-0.076.210.23NGC2162 7.........60027.09-634337.6323.06.8-1.247.490.148.........60028.17-634328.7318.16.9-1.207.400.259.........60032.04-634318.0320.46.8-2.528.780.1310........60029.47-634302.1335.66.8-1.888.390.1311........60027.80-634251.7315.96.80.077.520.49NGC2203 7.........60448.16-752648.1244.66.7-1.818.180.128.........60440.98-752639.8251.26.8-1.788.150.129.........60435.08-752629.2246.26.7-1.808.600.1410........60441.31-752621.2249.76.8-1.718.390.1211........60433.33-752613.1240.16.8-2.078.720.7912........60445.42-752604.0247.96.8-1.688.100.1213........60449.22-752555.6247.76.8-2.849.550.1414........60443.24-752545.6242.26.8-0.747.710.1916........60450.33-752524.2239.86.8-1.257.710.14

PAGE 149

135 Table3–6—Continued IDR.A.DeclRV s RV V V HB S W s S W (J2000.0)(J2000.0)(kms 1 )(kms 1 )(mag)( A)( A) NGC2193 6.........60620.10-650608.0288.46.8-2.088.710.107.........60620.74-650558.6287.96.7-1.557.780.118.........60618.03-650548.0298.86.8-1.638.200.159.........60615.56-650532.3290.06.7-0.186.580.2510........60616.58-650524.1291.26.8-0.086.970.31NGC2213 7.........61041.62-713219.0238.66.7-1.517.840.118.........61045.00-713200.1247.16.8-2.188.660.119.........61045.68-713147.1244.16.8-1.538.050.1310........61043.45-713136.8240.26.8-0.537.080.2311........61039.14-713133.7242.77.1-1.958.200.1112........61044.25-713109.4243.46.8-0.516.740.20Hodge11 3.........61431.61-694936.5245.26.8-1.474.150.124.........61430.51-694951.1251.36.9-0.713.360.177.........61421.85-694957.6246.66.8-2.014.160.108.........61424.80-695016.6247.57.0-0.873.180.239.........61424.55-695028.4243.56.8-2.875.570.1710........61421.99-695033.4240.86.8-3.035.840.1011........61416.88-695028.0241.97.0-0.383.740.2712........61417.20-695043.8241.66.8-2.134.540.0813........61417.88-695054.3244.66.8-2.034.670.0814........61422.57-695118.8242.07.3-1.144.100.1815........61407.08-695046.5250.97.2-0.383.710.2418........61408.43-695122.9245.28.0-0.142.820.26SL869 7.........61439.08-694730.6262.56.8-1.377.890.149.........61440.73-694759.6256.36.8-1.177.930.1311........61444.28-694846.4256.46.7-1.278.210.14NGC2231 3.........62041.59-673145.3274.56.8-1.097.230.144.........62043.41-673134.7272.66.8-0.326.750.225.........62038.01-673118.3274.56.7-2.218.730.116.........62045.55-673106.2279.46.7-0.537.400.287.........62045.24-673057.3282.76.8-1.507.940.128.........62044.94-673046.1278.66.7-1.227.390.209.........62048.86-673036.2285.16.8-0.517.270.1810........62040.00-673024.2274.06.8-1.948.270.1211........62040.63-673002.9277.26.7-0.907.390.18NGC2257 3.........63009.09-642033.3302.97.0-0.373.940.164.........63014.36-642023.0301.16.9-2.305.480.085.........63014.94-642010.0300.66.9-1.194.760.126.........63014.45-642001.9303.56.9-0.994.730.137.........63007.92-641953.0299.97.0-0.414.130.168.........63012.16-641943.5308.06.8-1.144.630.189.........63014.78-641932.7296.67.0-1.904.880.0810........63010.73-641924.3303.47.0-0.214.020.2711........63009.22-641915.4297.66.9-0.804.580.1612........63006.66-641906.8297.56.9-0.364.100.2113........63014.97-641858.5298.96.8-1.855.170.0914........63011.03-641846.6307.17.0-1.174.740.1415........63016.85-641833.8302.86.9-0.804.080.1417........63027.48-641807.5300.76.9-0.824.190.1518........63016.26-641753.8304.77.1-0.204.120.2219........63011.25-641744.1299.77.3-0.214.210.24

PAGE 150

136 Table3–7.PositionsandMeasuredValuesforFieldStars IDR.A.DeclRV s RV V V HB S W s S W (J2000.0)(J2000.0)(kms 1 )(kms 1 )(mag)( A)( A) SL4Primary 1.........4 h 32 m 47 : s 40-72 22 0 10 : 00 3216.86.9-0.317.180.37 3.........43252.69-722138.5198.86.8-1.427.360.174.........43235.49-722128.4237.66.9-0.216.470.465.........43236.28-722115.9204.06.80.485.080.838.........43247.72-722041.05.67.00.572.150.2010........43237.17-722021.6203.66.8-1.958.770.1413........43229.98-721951.7215.16.80.085.500.6414........43249.14-721943.4237.66.8-0.938.120.2715........43251.46-721929.3251.46.80.015.650.4816........43244.49-721914.2228.16.8-1.266.940.1617........43249.35-721855.8229.96.8-1.808.110.1418........43301.62-721845.3263.97.3-0.224.670.4919........43242.09-721831.8206.37.3-0.394.210.38SL4Secondary 1.........43221.15-722516.9233.46.7-1.578.030.152.........43257.67-722505.2235.67.0-0.497.130.273.........43256.66-722453.4250.36.7-0.975.420.204.........43227.22-722429.8-3.46.8-1.765.750.105.........43305.92-722409.5201.66.7-1.637.110.106.........43252.96-722357.7216.66.90.017.550.257.........43217.81-722348.433.26.8-1.766.820.088.........43303.14-722326.7237.36.9-0.296.470.319.........43258.19-722317.8251.96.70.138.640.5710........43240.69-722308.6196.26.90.075.510.4911........43234.49-722252.5253.56.9-0.096.670.5112........43215.50-722239.3239.16.80.167.090.44ReticulumPrimary 1.........43606.13-585307.0220.88.40.974.430.714.........43615.46-585230.6247.86.7-0.255.170.257.........43601.15-585157.3254.17.50.604.370.4110........43606.43-585120.9289.99.80.813.560.76ReticulumSecondary 1.........43558.36-585556.539.510.41.135.191.192.........43616.02-585427.6239.67.20.544.590.423.........43616.50-585341.8251.16.9-0.274.140.19NGC1651Primary 1.........43727.70-703644.7216.96.7-1.127.210.172.........43740.95-703628.7228.96.8-1.247.700.153.........43749.87-703618.1237.06.9-0.226.530.404.........43728.44-703608.9270.06.8-0.787.260.255.........43736.64-703559.7255.56.9-0.116.991.117.........43728.94-703542.0239.56.8-1.217.340.199.........43730.92-703521.8236.16.7-1.027.990.2014........43737.25-703444.5232.27.0-0.807.770.2918........43726.32-703401.6240.07.50.037.510.4519........43737.50-703352.1216.46.8-1.348.260.1420........43750.02-703337.8243.06.7-0.997.020.2421........43717.16-703323.8294.87.8-0.187.020.5522........43736.05-703314.3249.36.7-0.446.840.2623........43736.56-703302.4232.97.20.127.260.48

PAGE 151

137 Table3–7—Continued IDR.A.DeclRV s RV V V HB S W s S W (J2000.0)(J2000.0)(kms 1 )(kms 1 )(mag)( A)( A) NGC1651Secondary 1.........43737.99-703944.434.26.7-0.325.950.462.........43754.69-703926.7233.86.90.047.540.603.........43743.66-703916.5223.86.7-0.496.230.274.........43743.93-703856.4227.76.7-1.137.600.215.........43736.47-703844.6227.47.4-0.365.570.776.........43717.01-703826.8196.76.7-1.617.550.107.........43712.75-703810.8272.96.8-1.847.950.088.........43731.93-703757.3197.37.20.056.290.699.........43729.96-703740.1228.86.8-0.577.140.2810........43755.03-703727.8219.06.70.116.840.7111........43753.66-703711.8251.86.8-0.686.970.25NGC1652Primary 1.........43846.08-684152.6281.66.8-0.856.990.302.........43846.45-684143.9217.86.8-1.287.780.273.........43819.28-684130.8255.77.80.126.180.614.........43815.28-684118.3273.97.1-0.046.260.7711........43823.52-683958.8250.46.8-2.008.300.1313........43827.56-683930.4259.66.8-2.769.630.1314........43837.40-683911.1197.36.8-1.076.910.1915........43825.42-683858.2292.77.1-0.963.440.2716........43829.37-683841.2242.26.7-1.146.750.1817........43835.84-683832.6236.56.8-1.355.650.1718........43846.56-683819.3239.56.8-1.727.800.2019........43819.76-683804.9273.46.8-0.875.630.26NGC1652Secondary 1.........43834.98-684459.9221.66.7-0.478.030.302.........43833.51-684444.6186.76.8-2.047.500.123.........43831.55-684426.6227.66.8-0.647.350.214.........43817.46-684410.3244.66.8-0.486.620.375.........43823.79-684357.9214.46.8-1.636.990.196.........43817.41-684345.1209.96.8-0.637.240.227.........43844.01-684330.2249.97.0-0.057.140.498.........43836.65-684318.8283.16.7-0.317.560.289.........43806.39-684309.5223.17.00.256.940.6210........43825.17-684254.5279.76.8-0.918.120.1911........43827.71-684234.0260.66.8-2.299.520.2112........43842.44-684218.2278.96.8-1.828.020.13NGC1841Primary 5.........44618.75-840042.5221.77.0-0.932.910.267.........44443.30-840020.3209.26.8-1.773.160.1118........44537.73-835820.6209.97.7-0.583.940.42NGC1841Secondary 1.........44522.92-840427.7204.36.8-2.564.460.092.........44447.46-840353.247.26.7-1.656.740.163.........44423.46-840316.4133.97.80.295.260.654.........44518.81-840246.3-1.46.8-0.846.450.265.........44541.78-840213.0208.96.9-2.023.790.106.........44507.91-840202.2209.46.8-1.633.670.137.........44510.56-840146.0188.47.80.452.640.67

PAGE 152

138 Table3–7—Continued IDR.A.DeclRV s RV V V HB S W s S W (J2000.0)(J2000.0)(kms 1 )(kms 1 )(mag)( A)( A) SL41Primary 1....44723.47-723634.4232.56.8-1.017.410.112....44739.69-723621.5261.36.8-0.326.670.213....44734.44-723604.7211.86.8-2.229.010.0910...44734.60-723456.0215.16.9-0.086.000.3311...44737.60-723446.6208.26.7-1.137.600.1112...44731.28-723434.1274.06.7-2.087.900.0713...44725.00-723426.2213.26.8-0.427.200.1914...44720.74-723414.9218.96.8-1.127.310.1115...44756.17-723405.8267.46.7-1.758.240.0916...44752.84-723345.6228.76.8-2.118.330.0817...44727.14-723332.9247.86.8-1.407.950.1118...44727.93-723318.4219.96.7-1.207.790.15SL41Secondary 1....44737.25-724004.2250.06.7-0.837.360.182....44757.50-723933.8167.87.2-1.216.990.183....44737.28-723923.414.26.8-2.575.330.054....44741.99-723912.7244.26.7-2.608.420.075....44733.90-723851.1270.66.80.035.750.346....44736.32-723831.5241.16.9-0.786.920.477....44743.25-723812.949.26.8-1.856.070.078....44720.07-723801.7220.67.0-1.357.340.669....44744.56-723737.8217.26.8-2.639.260.1210...44754.41-723727.2227.17.2-0.126.700.3411...44727.35-723716.7180.26.8-2.688.570.13SL61Primary 1....45042.65-753340.1212.56.8-1.087.620.182....45028.13-753320.3238.66.8-1.738.100.143....45019.94-753301.5217.76.7-0.757.580.315....45045.00-753233.7247.86.8-2.358.920.126....45050.57-753225.9257.86.7-0.045.320.717....45028.13-753216.1211.96.7-1.437.770.1815...45049.94-753057.4231.27.40.026.100.5916...45041.74-753045.6216.56.8-2.218.790.1217...45015.63-753028.1201.06.8-0.594.510.4818...45015.79-753017.0217.66.8-0.887.880.2719...45039.87-753003.2222.37.10.005.860.46SL61Secondary 1....45049.38-753650.0197.06.8-1.527.450.142....45047.60-753637.7211.46.8-0.497.710.283....45038.08-753500.7189.76.8-1.296.270.134....45040.10-753429.0195.16.80.016.260.585....45102.61-753404.2240.76.90.176.791.05

PAGE 153

139 Table3–7—Continued IDR.A.DeclRV s RV V V HB S W s S W (J2000.0)(J2000.0)(kms 1 )(kms 1 )(mag)( A)( A) NGC1718Primary 1.........45220.98-670448.6259.66.9-0.115.440.342.........45213.97-670439.7287.56.8-1.727.820.123.........45236.17-670430.3265.36.8-0.327.210.304.........45225.83-670414.5280.96.8-1.718.550.125.........45232.17-670403.6273.36.9-0.087.620.456.........45220.25-670352.7278.66.8-0.987.970.197.........45230.12-670335.9273.86.8-0.737.460.239.........45222.52-670317.6278.56.7-0.997.730.1911........45222.50-670300.8284.96.8-1.358.330.1712........45224.93-670248.3276.86.8-0.927.810.2214........45227.93-670228.4270.77.10.097.590.7115........45221.99-670217.9285.16.8-1.307.850.1416........45227.35-670203.6291.17.00.096.910.4417........45227.07-670148.1275.36.8-0.627.870.2118........45229.56-670137.8288.86.90.126.540.4719........45236.45-670129.4224.06.7-0.057.090.3520........45241.24-670110.8266.57.0-0.066.980.44NGC1718Secondary 1.........45205.37-670753.8244.16.9-1.005.790.222.........45234.07-670737.6288.16.7-0.775.670.193.........45209.87-670727.887.66.8-0.427.040.304.........45232.80-670710.3232.16.7-1.337.600.125.........45218.21-670657.1266.66.70.066.810.316.........45204.62-670641.1281.66.7-1.708.270.127.........45207.39-670625.2260.36.7-0.827.080.228.........45241.73-670610.5268.36.8-0.357.120.219.........45220.06-670554.5234.86.8-0.467.130.2310........45242.36-670541.6269.56.8-0.177.070.3011........45242.10-670527.5264.36.7-0.747.050.2812........45240.78-670512.0302.26.9-0.316.430.46NGC1751Primary 1.........45403.66-695003.2232.06.7-2.709.350.122.........45425.29-694951.8241.36.7-0.477.220.473.........45419.05-694943.9278.86.8-0.646.590.354.........45409.81-694932.5263.96.7-2.749.300.127.........45403.93-694859.4224.76.7-1.828.060.1312........45417.30-694759.0218.56.8-1.017.360.2013........45355.79-694749.0247.16.9-0.847.920.1815........45423.81-694727.7244.86.7-2.148.410.1216........45408.29-694716.0278.06.8-0.636.060.3317........45412.57-694707.0223.76.8-0.286.980.3818........45400.14-694659.4231.97.5-0.196.100.6019........45423.75-694650.1242.36.8-2.919.060.1020........45405.47-694635.8263.96.9-1.247.380.1821........45357.39-694625.5266.56.9-0.277.520.63

PAGE 154

140 Table3–7—Continued IDR.A.DeclRV s RV V V HB S W s S W (J2000.0)(J2000.0)(kms 1 )(kms 1 )(mag)( A)( A) NGC1751Secondary 1.........45409.20-695318.7206.96.8-0.306.860.422.........45418.92-695309.0240.46.8-0.217.550.413.........45407.36-695257.8261.86.8-0.367.290.464.........45417.73-695248.2212.26.9-1.095.440.365.........45429.81-695235.6225.86.8-0.616.950.346.........45412.57-695226.1292.06.6-1.117.130.227.........45407.20-695209.9252.86.8-0.225.360.718.........45409.88-695158.8241.26.8-1.237.360.159.........45432.41-695150.1207.86.7-1.307.410.1410........45427.75-695133.6216.46.8-2.319.140.1111........45408.40-695120.6212.56.7-0.646.370.3012........45406.16-695109.4222.86.8-2.048.540.1013........45433.79-695059.9270.26.9-0.016.470.5714........45417.06-695050.8226.96.8-0.967.560.1815........45427.74-695035.9258.37.1-0.435.470.30NGC1846Primary 1.........50736.94-672907.7273.56.8-0.707.340.183.........50740.90-672841.7208.76.7-2.498.990.107.........50738.36-672803.0250.26.7-2.358.850.1019........50728.79-672621.4264.16.7-0.377.310.2122........50741.17-672548.6203.56.8-1.247.160.1323........50736.78-672537.0260.26.8-0.417.590.2624........50739.96-672529.1238.16.8-2.439.330.10NGC1846Secondary 1.........50727.07-673221.1255.46.7-1.067.200.162.........50725.96-673212.0249.66.7-0.497.510.293.........50723.56-673202.2231.16.6-0.436.960.244.........50724.15-673149.8260.46.8-1.057.430.165.........50724.71-673136.4306.56.7-2.448.420.126.........50739.56-673124.6239.46.7-1.307.660.117.........50739.46-673114.4230.46.8-0.206.540.298.........50736.52-673051.6262.86.7-1.038.140.199.........50733.37-673043.5273.66.7-0.837.750.1610........50732.65-673035.6270.46.9-0.787.340.2411........50740.05-673024.0255.06.9-0.236.820.2812........50727.32-673010.8242.26.8-1.618.520.1613........50726.91-672957.6278.46.6-0.317.730.2514........50727.21-672947.5236.76.8-0.277.630.2815........50730.31-672934.4234.66.7-1.527.900.12SL396Primary 1.........51910.80-730736.3254.96.8-0.656.390.202.........51913.34-730727.2248.27.0-0.079.330.473.........51919.84-730723.9-16.16.8-1.276.730.144.........51937.63-730741.2259.46.9-0.216.840.215.........51928.84-730709.8266.86.8-0.967.930.166.........52004.21-730809.9191.67.1-0.246.370.347.........51928.32-730647.4250.86.8-0.637.020.1912........51938.57-730609.6251.66.8-0.777.300.1414........51948.89-730604.1263.26.8-0.907.830.1815........51940.20-730534.5257.76.7-0.637.750.1816........51951.32-730544.9178.36.8-1.087.210.1317........51943.31-730515.9209.07.0-0.175.740.2718........51940.72-730451.6223.36.7-1.147.510.11

PAGE 155

141 Table3–7—Continued IDR.A.DeclRV s RV V V HB S W s S W (J2000.0)(J2000.0)(kms 1 )(kms 1 )(mag)( A)( A) SL396Secondary 1.........51916.47-731051.7323.16.8-2.566.900.072.........51928.74-731103.2250.36.70.046.340.283.........51916.57-731026.4257.16.7-1.588.300.124.........51929.12-731039.2251.26.8-0.837.430.195.........51914.77-730955.1228.56.7-1.177.880.156.........51937.09-731024.7254.96.7-0.806.940.197.........51930.11-730955.2250.46.9-0.404.280.228.........51908.80-730858.4234.67.1-0.486.800.279.........51916.78-730857.1208.56.8-1.017.040.1010........51922.32-730851.6193.16.9-1.264.600.1611........51927.00-730847.0233.66.8-0.126.310.3112........51956.49-730933.4207.96.7-1.228.050.13NGC1942Primary 1.........52442.68-635749.0291.46.90.036.920.232.........52432.71-635737.0267.46.8-0.017.310.263.........52455.44-635726.6285.86.9-0.226.440.194.........52458.42-635716.6291.36.8-0.236.450.315.........52451.28-635701.2316.16.8-1.688.510.1114........52454.06-635527.6281.46.8-0.016.240.4815........52502.09-635501.0290.56.9-1.157.760.1816........52436.74-635450.8267.06.8-1.528.460.1117........52446.60-635440.8302.36.8-0.296.740.2218........52442.99-635424.7245.26.7-1.616.760.0819........52452.52-635413.2291.66.8-1.658.450.1520........52445.50-635404.5279.37.0-0.187.200.29NGC1942Secondary 1.........52452.44-640028.0325.26.7-0.937.430.132.........52441.59-640019.4299.36.7-0.917.740.183.........52438.43-640009.7297.06.8-1.097.590.134.........52458.30-635950.4347.77.1-0.017.540.495.........52435.19-635940.0251.96.7-0.506.170.156.........52447.03-635921.0291.16.7-0.577.750.217.........52453.96-635908.3287.26.8-0.267.000.348.........52430.35-635858.7282.46.8-0.397.160.299.........52441.30-635834.1284.46.6-0.277.320.2310........52438.54-635810.3297.56.7-0.146.930.22NGC2019Primary 1.........53153.22-701116.0246.76.8-1.658.160.122.........53208.39-701107.7236.26.8-1.818.620.143.........53201.68-701057.1256.26.7-1.715.390.134.........53159.39-701048.7247.76.8-2.459.780.125.........53155.62-701042.5272.27.4-3.088.310.606.........53155.38-701039.6254.66.7-1.368.100.167.........53201.57-701029.2248.16.8-1.007.150.269.........53157.23-701010.3266.46.8-2.909.610.1314........53159.69-700926.4243.66.8-1.788.500.1215........53205.69-700915.1318.36.8-1.317.750.1416........53204.42-700904.5295.86.8-0.966.810.2217........53159.15-700854.0248.36.8-1.078.410.1918........53207.05-700848.0221.67.10.146.820.5419........53207.09-700845.1254.66.8-0.176.250.4320........53154.66-700836.2219.76.8-1.206.850.2221........53214.47-700825.1214.46.7-0.957.060.2522........53141.26-700815.0258.96.8-1.077.470.1623........53141.43-700805.2253.46.7-1.057.420.3024........53144.44-700756.2269.86.8-2.409.180.1225........53200.65-700747.8238.16.8-0.696.140.2526........53139.74-700739.3261.96.8-1.036.460.24

PAGE 156

142 Table3–7—Continued IDR.A.DeclRV s RV V V HB S W s S W (J2000.0)(J2000.0)(kms 1 )(kms 1 )(mag)( A)( A) NGC2019Secondary 1.........53219.47-701425.3281.06.7-1.788.400.112.........53217.16-701416.8256.07.0-1.266.250.203.........53150.36-701408.6243.26.8-2.148.910.144.........53206.91-701358.7273.66.7-1.218.440.155.........53206.01-701350.5237.76.7-2.308.140.106.........53211.84-701338.9274.06.7-0.666.560.267.........53204.93-701323.3262.96.7-2.008.600.148.........53153.19-701314.1215.76.7-2.787.140.1210........53145.04-701258.4268.16.8-2.209.190.1311........53150.84-701248.3279.26.7-1.938.610.4212........53147.30-701239.2245.26.7-2.408.440.1413........53200.64-701230.8238.36.8-1.819.420.1514........53157.41-701221.9260.16.9-2.407.660.3115........53207.21-701211.0229.06.8-1.856.850.1116........53209.49-701202.4257.56.8-1.868.540.1817........53159.62-701148.5224.56.8-1.287.300.1618........53157.88-701138.1258.76.7-0.487.480.28Hodge4Primary 1.........53235.94-644551.3319.06.8-0.717.090.192.........53220.35-644542.1310.57.0-0.086.770.363.........53217.83-644524.9282.96.8-1.037.710.184.........53218.35-644514.511.26.8-2.126.440.125.........53217.18-644505.3272.46.8-0.046.580.2611........53227.81-644409.5283.86.8-0.724.910.1614........53221.95-644337.1303.27.00.047.450.3115........53238.18-644324.3289.26.8-1.236.480.1416........53239.65-644312.1303.46.8-1.307.670.1417........53219.80-644259.2308.66.8-0.207.510.3218........53222.77-644250.6305.66.7-0.096.480.3819........53230.89-644225.8306.17.0-0.284.960.4120........53231.86-644212.0288.46.9-0.176.550.32Hodge4Secondary 1.........53208.97-644859.8266.56.7-0.327.680.292.........53236.54-644847.4313.97.2-0.066.830.413.........53237.92-644839.2286.76.8-0.828.080.214.........53208.70-644831.3296.06.7-0.757.220.215.........53210.17-644819.2309.46.8-2.709.180.126.........53237.27-644807.6320.06.8-0.297.330.257.........53212.71-644755.8308.76.8-1.458.260.128.........53216.67-644745.0287.76.80.036.620.319.........53217.61-644734.4307.06.8-0.296.590.3110........53226.30-644720.2286.66.8-0.206.710.2311........53232.42-644708.0327.47.1-0.065.130.3912........53211.22-644657.3288.76.8-1.437.530.1313........53236.59-644645.6279.46.8-1.588.150.1014........53236.66-644627.7277.66.8-1.255.360.3015........53215.60-644616.6316.36.8-2.288.400.15

PAGE 157

143 Table3–7—Continued IDR.A.DeclRV s RV V V HB S W s S W (J2000.0)(J2000.0)(kms 1 )(kms 1 )(mag)( A)( A) Hodge3Primary 1.......53305.77-681040.4269.76.9-0.097.350.532.......53331.96-681030.1257.06.8-1.067.250.173.......53314.04-681019.5268.56.7-0.916.480.154.......53318.22-681006.3219.86.8-1.917.360.106.......53328.11-680940.1352.18.8-0.884.960.349.......53319.46-680916.4277.07.0-0.018.310.5014......53303.98-680824.9330.67.0-0.215.020.4815......53307.28-680815.4345.011.0-0.277.320.4916......53309.02-680806.1300.86.8-2.278.140.1017......53324.31-680757.5276.36.8-0.647.020.3918......53331.30-680746.6306.86.7-1.397.890.1719......53304.61-680733.9290.86.9-0.196.320.5020......53323.45-680716.7280.56.7-1.207.550.1921......53331.97-680706.0290.16.8-0.136.320.4422......53323.52-680657.0315.46.8-1.177.160.20Hodge3Secondary 1.......53307.71-681343.3236.16.8-2.459.270.112.......53314.66-681333.3297.17.1-0.877.400.273.......53310.14-681325.2298.86.8-0.346.960.344.......53322.66-681313.9297.06.8-0.637.530.285.......53331.12-681303.3320.66.8-1.046.640.166.......53319.20-681254.8239.16.8-1.217.160.197.......53318.20-681245.1308.66.8-1.296.670.208.......53302.97-681231.9295.66.7-1.047.960.169.......53314.05-681221.0282.76.9-0.435.290.3410......53313.83-681209.1254.56.7-1.618.270.1511......53323.05-681159.5296.16.8-2.108.170.0912......53326.26-681151.3268.26.7-2.248.950.1213......53327.95-681139.8239.16.7-0.867.110.2214......53311.69-681128.0245.26.7-0.807.640.2615......53305.46-681117.1297.96.8-2.839.120.1516......53305.48-681112.8311.76.9-0.456.930.6617......53324.56-681101.7260.36.7-0.216.880.43IC2146Primary 1.......53817.64-744840.8223.16.8-1.007.110.226.......53756.11-744750.5208.86.8-1.918.660.1220......53722.52-744535.1212.56.9-0.167.510.5222......53740.54-744503.7210.96.8-0.248.850.36IC2146Secondary 1.......53738.85-745131.8208.77.2-0.927.690.172.......53815.24-745112.4215.46.8-2.359.110.123.......53757.61-745100.6239.06.9-1.127.210.244.......53815.99-745046.0226.46.8-1.788.580.145.......53748.06-745036.5205.37.3-0.804.082.026.......53809.62-745010.8234.76.8-1.018.640.467.......53746.94-745001.3213.76.8-1.017.810.178.......53809.04-744943.0241.06.9-0.386.940.289.......53742.74-744931.2213.06.8-1.257.580.1310......53745.53-744919.0218.46.8-0.197.170.3911......53736.91-744906.1209.76.8-2.128.720.12

PAGE 158

144 Table3–7—Continued IDR.A.DeclRV s RV V V HB S W s S W (J2000.0)(J2000.0)(kms 1 )(kms 1 )(mag)( A)( A) SL663Primary 1.........54228.27-652320.3274.06.8-1.468.030.142.........54245.67-652309.2329.86.8-0.136.920.333.........54223.75-652300.5317.06.7-0.346.630.274.........54245.25-652246.8290.66.8-1.457.730.136.........54219.27-652225.5310.76.7-1.347.970.137.........54232.80-652211.9300.16.8-0.446.220.2615........54216.02-652041.3297.67.0-1.788.500.1416........54249.09-652012.3282.36.8-2.178.150.0917........54235.87-652002.1293.36.8-1.247.830.1518........54235.06-651950.3300.56.8-0.727.210.19SL663Secondary 1.........54239.66-652619.6331.76.8-2.019.070.112.........54212.05-652608.8330.26.7-1.527.490.143.........54214.81-652543.7311.96.7-1.428.150.144.........54218.17-652532.6282.86.8-0.687.390.255.........54211.50-652508.5314.37.0-0.195.960.276.........54229.07-652458.5293.36.8-0.677.270.257.........54241.48-652438.3263.66.7-0.966.850.178.........54244.30-652421.4310.56.7-0.546.590.239.........54241.64-652411.4314.96.7-0.736.580.1610........54216.20-652348.2260.56.7-0.707.470.1811........54245.06-652337.7310.46.7-1.468.110.14NGC2121Primary 1.........54758.81-713029.7249.76.8-1.208.020.143.........54810.24-713006.2298.76.8-2.529.020.124.........54814.44-712954.6266.06.7-0.787.380.225.........54805.24-712944.3251.17.0-0.657.630.226.........54816.56-712934.8267.26.7-1.177.220.1713........54804.65-712820.4229.06.8-0.275.620.2818........54758.17-712734.342.56.8-2.906.030.0819........54810.00-712726.3269.16.8-0.657.080.2121........54830.26-712700.1278.26.8-1.237.500.1422........54814.14-712650.0243.66.8-1.607.950.1623........54822.50-712641.7248.56.8-0.317.480.30NGC2121Secondary 1.........54808.89-713331.5265.76.7-0.967.830.232.........54832.46-713318.8256.56.7-0.587.490.213.........54750.59-713307.6255.16.8-2.309.140.104.........54759.45-713257.0285.76.8-1.527.850.135.........54816.96-713246.9264.56.7-1.167.360.236.........54832.55-713231.8203.46.8-2.608.390.097.........54802.79-713218.1231.86.8-2.608.850.118.........54809.08-713205.3267.06.7-1.788.030.119.........54755.95-713154.6232.36.7-2.118.470.0910........54816.04-713142.3222.36.7-0.775.830.1511........54823.21-713132.7254.86.7-1.567.530.1112........54747.87-713120.8237.46.9-0.466.450.3113........54825.82-713110.8232.66.9-0.146.750.3214........54829.21-713100.4196.56.8-2.588.930.1315........54807.16-713048.7228.56.8-2.219.150.12

PAGE 159

145 Table3–7—Continued IDR.A.DeclRV s RV V V HB S W s S W (J2000.0)(J2000.0)(kms 1 )(kms 1 )(mag)( A)( A) NGC2173Primary 1.........55743.91-730019.0228.66.7-1.608.060.222.........55754.31-730003.1232.76.7-2.188.680.113.........55755.75-725951.3228.26.8-0.676.960.264.........55825.85-725940.7209.16.7-0.897.880.215.........55825.61-725922.0240.46.8-0.826.850.227.........55749.71-725901.8211.36.7-0.236.090.2812........55757.75-725812.0212.96.8-2.048.200.1013........55746.24-725802.3259.46.8-0.896.640.2415........55756.81-725744.8261.57.3-1.157.140.2116........55740.83-725736.0233.66.8-1.137.110.2017........55747.49-725728.1242.36.7-0.487.360.2518........55807.13-725717.3233.16.8-1.167.490.1819........55800.39-725707.0232.46.8-1.076.710.1920........55816.86-725646.7289.86.8-2.047.250.10NGC2173Secondary 1.........55743.07-730321.1203.06.8-0.097.020.572.........55804.20-730311.2251.96.7-0.927.780.213.........55749.43-730300.6250.86.7-0.086.930.414.........55741.56-730236.7235.66.7-0.347.060.325.........55818.78-730216.2249.46.8-0.235.210.526.........55813.89-730204.4202.26.7-0.316.780.367.........55730.58-730148.9234.76.7-1.938.390.138.........55747.14-730135.4227.76.8-0.425.910.399.........55749.84-730126.9251.76.7-0.785.510.26NGC2155Primary 1.........55845.87-653005.4307.86.8-0.476.960.252.........55848.19-652947.7314.36.8-2.238.180.188.........55836.12-652846.4327.76.9-1.037.850.1411........55836.28-652820.2326.46.8-0.266.740.2312........55832.84-652803.6304.26.8-0.085.700.3113........55832.69-652750.5346.96.8-0.845.920.1715........55829.79-652733.5260.16.8-0.695.490.1916........55819.83-652700.2307.66.8-0.746.060.1517........55849.04-652640.5286.86.9-1.017.720.17NGC2155Secondary 1.........55838.38-653316.4284.56.7-0.857.170.182.........55819.51-653306.3285.96.8-1.377.640.173.........55822.51-653243.8286.96.8-2.499.490.124.........55837.95-653234.7275.06.8-0.056.910.265.........55831.18-653202.3308.76.7-0.146.860.236.........55822.22-653142.9292.56.8-0.096.060.297.........55837.84-653123.0299.16.7-0.356.570.208.........55822.93-653114.8311.86.7-0.077.610.279.........55817.63-653101.4308.16.8-0.066.260.2610........55845.76-653042.6274.16.8-0.777.230.1411........55822.96-653033.1286.46.8-1.407.230.12

PAGE 160

146 Table3–7—Continued IDR.A.DeclRV s RV V V HB S W s S W (J2000.0)(J2000.0)(kms 1 )(kms 1 )(mag)( A)( A) NGC2162Primary 1.........60046.34-634449.8335.06.7-0.896.540.252.........60031.45-634439.9328.77.00.075.940.413.........60023.36-634425.5327.66.8-0.326.900.264.........60025.82-634416.2318.86.9-0.977.130.165.........60029.27-634402.8255.16.8-1.847.710.096.........60026.14-634346.6341.47.0-0.306.420.2512........60028.68-634243.8318.97.1-0.085.990.4313........60025.72-634232.2313.67.1-0.067.940.5014........60029.61-634220.4320.46.9-1.177.090.4215........60025.67-634209.9320.87.00.037.960.3316........60028.19-634154.2314.46.70.065.990.5017........60038.67-634139.6329.66.8-2.908.220.0918........60041.98-634122.0315.36.8-2.196.530.0919........60018.46-634112.4323.86.9-0.247.130.75NGC2162Secondary 1.........60040.69-634801.6276.96.8-0.546.510.182.........60029.65-634749.2313.16.9-0.026.790.213.........60026.70-634729.8315.06.8-0.087.220.314.........60026.10-634703.1306.56.7-1.447.670.155.........60041.59-634646.0338.16.90.037.270.346.........60010.49-634630.9314.96.7-0.356.920.287.........60042.64-634613.3296.66.7-0.165.540.378.........60040.02-634553.6310.86.9-0.707.000.259.........60017.77-634546.1242.06.90.036.120.3110........60042.67-634536.0320.36.9-0.336.020.2711........60031.52-634527.6324.36.9-0.156.680.24NGC2203Primary 1.........60441.53-752800.1248.86.7-0.717.020.262.........60448.82-752743.4242.06.8-2.679.570.123.........60453.27-752730.2249.26.9-0.536.780.244.........60446.49-752718.0244.96.8-0.876.820.185.........60438.31-752709.1252.06.7-0.917.040.226.........60435.76-752700.4249.06.8-0.886.720.1915........60427.05-752534.1249.16.8-0.866.690.2417........60448.29-752515.8255.16.9-0.225.710.3318........60427.70-752503.6240.06.8-2.569.200.1319........60443.00-752447.7246.46.8-0.877.530.2120........60501.32-752414.9252.06.8-1.117.800.18NGC2203Secondary 1.........60434.04-753104.0-34.76.8-0.816.730.172.........60516.08-753043.7252.96.8-2.158.490.123.........60515.85-753008.9258.16.7-1.447.540.144.........60411.96-752951.6242.16.8-0.426.600.265.........60458.24-752923.791.16.8-2.545.340.076.........60442.96-752905.9248.16.8-0.547.580.227.........60438.16-752844.2-9.46.8-1.096.230.15NGC2193Primary 1.........60621.06-650730.2317.36.8-1.377.730.122.........60602.62-650720.9310.56.8-0.747.480.253.........60624.04-650656.3325.26.8-1.325.190.124.........60617.64-650639.9340.06.9-0.146.250.265.........60606.57-650626.9309.06.8-1.758.390.1011........60629.37-650507.5284.36.7-0.147.170.2612........60622.49-650450.8309.86.8-1.668.130.1213........60615.95-650436.3294.76.8-0.476.920.2214........60628.35-650415.2313.56.8-1.518.000.13

PAGE 161

147 Table3–7—Continued IDR.A.DeclRV s RV V V HB S W s S W (J2000.0)(J2000.0)(kms 1 )(kms 1 )(mag)( A)( A) NGC2193Secondary 1.........60632.39-651039.829.96.8-2.146.720.162.........60616.45-651030.8308.06.8-2.308.560.113.........60615.41-651019.0309.36.7-1.367.800.144.........60614.60-651007.6275.36.8-1.877.710.105.........60559.84-650954.9307.76.7-0.717.940.176.........60628.64-650923.7294.56.8-2.759.120.117.........60629.26-650904.634.66.8-1.086.490.168.........60633.83-650838.1308.96.7-1.868.810.149.........60620.54-650825.5304.76.8-0.646.060.19NGC2213Primary 1.........61044.50-713329.3252.96.7-2.016.810.082.........61101.41-713230.0261.86.7-2.409.090.143.........61101.39-713218.6252.56.8-0.266.580.184.........61107.40-713150.0233.36.8-0.596.770.185.........61028.90-713317.8237.76.8-0.266.510.486.........61057.04-713148.9240.66.8-0.837.330.1913........61055.54-713016.2242.16.9-0.537.340.2114........61027.99-713115.4239.56.8-2.198.570.1015........61030.92-713029.7260.76.7-1.016.600.15NGC2213Secondary 1.........61118.41-713522.4227.06.9-0.257.200.252.........61047.88-713622.5249.86.7-0.676.940.213.........61124.35-713412.8246.56.9-0.926.840.164.........61117.64-713345.8264.96.8-1.916.280.085.........61107.14-713336.4239.66.7-2.137.860.076.........61107.18-713740.3257.56.7-1.646.410.097.........61108.93-713724.5239.86.8-0.327.350.36Hodge11Primary 1.........61413.21-694809.5309.26.9-0.206.250.342.........61433.45-694921.0262.46.9-0.057.190.235.........61430.76-695003.9276.56.8-1.848.570.126.........61425.27-694957.0284.86.80.067.340.3516........61401.58-695042.1269.76.8-0.727.180.1717........61358.62-695044.0285.86.8-1.437.940.1319........61406.62-695129.1243.66.80.067.110.4020........61417.72-695212.5256.36.80.113.910.3921........61423.49-695239.7245.57.3-0.783.160.1822........61414.32-695232.4289.47.00.476.360.37SL869PrimaryField 1.........61441.13-694603.5284.36.8-0.126.940.252.........61501.43-694714.1234.46.8-1.468.230.123.........61436.23-694610.4250.56.8-0.027.230.304.........61501.09-694735.2258.26.7-0.647.030.205.........61441.47-694650.3293.76.8-0.097.320.266.........61445.70-694737.0284.96.8-0.316.290.258.........61439.98-694744.7263.16.8-0.707.010.2210........61442.60-694823.5292.56.7-1.597.550.1012........61448.47-694925.0261.96.8-2.297.980.13NGC2231Primary 1.........62038.06-673224.4274.26.8-0.316.550.332.........62032.59-673208.5273.86.8-1.648.390.1512........62053.15-672936.8309.96.7-0.577.840.2413........62024.63-672915.4273.06.9-0.166.020.2814........62038.17-672851.2287.86.8-1.037.250.14

PAGE 162

148 Table3–7—Continued IDR.A.DeclRV s RV V V HB S W s S W (J2000.0)(J2000.0)(kms 1 )(kms 1 )(mag)( A)( A) NGC2231Secondary 1.........62027.84-673545.1302.36.8-0.157.210.282.........62050.46-673520.0310.36.8-1.628.370.113.........62105.08-673508.4303.76.8-1.948.520.144.........62056.06-673447.9299.26.8-1.098.040.145.........62054.19-673432.4305.17.0-0.604.950.186.........62047.46-673405.8283.16.8-0.477.670.197.........62103.47-673345.2279.76.8-0.236.240.298.........62035.64-673330.0307.16.8-1.337.940.149.........62105.17-673322.0265.16.7-0.797.910.1610........62101.86-673310.0298.56.8-0.997.670.1711........62046.36-673300.6306.67.0-0.317.220.81NGC2257Primary 1.........63009.42-642102.849.46.8-1.986.600.132.........63027.04-642054.020.66.8-0.535.540.1916........63011.78-641823.0307.76.8-1.837.250.0820........62957.91-641735.7316.46.8-0.397.150.15NGC2257Secondary 1.........63003.89-642407.263.76.8-1.726.430.092.........62955.32-642351.2308.76.7-0.087.460.323.........63005.30-642331.3259.96.8-0.947.070.124.........63017.33-642234.39.66.8-2.267.020.105.........63031.03-642133.9-5.36.8-1.516.670.11

PAGE 163

CHAPTER4 DISTANCESANDDISTRIBUTIONOFPOPULOUSLMCCLUSTERS 4.1Introduction Interactionsandmergereventscandominatetheformationh istoriesofgalaxies, bothlargeandsmall,athighandlowredshift( Abraham1999 ; Schweizer1999 )and theMilkyWay(MW)anditssatellitegalaxiesareanexcellen texampleofthis.The LargeMagellanicCloud(LMC)isanearbysatellitegalaxyth atisdynamicallyactive; itexhibitsmanyepochsofstarformation(includingcurren tstarformation)whilealso sufferingfromtidalinteractionswiththeSmallMagellani cCloud(SMC)andtheMW. Givenitsproximity,stellarpopulationsintheLMCareeasi lyresolved,allowingus toobtaininformationsuchasages,chemicalabundances,ki nematicsanddistancesto individualstars.Thus,theLMCoffersusanexcellentlocal laboratoryinwhichto studytheeffectsofgravitationalforcesontheevolutiono fasatellitegalaxy. Traditionally,theLMChasbeentreatedasaplanargalaxyth at,despiteits proximity,canbeassumedtolieatasingledistancefromus. Incontrast,using distancestoeldCepheidvariables, Caldwell&Coulson ( 1986 )rstshowedthatthe diskoftheLMCisinclinedwithrespecttothesky.Morerecen tstudiesofeldstars haveconrmedthisnding.Forexample, vanderMarel&Cioni ( 2001 )combined nearinfraredphotometryfromtheDeepNear-InfraredSouth ernSkySurvey(DENIS) andtheTwoMicronAll-SkySurvey(2MASS)tostudythedistri butionofeldstarsin theLMCouttoaradiusof 7 .UsingboththetipoftheRGBandasymptoticgiant branchasrelativedistanceindicators,theyfoundan I -bandpeak-to-peaksinusoidal brightnessvariationof 0.25magthatchangesasafunctionofpositionangleonthe sky,withstarsinthenortheastportionoftheLMCbrightert hanstarsinthesouthwest. Attributingthisvariationinbrightnesstoadifferencein distance,theycalculatedan 149

PAGE 164

150 inclinationof i = 34 : 7 6 : 2forthediskoftheLMC(where0 isfaceon)andtheline ofnodespositionangle(theintersectionoftheplaneofthe galaxywiththeplaneof thesky)of Q = 122 : 5 8 : 3.Inanapproachsimilarto vanderMarel&Cioni ( 2001 ), Olsen&Salyk ( 2002 )usetheapparent I -bandmagnitudeofRCstarstoexplorethe structureofthedisk.Calculatingrelativedistancesfor5 0eldsspreadacrossa6 6 areaoftheLMC,theynd i = 35 : 8 2 : 4and Q = 145 4 ,inagreementwith the vanderMarel&Cioni ( 2001 )results.Inadditiontotheinclination,theLMC's geometrybecomesevenmorecomplexwhenweconsiderthatits disk( v = s = 2 : 9 0 : 9) isthickerthantheMW'sthickdisk( v = s 3 : 9, vanderMareletal.2002 )andthatthe diskisared( Alves&Nelson2000 )andalsopossiblywarped( Olsen&Salyk2002 ; Nikolaevetal.2004 )asaresultofinteractionswiththeSMCandMW.Evenwithall oftheknowledgeoftheLMC'sstructurefromeldstarstudie s,thespatialdistribution ofpopulousclustersintheLMCremainsrelativelyunexplor ed. Schommeretal. ( 1992 ,seealso Grocholskietal.2006 )showedthattheLMCclustershavedisk-like kinetmatics,however,onlyrecentlyhasaplanargeometryb eenillustratedfortheLMC clustersystem( Kerberetal.2006 ). DistancestostellarpopulationsintheLMChavebeencalcul atedusingavariety ofstandardcandles,includingtheperiod-luminosity(P-L )relationofCepheidvariables (e.g., Macrietal.2006 ; Gierenetal.1998 ),themeanabsolutemagnitude-metallicity relationshipforRRLyraes(e.g., Walker1985 ),andcolormagnitudediagram(CMD) featureslikethetipoftheredgiantbranch(RGB,e.g., Cionietal.2000 ),corehelium burningredclump(RC)stars(e.g., Udalski2000 ; Sarajedinietal.2002 ),ormain sequenceturnoff(MSTO; Kerberetal.2006 ).Onestandardcandlethathasyetto befullyexploited,andisgearedtowardstudyingclusters, isthe K -bandluminosityof theRC.Intheirwork, Grocholski&Sarajedini ( 2002 ,hereafterGS02)use2MASS JK S photometryof14Galacticopenclustersthatposessinterna llyconsistentages, metallicities,andMSTOttingdistancestocalibratethea bsolute K -bandmagnitude

PAGE 165

151 oftheRC( M RC K )asafunctionofageandmetallicity.Animportantresultfr omtheir studyisthat,whilevariationsintheRCbrightnessaresmal lerinthe K -bandthan whatisseeninthe V -or I -bands, M RC K variesasafunctionofbothageandmetallicity and,foryoungages( < 3Gyr), M RC K canvarybyuptoamagnitude.Therefore, knowledgeoftheabundances and agesofRCstars,somethingthatcanonlybe unequivicallygleanedfromclusters,isnecessarytoprope rlyemploytheRCasa standardcandle.Sincethismethodprovidesanabsolutedis tance,itsapplicationallows thedeterminationofboththespatialdistributionofclust ersandthedistancetothe LMC. ThedistancetotheLMChasbeenofconsiderableinterestinr ecentyears,largely duetoitsuseasthezeropointfortheextragalacticdistanc escale.TheHSTKey Projecttodetermine H 0 (see Freedmanetal.2001 fornalresultsontheproject) usedasampleofCepheidvariablesintheLMC,alongwithanad opteddistanceof ( m M ) 0 = 18 : 5 0 : 1( Madore&Freedman1991 ),todenetheducialCepheidP-L relation. Freedmanetal. ( 2001 )thenusedthisnewP-Lrelationtocalculatedistances toalargenumberofgalaxies,therebyallowingthecalibrat ionofsecondarystandard candles(TypeIaandTypeIIsupernovae,Tully-Fisherrelat ion,surfacebrightness uctuations,fundamentalplane)thatliefurtheruptheext ragalacticdistanceladder. Thus,theaccuracyofthetheirvalueof H 0 = 72 8kms 1 Mpc 1 isultimately determinedbytheaccuracyofthedistancetotheLMC;itturn soutthatthedistance errortakesup6.5%oftheir9%errorbudget.Theiradopteddi stance,however,was basedonpreviouslypublisheddistancesand,untilrecentl y,therehavebeenrather largediscrepenciesbetweendifferentmethodsandsometim esevenamongdistances calculatedusingthesamemethod(particularlywithoptica lphotometryoftheRC). Ingeneral,theLMCdistancescanbesplitupintoa“long”dis tanceof 18.5-18.7 mag,usuallyfoundwithPopulationIindicators,anda“shor t”distanceof 18.3 mag,calculatedprimarilyfromRRLyraevariables. Clementinietal. ( 2003 )review

PAGE 166

152 theLMCdistancesandmethodsindetailandndthatthelonga ndshortdistance scalescanbereconciled,atleasttowithintheerrors,with improvedphotometry and/orreddeningestimates.Fromthedistancestheyhaveco llected(andcorrected), Clementinietal. ( 2003 )ndameanLMCdistanceof ( m M ) 0 = 18 : 515 0 : 085,in goodagreementwiththevalueadoptedby Freedmanetal. ( 2001 ). InanefforttodeterminethespatialdistributionoftheLMC clustersystemand improvetheaccuracyofthedistancetotheLMC,weapplythea pproachof GS02 to calculatingabsolutedistancesto17populousclustersint heLMC.Clusterdistances, combinedwiththegeometryoftheclustersystemallowustod etermineanaccurate distancetothecenteroftheLMC.In x 4.2 wediscussthenear-infrareddataacquisition, reduction,andphotometry.Theclusteragesandabundances necessaryforaccurately determining M RC K arepresentedin x 4.3 andin x 4.4 wecalculate K RC and M RC K forour clustersample.Finally,in x 4.5 ,clusterdistancesandthedistancetothecenterofthe LMCaregiven,withacomparisontoselectedpreviousworksi n x 4.6 .Ourresultsare summarizedin x 4.7 4.2Data 4.2.1Observations Wehaveobtainednearinfraredimagesofasampleofpopulous LMCclusters overthecourseofsixnights(20-22January2003and06-08Fe bruary2004)atthe CerroTololoInter-AmericaObservatoryBlanco4mtelescop e.Alldataweretakenwith theInfraredSidePortImager(ISPI),whichutilizesa2048 2048HAWAII2HgCdTe array.Inthef/8conguration,ISPIhasaeldofviewof 10 0 10 0 withaplatescale of 0 : 00 33pixel 1 .Atthetimeofourobservations,ISPIwasequippedwith J (1.25 m), H (1.64 m),and K 0 (2.12 m)ltersonloanfromGeminiandallclusterswere imagedinthe J -and K 0 -bandswithabouthalfoftheclustersalsohaving H -banddata. Averageseeingforallsixnightswas 1 : 2 00

PAGE 167

153 Table4–1.ExposureTimesatEachDitherPoint DatesJHK' 20-22Jan200360s15s 610s 9 06-08Feb20044s,20s,36s4s,15s 64s,10s 9 Eachclusterwasobservedwithanine-pointditherpattern, centeredonthecluster, withditheroffsetsrangingbetween30 00 and120 00 ,dependingonthesizeanddensity ofthetarget.Totalexposuretimeineachbandwasasfollows : J -540s; H -846s; K 0 -846s.Fortherstrun, H -and K 0 -bandimagesweresplitupintoshorterexposures toamelioratetheeffectsofskybrightnessinthenear-infr ared.Asweweretherst scienceusersofISPI,abetterunderstandingoftheinstrum ent,alongwithchangesin theelectronicsbetweenobservingruns,resultedinourgro upadjustingtheexposure timesplitsforthesecondobservingrun.Specically,duet otheshortrangeoverwhich theISPIdetectorislinear,wediscoveredtheneedtosplitu pthe J -bandimagesinto shorterexposuresinordertokeepmanyofthestarsfromfall ingintothenon-linear regime.Inaddition,forallthreebands,shortexposures(4 sateachditherpoint)were neededtoavoidsaturatingthebrighteststarsintheframe. InTable 4–1 ,wedetailthe exposurestimesforeachbandandobservingrunandinTable 4–2 welistourtarget clustersalongwiththeirpositionsonthesky,theltersin whichtheywereobserved, andtherunduringwhicheachclusterwasimaged.Forallbuto neoftheclusters observedduringbothruns,onlytheshort(4s)exposureswer etakenduringthesecond run;theexceptiontothisisNGC2155,forwhichtheentirese tof K 0 exposureswas obtainedduringthesecondrun.4.2.2Reduction Wehaveprocessedourdatausingstandarddatareductiontec hniques.Allimages havebeendarksubtracted,skysubtractedandthenatelde dusingon-offdomeats. Foreachtarget,skyframeswerecreatedbymediancombining theditheredcluster images,thuseliminatingthestarsandleavingonlytheskyi nthenalcombined

PAGE 168

154 Table4–2.LMCClusterSampleInformation ClusterAlternateR.A.Decl.FiltersRun Name(J2000.0)(J2000.0) NGC1651SL7,LW1243733 703508 JHK 0 1,2 SL61LW7945045 753200 J ::: K 0 2 NGC1783SL14845909 655914 J ::: K 0 2 NGC1846SL24350735 672731 J ::: K 0 2 NGC1978SL50152845 661409 JHK 0 1,2 Hodge4SL556,LW23753225 644412 JHK 0 1,2 IC2146SL632,LW25853746 744700 J ::: K 0 2 SL663LW27354229 652148 J ::: K 0 2 NGC2121SL725,LW30354812 712852 JHK 0 1,2 NGC2173SL807,LW34855758 725841 J ::: K 0 2 NGC2155SL803,LW34755833 652835 JHK 0 1,2 NGC2162SL814,LW35160030 634319 J ::: K 0 2 ESO121-0360203 603126 JHK 0 1,2 NGC2203SL836,LW38060443 752618 J ::: K 0 2 NGC2193SL839,LW38760618 650557 JHK 0 1,2 SL869LW44161441 694807 JHK 0 2 SL896LW48062958 692000 JHK 0 1,2 Note.—Unitsofrightascensionareinhours,minutes,andse condsand unitsofdeclinationareindegrees,arcminutes,andarcsec onds. skyframe.Beforeshiftingandcombiningourclusterimages wehadtoaddressthe problemofgeometricdistortions.ISPI'slargeeldofview causesimagestobecurved atthefocalplaneand,ifnotcorrected,nalframescreated byshiftingandcombining theditheredimageswillhaveseverelydegradedimagequali tyacrossmuchofthe frame.Thisproblemwasexacerbatedbythelargeoffsetsino urditherpattern.Using GalacticbulgestardatakindlyprovidedbyA.Stephens(200 3,privatecommunication), wecreatedandappliedahighorderdistortioncorrectionto ourimagesusingtheIRAF tasks geomap and geotran .Correctedimageswerethenaligned,shifted,andaverage combinedandbadpixelsweremaskedtocreateanalsciencei mageforeachcluster andlter.Thenalimagequalitywasexcellentandonlystar snearthecornersofthe frameexhibitedanysignsofdistortion.Wenotethatforeac hcluster,wehavecreated twoscienceimagesineachband;ashortexposureimage,crea tedbycombiningonly the4sexposuresfromeachditherpoint,andalongexposuret hatisacombinationof alldataforagivencluster.Asmentionedin x 4.2.1 ,theshortexposureswerenecessary foraccuratephotometryofthebrightRGBstars.InFig. 4–1 ,wepresent K 0 -band imagesofan 4 0 4 0 regionaroundeachofourtargetclusters.Wehaveusedthen al

PAGE 169

155 NGC 2121 SL 663 IC 2146 Hodge 4 NGC 1978 NGC 1846 NGC 1783 SL 61 NGC 1651 E N Figure4–1: K 0 -bandimagesforalltargetclusters.Wehaveusedthenalco mbined longexposuresandselectedaregion 4 0 4 0 insizearoundeachcluster. Inallframes,clustersarelabeledandtheorientationissu chthatnorthis upandeastistotheleft. combinedlongexposureforeachcluster,andinallframesno rthisupandeastisto theleft;clustersarelabeledinthelowerrighthandcorner 4.2.3Photometry UsingacombinationofDAOPHOTandALLSTAR( Stetson1987 ),wehave photometeredourimageswiththefollowingmethod.AroughP SFwascreatedfrom

PAGE 170

156 NGC 2193 SL 896 SL 869 NGC 2203 ESO 121 NGC 2162 NGC 2155 NGC 2173 E N Figure4–1: K 0 -bandimagesforalltargetclusters-Continued. thebrightest 200starsineachimage;wehavemadesuretoonlychoosestars that wereinthelinearregimeofthedetector.ThisroughPSFwast henusedtoremove neighborsfromaroundthefullsetof 50-150PSFstars(dependingoncluster),which allowedustocreateamorerobustPSFfromthecleanedimage. Next,ALLSTAR wasusedtottheimprovedPSFtoallstarsthatweredetected inthescienceframes. Inanefforttodetectandphotometerfaintstarsand/orcomp anions,weperformed asingleiterationwherewesubtractedallstarsphotometer edintherstALLSTAR

PAGE 171

157 pass,searchedforpreviouslyundetectedstars,andthenme asuredallofthenew detectionsandaddedthemtothephotometrylist.Aperturec orrections,calculatedfor eachscienceframe,werethenappliedtothePSFphotometry. Lastly,wecombined theaperturecorrectedphotometrylistsforeachlterwith therequirementthata starbedetectedinallavailablebandsforittobekeptinthe nalcombinedlistof instrumentalmagnitudes. Finally,tocalibratetheinstrumentalphotometryforeach cluster,webeganby matchingstarsincommonbetweenourlongandshortexposure s,thenthrowingout starsthatarenon-linearorsaturated(arebright)inthelo ngexposuresorhavelarge errors(arefaint)intheshortexposures.Typically,weare leftwithintermediate brightnessstarscoveringarangeof 2magoverwhichwecalculatetheoffset necessarytobringthelongexposurephotometryontothe`sy stem'oftheshort exposures.Afteroffsettingthelongexposurephotometry, wecombinethelongand shortphotometryinthreepieces;thebrightstarphotometr yistakenfromonlythe shortexposures(longexposuresarenon-linearorsaturate d)whilethefaintstarscome onlyfromtheoffsetlongexposurephotometry(starshavela rgeerrorsorarenot detectedintheshortexposures).Theintermediatebrightn essstars,whichhavegood photometryfromboththelongandshortexposures,areavera gedtogetherforthenal catalogofeachcluster.Toputourphotometryontoastandar dsystem,wematchour starswiththoseintheAll-SkyDataReleaseoftheTwoMicron AllSkySurvey 1 (2MASS).Wehaverestrictedthe2MASSselectiontoonlythos estarspossessingeither apertureorPSFttingphotometryandhavingerrorslesstha t0.1mag.Zeropoint offsetsforeachbandarethencalculatedandappliedtoourp hotometry.Inthelast stepofourcalibration,wefollowtheapproachof GS02 andconvertourphotometry (onthe2MASSsystem)tothe Bessell&Brett ( 1988 )systemusingtheconversions 1 http://www.ipac.caltech.edu/2mass/releases/allsky

PAGE 172

158 Table4–3.LMCClusterAgesandMetallicities Cluster[Fe/H] a s [ Fe = H ] a LogAgeAge(Gyr)CMDRef. NGC1783 0 : 47 b 0.14 b 9.08 d 1.20 NGC1846 0 : 490.039.101.267 NGC2162 0 : 460.079.151.411 NGC2203 0 : 410.039.151.417 SL869 0 : 400.049.151.416 SL61 0 : 350.049.181.514 NGC2173 0 : 420.039.201.581 IC2146 0 : 410.029.251.787 NGC1978 0 : 38 c 0.07 c 9.27 d 1.86 NGC1651 0 : 530.039.281.911 NGC2193 0 : 490.059.302.003 Hodge4 0 : 550.069.332.145 SL896 0 : 48 e 0.09 e 9.332.146 NGC2155 0 : 500.059.452.821 SL663 0 : 540.059.452.821 NGC2121 0 : 500.039.483.025 ESO121-03 0 : 91 f 0.16 f 9.958.912 Note.—OpticalphotometryusedtoconstructtheCMDscomesf romthe followingsources:(1) Brocatoetal. ( 2001 );(2) Bicaetal. ( 1998 );(3)HST GO-5475;(4) Mateo&Hodge ( 1985 );(5) Sarajedini ( 1998 );(6) Piattietal. ( 2002 );(7)Grocholskietal.(2007,inprep) a From Grocholskietal. ( 2006 ),unlessnoted. b FromColeetal.(inprep) c From Ferraroetal. ( 2006 ) d Agesadjustedfrom Geisleretal. ( 1997 ) e Meanvalueoftheintermediate[Fe/H]clustersfrom Grocholskietal. ( 2006 ) f From Hilletal. ( 2000 ) presentedby Carpenter ( 2001 ,theirEqs.A1-A4).Thisstepisnecessaryasitplaces ourphotometryonthesamesystemasthe Girardi&Salaris ( 2001 )models(see x 4.4 ). Wenotethatwehavenottanycolortermsinourcalibrationd uetothesmallrange incolor( 0.5mag)coveredbytheRGBinadditiontothesimilarityofth eISPIand 2MASSltersystems. 4.3ClusterAgesandAbundances Asmentionedin x 4.1 GS02 showedthatknowledgeofapopulouscluster'sage andmetallicityisimperativetoaccuratelypredicting M RC K ,andthusdeterminingthe cluster'sdistance.Thisisespeciallytrueforclusterswi thlog(Age) < 9.3or[Fe/H] < 0 : 4,tworegionsofparameterspacewhere M RC K canvaryrapidly(seeFigs.5and6 in GS02 )andinwhichmanyLMCclustersreside.

PAGE 173

159 Fortheclustermetallicities,weturnprimarilytotherece ntworkof Grocholskietal. ( 2006 ).Intheirpaper,theypresent[Fe/H]for28populousLMCclu sters,derivedfrom thestrongnearinfraredabsorptionlinesoftheCa II triplet;allbutfouroftheclusters inoursample(ESO121-03,NGC1783,NGC1978,andSL896)have metallicities in Grocholskietal. ( 2006 ).RedgiantsinNGC1783werestudiedbyA.A.Coleet al.(2007,inpreparation)usingtheCa II tripletinanalmostidenticalapproachtothat of Grocholskietal. ( 2006 ),soweadopttheirmetallicity( 0 : 47 0 : 14dex)forthis cluster.ForNGC1978,weusethemetallicitycalculatedby Ferraroetal. ( 2006 ), whichisbasedonhighresolutionspectraof11redgiantstar s.Wenotethattheirvalue of 0 : 38 0 : 07dexisingoodagreementwiththeresultsofA.A.Coleetal. (2007, inpreparation),whond[Fe/H]= 0 : 35 0 : 07.UsingUVESontheVLT, Hilletal. ( 2000 )obtainedhighresolutionspectrafortwogiantstarsinESO 121-03andfound [Fe/H]= 0 : 91 0 : 16,whichwewilladoptforthispaper.Finally,whilethesma ll clusterSL896hasnopreviouslypublishedspectroscopical lyderived[Fe/H]available, theresultsof Grocholskietal. ( 2006 )showthattheintermediatemetallicityLMC clustershaveaverytightspreadinmetallicity( s = 0 : 09),withameanmetallicity of 0 : 48dex.Thus,weadoptthesevaluesasthemetallicityanderr orforSL896. Clustermetallicitiesanderrorsarepresentedincolumns2 and3ofTable 4–3 Themostreliablewaytodetermineclusteragesisbycompari ngthepredictions oftheoreticalisochronestotheluminosityofacluster'sm ainsequenceturnoff. However,nolargescale,homogeneousdatabaseofmainseque ncetting(MSF)ages existsforLMCclusters.Toaddressthisshortcoming,wehav ebeguntocompile opticalphotometrythatreachesbelowthemainsequencetur noff(MSTO)foralarge numberofLMCclusters.Whiletheentirestudywillbepresen tedinafuturepaper (Grocholskietal.2007,inpreparation),wehereinprovide abriefdescriptionofthe datasetandttingmethodthatareusedtoderiveclusterage s,aswellaspresent agesforasub-sampleofclusters.Opticalphotometrywasta kenprimarilyfromthe

PAGE 174

160 literatureandincolumn7ofTable 4–3 ,welisttheCMDsources.Inafewcases, wehaveusedunpublishedopticalimages,obtainedwitheith erVLTFORS2(NGC 1846,NGC2203,IC2146;see Grocholskietal.2006 )or HST WFPC2(NGC2193; programnumberGO-5475).Forthethreeclusterswith V and I bandVLTFORS2 images,starswereidentiedandphotometeredwiththeaper turephotometryroutinesin DAOPHOT( Stetson1987 )andthenmatchedtoformcolors.Currently,thephotometry forthesethreeclustersisuncalibrated;however,thecolo rtermsfortheFORS2array aresmall( 0.03in V I )andthushavelittleeffectontheshapeoftheMSTO/RC region,whichspansacolorrangeofonly 0.6magin V I .RegardingNGC2193, theoneclusterinourinitialsamplewithunpublished HST WFPC2photometry,we retrievedF450WandF555WimagesfromtheHSTarchive.These pipelineprocessed imageswerephotometeredviatheprocedureoutlinedby Sarajedini ( 1998 ),including the Holtzmanetal. ( 1995 )transformationcoefcients.Sincethephotometriczero pointsforWFPC2arerelativelyuncertain,andtheFORS2dat aisuncalibrated,we proceedwithMSFasfollows.UtilizingtheZ=0.008([Fe/H] 0.4)andZ=0.004 ([Fe/H] 0.7)theoreticalmodelsfromthePadovagroup( Girardietal.2002 ),which includetreatmentforcoreovershoot,werstshifttheisoc hronesverticallytomatch thebrightnessoftheRCandthenmovethemhorizontallytoma tchthecolorofthe unevolvedmainsequence.Forillustrativepurposes,NGC16 51andNGC2173are showninFig. 4–2 ,withtheZ=0.008isochronesoverplottedforlog(Age)=9.2 5 and9.30forNGC1651and9.15,9.20,and9.25forNGC2173;bas edonthesets, weadoptagesoflog(Age)=9.28(1.91Gyr)and9.20(1.58Gyr) forNGC1651and NGC2173,respectively,andweestimatetheerrorinourtst obe 0.05intermsof log(Age).Table 4–3 givesMSFagesforallclustersinourpreliminarysamplewit h availableopticalphotometry.WhileneitherNGC1783norNG C1978hasreliable photometryavailableintheliterature,bothhaveagesdete rminedby Geisleretal. ( 1997 ),whousedthedifferencein V -bandmagnitudebetweenthecluster'sRCand

PAGE 175

161 Figure4–2:OpticalphotometryforNGC1651andNGC2173,ove rplottedwiththeZ =0.008theoreticalisochronesfrom Girardietal. ( 2002 );isochroneages arelistedinthegure.TheseplotsillustrateourMSFmetho dwherewe matchisochronestothebrightnessoftheRCandcoloroftheu nevolved mainsequencetodetermineclusterages. mainsequenceturnofftoestimateclusterages.Forcluster sincommon,wendan offsetof0.03inlog(Age),whereourMSFagesareyoungertha ntheirages.Therefore, forNGC1783andNGC1978,weoffsetthevaluesin Geisleretal. ( 1997 )andadopt theseastheagesforNGC1783andNGC1978. 4.4ApparentandAbsolute K -bandRCMagnitudes TocalculatetheapparentandabsoluteRCmagnitudes,wegen erallyfollowthe methodprescribedby GS02 .Theydeterminetheapparent K -bandmagnitudeofthe RC( K RC )byplacingastandardsizedbox(0.8magin K and0.2magin J K )around theRC;themedianvalueofallstarswithinthisboxistakena s K RC .Aconstant boxsizeisusedinconjunctionwiththemedianmagnitudeoft heRCinaneffortto eliminateanyselectioneffectsthatmayoccurinchoosingt helocationofthebox, aswellastolimittheeffectsofoutlierson K RC .Inafewcases,wehavehadto shifttheboxcenterslightyincolorsoastoavoidcontamina tionfromRGBstars.

PAGE 176

162 ForpredictingtheabsoluteRCmagnitude( M RC K ), GS02 combinedavailable2MASS photometry( JK S )for14Galacticopenclusters,whichalsohaveinternallyc onsistent ages,abundances,anddistances,withaninterpolationrou tinebasedonloworder polynomials.Theinterpolationovertheopenclustersallo wsthepredictionof M RC K for atargetclusterwithaknownageand[Fe/H].Thismethodwasa ppliedtoNGC2158 by GS02 andtoHodge4andNGC1651by Sarajedinietal. ( 2002 ),allwithpromising results. GivenISPI'slargeeldofview,beforewecanmeasure K RC wemustseparate theclusterstarsfromtheeldbyperformingradialcutsono urdata.Whereavailable, weusetheclusterradiiasdeterminedby Grocholskietal. ( 2006 ,valuesunpublished), whichwerebasedonthekinematicsofindividualstars;typi cally,thefartheststar fromtheclustercenterthatismovingatthevelocityofthec lusterdenotestheadopted radius.Forthefourclustersnotincommonwiththeirstudy, radialcutswerechosen byeye,usingacombinationofclusterimagesandourphotome triccatalogs.Wenote thatsmallvariationsintheadoptedclusterradiihavenoap preciableeffectonour results.InFig. 4–3 ,wepresenttheresulting K vs. J K clusterCMDs,whichextend fromthetipoftheRGBto 1.5magbelowtheheliumburningRC;thestandardsize boxusedincalculating K RC isshown.Foreachcluster,themeasuredvalueof K RC is givenincolumn2ofTable 4–4 ,alongwiththestandarderrorofthemedian(column 3)andnumberofRCstarsineachbox(column4). Ideally,wewouldliketopredict M RC K usingtheopenclusterdatapresented in GS02 .Inpractice,however,thisisdifcultsinceourLMCcluste rsamplefalls outsideoftheparameterspace(inmetallicity)coveredbyt heopenclusters;tests ofanextrapolationtothetargetclusterabundancesproved tobequestionableat best.Instead,weturntothetheoreticalmodelsof Girardi&Salaris ( 2001 ,seealso Girardietal.2000 ),whichprovideexpectedvaluesof M RC K thatspanalargerange ofagesandmetallicitiesandencompassourLMCtargetclust ers. GS02 testedtheir

PAGE 177

163 Table4–4.CalculatedRedClumpValuesandClusterDistance s Cluster K RC s K RC nM RC K s M RC K E ( B V ) A K ( m M ) 0 s ( m M ) 0 D s D NameStars (kpc)(kpc) NGC165116.930.0293 1.560.020.100.03418.460.0349.10.6 SL6117.010.0322 1.520.080.110.03818.490.0949.92.1 NGC178316.930.01384 1.100.180.020.00718.020.1840.23.4 NGC184616.980.01301 1.170.190.060.02018.130.1942.33.8 NGC197816.860.01231 1.560.020.050.01718.400.0247.90.5 Hodge416.810.0248 1.570.020.040.01418.370.0347.10.6 IC214617.010.0272 1.560.020.120.04118.530.0350.80.8 SL66316.840.0429 1.520.020.040.01418.350.0446.70.9 NGC212116.830.02184 1.510.020.100.03418.310.0245.80.5 NGC217316.940.0362 1.530.040.100.03418.440.0448.71.0 NGC215516.780.0263 1.530.020.030.01018.300.0345.70.7 NGC216217.100.0372 1.490.180.030.01018.580.1852.04.5 ESO12116.930.0320 1.200.060.030.01018.120.0642.11.3 NGC220316.970.02128 1.480.160.110.03818.410.1748.13.8 NGC219316.880.0428 1.580.010.040.01418.450.0448.90.9 SL86917.120.0615 1.480.160.100.03418.570.1751.74.3 SL89616.890.077 1.580.010.090.03118.440.0748.71.6 Note.—Allnumbersaregiveninmagnitudesunlessotherwise noted. openclusterdataagainstthesetheoreticalmodelsandfoun dgoodagreement,withall clusterslyingwithin1.5 s oftheappropriatemodelandnosystematicoffset.Since theircomparisonwasbasedondatafromtheSecondIncrement alDataReleaseofthe 2MASSPointSourceCatalog,wehaverecomparedthemodelsan dthedata,using theupdated2MASSAllSkyDataRelease.Withthenew2MASSpho tometry,we stillndgoodagreementwiththemodels,however,thereisn owanoffsetof0.08 mag,inthattheobservedRCvaluesarebrighterthanwhatisp redictedbythemodels. Wediscussthisinmoredetailin x 4.5.4 .Giventheagesandmetallicitieslistedin Table 4–3 ,weareabletodetermine M RC K foreachLMCclusterbyinterpolatingover the Girardi&Salaris ( 2001 )models;predictedvaluesof M RC K arepresentedinTable 4–4 .Thequotederrorin M RC K iscalculatedbyaddinginquadraturetheeffectsofage andabundanceerrorsonthepredictedabsoluteRCmagnitude .Wenotethattheve youngestclustersinoursamplehaverelativelylargeerror barsduetothefactthattheir agesplacetheminaregionwheretheRCbrightensrapidlywit hincreasingage(see Fig.4in GS02 );thus,smallerrorsinageresultinlargeerrorsin M RC K

PAGE 178

164 Figure4–3:Near-infraredCMDsforthe17clustersinoursam ple.ClusterRCsare denotedbytheboxandallstarswithinthisboxareusedincal culating M RC K

PAGE 179

165 Figure4–3:Near-infraredCMDs-Continued

PAGE 180

166 Figure4–3:Near-infraredCMDs-Continued

PAGE 181

167 4.5ClusterDistancesandtheDistancetotheLMC 4.5.1AbsoluteDistanceModuli With K RC and M RC K inhand,clusterreddeningsareallthatisneededtocalcula te absolutedistancemoduli.Theextinctionmapsofboth Burstein&Heiles ( 1982 )and Schlegel,Finkbeiner,&Davis ( 1998 )covertheentireLMC;however, Schlegeletal. ( 1998 )werenotabletoresolvethetemperaturestructureinthein nerportionsofthe LMCand,therefore,couldnotestimatethereddeningreliab ly.Formostclusters,the tworeddeiningmapsgivevaluesingoodagreement,although assomeofourclusters lieintheunresolvedregion,weadopt E ( B V ) valuessolelyfrom Burstein&Heiles ( 1982 )andassumeanerrorof20%.Reddeningsareconvertedto A K usingthe extinctionlawof Cardelli,Clayton,&Mathis ( 1989 ),where R V = 3 : 1and A K = 0 : 11 A V .Wenotethat,since A K isapproximatelyonethirdof E ( B V ) ,anydifferences betweenthetwoextinctionmapsareultimatelynegligible. InTable 4–4 wegive E ( B V ) and A K fortheclustersample.WithabsoluteandapparentRCmagnit udes andreddeningsforeachcluster,theabsolutedistancemodu li, ( m M ) 0 ,arereadily calculatedas ( m M ) 0 =( m M ) K A K ; (4–1) where ( m M ) K = K RC M RC K .TheclusterdistancemoduliarelistedinTable 4–4 alongwiththeirerrors,whicharefoundbyaddinginquadrat uretheerrorsin K RC M RC K ,and E ( B V ) .Wealsoconvertthesenumberstoaphysicaldistanceusingt he relation, D = 10 [( m M ) 0 + 5 ] = 5 ,where D isinparsecs. 4.5.2LMCClusterDistribution IthaslongbeenknownthatthediskoftheLMCisinclinedwith respectto theplaneofthesky(seee.g., Caldwell&Coulson1986 ),andthisinclinationisan importanteffectwhenusingindividualstars(orclusters) todeterminethedistance totheLMCcenter.Recentworkusingeldstarsasatraceroft hedisk(tipofthe RGBandAGB, vanderMarel&Cioni2001 ;eldRCstars, Olsen&Salyk2002 ;

PAGE 182

168 carbonstars, vanderMareletal.2002 ;Cepheidvariables, Nikolaevetal.2004 )has shownthattheLMC'sinclination, i ,is 31 36 ,withapositionangleofthelineof nodes, Q ,between120 and150 ;bothofthesequantitieshavethestandarddenitions where i = 0 forafaceondiskand Q ismeasuredcounterclockwisefromnorth.The LMCcentersadoptedbyeachoftheseauthors,inadditiontot heirderivedvaluesfor Q and i ,aregiveninTable 4–7 .InFig. 4–4 weplotthepositionsontheskyofour targetclustersaswellastheLMCcentersadoptedby vanderMarel&Cioni ( 2001 lledsquare ), vanderMareletal. ( 2002 lledtriangle ),and Olsen&Salyk ( 2002 lledstar ).Thesolidlinespassingthroughthesepointsshoweachaut hor'sposition angleofthelineofnodes.Wenotethat,forclarity,wehaven otplottedthecenterand positionangleofthelineofnodesfrom Nikolaevetal. ( 2004 )astheyareverysimilar tothevaluesin Olsen&Salyk ( 2002 ).Forreference,the2 near-infraredisopleth ( vanderMarel2001 ),whichroughlyoutlinestheLMCbar,isplottedasthedashe d ellipse.ConversiontoCartesiancoordinatesfromrightas censionanddeclinationwas performedusingazenithalequidistiantprojection(e.g., vanderMarel&Cioni2001 theireqs.[1]-[4]);linesofrightascensionanddeclinati onhavebeenmarkedwith dottedlines.Ingeneral,thesegeometriestellsusthatthe northeastportionoftheLMC isclosertousthanthesouthwest.Morespecically,sincep ointsalongthelineof nodesareequidistantfromtheobserver,inthedirectionpe rpendiculartothelineof nodeswewouldexpecttoseeamaximumgradientinclusterdis tance. Tocompareourclusterdistributionwiththegeometryofthe LMC,inFig. 4–5 weplotclusterdistanceasafunctionofradialdistancealo ngthelineofmaximum gradient.Whilewehaveusedthegeometryof vanderMarel&Cioni ( 2001 )to determinethepositionofthelineofmaximumgradient,thec hoiceinLMCgeometry betweenthesethreerecentstudieshaslittleeffectonther esults(see x 4.5.3 ).Inthe toppanel,clustersarelabeledforreferenceandinthebott ompanelwehaveincluded the1 s distanceerrors.Inaddition,thedashedlinerepresentsth ediskoftheLMC,

PAGE 183

169 Figure4–4:Schematicdiagramshowingthepositionsonthes kyofourtargetclusters.Thedashedellipserepresentsthe2 near-infraredisoplethfrom vanderMarel ( 2001 ),whichroughlyoutlinestheLMC'sbar.Alsoshown aretheLMCcentersusedby vanderMarel&Cioni ( 2001 lledsquare ), vanderMareletal. ( 2002 lledtriangle ),and Olsen&Salyk ( 2002 lledstar ).Thepositionangleofthelineofnodesderivedbyeachof theseauthorsisplottedasthesolidlinepassingthroughth eappropriate LMCcenter.

PAGE 184

170 Figure4–5:Clusterdistancesasafunctionoftheirpositio nalongthelineofmaximum gradient(see x 4.5.2 ).Opencirclesmarktheoldglobularclustersfrom Walkerwhilethelledcirclesrepresentthepopulousclust ersinourstudy. Inthebottompanel,thedashedlinemarkstheLMC'sdiskwith i = 34 : 7 and D 0 = 47 : 9kpc(at x =0),andthedottedlinesrepresentadiskthicknessof 1kpc;thelledsquaredenotesthecenteroftheLMC.Thisplo t illustratesthattheboththeoldandintermediateageclust ersaredistributed alongthediskoftheLMC.

PAGE 185

171 wheretheLMCcenter( x = 0)hasadistanceof47.9kpc(see x 4.5.3 )and i = 34 : 7 ( vanderMarel&Cioni2001 );thedottedlinerepresentsaconstantdiskthickness of 1kpc.Whileaareddiskmodel( Alves&Nelson2000 )isprobablyamore correctrepresentationoftheLMC'sdisk,forthepurposeso fourcomparisonaconstant thicknessdiskmodelisadequate.Regardless,Fig. 4–5 showsthat,withtheexception oftheyoungestclusters,whichhaveinherentlyuncertaind istances,ourresultsarein excellentagreementwiththeideathattheLMCclustersliei nthesameinclined,thick diskasdenedbyavarietyofeldpopulations. Adisk-likeclusterdistributionisasexpected,basedonth ekinematicsofthe clustersystem( Schommeretal.1992 ),butthisisthersttimeithasbeendemonstratedthattheclustersandeldstarsresideinthesamedi sk.Thisresultisincontrast totherecentndingsof Kerberetal. ( 2006 ),whousedtheMSTOtocalculatedistancesfor15LMCclusters.Fromtheirdatatheyfoundadisklikedistributionfor theirclusters,alongwithaninclinationof39 7 ,whichis 8 steeperthanthe 30 : 7 1 : 1diskinclinationthat Kerberetal. ( 2006 )adoptedfrom Nikolaevetal. ( 2004 ). Kerberetal. ( 2006 )interpretedthisinclinationdifferenceasbeingindicat ive oftheLMCintermediate-ageclustershavingformedinadiff erentdiskthantheeld stars.However,theydiscussneithertheresultsof vanderMarel&Cioni ( 2001 )nor Olsen&Salyk ( 2002 ),whonddiskinclinationsof34 : 7 6 : 2and35 : 8 2 : 4, respectively,bothinagreementwiththeclusterdiskincli nationfoundby Kerberetal. ( 2006 ). Wenoteinpassingthat Olsen&Salyk ( 2002 )foundwhatappearstobeawarp inthesouthwestportionoftheLMC.Theireldsinthisregio narebrighterthan expected,givingtheimpressionthattheyhavebeenpulledt owardtheMW.Thereis, however,apossibleproblemwiththereddeningcorrections that Olsen&Salyk ( 2002 ) haveappliedtotheseeld,whichmayexplaintheapparentwa rp.Asonlytwoofour

PAGE 186

172 targetclusters,NGC1651andSL61,lieinthewarpedarea,we arenotinaposition tocommentontheirresult. SincewehaveshownthatourclusterslieinthediskoftheLMC ,itispossibleto usetheiragestoinfertheageofthedisk.ESO121,thecluste rinourIRsamplethat isbothfarthestfromtheLMCcenterandtheoldest,appearst oresideinthedisk.At itsdistancefromtheLMCcenter,ESO121islikelytohavehad veryfew,ifany,close encounterswithothermassiveobjects.Hadanencounterlik ethisoccurred,wewould expectESO121tohavebeenkickedoutofthediskandtakeonha lo-likekinematics. SinceESO121,liketheentireLMCclustersystem,exhibitsd isk-likekinematics ( Schommeretal.1992 ),itcanbereasonablyassumedthatESO121remainsvirtuall y unperturbedfromitsoriginalorbit.Therefore,ifwecanal soassumethatESO121is thelastsurvivorofapopulationofclusterswithsimilarag es,andisrepresentativeof starsthatformedaroundthesametime,thenitspositionimp liesthatthediskofthe LMCisatleast 9Gyrold. AsESO121iswellknowntobetheonlyclusterintheLMCwithan age betweenapproximately3Gyrand13Gyr,tofurtherexploreth eageofthedisk,we turntotheLMC's bonade old( 13Gyr)globularclusterpopulationandtheoptical photometryofA.R.Walker(see Walker1985 ; Walker&Mack1988 ; Walker1989 1990 1992b c 1993 ).Walkermeasuredthemeanapparent V -bandmagnitude( V RR )of RRLyraestarsinsevenLMCglobularclustersand,usingthei rpulsationalproperties, wasabletoestimateclustermetallicities.Giventhemetal licityofacluster,themean absoluteRRLyraemagnitudeisdeterminedby M RR V = 0 : 23 [ Fe = H ]+ c ( Chaboyer 1999 ),andbyadoptingreddeningsfrom Burstein&Heiles ( 1982 )wecanreadily calculatedistancesforthesesevenclusters.Thezeropoin t, c ,intheaboverelationis chosensuchthatNGC1835liesonthedashedline.Clusterinf ormationisgivenin Table 4–5 ,andthesenewdatapointsareplottedinFig. 4–5 asopencircles,alongwith their1 s errors.Theerrorsin[Fe/H]and V RR aretakenfromWalkerandweassume

PAGE 187

173 Table4–5.LMCGlobularClusterInformation ClusterR.A.Decl.[Fe/H] V RR E ( B V ) D (Name)(J2000.0)(J2000.0)(dex)(mag)(mag)(kpc) NGC1466034433.35 714017.7 1 : 9 0 : 119.33 0.020.0551.8 1.0 Reticulum043611.00 585140.0 1 : 7 0 : 119.07 0.010.0047.3 1.5 NGC1841044523.83 835949.0 2 : 2 0 : 219.31 0.020.1147.6 1.9 NGC1786045907.82 674442.8 2 : 3 0 : 219.27 0.030.0650.8 1.6 NGC1835050506.58 692413.9 1 : 8 0 : 219.38 0.050.0948.5 2.0 NGC2210061131.36 690717.0 1 : 9 0 : 219.12 0.020.0943.5 1.5 NGC2257063013.00 641929.1 1 : 8 0 : 119.03 0.020.0444.4 0.8 Note.—Unitsofrightascensionareinhours,minutes,andse condsandunitsofdeclinationarein degrees,arcminutes,andarcseconds. a20%errorin E ( B V ) forallclustersexceptReticulum,forwhichweadopt0.02 mag.Fig. 4–5 showsthat,liketheintermediateageclusters,theoldglob ularclusters aredistributedinamannerthatisconsistentwiththethick ,inclineddiskgeometryof theLMCeldstars.Withtheagreementbetweentheoldcluste rsandthedisk,and giventheirdistancesfromthecenter,theargumentcanbema dethatclusterslikeNGC 2257andNGC1466musthaveformedin,andhaveremainedin,th ediskoftheLMC. Thus,thediskisatleastthesameageastheglobularcluster s, 13Gyrold. Lastly,wenotethepositionofNGC1841.Thisclusterreside s 12kpcfromthe LMCcenter(tothesouth),whichplacesitnearthetidalradi us( r t = 15 : 0 4 : 5kpc, vanderMareletal.2002 )oftheLMC,and,ascanbeenseeninFig. 4–5 ,itsitswell outoftheplaneofthedisk,inthedirectionoftheMilkyWay. Thus,NGC1841is likelytohaveeitherbeenpulledoutofthedisk,orstripped fromtheLMCaltogether, inacloseencounterwiththeMilkyWay.4.5.3TheDistancetotheLMCCenter Foranygivenpoint, P ,thatresidesinthediskoftheLMC,thedistance, D ,of thatpointisrelatedtothedistancetothecenteroftheLMC, D 0 ,by D = D 0 = cos i = [ cos i cos r sin i sin r sin ( f q )] ; (4–2) where i istheinclinationofthediskand q = Q + 90(see vanderMarel&Cioni2001 foradetaileddiscussionofequations 4–2 4–5 ).Theangularcoordinate r isdened

PAGE 188

174 astheangularseparationontheskybetween P andtheLMCcenter,while f isthe positionangleof P relativetothecenter.Typically, f ismeasuredcounterclockwise fromtheaxisthatrunsinthedirectionofdecreasingrighta scensionandpasses throughtheLMCcenter.Thesecoordinates( r f )canbeuniquelydenedbythe cosineandsineruleofsphericaltrigonometryandtheanalo gformula,whichgive cos r = cos d cos d 0 cos ( a a 0 )+ sin d sin d 0 ; (4–3) sin r cos f = cos d sin ( a a 0 ) ; (4–4) and sin r sin f = sin d cos d 0 cos d sin d 0 cos ( a a 0 ) : (4–5) Inequations 4–3 4–5 a 0 and d 0 aretherightascensionanddeclinationofthe LMCcenterwhile a and d markthepositionontheskyof P .Therefore,sinceitis reasonabletoassumethatourtargetclusterslieinthedisk oftheLMC,asdenedby theeldstars( x 4.5.2 ),wecanusethedistancesofourclustersinconjunctionwit hthe LMCgeometrytocalculatethedistancetothecenteroftheLM C. Asanexample,weadopt i = 34 : 7and Q = 122 : 5( vanderMarel&Cioni 2001 ),andcalculatevaluesfortheLMCcenterdistancebasedont hedistanceand positionofeachofour17targetclusters.Rawclusterdista ncesfromTable 4–4 and thecorrespondingLMCdistancearegiveninTable 4–6 withtheLMCdistanceerrors calculatedbypropogatingtheerrorsin i Q ,and D throughequation 4–2 .Finally,we calculatethedistancetotheLMCasthemeanoftheindividua lcenterdistances,for whichwend D 0 = 47 : 9 0 : 9kpc,or ( m M ) 0 = 18 : 40 0 : 04;thequotederroris thestandarderrorofthemean.Wenotethat,whilecalculati ngthestraightmeandoes includetheyoungclusters,whichhaveuncertaindistances ,wehavefoundthatthe mean,median,weightedmean,and2 s clippedmeanallgivedistanceswithin0.01mag ofeachother,thuswehavechosentosimplyadoptthemeanaso urnaldistance.In additionto vanderMarel&Cioni ( 2001 ),wealsousethegeometryof Olsen&Salyk

PAGE 189

175 Table4–6.LMCCenterDistances Cluster D s D D 0 s D 0 Name(mag)(mag)(mag)(mag) NGC165118.460.0318.350.04SL6118.490.0918.300.10NGC178318.020.1818.050.18NGC184618.130.1918.140.19NGC197818.400.0218.470.03Hodge418.370.0318.470.04IC214618.530.0318.410.04SL66318.350.0418.450.05NGC212118.310.0218.280.03NGC217318.440.0418.380.05NGC215518.300.0318.420.04NGC216218.580.1818.730.18ESO12118.120.0618.330.08NGC220318.410.1718.290.17NGC219318.450.0418.580.05SL86918.570.1718.600.17SL89618.440.0718.490.07 Table4–7.EffectofLMCGeometry GeometryR.A.Decl. Q i ( m M ) 0 D 0 (Reference)(J2000.0)(J2000.0)(deg)(deg)(mag)(kpc) vanderMarel&Cioni ( 2001 )52900 693000122.5 8.334.7 6.218.40 0.0447.9 0.9 Olsen&Salyk ( 2002 )51938.0 692705.2145 435.8 2.418.41 0.0448.0 0.9 vanderMareletal. ( 2002 )52736 695212129.9 6.034.7 6.218.40 0.0447.9 0.9 Note.—Unitsofrightascensionareinhours,minutes,andse condsandunitsofdeclinationareindegrees,arcminutes,andarcseconds.DistancesgivenarefortheLMCcenter ,calculatedbycombiningourclusterdistanceswiththegiv en LMCgeometry. ( 2002 ), vanderMareletal. ( 2002 ),and Nikolaevetal. ( 2004 )tocalculatethedistance totheLMC,withallfourmeandistancesgiveninTable 4–7 .Thenaldistances, D 0 = 47 : 9 0 : 9kpc,48 : 1 0 : 9kpc,47 : 9 0 : 9kpc,and48 : 1 0 : 9kpc,areallinexcellent agreement,whichshowsthatthechoiceofgeometrybetweent hesefourauthorshas littleeffectonthedistancetotheLMC.4.5.4SystematicErrors Ananalysisofourapproachtocalculatingclusterdistance sgivestwopossible sourcesofsystematicerrors.Therstsourceoferrorinour calculationsarisesfrom ourinterpolationmethod.Asdiscussedin x 4.4 ,duetothelocationofourtarget clustersintheage-metallicityparameterspace,wearenot abletointerpolateover

PAGE 190

176 theopenclusterdatain GS02 .Instead,wehavehadtousethetheoreticalmodelsof Girardi&Salaris ( 2001 )forourinterpolation.Whilethemodelsareingoodagreeme nt withtheopenclusterdata,theypredictabsolutemagnitude sthatare,onaverage,0.08 magfainterthanwhatisobserved.Anadditionalsystematic errormayarisefromour choiceofreddeningmap. Burstein&Heiles ( 1982 )zeropointtheirreddeningmaps toanareanearthenorthgalacticpolewhichwaslongbelieve dtobeadirectionof zeroreddening. Schlegeletal. ( 1998 ),however,nd E ( B V )= 0 : 02magforthe samelocationonthesky.Thesetwosystemticerrorsworkino ppositedirections;if weappliedacorrectionfortheinterpolationerror,cluste rswouldmove closer ,while acorrectionforthereddeningerrorwouldmakethemappearf artheraway.However, since A K = 0 : 341 E ( B V ) ,thesystematicreddeningerrorissmallandisdominated bythesystematicerrorduetoourinterpolation.Therefore ,weadopt0.08magasour systematicerror. 4.6ComparisontoPreviousDistances SinceanextensivereviewofLMCdistancesdeterminedbyava rietyofstandard candlescanbefoundin Clementinietal. ( 2003 ),hereinwerestrictourcomparisonto onlyacouplerecentdistancecalculations.Theonlyprevio usLMCclusterdistances basedonthe K -bandluminosityoftheRCarepresentedin Sarajedinietal. ( 2002 ) and,usingtheapproachdescribedin GS02 ,theynd ( m M ) 0 = 18 : 55 0 : 12and 18 : 52 0 : 17forNGC1651andHodge4,respectively.Bothdistancesare farther thanwhatwendforthesameclusters,dueprimarilytotheir photometriccalibration. Forbothclusters, Sarajedinietal. ( 2002 )measure K RC tobe 0.1magfainterthan ourvalues.Giventhesmallnumberofstandardstarsusedby Sarajedinietal. ( 2002 ) alongwiththeirsmalleldofview,whichprovidedonlyahan dfulofstarsforaperture correction,thisdifferenceinphotometriczeropointisno tunexpected. Mostrecently, Macrietal. ( 2006 )observedCepheidvariablesintwoeldsin themaser-hostgalaxyNGC4258.BycomparingtheLMC'sCephe idP-Lrelation

PAGE 191

177 totheirobservationsofvariablesinNGC4258, Macrietal. ( 2006 )wereableto calculatea relative distancebetweenthesetwogalaxiesof D ( m M ) 0 = 10 : 88 0 : 04 (random) 0 : 05(systematic).Beingamaser-hostgalaxy,NGC4258hasana ccurate geometricdistance(29 : 29 0 : 09 0 : 12mag)that,combinedwiththeCepheidbasedrelativedistance,allowed Macrietal. ( 2006 )tocalculatethedistancetothe LMC.Theynd ( m M ) 0 = 18 : 41 0 : 10 0 : 13,inexcellentagreementwithour results.AsdiscussedbyMarcietal.(2006),thisimprovedd istancehasimplications forcalculationsof H 0 .The HST KeyProjecttodeterminetheHubbleconstant(see Freedmanetal.2001 )adopted ( m M ) 0 = 18 : 5 0 : 1astheirdistancetotheLMC, whichactsasthezeropointfortheextragalacticdistances cale.Usingthislonger distance, Freedmanetal. ( 2001 )nd H 0 = 72 8kms 1 Mpc 1 .Inrecalculating H 0 Macrietal. ( 2006 )ndthattheshorterLMCdistanceincreasestheHubble constant 3%.However,theyndthattheirnewcoefcientofmetallici tydependence forCepheidvariableshastheoppositeeffect,changing H 0 by 2%.Thus,the cumulativeeffectresultsinonlyasmallchangeintheHubbl econstant.Withtheirnew results,theycalculate H 0 = 74 3 6kms 1 Mpc 1 4.7Summary Inthischapterwehavepresentedresultsofanear-infrared photometricstudy ofpopulousclustersintheLMC.UsingISPIontheCTIO4mweob tained JK 0 photometrydownto 1.5magbelowthecoreheliumburningredclumpstarsin17 clusters,allowingustoaccuratelymeasurethe apparentK -bandmagnitudeoftheRC. Inasimilarapproachtothatof GS02 ,wecombineclusteragesandmetallicitieswith theoreticalmodelstopredictthe absoluteK -bandRCmagnitudeforeachofthese clusters.Thus,weareabletodetermineaccurateclusterdi stancesandexplorethe 3-dimensionalclusterdistributionaswellascalculateth edistancetothecenterofthe LMC.Themainresultsofourpaperareasfollows:

PAGE 192

178 1)Wehavecompileddeepopticalphotometry(belowtheMSTO) for15ofour clusters.Bycombiningthesedatawithpreviouslypublishe dmetallicities,weareable tobreakthewellknownage-metallicitydegeneracyandcalc ulateaccurateclusterages viaMSTOttingwiththeoreticalisochronesthatincludetr eatmentforcoreovershoot. Theintermediateageclustersrangeinagefromonly 1-3Gyr;thus,theseMSFages donotclosetheLMC'sclusteragegap.WeconrmthatESO121, theonlyLMC clusterknowntohaveanagebetween 3-13Gyr,formedapproximately9Gyrago. 2)Bycombining K RC measuredfromournearIRphotometrywiththevaluesof M RC K predictedbytheoreticalmodels,wehavedeterminedaccura tedistancesforall17 clustersinoursample;ouraveragestandarderrorofthemea ndistanceis0.08mag, or1.8kpc.ThisworkrepresentsthelargestsampleofLMCclu sterswithdistances derivedinaninternallyconsistentway. 3)Theclusterdistancesallowustoexplorethespatialdist ributionoftheLMC clustersystem.PreviousworkhasshownthattheLMCeldpop ulationslieina thick,inclineddiskandourresultsillustratethattheclu stersaredistributedinthe samemanner.Adisk-likedistributionforallLMCclustersh asbeeninferredfromthe kinematicsoftheclustersystem,however,ourresultsmark thersttimethatithas beendemonstratedthattheclustersandeldstarslieinthe sameplane. 4)PreviouslypublishedRRLyraedataforsevenoldglobular clustershave allowedustocalculatedistancesfortheseclustersandcom paretheirdistributiontothe geometryoftheLMC.Liketheintermediateageclusters,the globularclustershavea distributionthatisconsistentwithresidenceinthedisko ftheLMC. 5)ClustersthatliewellawayfromthecenteroftheLMChavep robablyseen few,ifany,encounterswithothermassiveobjectsandareth ereforeunlikelytohave beenperturbedfromtheiroriginalorbits.Thus,clustersw ithlargegalactocentricradii thatarecurrentlylocatedinthedisk(e.g.,NGC2257,NGC14 66,ESO121)likely formedinthedisk.Wenotethatthedisk-likekinematicsoft heseclusterssuggestthat

PAGE 193

179 thepositionsoftheseclustersarenotjustachancealignme ntofhaloclusterswiththe planeofthegalaxy.Fromthis,weinferthatthediskoftheLM Cmusthaveformedat aboutthesametimeastheoldglobularclusters, 13Gyrago. 6)TheoldglobularclusterNGC1841liesneartheLMC'stidal radiusandwell outoftheplaneofthedisk,inthedirectionoftheMilkyWay. Itspositionsuggests thatitwaspulledoutofthedisk,orpossiblystrippedfromt heLMC,inaclose encounterwiththeMilkyWay. 7)TakingtheinclinedgeometryoftheLMCintoaccount,wen dthemean distancetothecenterofthisnearbygalaxytobe ( m M ) 0 = 18 : 40 0 : 04 0 : 08or D 0 = 47 : 9 0 : 9 1 : 8kpc.Ourresultisinexcellentagreementwiththerecentwo rkof Macrietal. ( 2006 )whofound ( m M ) 0 = 18 : 41 0 : 1 0 : 13bycomparingCepheid variablesinthemaser-hostgalaxyNGC4258withthoseinthe LMC.Thisdistance, however,is 0.1magshorterthanthecommonlyaccepteddistanceof18 : 5 0 : 1mag, whichwasusedinthe HST KeyProjecttocalculate H 0 (see Freedmanetal.2001 ). TheshorterLMCdistancehastheeffectofincreasing H 0 by3%. Macrietal. ( 2006 ) usethenewdistancetorecalculatetheHubbleconstant,how ever,theyfoundthattheir improvedCepheidcalibrationmitigatesthedistanceeffec tsomewhat,resultingina valueof H 0 = 74 3 6kms 1 Mpc 1

PAGE 194

CHAPTER5 SUMMARY Asdiscussedin x 1 ,theLargeMagellanicCloudisanimportantgalaxyfora varietyofreasons,nottheleastofwhichisitsproximityto MilkyWayaswellasthe factthatitissufferingfromgravitationalinteractionsw ithboththeGalaxyandSmall MagellanicCloud.Theabilitytoresolvestellarpopulatio nsintheLMCenablesus todeterminethephysicalpropertiesof individual stars,somethingthatisnoteasily accomplishedinmoredistantgalaxies.Thus,theLMCisamuc hneededpiecein solvingthepuzzleofhowtidalforcesshapetheformationhi storyofasatellitegalaxy. TostudythecharacteristicsofstellarpopulationsintheL MC,wehavefocused ourattentiononitspopulousstarclusters.Theseobjectsp reservearecordoftheir hostgalaxy'schemicalabundancesatthetimeoftheirforma tion,containstarsthatare coeval,andthestarsineachclustercanbeassumedtoalllie atthesamedistancefrom us.Thesetraitsmakestarclustersanexcellentprobeofthe irhostgalaxy'sevolutionary history. FortherststepinourinvestigationoftheLMC,wehaveesta blishedthe K bandluminosityofcoreheliumburningredclumpstarsasast andardcandle( x 2 ; Grocholski&Sarajedini2002 ).Bycombiningnear-infraredphotometryfrom2MASS withabundances,ages,anddistancesfor14Galacticopencl ustersandtwoglobular clusters,wecalibratedtheabsolute K -bandmagnitudeoftheredclumpasafunction ofageandmetallicity.Ananalysisofthesedataleadsustot hefollowingconclusions. Ingeneral,while M RC K islesssensitivetochangesinageandmetallicitythaneith er M RC I or M RC V ,thereisstillastatisticallysignicantrangeof M RC K valuesamong ourclusters; s M RC K =0.22mag,whilethemeanerrorin M RC K is0.13mag.Thisis especiallyapparentforclustersyoungerthan 2GyrinwhichtheRCbrightnesscan 180

PAGE 195

181 changebyuptoamagnitudeasafunctionofage.Forolderclus ters, M RC K showslittle dependenceonage,butvariesasafunctionof[Fe/H]withlow ermetallicityclusters ( < 0 : 4dex)showingthelargestvariation.Byinterpolatingover theobservational data,wendthat,giventheageandabundanceofatargetclus ter,wecanpredict M RC K forthatcluster.Weappliedthistechniquetocalculatingt hedistancetotheopen clusterNGC2168aswellastheLMCclustersNGC1651andHodge 4,allwith promisingresults.Inaddition,comparingournear-infrar eddatatothetheoreticalRC modelsof Girardi&Salaris ( 2001 ),wendgoodagreementbetweentheobservations andthemodels,withallofourclusterslyingwithin1.5 s oftheappropriatemodel. Thissuggeststhatthemodels,whichcoveralargerparamete rspacethantheopen clusterdata,couldbeusedforpredicting M RC K inplaceoftheobservationaldata. Next,toapplyourRCcalibrationtotheLMC,itwasnecessary todetermine abundancesandagesforasampleofpopulousclusters.Using FORS2ontheVLT, weobtainednear-infraredspectraoftheCa II tripletlinesinRGBstarsin28LMC clusters( x 3 ; Grocholskietal.2006 ).AstheCaTlineshavebeenshowntobeagood tracerof[Fe/H](e.g., Coleetal.2004 ),weusedthesummedequivalentwidthofthese threelinestocalculatestellarabundances.Inaddition,t hroughacross-correlation withtemplatespectra,wedeterminedstellarradialveloci ties.Thesedatahaveallowed ustoimproveontheonlypreviouslargescalespectroscopic studyoftheLMC ( Olszewskietal.1991 )andidentifyanaverageof8clustermemberspertargeteld andcalculateclusterabundancesandvelocitieswithsmall randomerrors(0.04dexand 1.6kms 1 ).FortheclustersSL4,SL41,SL396,SL663,SL869,andHodge 3,we reporttherstspectroscopicallyderivedvelocitiesanda bundances.Inaddition,NGC 1718andNGC2193havenopreviouslyreportedspectroscopic [Fe/H]values. Thekinematicsofourclustersareinexcellentagreementwi ththeresultsof Schommeretal. ( 1992 ),whichshowthattheLMCclustersexhibitdisk-likerotati on withnoobviouspressuresupportedhalopopulation.Instar kcontrasttothestellar

PAGE 196

182 populationsofM33( Tiedeetal.2004 )andtheMW( Frieletal.2002 ),forwhich [Fe/H]decreasesasgalactocentricradiusincreases,theL MCclustersystemshowsno evidenceofametallicitygradientasafunctionofeitherra diusorpositionangle;the LMC'sstellarbarismostlikelyresponsibleforthemixingo fthegaspriortocluster formation.Aninspectionofthemetallicitydistributiono fourintermediate-ageclusters showsamuchtighterdistributionthanwasfoundby Olszewskietal. ( 1991 ),with notailofmetallicitiesstretchingtowardsolarvalues.Th isresultsisimportantinthat ourmean( 0 : 48dex)andspread(0.09dex)suggestthattheformationhist oryofthe intermediate-ageclustersisverysimilartothatofthebar ([Fe/H]= 0 : 37 0 : 15; Coleetal.2005 ).DynamicalmodelsoftheLMC-SMC-MWsystemby Bekkietal. ( 2004 )indicatethattherstverycloseencounterbetweentheLMC andSMC( 4 Gyrago)wouldhavebeensufcienttocausetheformationoft hebaraswellasto induce“dramaticgascloudcollisions,”therebycausingth erestartofclusterformation intheLMC.Thus,theobservedagreementbetweenourcluster metallicitydistribution andthatofthebararepredictedbythe Bekkietal. ( 2004 )model.Oftheclustersin oursample,NGC1718([Fe/H]= 0 : 8)istheonlyonethatfallsintherange 1 : 3 [Fe/H] 0 : 6,whichcorrespondswiththewellknown3-13GyrLMCagegap. Using archival HST WFPC2photometry,wendanageof 2Gyr,placingNGC1718in apositiontobeoneofthemostmetal-poorintermediate-age clustersintheLMCand leavingESO121( 9Gyr, 0 : 91dex)astheonlyknownclustertoresideinthe agegap.ThelowmetallicityandrelativelyyoungageofNGC1 718areintriguing sincetheytellusthat,eventhoughthegasthatwentintofor mingtheintermediateageclusterswaswellmixedbythebar,somepocketsofmetalpoorgasmusthave remainedintactduringthemixing.Acomparisonofresultsf orfourofourclustersto abundancesderivedthroughhigh-resolutionspectroscopy showsthattwooftheclusters areingoodagreement.Theothertwoclusters,alongwithpre liminaryresultsforan additionalpairofclusters,haveabundancesderivedfromt heCaTthatare 0.3dex

PAGE 197

183 moremetal-richthanwhatisfoundwithhigh-resolutionspe ctra.Thisdifferencehas beenattributedtovariationsin[Ca/Fe]betweentheMWandL MCclusters.However, high-resolutionworkintheLMCshows[Ca/Fe]thatislowert hanforMWgiantswith thesame[Fe/H].Alower[Ca/Fe]valuewouldmakeLMCgiantsa ppeartobemore metal-poorthanMWgiants,whichistheoppositedirectiono fwhatisobserved. Regardingclusterages,wehavecompileddeepopticalphoto metryfromthe literaturealongwithunpublishedVLTFORS2and HST WFPC2imagesof15populousclusters.Asthesedatareachbelowthemainsequencet urnoff,weareableto determineclusteragesviamainsequencetting.Wenotetha ttheavailablecluster abundanceshelptobreakthewellknownage-metallicitydeg eneracythatoftenplagues themainsequencettingmethod.Clusteragesaresimilarto whathasbeenfound previously,withnointermediate-agesclustersolderthan 3Gyr,andweconrmthat ESO121hasanageof 9Gyr. Finally,wecombinedourRCcalibration,clusterages,andc lustermetallicities withnear-infraredphotometrytocalculatedistancestoas ampleofclusters( x 4 ). UsingtheInfraredSidePortImagerontheCTIO4mweimaged17 populousLMC clustersinthe J -and K -bands,withtheresultingphotometryreaching 1.5mag belowtheRC.Thesedataallowedustoaccuratelymeasure K RC and,combined with M RC K predictedfromourRCcalibration,determinedistancestoe achofthese clusters.Exploringthespatialdistributionofclusters, wendgoodagreementwiththe geometryoftheLMCascalculatedbyeldstars(e.g., vanderMarel&Cioni2001 ); bothpopulationslieinathick,inclineddisk.Residenceof theclustersinthediskof theLMChasbeenassumedbasedontheirkinematics,however, ourresultrepresents thersttimethishasbeendemonstratedusingclusterdista nces.WecalculateRR Lyraebaseddistancesforsevenoldglobularclustersandn dthattheseclustersare alsoconsistentwiththethick,inclineddiskgeometry.Giv enthedisk-likekinematicsof theentireclustersystem,itisunlikelythattheagreement ofeithertheintermediate-age

PAGE 198

184 oroldclustersisachancealignmentwiththedisk.Sincesom eoftheoldclusterslie wellawayfromthecenteroftheLMCtheyareunlikelytohaveh adanyencounters thatwouldhaveperturbedthemfromtheiroriginalorbits.T hus,theirlocationsuggests thattheyformedinthedisk,implyingthatthediskmustbero ughlythesameageas theglobularclustersor 13Gyrold. AsourresultsshowthattheLMCclusterssharethesamedistr ibutionastheeld stars,wecanthereforeusethegeometryofthediskinconjun ctionwithourcluster distancestocalculatethedistancetothecenteroftheLMC. Usingall17ofourtarget clusters,wendameanLMCdistanceof ( m M ) 0 = 18 : 40 0 : 04 ran 0 : 08 sys or D 0 = 47 : 9 0 : 9 1 : 8kpc,withallthreerecentgeometries( vanderMarel&Cioni 2001 ; Olsen&Salyk2002 ; vanderMareletal.2002 )givingsimilarresults.This distanceisinexcellentagreementwiththeworkof Macrietal. ( 2006 )whofound ( m M ) 0 = 18 : 41 0 : 1 0 : 13throughacomparisonofCepheidvariablesinthe maser-hostgalaxyNGC4258withCepheidsfoundintheLMC.Ho wever,thisdistance isshorterby 0.1magthanthecommonlyacceptedLMCdistance, ( m M ) 0 = 18 : 5 0 : 1,whichwasadoptedby Freedmanetal. ( 2001 )intheir HST KeyProject todetermine H 0 .RecalculatingtheHubbleconstantusingtheshorterLMCdi stance, Macrietal. ( 2006 )foundthat H 0 increasedbyabout3%.However,theirimproved Cepheidvariablecalibrationmitigatesthiseffectslight lyandresultsinanewvalueof H 0 = 74 3 6kms 1 Mpc 1 Asmentionedabove,understandingthephysicalproperties ofstellarpopulations intheLMCcangivecluesastohowtidalforcesaffecttheform ationhistoryofa satellitegalaxy.Inthisdissertation,wehaveshownthats omeofthesignaturesof interactionsarereadilyvisibleintheLMC.Forinstance,u singabsolutedistances wefoundthat,whilemostclustersinoursampleappeartores ideinthediskofthe LMC,NGC1841hasadistancethatplacesitwellawayfromtheL MC'sdisk, 8 kpcclosertotheMWthanexpected.Givenitslocationnearth etidalradiusofthe

PAGE 199

185 LMC,thepositionofNGC1841suggeststhatithasbeenpulled outofthediskasa resultofthegravitationalpulloftheMW.Anadditionalsig natureoftidalinteractions isfoundinthemetallicitydistributionoftheintermediat e-ageclustersandthatofthe LMC'sbar.Thesimilarityinabundanceandspreadfortheset wostellarpopulations suggestasimilarstarformationhistory,ingoodagreement withthedynamicalmodels of Bekkietal. ( 2004 ),whichshowthattheformationofboththeLMC'sbarand intermediate-ageclustersareadirectresultofaveryclos eencounterbetweentheLMC andSMC 4Gyrago.Thus,thesesignaturesshowthattidalinteractio nswiththe SMCandMWareadrivingfactorintheboththestarformationh istoryandstructure oftheLMC.

PAGE 200

REFERENCES Abraham,R.G.1999,inIAUSymp.186:GalaxyInteractionsat LowandHigh Redshift,ed.J.E.Barnes&D.B.Sanders,11 Alcaino,G.1976,A&AS,26,359Alcaino,G.,Liller,W.,Alvarado,F.,Kravtsov,V.,Ipatov ,A.,Samus,N.,&Smirnov, O.1996,AJ,112,2020 Alves,D.R.2000,ApJ,539,732Alves,D.R.&Nelson,C.A.2000,ApJ,542,789Armandroff,T.E.&DaCosta,G.S.1991,AJ,101,1329Armandroff,T.E.&Zinn,R.1988,AJ,96,92Beasley,M.A.,Hoyle,F.,&Sharples,R.M.2002,MNRAS,336, 168 Bekki,K.,Couch,W.J.,Beasley,M.A.,Forbes,D.A.,Chiba, M.,&DaCosta,G.S. 2004,ApJ,610,L93 Bertelli,G.,Bressan,A.,Chiosi,C.,Fagotto,F.,&Nasi,E .1994,A&AS,106,275 Bertelli,G.,Nasi,E.,Girardi,L.,Chiosi,C.,Zoccali,M. ,&Gallart,C.2003,AJ,125, 770 Bessell,M.S.&Brett,J.M.1988,PASP,100,1134Bica,E.,Claria,J.J.,Dottori,H.,Santos,J.F.C.,&Piatt i,A.E.1996,ApJS,102,57 Bica,E.,Geisler,D.,Dottori,H.,Claria,J.J.,Piatti,A .E.,&Santos,Jr.,J.F.C.1998, AJ,116,723 Bolte,M.1987,ApJ,315,469Borissova,J.,Minniti,D.,Rejkuba,M.,Alves,D.,Cook,K. H.,&Freeman,K.C. 2004,A&A,423,97 Brocato,E.,Castellani,V.,&Piersimoni,A.M.1994,A&A,2 90,59 Brocato,E.,DiCarlo,E.,&Menna,G.2001,A&A,374,523Burki,G.&Meylan,G.1986,A&A,156,131Burstein,D.&Heiles,C.1982,AJ,87,1165 186

PAGE 201

187 Caldwell,J.A.R.&Coulson,I.M.1986,MNRAS,218,223Cardelli,J.A.,Clayton,G.C.,&Mathis,J.S.1989,ApJ,345 ,245 Carpenter,J.M.2000,AJ,120,3139—.2001,AJ,121,2851Carretta,E.&Gratton,R.G.1997,A&AS,121,95Cenarro,A.J.,Cardiel,N.,Gorgas,J.,Peletier,R.F.,Vaz dekis,A.,&Prada,F.2001, MNRAS,326,959 Cenarro,A.J.,Gorgas,J.,Cardiel,N.,Vazdekis,A.,&Pele tier,R.F.2002,MNRAS, 329,863 Chaboyer,B.1999,inASSLVol.237:Post-HipparcosCosmicC andles,ed.A.Heck& F.Caputo,111 Christian,C.A.,Heasley,J.N.,&Janes,K.A.1985,ApJ,299 ,683 Cioni,M.-R.L.,vanderMarel,R.P.,Loup,C.,&Habing,H.J. 2000,A&A,359,601 Clementini,G.,Gratton,R.,Bragaglia,A.,Carretta,E.,D iFabrizio,L.,&Maio,M. 2003,AJ,125,1309 Cole,A.A.1998,ApJ,500,L137Cole,A.A.,Smecker-Hane,T.A.,Tolstoy,E.,Bosler,T.L., &Gallagher,J.S.2004, MNRAS,347,367 Cole,A.A.,Tolstoy,E.,Gallagher,J.S.,&Smecker-Hane,T .A.2005,AJ,129,1465 Cˆote,P.,Marzke,R.O.,West,M.J.,&Minniti,D.2000,ApJ ,533,869 Crowl,H.H.,Sarajedini,A.,Piatti,A.E.,Geisler,D.,Bic a,E.,Claria,J.J.,&Santos, J.F.C.2001,AJ,122,220 DaCosta,G.S.1991,inIAUSymp.148:TheMagellanicClouds, ed.R.Haynes& D.Milne,183 DaCosta,G.S.2002,inIAUSymposium,83–93DaCosta,G.S.&Armandroff,T.E.1995,AJ,109,2533DaCosta,G.S.&Hatzidimitriou,D.1998,AJ,115,1934Feitzinger,J.V.&Weiss,G.1979,A&AS,37,575Ferraro,F.R.,FusiPecci,F.,Testa,V.,Greggio,L.,Corsi ,C.E.,Buonanno,R., Terndrup,D.M.,&Zinnecker,H.1995,MNRAS,272,391

PAGE 202

188 Ferraro,F.R.,Mucciarelli,A.,Carretta,E.,&Origlia,L. 2006,ApJ,645,L33 Freedman,W.L.,Madore,B.F.,Gibson,B.K.,Ferrarese,L., Kelson,D.D.,Sakai,S., Mould,J.R.,Kennicutt,R.C.,Ford,H.C.,Graham,J.A.,Huc hra,J.P.,Hughes, S.M.G.,Illingworth,G.D.,Macri,L.M.,&Stetson,P.B.200 1,ApJ,553,47 Friel,E.D.&Janes,K.A.1993,A&A,267,75Friel,E.D.,Janes,K.A.,Tavarez,M.,Scott,J.,Katsanis, R.,Lotz,J.,Hong,L.,& Miller,N.2002,AJ,124,2693 Geisler,D.,Bica,E.,Dottori,H.,Claria,J.J.,Piatti,A. E.,&Santos,J.F.C.1997,AJ, 114,1920 Geisler,D.,Piatti,A.E.,Bica,E.,&Claria,J.J.2003,MN RAS,341,771 Geisler,D.,Piatti,A.E.,Claria,J.J.,&Minniti,D.1995, AJ,109,605 Gieren,W.P.,Fouque,P.,&Gomez,M.1998,ApJ,496,17Girardi,L.,Bertelli,G.,Bressan,A.,Chiosi,C.,Groenew egen,M.A.T.,Marigo,P., Salasnich,B.,&Weiss,A.2002,A&A,391,195 Girardi,L.,Bressan,A.,Bertelli,G.,&Chiosi,C.2000,A& AS,141,371 Girardi,L.&Salaris,M.2001,MNRAS,323,109Grocholski,A.J.,Cole,A.A.,Sarajedini,A.,Geisler,D., &Smith,V.V.2006,AJ, 132,1630 Grocholski,A.J.&Sarajedini,A.2002,AJ,123,1603Hill,V.2004,inOriginandEvolutionoftheElements,205–2 19 Hill,V.,Francois,P.,Spite,M.,Primas,F.,&Spite,F.20 00,A&A,364,L19 Holtzman,J.A.,Burrows,C.J.,Casertano,S.,Hester,J.J. ,Trauger,J.T.,Watson, A.M.,&Worthey,G.1995,PASP,107,1065 Holtzman,J.A.,Gallagher,J.S.,Cole,A.A.,Mould,J.R.,G rillmair,C.J.,Ballester, G.E.,Burrows,C.J.,Clarke,J.T.,Crisp,D.,Evans,R.W.,G rifths,R.E.,Hester, J.J.,Hoessel,J.G.,Scowen,P.A.,Stapelfeldt,K.R.,Trau ger,J.T.,&Watson, A.M.1999,AJ,118,2262 Ibata,R.A.,Gilmore,G.,&Irwin,M.J.1994,Nature,370,19 4 Jrgensen,U.G.,Carlsson,M.,&Johnson,H.R.1992,A&A,25 4,258 Johnson,J.A.,Bolte,M.,Stetson,P.B.,Hesser,J.E.,&Som erville,R.S.1999,ApJ, 527,199

PAGE 203

189 Johnson,J.A.,Ivans,I.I.,&Stetson,P.B.2006,ApJ,640,8 01 Kerber,L.O.,Santiago,B.X.,&Brocato,E.2006,ArXivAstr ophysicse-prints Kim,S.,Staveley-Smith,L.,Dopita,M.A.,Freeman,K.C.,S ault,R.J.,Kesteven, M.J.,&McConnell,D.1998,ApJ,503,674 Koch,A.,Grebel,E.K.,Wyse,R.F.G.,Kleyna,J.T.,Wilkins on,M.I.,Harbeck, D.R.,Gilmore,G.F.,&Evans,N.W.2006,AJ,131,895 Koornneef,J.1983,A&AS,51,489Kraft,R.P.&Ivans,I.I.2003,PASP,115,143Leonardi,A.J.&Rose,J.A.2003,AJ,126,1811Luck,R.E.,Moffett,T.J.,Barnes,T.G.,&Gieren,W.P.1998 ,AJ,115,605 Macri,L.M.,Stanek,K.Z.,Bersier,D.,Greenhill,L.,&Rei d,M.2006,ApJ,652 Madore,B.F.&Freedman,W.L.1991,PASP,103,933Majewski,S.R.,Law,D.R.,Polak,A.A.,&Patterson,R.J.20 06,ApJ,637,L25 Maraston,C.2005,MNRAS,362,799Mateo,M.&Hodge,P.1985,PASP,97,753Mathieu,R.D.,Latham,D.W.,Grifn,R.F.,&Gunn,J.E.1986 ,AJ,92,1100 Mayor,M.,Imbert,M.,Andersen,J.,Ardeberg,A.,Baranne, A.,Benz,W.,Ischi,E., Lindgren,H.,Martin,N.,Maurice,E.,Nordstrom,B.,&Prev ot,L.1983,A&AS,54, 495 Mould,J.R.,Han,M.,Stetson,P.B.,Gibson,B.,Graham,J.A .,Huchra,J.,Madore, B.,&Rawson,D.1997,ApJ,483,L41 Nikolaev,S.,Drake,A.J.,Keller,S.C.,Cook,K.H.,Dalal, N.,Griest,K.,Welch, D.L.,&Kanbur,S.M.2004,ApJ,601,260 Olsen,K.A.G.,Hodge,P.W.,Mateo,M.,Olszewski,E.W.,Sch ommer,R.A., Suntzeff,N.B.,&Walker,A.R.1998,MNRAS,300,665 Olsen,K.A.G.&Salyk,C.2002,AJ,124,2045Olszewski,E.W.,Schommer,R.A.,Suntzeff,N.B.,&Harris, H.C.1991,AJ,101, 515 Osterbrock,D.E.&Martel,A.1992,PASP,104,76Paczynski,B.&Stanek,K.Z.1998,ApJ,494,L219

PAGE 204

190 Pagel,B.E.J.,Edmunds,M.G.,Fosbury,R.A.E.,&Webster,B .L.1978,MNRAS, 184,569 Persson,S.E.,Aaronson,M.,Cohen,J.G.,Frogel,J.A.,&Ma tthews,K.1983,ApJ, 266,105 Piatti,A.E.,Sarajedini,A.,Geisler,D.,Bica,E.,&Clari a,J.J.2002,MNRAS,329, 556 Putman,M.E.,Staveley-Smith,L.,Freeman,K.C.,Gibson,B .K.,&Barnes,D.G. 2003,ApJ,586,170 Reid,I.N.1999,ARA&A,37,191Rolleston,W.R.J.,Trundle,C.,&Dufton,P.L.2002,A&A,39 6,53 Rutledge,G.A.,Hesser,J.E.,&Stetson,P.B.1997a,PASP,1 09,907 Rutledge,G.A.,Hesser,J.E.,Stetson,P.B.,Mateo,M.,Sim ard,L.,Bolte,M.,Friel, E.D.,&Copin,Y.1997b,PASP,109,883 Sarajedini,A.1998,AJ,116,738—.1999,AJ,118,2321Sarajedini,A.,Grocholski,A.J.,Levine,J.,&Lada,E.200 2,AJ,124,2625 Schlegel,D.J.,Finkbeiner,D.P.,&Davis,M.1998,ApJ,500 ,525 Schommer,R.A.,Suntzeff,N.B.,Olszewski,E.W.,&Harris, H.C.1992,AJ,103, 447 Schweizer,F.1999,inIAUSymp.186:GalaxyInteractionsat LowandHighRedshift, ed.J.E.Barnes&D.B.Sanders,1 Searle,L.&Zinn,R.1978,ApJ,225,357Seidel,E.,Demarque,P.,&Weinberg,D.1987,ApJS,63,917Smecker-Hane,T.A.,Cole,A.A.,Gallagher,J.S.,&Stetson ,P.B.2002,ApJ,566, 239 Smith,V.V.,Hinkle,K.H.,Cunha,K.,Plez,B.,Lambert,D.L .,Pilachowski,C.A., Barbuy,B.,Melendez,J.,Balachandran,S.,Bessell,M.S. ,Geisler,D.P.,Hesser, J.E.,&Winge,C.2002,AJ,124,3241 Stanek,K.Z.&Garnavich,P.M.1998,ApJ,503,L131Staveley-Smith,L.,Kim,S.,Calabretta,M.R.,Haynes,R.F .,&Kesteven,M.J.2003, MNRAS,339,87

PAGE 205

191 Stetson,P.B.1987,PASP,99,191—.1994,PASP,106,250Suntzeff,N.B.,Schommer,R.A.,Olszewski,E.W.,&Walker, A.R.1992,AJ,104, 1743 Tiede,G.P.,Martini,P.,&Frogel,J.A.1997,AJ,114,694Tiede,G.P.,Sarajedini,A.,&Barker,M.K.2004,AJ,128,22 4 Tolstoy,E.,Irwin,M.J.,Cole,A.A.,Pasquini,L.,Gilmozz i,R.,&Gallagher,J.S. 2001,MNRAS,327,918 Tonry,J.&Davis,M.1979,AJ,84,1511Twarog,B.A.,Ashman,K.M.,&Anthony-Twarog,B.J.1997,AJ ,114,2556 Udalski,A.2000,ApJ,531,L25vanderMarel,R.P.2001,AJ,122,1827vanderMarel,R.P.,Alves,D.R.,Hardy,E.,&Suntzeff,N.B. 2002,AJ,124,2639 vanderMarel,R.P.&Cioni,M.-R.L.2001,AJ,122,1807VandenBerg,D.A.2000,ApJS,129,315Walker,A.R.1985,MNRAS,212,343—.1989,AJ,98,2086—.1990,AJ,100,1532—.1992a,AJ,103,1166—.1992b,AJ,103,1166—.1992c,AJ,104,1395—.1993,AJ,105,527Walker,A.R.&Mack,P.1988,AJ,96,1362Weinberg,M.D.2000,ApJ,532,922Zaritsky,D.,Kennicutt,R.C.,&Huchra,J.P.1994,ApJ,420 ,87 Zentner,A.R.&Bullock,J.S.2003,ApJ,598,49Zinn,R.1985,ApJ,293,424Zinn,R.&West,M.J.1984,ApJS,55,45

PAGE 206

192 Zoccali,M.,Renzini,A.,Ortolani,S.,Bragaglia,A.,Bohl in,R.,Carretta,E.,Ferraro, F.R.,Gilmozzi,R.,Holberg,J.B.,Marconi,G.,Rich,R.M., &Wesemael,F.2001, ApJ,553,733

PAGE 207

BIOGRAPHICALSKETCH AaronGrocholskiwasbornonMay15,1978,inTampa,Florida, andgrewup onthenorthsideoftowninaquietgolfcourseneighborhoodk nownasForestHills. MuchofhischildhoodwasspentplayingwithConstruxandLeg osorridinghisbike aroundtheusuallycar-lessneighborhoodstreets.Aaronwa salsointerestedinsports, particularlyenjoyingplayingbaseballandgolf,andthank stohisverysupportive parents,JoeandKatha,hewasabletoparticipateasmuchash ewanted.Whilehe hadaninterestinbecomingaprofessionalathlete,Aaron's originalcareergoalwas tobecomeagarbageman;hequitelikedtheideaofridingonth ebackofthetruck. Fortunatelyforallinvolved,thatideapassedrelativelyq uickly.OnceAaronrealized thattheinabilitytohitacurveballandnotbeingabletoput twouldkeephimfrom acareerinprofessionalbaseballorgolf,hedecidedtofocu sonadifferentsortof job.Science,andinparticulartheprocessofdiscovery,ha dalwaysinterestedhim, especiallysincehis8thand9thgradescienceclasseswithM r.Nevsimal,whoshowed aneverendingenthusiasmforexperimentation.Astronomyw assomethingthatalso interestedAaron.Manyanightwasspentwithhisfamilyinth eirdrivewaylooking attheringsofSaturn,actingouttheorbitoftheMoongoinga roundtheEarthgoing aroundtheSun,orwatchingtheSpaceShuttlegetcloserandc losertotheHubble SpaceTelescopesothatitcouldrepairthefaultymirror.Ho wever,thediscoveryofthe rstextrasolarplanetin1996,Aaron'ssenioryearinhighs chool,sealedhisfate.He decidedtobeanastronomer. ThecallingofhighereducationtookAarontothesmalltowno fStatesboro, Georgia,whereheattendedGeorgiaSouthernUniversity.Wh ileatGSU,hemajored inphysics,wasaphysicstutor,anastronomylabassistant, workedinthePlanetarium, 193

PAGE 208

194 andplayedcollegiategolfforoneseason.Dueprimarilytot heconnectionsof GSU'sastronomyfaculty,Aaronspentthesummerfollowingh issophomoreyear (1998)workingattheSpaceTelescopeScienceInstituteinB altimoreasapartof itsundergraduateresearchprogramwherehelearnedthejoy sofdataprocessing. NovemberofthatyearfoundhimonhisrstobservingtriptoK ittPeakNational Observatory.Duringthistime,Aaronfoundoutrsthandtha ttheweatherdoesnot careifastronomersstillhavemoredatatotake.Thesummero f1999wasspentin Laramie,WyomingasaparticipantintheSummerUndergradua teResearchAssistance Program.ItwasinWyomingthatAaronlearnedthattherearep laceswhereitwill snowatleastonceineverymonthoutoftheyear,thatasmanyn ationalparksshould bevisitedaspossible,andthatobservingJupiterinthemid -infraredcanbedone24 hoursaday,justsolongasthetelescopeisnotpointingtooc losetotheSun. AftergraduatingfromGSUin2000,Aaronreturnedtothestat eofFloridato attendgraduateschoolattheUniversityofFlorida.Bythee ndofhisrstyearin graduateschool,AaronbeganworkingwithDr.AtaSarajedin istudyingintermediateagestarsintheMilkyWayandLargeMagellanicCloud.Whilei ngraduateschool hefoundanappreciationforcartoonsandanime,withshowsl ike Futurama,Family Guy,CowboyBebop,GhostintheShell and SpongebobSquarepants makingregular appearancesinhisTVwatchingscheduleand,ultimately,he lpingtokeephimsane. Tokeepactive,Aaronmadeapracticeofplayinggolfandintr amuralsoftballwheneverpossible,butnotnearlyasoftenashewouldhaveliked. Evenwithallofthe extracurricularactivities,mostofhissixandahalfyears inGainesvillewerespent workingonhisresearch,theresultsofwhichcomprisethepr eceding200orsopages ofthisdissertation(althoughitismostlyguresandtable s).Afterskippinggraduation ceremoniesinfavorofanastronomymeetinginSpain,Aaroni shopingtondajob. Whilehewillendupgoingwhereverhegetshired,heishoping forsomethingnottoo cold,withnicegolfcoursesnearby.


Permanent Link: http://ufdc.ufl.edu/UFE0017420/00001

Material Information

Title: Metallicity, Distance and Distribution of Populous Clusters in the Large Magellanic Cloud
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0017420:00001

Permanent Link: http://ufdc.ufl.edu/UFE0017420/00001

Material Information

Title: Metallicity, Distance and Distribution of Populous Clusters in the Large Magellanic Cloud
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0017420:00001


This item has the following downloads:


Full Text











METALLICITY, DISTANCE, AND DISTRIBUTION OF POPULOUS
CLUSTERS IN THIE LARGE MVAGEtLLANIC CLOUD
















By
AARON J. GROCHOLSKI
















Ai DISSERTATION PR i : 11 i TED) TO THE GRADUATE SCHOOL,
OF TH-IE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THIE REQUIIR~I, blt1TS FOR TH-IE DEGREE OF
DOCTOR OF PHILOSOPHY-


UNIVERSITY OF FLORIDA


































Copyright:

by
A~aron J. Grocholski

























""Space... it seems to go on and on forever. But then you get to
the end and a gorilla starts throwing barrels at you."
i i i J. Fry
















P. :IENOWILEDC. Iill ITS

First and foremost, I would like to thank my adviser, Dr. Ata Sarajedini. Through

incredible patience with my questions (and my forgetting to write down the answers)

and giving me: the <.iil :i to travel to many meetings and'l 1.1 ...|.. he has made:

me the astronomer that I amn today. I feel that this dissertation is as much a reflection

of his abilities as it is of mine (so, hopefully it is good).

In doing the research that went in to this dissertation, I have had the ..1

to work with a number of astronomers. Dr. A~ndrew Cole served as a bit of a second

adviser, teaching me how to process stellar spectra and better understand why they look

the way they do. Drs. Doug Geisler and Verne Smith also contributed greatly to ( i: :

ter 3, providing the rawt~ data as well as many useful .. i .. on the numerous drafts

of that i .:i : For ::i:.:i i :- 4, Dlrs. Knut Olsen and Glenn Tiede offered considerable

input on the: target list, data ?.| i: li..:: and i:i :i : 1il ?ir : Conor Mancone, with his

hard work on the cluster ages, helped to make the results of that chapter considerably

more accurate than they would have been otherwise.

I would also like to thank my dissertation committee (Drs. : c. I~ r II ~F~red

H-amnann, Elizabeth Lada, and Vicki Sarajedini) for .::r;::: up with my many e-mails

and changes in scheduling my defense as i i as reading through / I i i .. I 1. quite

thoroughly) my dissertation.

Joanna Levine... or rather, Dr. Joanna Levine, was an iI i dissertation buddy.

Being able to share the fun and excitement of : i-:: submitting, and i; i=::.1i:::u (only

twno days apart) wMith her made the: whole process a little more bearable. She also

talked my adviser in to sending me with her to Chile on my first real 1 .: : I::. run,

where: I got to see the LMC and SMC in person for the: first time. On that trip, I also










learned from a flight attendant that Joanna and I were living in sin and that they have:

special customs forms for that.

The: future: Dr. I'. 1:i Barker has been a great office late, never complaining
about my .: 7 basic astronomy questions or random inr:" 1.:. ... 1...

I r: -::- -. etc, when I'm lost in thought or annoyed at my data, .. :::1.::r 1. or the -;::i-I-

'L' template. iir doubtful that we will ever be at the same institution .: :r. but

I know that if I ever have any questions about _-.i I or synthetic color-magnitude

diagrams, he is the first person I'll ask.

I would like to not thank Jeff Julian and M~faj. Dr. Steve Novotny, USAF. Without

them (7. 1:. me to lunch at Sonny's for all you can eat ribs so often, I probably

would have finished my dissertation a long time ago. But, the ribs were tasty...

And, finally, I want to thank my parents. In 28 years (and I i: for many

more), I have had nothing but :: -: t from them, whatever path I chose to take. Even

with all the: big words I have learned while writing up my dissertation, I still cannot

find the words to i what this has meant to me. i 1,:- -1 you.

My research ( i: .;: ri'i :l my salary, travel, and publication money) was supported

by: i CAI~iERE grant A i :: --48 to Ata Sarajedini. This dissertation is brought to

you by Thomnpson's Teeth. The only teeth strong enough to eat other teeth.



















TABLE OF CONTENTS



ACKNOWLEDGMENTS . . iv

LIST OF TABLES . . viii

LIST OF FIGURES . . ix

ABSTRACT . ........................... . xiii

CHAPTER

1 INTRODUCTION . 1

2 K-BAND RED CLUMP MAGNITUDE AS A DISTANCE INDICATOR 7

2.1 Introduction . . 7
2.2 The Data . . 9
2.2.1 Open Clusters .. . . 9
2.2.2 Globular Clusters . . 15
2.3 Results and Discussion . . 17
2.3.1 Cluster Data . . 17
2.3.2 Field Star Data . . . 17
2.3.3 Comparison With Theoretical Models . . 19
2.4 Application As a Distance Indicator . . 23
2.5 Conclusions . .............. ...... . 25
2.6 Beyond Grocholski & Sarajedini (2002) . . 26

3 ABUNDANCES AND VELOCITIES OF A SAMPLE OF LMC CLUSTERS 31

3.1 Introduction . . 31
3.2 D ata . . . . . . ... . 36
3.2.1 Target Selection . . 36
3.2.2 Acqui siti on . . 39
3.2.3 Processing . . 41
3.2.4 Radial Velocities ......... ... 42
3.2.5 Equivalent Widths and Abundances . . 45
3.3 Analysis . . 49
3.3.1 Cluster Membership. . . 50
3.3.2 Cluster Properties . . 56
3.3.2.1 Metallicities . . 57
3.3.2.2 Kinematics . . 62












3.4 Comparison with Previous Work . . 63
3.5 Summary . .................. . 71
3.6 Notes on Individual Clusters . . 73
3.6.1 NGC 1718 . . 73
3.6.2 NGC 1846. .. .... ......_ 75
3.6.3 NGC 1861. . . 75

4 DISTANCES AND DISTRIBUTION OF POPULOUS LMC CLUSTERS 149

4.1 Introduction . . 149
4.2 D ata . . . . . . ... . .152
4.2.1 Observations . . 152
4.2.2 Reduction . . 153
4.2.3 Photometry . . 155
4.3 Cluster Ages and Abundances . . 158
4.4 Apparent and Absolute K-band RC Magnitudes . . 161
4.5 Cluster Distances and the Distance to the LMC . . 167
4.5.1 Absolute Distance Moduli . . 167
4.5.2 LMC Cluster Distribution . . 167
4.5.3 The Distance to the LMC Center . . 173
4.5.4 Systematic Errors . . 175
4.6 Comparison to Previous Distances . . 176
4.7 Summary . . 177

5 SUMMARY . ......................... .. .180

REFERENCES . . 186

BIOGRAPHICAL SKETCH . . 193




















LIST OF TABLES

Table


2-1 Open and Globular Cluster Information .....

2-2 LMC Cluster Information ....


3-1 LMC Target Cluster Information .....

3-2 CaT Line and Continuum Bandpasses .....

3-3 Derived LMC Cluster Properties .....

3-4 Published LMC Cluster Metallicities ....


3-5 Metallicities of Young and Intermediate-Age Stellar Populations .

3-6 Positions and Measured Values for Cluster Members ....

3-7 Positions and Measured Values for Field Stars ....


4-1 Exposure Times at Each Dither Point ....

4-2 LMC Cluster Sample Information ....

4-3 LMC Cluster Ages and Metallicities .....

4-4 Calculated Red Clump Values and Cluster Distances .....

4-5 LMC Globular Cluster Information ....

4-6 LMC Center Distances .....


4-7 Effect of LMC Geometry ....


. 17

. 28

. 38

. 45

. 57

. 66

. 71

131

136

153

154

158

163

173

175

175



















LIST OF FIGURES


Figure

2-1 Comparison of open cluster ages ....

2-2 Near-IR open cluster CMDs ....

2-3 Near-IR globular cluster CMDs ....

2-4 Comparison of MRC and M,RC ....

2-5 Effects of age on MRC ....

2-6 Effects of [Fe/H] on MRIC ....

2-7 Age effects on solar neighborhood RC stars .

2-8 Intrinsic red clump color .....

2-9 Near-IR CMDs for Hodge 4 and NGC 1651

3-1 Schematic diagram of the LMC ....

3-2 Sample of spectra from RGB stars in our targ~

3-3 The xy positions of our target stars in the Hod

3-4 Radial velocities for our spectroscopic targets

3-5 Hodge 11 target star metallicities ....

3-6 CW vs. V VHB for Hodge 11 .....

3-7 CMD for the entire Hodge 11 field ....

3-8 Positions on the sky and derived metallicities :

3-9 Cluster metallcity vs. position angle .....

3-10 Cluster metallicity vs radial distance ....

3-11 Cluster radial velocity vs. position angle ..

3-12 Metallicity comparison with OSSH ....

3-13 Metallicity distribution of LMC clusters ...


. 11

. 13

. 15

. 18

... 20

. 21

. 22

. 25

. 30

. 37

et clusters . . 43


ge 11 field . . 51

in Hodge 11 . . 52

. 53

. 54

. 55

for our target clusters . 59

. 60

. 61

. 64

. 67

. 70











3-14 Cluster CMD for NGC 1718 .....

3-15 IC 2146 cluster member selection ...

3-16 IC 2146 cluster and field CMD ....

3-17 NGC 1651 cluster member selection .


... 74

77

. 78

. 79

. 80

. 81

. 82

. 83

. 84

. 85

. 86

. 87

. 88

. 89

. 90

. 91

. 92

. 93

. 94

. 95

. 96

. 97

. 98

. 99

. 100

. . 101

. 102


-18 NGC

-19 NGC

-20 NGC

-21 NGC

-22 NGC

-23 NGC

-24 NGC

-25 NGC

-26 NGC

-27 NGC

-28 NGC

-29 NGC

-30 NGC

-31 NGC

-32 NGC

-33 NGC

-34 NGC

-35 NGC

-36 NGC

-37 NGC

-38 NGC

-39 NGC

-40 NGC


1651

1652

1652

1718

1718

1751

1751

1841

1841

1846

1846

1942

1942

2019

2019

2121

2121

2155

2155

2162

2162

2173

2173


cluster and field CMD ...

cluster member selection .

cluster and field CMD ...

cluster member selection .

cluster and field CMD ...

cluster member selection .

cluster and field CMD ...

cluster member selection .

cluster and field CMD ...

cluster member selection .

cluster and field CMD ...

cluster member selection .

cluster and field CMD ...

cluster member selection .

cluster and field CMD ...

cluster member selection .

cluster and field CMD ...

cluster member selection .

cluster and field CMD ...

cluster member selection .

cluster and field CMD ...

cluster member selection .

cluster and field CMD ...











-41 NGC

-42 NGC

-43 NGC

-44 NGC

-45 NGC

-46 NGC

-47 NGC

-48 NGC

-49 NGC

-50 NGC


2193

2193

2203

2203

2213

2213

2231

2231

2257

2257


cluster member selection .

cluster and field CMD ...

cluster member selection .

cluster and field CMD ...

cluster member selection .

cluster and field CMD ...

cluster member selection .

cluster and field CMD ...

cluster member selection .

cluster and field CMD ...


. 104

. . 105

. 106

. . 107

. 108

. . 109

. 110

. . 111

. 112

. . 113

. . 114

. . 115

116

117

118

119

120

. 121

122

. 123

124

125

126

. .. 127

128

. .. 129


-51 Reticulum cluster member selection ..

-52 Reticulum cluster and field CMD ...

-53 SL 396 cluster member selection ...

-54 SL 396 cluster and field CMD .....

-55 SL 41 cluster member selection ....

-56 SL 41 cluster and field CMD ....

-57 SL 4 cluster member selection .....

-58 SL 4 cluster and field CMD ....

-59 Hodge 4 cluster member selection ...

-60 Hodge 4 cluster and field CMD ....

-61 Hodge 3 cluster member selection ...

-62 Hodge 3 cluster and field CMD ....

-63 SL 61 cluster member selection ....

-64 SL 61 cluster and field CMD ....

-65 SL 663 cluster member selection ...

-66 SL 663 cluster and field CMD .....

-67 SL 869 cluster member selection ...











3-68 SL 869 cluster and field CMD . . 130

4-1 K'-band images for all target clusters . . 155

4-2 Optical photometry for NGC 1651 and NGC 2173 . .. ..161

4-3 Near-infrared CMDs . ...... .. .. 164

4-4 Schematic diagram . ...... ... 69

4-5 Cluster distances as a function of their position .... .. .. 170















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of ii :: i ..- ii:y

M\IETAILLICi i, DISTANCE, AND DISTRIBUTION OF POPULOUS
CLUSTERS IN TH(E ILARG~E M~AGELLANIC CLOUD

By

Alaron J. Grocholski

December 2~

Chair: Ata :.:
Major Department: Astronomy

The ILarge i' il.: 1 1 : Cloud ii li' i: ), with its proximity to the li' i y '.;..1, and

location well awvay from the Galactic }.7 :::- offers us an excellent ;- ..: i:: to study

the < ? i. : of tidal interactions on the formation and evolution of :il i populations in

a satellite galaxy. In this work we i:: :::1 results from a program aimed at determining

the ages, kinematics, metallicities, distances and spatial distribution of populous

clusters in the LMC.

To further our understanding of the LMCI cluster system, wet have: acquired near-

infrared; 1e i )i photometry and spectroscopy and combined these data with optical

and NIR photomnetry from the: literature. WVith FORS2 on the: VLT, wne obtained NIR

spectra for more than 800 stars in and around 29 LMC clusters that span a large

range of ages (~ 1-13 Gyr) and metallicities (--0.3 2 T Ir rr -2.0o). Wie use:

these spectra to calculate r0 i1 :: velocities and metallicities, and identify more than

members in 28 clusters. U I:--.published id:..(. ::: ;:y, VLT FORS2 images, and

archival HST WJ\FPC2 images, we compiled deep optical photometry for 15 clusters.

These data extend below each cluster's main : i::: :: turnoff and, combined with

our abundances, l l. .. us to break the well known age. : .< : i .. : degeneracy and










determine ..... ..1 .>cal ages via main sequence -i.::::, (:"l .12). As the first step in our

i .:: calculations, we use JK~, photometry of 141 Galactic open clusters from the

-.1 rlSS to calibrate: the K-band luminosity of core helium burning red clumnp stars

: ) as a function of age and metallicity. Next, with ISP"I on the : i ii:0 4m telescope,

we imaged 17 ILMC clusters in the NIR (JK') down to K' ~ 18._5, or about 1.5

belowv the RiC, allowing us to measure the apparent K-band RiC magnitude of each

cluster. WZe combine the LMC cluster ages and abundances with our RC calibration to

predict ': for each cluster and thereby calculate accurate distances and i I.T .-e the

geometry of the cluster system.

velocities are in good argeemetnt wMith previous results and showv that

the ILMCL clusters rotate with diski-like kiinematics. Our abundances indicate that the

: 11. :j i: :::i.:: .. :: of the: more: metal-rich clusters is much tighter than i : .:: 7y

believed, with no tail toward solar ::: -: :i i: :i:: The peak of this distribution is similar

to that of the bar, which is in good agreement wiith dynamical models that :: that

both the restart of cluster formation and the formation of the bar occurred as a result of

the first close encounter with the SMCI (~ 4 Gyr ago). Cluster ages derived from MISF

range from ~1-3 Gyr for all clusters in our sample except ESO 121-St: (~ 9 Gyr),

the only known cluster in the LMC: with an age between ~3-13 Gyr. The intermediate

age range, as i i as the age of ESO 121-SCO3, is similar to previous results.

Finally, we find that the .i:r ,7 distribution of the LMIC: cluster system is in

good agreement with thick, inclined disk geometry found from LMC field stars. In

.:iii :~ using RRE L~yrae based distances, we find that the old globular clusters are

also consistent wiith this geometry. Given the: .1: i -like kinemnatics of the entire cluster

system, this implies that the :' i: 's 1: i formed at about the same time as the old

clusters, ~ 13 Gyr ago. Combining the LMC geometry with cluster ii: : r:::. we:

calculate a distance to the center of the ii li' ii C of (m2 :'j, ')0 18.40 + 0.04 f 0.08, which

is 0.1 mag shorter than the commonly accepted ILMCI distance.















CHAPTER 1
INTRODUCTION

The current scenario of hierarchical formation suggests that large galaxies like the

Milky Way (MW), and in particular their spheroid components (bulge and halo), are

built up through the accretion of dwarf satellite galaxies and protogalactic fragments

(e.g., C~te: et al. 2000; Zentner & Bullock 2003). It is clear from an observational

standpoint that the chemical enrichment and star formation histories of galaxies in

general, whether large or small, at high or low redshift, are dominated by interactions

and merger events (Abraham 1999; Schweizer 1999). Thus, understanding the effects

of gravitational interactions on the evolution of satellite galaxies is an important piece

in the puzzle of large galaxy formation.

The MW and its satellite galaxies are a prime example of interactions and mergers

in action, with signatures of the many stages of hierarchical formation visible in and

around the Galaxy. The most striking evidence of this is the cannibalization of the

Sagittarius dwarf galaxy (Sgr, Ibata et al. 1994). Lying ~ 25 kpc from us and almost

directly behind the Galactic center, Sgr is elongated along its orbit around the MW

as a result of tidal stripping. Also visible throughout the MW are many tidal streams

(e.g., Majewski et al. 2006), all of which are remnants of dwarf galaxies that were

long ago disrupted by and accreted into our Galaxy. Even halo globular clusters that

have large Galactocentric radii (X 8 kpc) likely formed in satellite galaxies before

being absorbed by the MW (Searle & Zinn 1978). While signatures of past accretions

abound in the Galaxy, the fact that these systems were disrupted well in the past

makes it difficult if not impossible to uncover the formation history of their parent

galaxies. Similarly, the location of Sgr almost directly behind the Galactic bulge

causes widespread contamination by foreground stars, rendering observations of this










galaxy difficult to interpret. In contrast, the Large Magellanic Cloud (LMC) is a

nearby galaxy that shows signatures of interactions, but remains mostly intact, and is

relatively free from foreground contamination. Since its proximity allows us to resolve

individual stars and determine their physical properties, the LMC offers us an excellent

environment in which to study the effects of tidal forces on the formation history of a

satellite galaxy.

Using gas dynamical N-body simulations, Bekki et al. (2004) model the orbits of

the LMC and Small Magellanic Cloud (SMC) in their paths around the MW and pay

special attention to the effects of tidal interactions between the Clouds. Prior to 5 Gyr

ago, the LMC orbited the MW every ~ 2 Gyr with a highly eccentric orbit that ranged

in Galactocentric radius from ~ 50-150 kpc. Originally, the SMC orbited the MW on

a similar path to, but independent from, the LMC. Its smaller Galactocentric radius (~

50-100 kpc) resulted in a faster orbit, with the SMC completing one revolution around

the MW every 1.5 Gyr. Approximately 5 Gyr ago, the LMC and SMC passed within

25 kpc of each other, an encounter that caused a small decay in the LMC's orbit and

an increase in the radius of the SMC's orbit, which ultimately led to the Magellanic

Clouds becoming bound ~ 1 Gyr later. Since becoming bound, the LMC and SMC

have had a number of close encounters (g 10 kpc, see Fig. 1 in Bekki et al. 2004),

with their first close encounter (6.4 kpc, 3.6 Gyr ago) having a significant effect on the

star formation history of the LMC (see below).

Quite possibly, the most remarkable signature of interactions in the LMC-SMC-

MW system is seen at radio wavelengths; column density maps of H I show a complex

envelope of gas in and around the Magellanic Clouds. The LMC and SMC are

connected by a substantial bridge of material that was likely stripped from the SMC in

a previous encounter. In addition, both the short Leading Arm and longer Magellanic

Stream, which trails the Magellanic Clouds in their orbit around the MW and stretches









~ 100" across the sky, are the result of tidal stripping of material from the Magellanic

Clouds by the MW (Putman et al. 2003).

More subtle, but just as revealing, are the markers of interactions found in the

LMC's stellar populations. Studying carbon star kinematics, Alves & Nelson (2000)

showed that the disk of the LMC is flared, with the disk scale height increasing from

0.3 kpc at a R = 0.5 kpc to 1.6 kpc at R = 5.6 kpc. Using an expanded sample

of carbon stars, van der Marel et al. (2002) find similar results and also show that

the LMC's disk, with V/o a 2.9 + 0.9, is much thicker than the MW thin disk

(V/o a 9.8) and slightly thicker than the MW thick disk (V/o a 3.9). In addition,

both Olsen & Salyk (2002) and Nikolaev et al. (2004) use field stars as relative

distance indicators to show that the LMC disk may also be warped. These results are

in agreement with the N-body simulations by Weinberg (2000) which predict that the

Galaxy is a significant driver of the LMC's evolution and that tidal forces from the

MW will heat (thicken) and possibly warp the disk of the LMC.

However, the most impressive feature of the LMC that is likely linked to gravita-

tional forces is its cluster formation history. The LMC is known to have a population

of old, metal-poor globular clusters that formed ~ 13 Gyr ago and a more recent epoch

of intermediate metallicity cluster formation that began ~ 3 Gyr ago and has continued

to the present. In between these two epochs is the well known "age gap" in which

only one cluster, ESO 121-SCO3 (ESO 121; 9 Gyr), is known to reside (e.g., Da Costa

1991; Geisler et al. 1997; Da Costa 2002). Similar to the intermediate-age clus-

ters, stars in the LMC bar formed only within the past ~ 5 Gyr (Cole et al. 2005),

whereas star formation histories for the disk of the LMC show that field stars had a

constant, although low, star formation rate during the cluster age gap (Holtzman et al.

1999; Smecker-Hane et al. 2002). The aforementioned model of Bekki et al. (2004)

shows that, while the cause of the apparent end of cluster formation ~ 13 Gyr ago

is unknown, the first very close encounter between the LMC and SMC ~ 4 Gyr ago









would have caused the formation of the LMC bar as well as "dramatic gas cloud

collisions" that resulted in the restart of cluster formation in the LMC; continued

strong interactions have sustained the LMC's cluster formation. During the age gap,

weak interactions between LMC, SMC, and MW would have only been sufficient to

support star formation in the field. Thus, the star formation history of the LMC, and,

in particular, the star formation that has occurred in within the last ~ 4 Gyr, is a direct

result of tidal forces acting on the LMC.

As mentioned above, tracers of the LMC's field populations show that the spatial

distribution of the field stars has been affected by interactions; in contrast, the 3-

dimensional distribution of clusters has not been fully explored. Typically, the LMC

is treated as a planar galaxy that can be assumed to lie at a single distance from us.

However, the proximity of the LMC (~ 48 kpc) combined with the fact that it is

inclined appreciably with respect to the plane of the sky (e.g., Caldwell & Coulson

1986) leads to a significant distance gradient across the face of the LMC. Recently,

both van der Marel & Cioni (2001) and Olsen & Salyk (2002) have used field stars

to trace the geometry of the LMC, where they have assumed that variations in the

brightness of their observed fields were due to differences in distance (see Chapter 4

for more detail). Using this method, both authors find that the LMC field populations

lie in a disk that is inclined ~ 35" (0" is face on) with the northeast portion of the

LMC closer to the MW than the southwest. For stars lying 5 kpc from the center of

the LMC, this inclination can lead to difference in distance from the MW of ~ 8 kpc.

While relative distances have been used to show that the LMC field stars lie in a thick,

inclined disk, the distribution of the clusters has only been inferred from kinematics.

Schommer et al. (1992) calculated velocities for ~ 80 populous clusters and found that

the entire cluster system rotates with disk-like kinematics, while no clusters appear to

reside in a pressure supported halo. We note that, although both the cluster system and










the majority of field stars reside in the disk of the LMC, there does seem to exist a

tenuous halo of metal-poor RR Lyrae stars (Borissova et al. 2004).

Star clusters are an important tool for studying the structure and formation

history of a galaxy because they have one main advantage over field stars. Whereas

a sample of field stars may cover a wide range of ages, all stars in a given cluster

can be considered to be coeval, allowing the cluster's age to be readily determined

from deep color-magnitude diagrams. Thus, clusters place a much needed time-stamp

on a variety of events in the history of a galaxy. For example, since clusters contain

a record of their host galaxy's chemical abundances at the time of their formation,

they permit us to place tight constraints on the age-metallicity relation of the galaxy.

This is particularly important when we consider that tidal forces can result in a

galaxy experiencing infall or outflow of material, which may leave markers of these

interactions on the galaxy's age-metallicity relation. In addition, given knowledge of

the kinematics and distribution of the cluster system, it may be possible to determine

the timescale of formation of features, such as the disk of the galaxy. Finally, for

many standard candles, their absolute brightness varies as a function of both age and

metallicity. Due to the fact that the ages of field stars are difficult to determine, it is

usually only possible to calculate relative distances from field populations. Clusters, on

the other hand, with their available ages and abundances, enable us to properly apply a

standard candle and thereby calculate accurate absolute cluster distances. Therefore, we

can use a sample of clusters not only to explore their spatial distribution, but we can

combine this distribution with their absolute distances to determine an accurate distance

to their host galaxy. For the LMC, an accurate distance is of particular importance

due to its use as the zeropoint in the extragalactic distance scale (e.g., Freedman et al.

2001).

In this dissertation, we present the results from a program designed to better

understand the ages, kinematics, abundances, distances, and spatial distribution










of populous clusters in the LMC. First, in Chapter 2, we have developed the core

helium burning red clump (RC) stars as a standard candle. Specifically, we calibrated

the absolute K-band magnitude of the red clump as a function of cluster age and

metallicity for a sample of Galactic open clusters. Next, in Chapter 3, we obtained

moderate-resolution near-infrared spectra for a large number of stars in and around a

sample of LMC clusters. With these data we were able to identify cluster members

and subsequently determine cluster abundances and velocities. In Chapter 4, we have

acquired near-infrared (JK) images for a number of intermediate-age LMC clusters

and used the resulting photometry to measure the apparent K-band RC magnitude

for these clusters. We combine newly calculated ages from deep optical photometry

(see ~4.3) with the abundances from Chapter 3 and the RC calibration presented in

Chapter 2 to predict the absolute K-band magnitude of the RC for our LMC clusters.

Absolute cluster distances are then readily calculated and the spatial distribution of the

cluster system is explored, in addition to determining the absolute distance to the LMC.

Finally, in Chapter 5, we summarize our results.















CHAPTER 2
K-BAND RED CLUMP MAGNITUDE AS A DISTANCE INDICATOR

2.1 Introduction

During the past few years, the helium burning red clump (RC) has gained

considerable attention for its potential as a standard candle. The primary advantage

of the RC is the ease with which it can be recognized in the color-magnitude diagram

(CMD). However, there is currently a great deal of controversy in the literature

regarding the appropriate treatment of possible metallicity and age effects on the I-band

absolute magnitude of the RC ( 0C). There are two schools of thought on this issue.

The first assumes a constant value for 0R which is then used to facilitate a single-step

distance determination via knowledge of the apparent RC magnitude and the extinction

(e.g., Paczyliski & Stanek 1998; Stanek & Garnavich 1998) The second approach is

founded on the claim that both age and metal abundance have a significant influence on

the luminosity of RC stars (e.g., Cole 1998; Sarajedini 1999, hereafter, S99) and must

be accounted for in determining 0R and therefore the distance.

Both Paczyliski & Stanek (1998) and Stanek & Garnavich (1998) use Hipparcos

RC stars with parallax errors of less than 10% to calculate the I-band absolute magni-

tude of the solar neighborhood red clump. In their analysis, Paczyliski & Stanek (1998)

find that ~IC shows no variation with color over the range 0.8 < (V -I)o < 1.4 and,

from a Gaussian fit to the RC luminosity function, find ~IC = -0.28 + 0.09. Follow-

ing the same methodology and building upon the earlier work, Stanek & Garnavich

(1998) find a similar result with MRC = -0.23 + 0.03. With this calibration, a sin-

gle step calculation is then used to determine the distance to the Galactic center









(Paczyniski & Stanek 1998) and M31 (Stanek & Garnavich 1998). Both of these inves-

tigations found little or no variation in Myr of the RC stars with color; this was taken to

imply that MRC does not vary significantly with metallicity.
In contrast, theoretical models from Girardi & Salaris (2001) and the earlier

models of Seidel, Demarque, & Weinberg (1987, see also Cole 1998) show that MRC

is dependent on both age and metallicity, becoming fainter as both increase. These

models are in good agreement with the observations presented by S99. Using published

photometry for eight open clusters, S99's most important result is that while MRC iS

less sensitive to metal abundance than MR~C, both still retain a considerable dependence

on the age and metallicity of the stellar population. As a result, the single-step method

of applying the solar-neighborhood MRC to populations with a different age-metallicity
mix could be problematic.

Alves (2000) also uses the Hipparcos RC for his calibration; however, he relies

upon the K-band luminosity (MrK) of RC stars in the hope that, since the K-band is less

sensitive to extinction (and possibly metallicity as well) than the I-band, it might make

a better choice as a standard candle. Alves (2000) restricts his RC stars to those that

have metallicities from high resolution spectroscopic data. For this group of 238 RC

stars, he finds a peak value ofMRC = 1.61 +0.03 with no correlation between [Fe/H]

and Mr. However, he is not able to explore the effect of age on MRC due to the lack

of such information for the individual stars in his sample.

These previous works prompted us to combine the approaches of S99 and Alves

(2000) to investigate the influence of age and metal abundance on MRC for a number of

open clusters with well-known distances and metallicities. Our findings were originally

published in Grocholski & Sarajedini (2002, hereafter, GSO2), however, the results

relied on photometry from the Second Incremental Data Release of the Two Micron

All Sky Survey (2MASS). Since the original publication of our paper, the All Sky Data

Release of the 2MASS catalog has been made publicly available and, in this chapter,










we have updated the work presented GSO2, based on the newest 2MASS photometry.

Additionally, in @ 2.6, we have updated our initial application of the RC calibration to

the LMC (Sarajedini et al. 2002). We note that the improved photometry from the All

Sky Release of 2MASS has had very little effect on the calculations in either GSO2 or

Sarajedini et al. (2002) and has not changed any of the conclusions in these papers.

In @ 2.2 we discuss the observational data used in calibrating the K-band lumi-

nosity of the red clump. Section 2.3 compares our data with the results of theoretical

models and presents a discussion of the results. We test the utility of our results for

calculating the distance to a Galactic open cluster in @ 2.4 and our conclusions are

summarized in @ 2.5. Finally, in @ 2.6, we discuss the work of Sarajedini et al. (2002),

which sought to apply our RC calibration to determining the distance to a pair of LMC

clusters.

2.2 The Data

2.2.1 Open Clusters

In the present study, the most important criterion that the observational data

must fulfill is that of internal consistency. For example, we must ensure that the

distance moduli, reddenings, ages, and metallicities of all of the clusters in our sample

have been determined using the same techniques. In addition, it is imperative that

the infrared photometry we rely upon be measured and calibrated in a consistent

manner. For the former, we use the database of open cluster properties as measured by

Twarog, Ashman, & Anthony-Twarog (1997), supplemented by cluster ages from the

WEBDA database, and for the latter, we utilize the All Sky Release of the Two Micron

All Sky Survey (2MASS) Point Source Catalog. We now discuss each of these in more

detail.

Twarog et al. (1997), have compiled a list of 76 open clusters for which they

provide reddenings, distance moduli, and metallicities. For the purposes of the

present paper, we limit ourselves to distance moduli derived via the technique of









main sequence fitting (MSF) so as to remain independent of methods that rely on the

luminosity of the RC. Their metallicities have all been measured on the same system

and the reddenings have been determined using an internally consistent method. The

vast majority of these values are consistent with those found in the literature, except for

the reddening of NGC 6819 for which the Twarog et al. (1997) value is much higher

than other published values. As a result, we have decided to adopt the S99 reddening

for NGC 6819 instead of the apparently discrepant value tabulated by Twarog et al.

(1997). In addition, because the determination of the reddening and distance modulus

is coupled, we also adopt the S99 distance modulus for NGC 6819.

The ages of the open clusters have been obtained from WEBDA, which is a

compilation of open cluster data from various sources. To check the reliability of the

WEBDA ages, we compare them with the cluster ages determined by S99 in Fig. 2-1.

S99 presents isochrone-fitting ages for eight open clusters that have been determined

in a consistent manner using the Bertelli et al. (1994) theoretical isochrones. The left

panel of Fig. 2-1 plots the ages from S99 versus the ages in WEBDA, where both

axes are in log space and the dashed line represents a zero age difference between

the systems. From this plot, it is evident that there is a systematic offset between the

two systems with the WEBDA ages being younger than those of S99. The average

difference, Alog (Age) = 0.191, is used to shift the ages given in WEBDA onto the

S99 system. The right panel of Fig. 2-1 shows the ages from S99 plotted against

the shifted WEBDA ages; it is clear from Fig. 2-1 that the shifted ages are in better

agreement with those of S99; as a result, we will apply this shift to nine of the clusters

in our study and use the S99 ages for the five clusters that are common to both studies.

For the open clusters in the Twarog et al. (1997) study that possess MSF dis-

tances, we extracted JHKs photometry from the All Sky Release of the 2MASS

Point Source Catalog. As noted above, these data have been obtained using similar

instruments and reduced with the same pipeline techniques. For each cluster, we have














10.0 *~ / *



9,4



S9.2 -

9.0~ *

8.8 *

8,8 9.0 9.2 9.4 9.6 9.8 10.0 8.8 9.0 9.2 9,4 9.6 9.8 10.0
Log age (WEBDA ages) Log age (Shifted WEBDA ages)

Figure 2-1: Comparison of open cluster ages. Ages from WEBDA (left panel) plotted
against those from S99. Due to the systematic difference between the sys-
tems, we shift the WEBDA ages older by Alog (Age) = 0.191 (right panel)
to place them on the same system as S99.


utilized the same criteria for the 2MASS data retrieval. The field size is originally

set to 30' in radius and then reduced to fields as small as 5' in radius in an attempt to

isolate the cluster stars. The sources are limited to a brightness of 6th magnitude or

fainter due to saturation effects at the bright end (Carpenter 2000). Lastly, we have

extracted only the highest quality photometry from the 2MASS catalog, which provides

a read flag (rd~fg) indicating how the photometry of each star was measured. We have

chosen to exclude any source that has a read flag of zero in any band since this implies

that the source was not detected in that band and the magnitude given is an upper limit.

We note that the vast majority of the stellar magnitudes used in this study are based on

point-spread-function fitting (i.e., rd~fig = 2); however, in order to include the brighter

red clumps of more nearby clusters, we have had to use the aperture photometry in a

minority of cases.

The 2MASS program uses a K-short (Ks) filter for their observations. We have

chosen to convert these magnitudes to the K-band adopting the Bessell & Brett (1988,









hereafter, BB) system, which is also used in the theoretical models of Girardi et al.

(2000) and Girardi & Salaris (2001). The transformation equations are derived by

Carpenter (2001) and are adopted as follows:


(J -KBB) = [(J- Ks) (-0.01 1 +0.005)]/(0.972 +0.006) (2-1)

and

KBB = [Ks (-0.044 +0.003)] (0.000 +0.005) (J- KBB). (2-2)

We note, however, that transformation to the Koornneef (1983) K-band (used

by Alves 2000; @ 2.3.2) would have a negligible effect on our results. To cor-

rect for the interstellar reddening, we adopt the extinction law determined by

Cardelli, Clayton, & Mathis (1989), which, using their value of Rv = 3.1, gives

AK = 0.11Av and AJ = 0.28Av. From this, it is a simple matter to calculate the

absolute K-band magnitude and dereddened J-K color of the open cluster stars.

We have determined the RC luminosity for our clusters by taking the median

value ofM~K for all stars within a standard sized box placed around the RC. We use

the median value of MK along with a constant box size in an attempt to eliminate any

selection effects that may occur in choosing the location of the RC and to limit the

effect of outliers on My~. Fig. 2-2 shows the CMDs for all 14 clusters, focused on the

RC and main sequence turnoff. The box used to select the RC stars for each cluster

is also shown. We note that, where available, we have used published optical CMDs

for our clusters to help isolate the approximate RC location. The uncertainty in Mf is

calculated by combining the standard error about the mean K-magnitude for all stars

inside the RC boxes along with the errors in E(B V) and (m M),, all added in

quadrature. Except where otherwise noted, we adopt 20% of the value as the error in

E(B V) and 10% of the value as the error in (m -M),.





















NGC 752










oo








a



NGC 2099


= *




O


NGC 1817


*o *

O






co
o
o

0 0


1
O




d









8,

o
o
oo

c~o
a
o
o
o o


SNGC 2204


o @


1






















G
d


NGC 2360
0 0


Be 39


0 a

a .



o,







.: .








).0 0.5 1.0

(J-K)


O


O
O

O

9

.o
o
oo

a
oo
o


).0 0.5

(J-K)o


Figure 2--2: Near-IR open cluster CMI~s. Infrared CM13s for the 14 open clusters in

our sample are shown, with a box indicating the location of the cluster's

red 7:r:::.;. All stars within the box are: used in calculating the median K

::: ,l-, of the red 1



























oo

Co

o o o


40 Co a oc oo


D o -
2* o





o M oo





o o






-2o -
OO O

co


1, o

8 oo

2~O 3 co a .

0. 0.5 1. .
(J K (J-K)

Figr 2-2 Nea-I opncutrCo otne










































I I I 1
47 Tuc li,' NGC 362 O






~O O

OO O vO OO
8o
Og


-2


-1


0



1


2


Figure 2-2: Near-IR open cluster CMDs Continued.


0.0 0.5 1.0
(J-K)

Near-IR globular cluster CMDs.
clusters in our sample.


0.0



Same as Fig.


0.5 1.0
(J-K)o

2-2, but for the globular


Figure 2-3:


2.2.2 Globular Clusters

It is difficult to ensure that the distances, ages, and metallicities of globular clus-

ters with RCs are on the same system as those of the open clusters. Fortunately, the










ages and metallicities of the two globulars in our sample 47 Tuc and NGC 362 are

sufficiently different from the bulk of the open clusters that small systematic discrep-

ancies in these quantities should not be a significant hindrance to the interpretation of

the results. In any case, we have decided to adopt literature values for the basic cluster

parameters and use the globular cluster RCs as a consistency check.

In the case of 47 Tuc, we adopt the metallicity quoted by Carretta & Gratton

(1997) of [Fe/H] = -0.70 + 0.07, which happens to be very close to the Zinn & West

(1984) value. For the distance modulus and reddening, we average the published

values listed in Table 2 of Zoccali et al. (2001) to obtain (m -M); = 13.45 +0.21 and

E(B V) = 0.044 & 0.008, where the errors represent half of the range of tabulated

values. Lastly, for the age of 47 Tuc, we adopt the oldest age for which the models

predict the presence of a RC at its metallicity 12 Gyr.

For NGC 362, we adopt a similar approach. The metal abundance of [Fe/H] =

-1.15 + 0.06 is taken from Carretta & Gratton (1997), which is approximately 0. 1

dex more metal-rich than the Zinn & West (1984) value. Our search of the literature

has revealed distance moduli that range from (m -M);- = 14.49 (Zinn 1985) to 14.95

(Burki & Meylan 1986, see also Bolte 1987) and reddenings in the range E(B V)

= 0.032 (VandenBerg 2000) to 0.08 (Alcaino 1976) leading to adopted values of

14.70 + 0.23 and 0.048 + 0.024 for the apparent distance modulus and reddening of

NGC 362, respectively. Once again, we adopt an age of 12 Gyr.

The RCs of these globulars have been isolated in the 2MASS point source catalog

in the same way as for the open clusters. The [M~K, (J- K)o] CMDs for 47 Tuc and

NGC 362 are shown in Figure 2-3 along with the box used to define their RCs. All

of the relevant observational parameters for the open and globular clusters are listed in

Table 2-1.











Table 2-1. Open and Globular Cluster Information
Name Log Age (m-M~)Va E(B-F)a [Fe/H]a O([Fe/H])a h1g o(Mr() (J-K)o o(J-K)o
NGC 752 9.24 8.35 0.04 -0.088 0.018 -1.538 0.118 0.603 0.011
NGC 1817 8.80 12.15 0.26 -0.268 0.023 -1.875 0.181 0.565 0.028
NGC 2099 8.73 11.55 0.27 0.089 0.073 -2.111 0.185 0.555 0.029
NGC 2204 9.28b 13.30 0.08 -0.338 0.120 -1.607 0.114 0.612 0.010
Be 39 9. 88b 13.50 0.11 -0.177 0.032 -1.595 0.122 0.677 0.013
NGC 2360 8.94 10.35 0.09 -0.150 0.026 -1.177 0.120 0.604 0.012
NGC 2420 9.24 12.10 0.05 -0.266 0.017 -1.690 0.115 0.617 0.009
NGC 2477 9.04 11.55 0.23 0.019 0.047 -1.436 0.163 0.597 0.024
NGC 2506 9.24 12.60 0.05 -0.376 0.029 -1.573 0.107 0.619 0.007
NGC 2527 8.84 9.30 0.09 -0.080 0.090 -1.700 0.125 0.554 0.014
NGC 2539 8.76 10.75 0.09 0.137 0.028 -1.564 0.125 0.540 0.016
M 67 9.60b 9.80 0.04 0.000 0.092 -1.687 0.106 0.668 0.011
NGC 6791 9.98b 13.40 0.15 0.150 0.041 -1.422 0.133 0.687 0.016
NGC 6819 9.42b 12.44b 0.16b 0.074 0.035 -1.658 0.136 0.648 0.017
47 Tue 10.08 13.45C 0.044C -0.70d 0.07d -1.340 0.211 0.538 0.016
NGC 362 10.08 14.70" 0.048C -1.15d 0.06d -0.831 0.240 0.440 0.029

aFrom Twarog et al. (1997) unless otherwise noted
bFrom Sarajedini (1999)
CSee 6 2.2.2
dFrom Carretta & Gratton (1997)


2.3 Results and Discussion

2.3.1 Cluster Data

As described in @ 2.1, we are interested in exploring the dependence of Me on

[Fe/H] and age. Plotted in Figure 2-4 are the Mye values for the 14 open clusters

(open cirtles) and the two globulars (filled cirtles) in our sample versus the logarithm

of the age (top right panel) and the metallicity (top left panel). The red clumps start

out very bright at young ages and decrease in brightness by almost 1 mag as the cluster

ages approach 109 yr after which they brighten by ~ 0.5 mag. Then, the RCs become

slightly fainter as the cluster ages increase up to 1010 yr. The top two panels of Figure

2-4 also include MRC (open squares) from S99. Keeping in mind that the numbers of

clusters is small, over the age and metallicity range common to both studies, the K-

band absolute magnitude of the RC exhibits less sensitivity to age and metal abundance

than does MRq

2.3.2 Field Star Data

Alves (2000) reports K-band absolute magnitudes of solar-neighborhood stars

in the Hipparcos catalog along with parallaxes and proper motions. Following the


































1111111111111 III


I I I I I I I I I I I


MK (This Paper)
MI (S99)


MK (This Paper)
MI (S99)


-


-2.5

a, -2.0








-25






-2.0






--1.0


--0.5


O


a


*@)


9.5
Log Age


o Alves (2000)
e This paper


-1.0


-0.5
[Fe/H]


Figure 2-4: Comparison ofM c and MfRc. Upper panels show the variation of the
RC absolute magnitude as a function of [Fe/H] (top left panel) and age
(top right panel). The open circles represent K-band absolute magnitudes
(Mc) for the 14 open clusters while the filled circles signify Mc values
for the two globular clusters in the present sample. The open squares des-
ignate Mf~c values from S99. In the bottom panel, MK for Hipparcos Solar
neighborhood red clump stars from Alves (2000) (open circles) are com-
pared with Mrc for clusters in the present paper (filled cirles). These two
data sets show remarkable agreement in their mean K-band magnitudes.










analysis of Alves (2000), we have limited ourselves to stars with 2.2 < (V K)o < 2.5

and -2.5 < 2W < -0.8 in Table 1 of Alves' paper in order to isolate a sample of

nearby RC stars. We have plotted 2& versus [Fe/H] for these stars in the bottom panel

of Figure 2-4 (open circles). For comparison, the open and globular cluster data from

the present work (filled circles) are also shown. Keeping in mind that there are far

fewer open clusters than field stars in Figure 2-4, we find good consistency in the

locations of the two samples. Alves (2000) finds (2W(RC))= -1.61 +0.03, while the

open clusters in our sample give (2W(RC))= -1.61 +0.06. This is remarkable given

the fact that the open cluster distances are based on the main sequence fitting results

of Twarog et al. (1997) and the field stars are on the Hipparcos distance scale. Both

show mean K-band magnitudes of ~-1.6 and very little if any dependence on metal

abundance over the same range.

2.3.3 Comparison With Theoretical Models

Leo Girardi has kindly provided us with theoretical models that represent the

median magnitude of the red clump as a function of age and metal abundance

(Girardi & Salaris 2001; Crowl et al. 2001). Figures 2-5 and 2-6 show these the-

oretical models in the K-band compared with our open and globular cluster data. The

nine panels of Figure 2-5 display the K-band luminosity of the red clump as a function

of metallicity for a range of ages. The five panels in Figure 2-6 illustrate the variation

of the K-band luminosity with age for a range of metal abundances.

In both of these figures, clusters that are similar in metallicity or age to the model

plotted in each panel are represented by filled circles while the remaining clusters are

denoted by open circles. Figures 2-5 and 2-6 suggest that at ages younger than ~2

Gyr, the red clump luminosity is greatly dependent on the age of the cluster and shows

little effect from the metallicity whereas clusters older than ~2 Gyr show the exact

opposite, having little age dependence while still showing the effects of metallicity.














Log Age = 8.6 Log Age = 8.8 Log Age = 9











Log Age = 9.2 Log Age = 9.4 Log Age = 9.6







Il 11 l



Log Age = 9.8 Log Age = 10 Log Age = 10.1






I III I


-0.5


-2.5




-1.0

-0.5



-2.5







-0.5


-1.0 -0.5
[Fe/H]


0.0 -1.0 -0.5
[Fe/H]


0.0 -1.0 -0.5
[Fe/H]


Effects of age on MW. Plotted is the observed variation of M with the
logarithm of the age as compared with the predictions of theoretical mod-
els (Girardi et al. 2000) for the indicated metallicities. The filled circles
represent clusters with ages that are within +0. 1 dex of the model age
in each panel. For the upper left and lower right panels, the filled circles
represent clusters with log(Age) < 8.7 and log(Age) > 10.05, respectively.
The remaining clusters in each panel are marked by open circles.


Figure 2-5:


To facilitate a more detailed comparison between the models and the observations,

we utilize an interpolation routine based on low order polynomials to compare the

theoretical M~K values with the observed ones. As a test of the interpolation, we have

applied it to the observational data alone comparing the interpolated MPC values to














-2.5

-2.0


Y-1.0


-0.5


-2.5

-2.0


-1.0

-0.5





Figure 2-6:


9.0 9.5 1 0.0 9.0 9.5 1 0.0
Log Age Log Age

Effects of [Fe/H] on Mf. Plotted is the observed variation of My with
metal abundance as compared with the predictions of theoretical models
(Girardi et al. 2000) for specific ages. The filled circles denote the clusters
with [Fe/H];n,, < [Fe/H] < [Fe/H];;;a, where [Fe/H];ni,, and [Fe/H];na are
halfway between the model shown and the next lower and next higher
metallicity models, respectively. For the models of [Fe/H] = -1.3 and
[Fe/H] = 0.2, clusters having [Fe/H] < -1.0 and [Fe/H] > 0.1, respec-
tively, are marked with filled circles. The remaining clusters in each panel
are shown by open circles.


the actual values at the age and abundance of each cluster. We find that the rms of

the residuals is negligible in MC with no systematic trends as a function of age or

abundance. We have also tested the interpolation routine on the theoretical models with

similar encouraging results. The accuracy of the interpolation allows us to compare the

My values predicted by the models for a given age and metallicity to the observed

Mc for each cluster. From this comparison, we find that the rms deviation of the

theoretical models from the observations is 0.16 mag, with no systematic variation

in the residuals as a function of age or metallicity. This deviation is slightly larger















Log Age = 8.6 Log Age = 8.8 Log Age = 9
o o a






0 a ~ ~


O O







Log Age = 9.8 Log Age = 104 Log Age = 10.1

co a co ao
0 o a



I, I I I I II


-2.5

-2.0





S-2.5

-2.0



-0.5



-2.5

-2.0

IY -1.5

-1.0

-0.5


-0.6 -0.4 -0.2 0.0 -0.6 -0.4 -0.2 0.0 -0.6 -0.4 -0.2 0.0
[Fe/H] [Fe/H] [Fe/H]


Figure 2-7: Age effects on solar neighborhood RC stars. We plot the K-band absolute
magnitude of solar-neighborhood RC stars with Hipparcos parallaxes vs.
their metallicities. The solid lines represent the predictions of theoretical
models constructed by Girardi et al. (2000). The models suggest that the
vertical spread in M~K can be explained by variations in the ages of the
stars.


than the mean error in the MC values of the 16 clusters, which we find to be 0.13

mag. Given that the mean deviation of the models from the observations is roughly

consistent with the errors inherent in the latter, it is reasonable to conclude that the

models are generally consistent with the observational data.









Figure 2-7 shows the Girardi models plotted along with the Alves (2000) field

red clump star data. The models reinforce the conclusion drawn by Alves (2000) that

2& is insensitive to metallicity for nearby stars in this abundance range. Furthermore,

given that a typical 2& error in Alves (2000) data is 0.11 mag, this figure suggests

that the vertical spread in the 2A values is mainly the result of age effects among the

field stars. Both the 108.8 and 109.2-9.4 year isochrones agree with the majority of the

data. However, given the expectation that stars in the Solar neighborhood are likely to

be near Solar-age, most of the Hipparcos stars in Figure 2-7 probably have Log ages

between 9.2 and 9.6 (1.6 to 4.0 Gyr).

It is interesting to note that the ages of the solar neighborhood stars as predicted

by the models show a lack of stars around 109 yr (Figure 2-7). In contrast, using

a model of the solar neighborhood RC that assumes a constant star formation rate,

Girardi & Salaris (2001) expect an age distribution for the RC stars that peaks at 1 Gyr

with approximately 60% of the stars having this age. The discrepancy in this result

with the apparent ages of the Hipparcos RC stars likely indicates a non constant star

formation rate in the solar neighborhood; this is not surprising if the formation of stars

is triggered by density waves traveling through the solar neighborhood, which is an

intrinsically episodic process.

2.4 Application As a Distance Indicator

An important aspect of this study is the application of the K-band RC absolute

magnitude as a distance indicator. To optimize this application in the present work,

we seek a range of age and abundance over which variations in MPC are minimized.

Inspecting Figure 2-4, we see that if the age of the stellar population is in the range

2gAgeg6 Gyr and the metal abundance is between -0.5g[Fe/H]g0.0, then the

intrinsic variation in MFC is minimized suggesting that uncertainties in our knowledge

of these properties are inconsequential in the determination of the distance. On the

basis of these considerations, we have selected the open cluster NGC 2158. This









cluster possesses 2MASS photometry, and it is included in the study of Twarog et al.

(1997), so we have a metallicity value ([Fe/H] = -0.24 & 0.06) that is on the same

system as the other clusters in this study. The age shift described in @ 2.2.1 is also

applied to NGC 2158 giving us an age of 1.6 +0.2 Gyr. We note in passing that NGC

2158 was not included as part of our M~K(RC) calibration because the distance given

in Twarog et al. (1997) was determined using the magnitude of the RC and not main

sequence fitting.

For the reddening toward NGC 2158 we can utilize the data in Table 2-1 to

parameterize the intrinsic color of the RC [(J -K)o] in terms of the metal abundance

and age. Figure 2-8 shows (J-K)o versus [Fe/H] (left panel) and age (right panel)

for the clusters in our sample. Using the interpolation discussed in @ 2.3.3, we

can determine the intrinsic color of NGC 2158 given its metallicity and age, for

which we find (J -K)o = 0.618 +0.003. We calculate the error in (J -K)o by

determining the uncertainty resulting from oage and GT[Fe/H] and adding these in

quadrature. Comparing the implied intrinsic color of the RC with the apparent color,

(J -K) = 0.837 +0.005, we find E(J -K) = 0.219 +0.006. Converting this to a

color excess in the optical regime, we find E(B V) = 0.42 +0.012, which is in good

agreement with published values (e.g., Christian et al. 1985; Twarog et al. 1997). The

preceding method represents an internally consistent formalism which can be utilized to

estimate the reddening of a cluster.

The interpolation on M using only the open cluster data predicts MZK =

-1.67 + 0.09 for NGC 2158. Along with E(B V) = 0.42 and the apparent RC

K-band magnitude, K(RC) = 11.53 + 0.02, we find (m M); = 14.35 + 0.09. Our

distance modulus for NGC 2158 agrees within the errors with the main sequence fitting

modulus of (m -M);- = 14.4 & 0.2 found by Christian et al. (1985), but is slightly

lower than that determined by Twarog et al. (1997) of (m -M);- = 14.5.












0.4-



0.5







0.7: --


-1,0 -0.5 0.0 9.0 9.5 10.0
[Fe/H] Log Age

Figure 2-8: Intrinsic red clump color. The intrinsic color of the RC is plotted as a
function of [Fe/H] (left) and age (right), where the open circles represent
the open clusters while the filled circles are the globulars.


2.5 Conclusions

In this work, we have sought to establish the K-band absolute magnitude of the

helium burning red clump stars (Mf) as a distance indicator. To facilitate this, we

have utilized infrared photometry from the 2MASS catalog along with distances,

metallicities, and ages for 14 open clusters and two globular clusters. Our sample

encompasses an age range from 0.63 Gyr to 12 Gyr and metallicities from -1.15 to

0.15 dex. Based on an analysis of these data, we draw the following conclusions.

1. There is a statistically significant range ofMF values among the star clusters

in our sample. In particular, for the 14 open clusters, we calculate (MF)= -1.61 with

a standard deviation of 0.22 mag. In contrast, the mean error in these MW values is

0.13 mag.

2. Upon inspection of Figures 2-5 and 2-6, we find that for clusters younger than

~2 Gyr, MW is insensitive to metallicity but shows a dependence on age. In contrast,









for clusters older than ~2 Gyr, Mye is influenced primarily by the metallicity of the

population and shows little or no dependence on the age.

3. In general, MRC is less sensitive to age and metallicity than MRC over the

parameter range common to both this paper and Sarajedini (1999) from which the M,~RC
values are taken.

4. Over comparable metallicity and age ranges, our average MRC value of -1.61

mag is consistent with that of Alves (2000) which is based on solar-neighborhood

RC stars with Hipparcos parallaxes. We also suggest that the significant scatter in

the Alves (2000) M~K data is likely due to a range of ages between ~1.6 and ~4 Gyr

among these stars.

5. The theoretical RC models based on the formalism of Girardi et al. (2000)

agree reasonably well with our observational data, indicating that age plays an impor-

tant role in determining MRC for younger populations while metallicity mainly affects

older populations.

6. Using the K-band absolute magnitude of the RC, we are able to compute

the distance to the open cluster NGC 2158. Adopting an age of 1.6 + 0.2 Gyr and

[Fe/H] = -0.24 & 0.06, our calibration yields a distance of (m M), = 14.35 + 0.09.

7. When determining distances for star clusters having -0.5 < [Fe/H] < 0.0 and

109.2 < age < 109.9, one can ignore the interpolation discussed in @ 2.3.3 and simply

use (M~K(RC)) = -1.61 +0.04.

2.6 Beyond Grocholski & Sarajedini (2002)

As discussed in the introduction, we ultimately want to apply our RC calibration

to determining the distance to the LMC. In Sarajedini et al. (2002) we tested the

feasibility of applying the RC calibration presented in @ 2.3.3 to the stellar populations

of the LMC. Over three nights in December 2001, using OSIRIS on the CTIO 4m,

we obtained JKs photometry of the LMC clusters Hodge 4 and NGC 1651 down to

a photometric depth of Ks ~19, or about 2 magnitudes below the RC. Images were










processed using a number of IRAF scripts, which performed the following tasks. The

nine dithered images of each cluster were dark subtracted and median combined,

without shifting, so as to remove any objects from the combined frame, leaving only an

exposure of the sky. We created a flat-field image by normalizing this sky frame. The

original images were then sky subtracted and flat-fielded using these calibration frames.

After processing, spatial offsets in the dither pattern were calculated and the images

were shifted and average combined resulting in a pair of images for each cluster with

signal-to-noise ratios equivalent to a single 198 s exposure in J and a single 270 s

exposure in K.

Standard star images were combined in the same fashion as the science frames

and they were analyzed with the QPHOT software package in IRAF, which was used to

measure instrumental magnitudes using a 15 pixel radius aperture. Standard stars were

observed over the course of three nights and we have combined these observations by

offsetting the measured instrumental magnitudes to the same zero point using stars

in common between the three nights. The combined data set of seven standard star

observations was fitted using a least-squares algorithm to calculate the extinction

coefficients and zero points. This procedure yielded the following equations:


j J = (2.03 + 0.03) +t (0.06 + 0.02)XJ, (2-3)

ks Ks = (2.23 + 0.03) +t (0.09 + 0.02)XK, (2-4)


where the lower case letters represent the instrumental magnitudes, X is the air mass,

and the capital letters are the apparent magnitudes.

The combined JKs cluster images were measured using the aperture photometry

routines in DAOPHOT (Stetson 1994) and calibrated using equations 2-3 and 2-4. Ks

magnitudes are converted to the K-band using equations 2-1 and 2-2 and Figure 2-9

shows the resulting [K,(J-K)] CMDs. Both of these clusters show a well populated

red giant branch (RGB) and an easily identifiable helium burning RC. The box in each









Table 2-2. LMC Cluster Information
Cluster [Fe/H] Age (Gyr) K(RC) Alg(RC) E(J-K) (m-M~)o
Hodge 4 -0.17 10.04 1.7 10.3 16.90 +0.02 -1.64 10.17 0.03 10.01 18.52 +0.17
NGC 1651 -0.07 +0.10 1.8 +0.3 17.03 +0.02 -1.54+0.12 0.06 +0.01 18.53 +0.12


panel indicates the stars that were used in calculating the median apparent K-band

magnitude of each cluster RC, which we find to be 16.90 + 0.02 mag and 17.03 + 0.02

mag for Hodge 4 and NGC 1651, respectively. As mentioned in the previous section,

knowledge of a cluster's age and metallicity are necessary to determining its distance

using M Work by Tiede, Martini, & Frogel (1997) showed that the slope of a

cluster's RGB in the near infrared is related to it's [Fe/H]. In Figure 2-9 we mark

the cluster RGB stars with filled circles and the solid line denotes the linear fit to

these stars. Utilizing these fits with the calibration of Tiede et al. (1997), we find

[Fe/H] = -0. 17 + 0.04 for Hodge 4 and [Fe/H] = -0.07 + 0. 10 for NGC 1651.

Additionally, we used published optical photometry of the MSTO for these clusters

(Sarajedini 1998, Hodge 4 and Mould et al. 1997, NGC 1651) to calculate ages

via MSF with theoretical isochrones (Girardi et al. 2000). The resulting ages are

1.7 + 0.3 Gyr for Hodge 4 and 1.8 + 0.3 Gyr for NGC 1651. Combining ages and

metallicities with our RC calibration, we find for Hodge 4, MC = -1.64 +0. 17 and

for NGC 1651, MC = -1.54 & 0. 12. Since they are sufficiently close to each other,

we averaged the reddening values derived from both the Burstein & Heiles (1982) and

Schlegel, Finkbeiner, & Davis (1998) dust maps, for each cluster. Using the reddening

relations quoted by Schlegel et al. (1998), A,- = 3.1E(B V), AK = 0. 11A,-, and

AJ = 0.28Ar-, we convert E(B V) to E(J -K). These values, along with other derived

cluster parameters, are given in Table 2-2.

Finally, combining our apparent and absolute K-band RC magnitudes with the

derived reddenings, we calculate the absolute cluster distances to be (m -M~o =

18.52 + 0. 17 and (m M1o = 18.53 + 0. 12 for Hodge 4 and NGC 1651, respectively.

These numbers are in good agreement with the LMC distance of 18.515 + 0.085 mag






29



published by Clementini et al. (2003). This value is derived by averaging published

LMC distances determined through a variety of distance indicators. Additionally,

these cluster distances are in good agreement with the LMC geometry determined by

van der Marel & Cioni (2001) and Olsen & Salyk (2002), which indicates that Hodge

4, in the Northeast portion of the LMC, should be closer to us than NGC 1651, found

in the Southwest.




















14-





16-



O Oo

o OO oOO
ooo



.NGC 1651


12-





14-

o **Y


16 gs oo




18-o*$oqo o
OO" OO p p


0.0 0.5 1.0
J-K

Figure 2-9:) Near-IR ::- CMls f~or the : i:: clusters Hodge 41 and :: 1651. The
: : 1 are used to outline: the cluster R~s. All stars within these boxes
are used in .:1 ::1 :i::- the :i -::i- K--band RC magnitude. ( i::.i= :- RGB
stars are denoted by the filled circles with the solid i showing the
least-sqluares fits.















CHAPTER 3
ABUNDANCES AND VELOCITIES OF A SAMPLE OF LMC CLUSTERS

3.1 Introduction

In the current paradigm of galaxy formation, it is believed that the formation

history of spiral galaxy spheroids, such as the Milky Way (MW) halo and bulge, may

be dominated by the accretion/merger of smaller, satellite galaxies (e.g., Searle & Zinn

1978; Zentner & Bullock 2003). This type of galactic interaction is currently seen

in the Sagittarius dwarf galaxy (Sgr), which is in the midst of being cannibalized

by the MW. However, due to its location on the opposite side of the Galaxy from

us (Ibata et al. 1994), contamination by MW foreground stars makes it difficult to

study stellar populations in Sgr. In contrast, both the Large Magellanic Cloud (LMC)

and Small Magellanic Cloud (SMC), two satellite galaxies that may eventually be

consumed into the MW halo, suffer little from foreground contamination due to

their direction on the sky, which places them well out of the plane of the MW. In

addition, the relative proximity of these galaxies allows us to easily resolve stellar

populations in the Magellanic Clouds down below their oldest main sequence turnoffs

(MSTOs). Thus, the LMC and SMC offer us a golden opportunity to study the effects

of dynamical interactions on the formation and evolution of satellite galaxies; this

information plays an integral part in discovering the secrets of spiral galaxy formation.

One of the most direct ways to determine the chemical evolution history (CEH)

and star formation history (SFH) of a galaxy is through the study of its star clusters,

which preserve a record of their host galaxy's chemical abundances at the time of

their formation. The LMC star clusters continue to play a critical role in shaping

our understanding of the age-metallicity relation of irregular galaxies. The rich star

cluster system of the LMC is also a unique resource for many experiments in stellar










and galactic astronomy, largely due to the fact that the LMC harbors well populated

clusters that occupy regions of the age-metallicity plane that are devoid of MW

clusters. Thus, LMC clusters have been widely studied as a test of stellar evolution

models at intermediate metallicity and age (e.g., Brocato et al. 1994; Ferraro et al.

1995; Bertelli et al. 2003) and as empirical templates of simple stellar populations for

applications to population synthesis models of unresolved galaxies (e.g., Beasley et al.

2002; Leonardi & Rose 2003; Maraston 2005).

The LMC cluster system, however, is well known to show a puzzling age distri-

bution, with a handful of old (~ 13 Gyr), metal-poor globular clusters; a number of

intermediate age (1-3 Gyr), relatively metal-rich populous clusters; and, apparently,

only one cluster, ESO 121-SCO3 (~ 9 Gyr, hereafter ESO 121), that falls into the

LMC's so-called "age gap" (e.g., Da Costa 1991; Geisler et al. 1997; Da Costa 2002).

We note that the LMC bar seems to show a formation history very similar to that of

the clusters (Cole et al. 2005), while field SFHs derived from deep color-magnitude

diagrams (CMDs) suggest that stars in the LMC disk had a constant, albeit low, star

formation rate during the cluster age gap (Holtzman et al. 1999; Smecker-Hane et al.

2002). While the cause of the cessation of cluster formation (the beginning of the

age gap) is not known, dynamical simulations by Bekki et al. (2004) suggest that the

recent burst of cluster formation is linked to the first very close encounter between the

Clouds about 4 Gyr ago, which would have induced "dramatic gas cloud collisions,"

allowing the LMC to begin a new epoch of cluster and star formation; strong tidal

interactions between the Clouds have likely sustained the enhanced cluster formation.

Bekki et al. (2004) also find that the close encounter between the Clouds would have

been sufficient to cause the formation of the LMC bar around the time of the new

epoch of cluster formation, giving rise to the similar SFHs seen in the cluster system

and the bar. In addition to enhancing star formation, tidal forces can result in the infall

or outflow of material, thereby affecting the CEH of the LMC and, at the same time,










leaving behind a signature of the interaction. Thus, accurate knowledge of the ages

and metallicities of LMC clusters is necessary to fully understand the formation and

dynamical history of this galaxy.

While age and metallicity estimates from isochrone fitting to CMDs exist for

a large number of clusters, the degeneracy between age and metallicity makes these

estimates inherently uncertain in the absence of solid metallicity measurements based

on spectroscopic data. Integrated light has been used to measure [Fe/H] for many of

these clusters; however, these values are often problematic, since the cluster light can

be dominated by a few luminous stars, and the results are susceptible to small-number

statistical effects. In recent years, high spectral resolution studies of a few prominent

clusters have been undertaken, yielding, for the first time, detailed abundance estimates

of a wide variety of elements, including iron, for individual stars within these clusters

(Hill 2004; Johnson et al. 2006). This work is highly valuable, but because of the large

investment in telescope time necessary to obtain data of sufficiently high signal-to-noise

ratio (S/N), it has necessarily been limited to only a few stars in a few clusters; most

of these targets are very old, leaving the vast majority of young and intermediate age

clusters unmeasured.

Moderate-resolution studies are an excellent complement to high-resolution work

for a couple of reasons. First, the multi-object capability available for many moderate-

resolution spectrographs makes it possible to observe many potential cluster members

in a given field. This increases the probability of observing true cluster members and

facilitates their identification, even in sparse clusters. Second, less integration time is

needed to achieve the desired S/N at moderate resolution, allowing the observation of

many more targets in a given amount of time. Thus, with moderate-resolution spectra

we can observe a large number of targets in a short period of time and thereby create

an overview of a galaxy's global metallicity distribution, both spatial and temporal.









This approach is particularly important for the LMC, since its metallicity distribution is

very broad and the intrinsic shape is not very well known.

To date, the only large-scale spectroscopic metallicity determination for LMC clus-

ters based on individual cluster stars has been the landmark study by (Olszewski et al.

1991, hereafter, OSSH; Suntzeff et al. 1992). They obtained medium-resolution spectra

of red giant branch (RGB) stars in ~ 80 clusters at a wavelength of a 8600 A+, cen-

tered on the very prominent triplet of Ca II (CaT) lines. Their work was motivated by

the recognition that the CaT lines were proving to be a reliable metallicity indicator

for Galactic globular clusters (e.g., Armandroff & Zinn 1988; Armandroff & Da Costa

1991) Additionally, this spectral feature is easily measured in distant targets and at

medium resolution since the CaT lines are extremely strong and RGB stars are near

their brightest in the near-infrared. Using the CaT, OSSH calculated metallicities and

radial velocities for 72 of their target clusters. Analysis of the metallicity distribution

showed that the mean [Fe/H] values for all clusters in the inner (radius < 5 ) and outer

(radius > 5 ) LMC are almost identical (-0.29 + 0.2 and -0.42 + 0.2, respectively),

suggesting the presence of little, if any, radial metallicity gradient, in sharp contrast

to what is seen in the MW (e.g., Friel et al. 2002) and M33 (Tiede et al. 2004). Us-

ing radial velocities from the OSSH sample, Schommer et al. (1992) found that the

LMC cluster system rotates as a disk, with no indication that any of the clusters have

kinematics consistent with that of a pressure-supported halo.

However, the results of OSSH present some difficulties owing to the limitations

of technology at the time. The use of a single-slit spectrograph severely limited the

number of targets observed toward each cluster. Additionally, the distance of the LMC

paired with a 4m telescope required that they observe the brightest stars in the clusters.

Many of these stars are M giants, which have spectra contaminated by TiO (although

it may not be significant until spectral type MS or later), or carbon stars, neither of

which are suitable for using the CaT to determine [Fe/H]. Thus, the combination of a










single-slit spectrograph with a midsized telescope made it difficult for OSSH to build

up the number of target stars necessary to differentiate between cluster members and

field stars. Most of the resulting cluster values are based on only one or two stars; in

some cases, there are metallicity or radial velocity discrepancies between the few stars

measured, and it is unclear which of the values to rely on.

The interpretation of the OSSH results is further complicated by subsequent

advances both in knowledge of the globular cluster metallicity scale to which the

CaT strengths are referred (Rutledge et al. 1997a) and in the standard procedure used

to remove gravity and temperature dependencies from the CaT equivalent widths

(Rutledge et al. 1997b). It is not a simple matter to rederive abundances from the

equivalent widths of OSSH because of the lack of homogeneous photometry for many

of the clusters; mapping the OSSH abundances to a modern abundance scale (e.g., that

defined at the metal-poor end by Carretta & Gratton 1997 and near-solar metallicity by

Friel et al. 2002) is insufficient, because the transformation is nonlinear and random

metallicity errors tend to be greatly magnified (see, Cole et al. 2005).

In an effort to produce a modern and reliable catalog of LMC cluster metallicities,

we have obtained near-infrared spectra of an average of eight stars in each of 28 LMC

clusters. We have taken advantage of the multiplex capability and extraordinary image

quality and light-gathering power of the European Southern Observatory's (ESO)

8.2m Very Large Telescope (VLT) and of the great strides in the interpretation and

calibration of CaT spectroscopy made in the past 15 years to provide accurate cluster

abundances with mean random errors of 0.04 dex. Here we present our derived cluster

metallicities and radial velocities and compare these results to previously published

spectroscopic metallicities. The metallicity distribution of several hundred non-cluster

LMC field stars will be presented in a forthcoming paper (A. A. Cole et al., in prepara-

tion). The current chapter is laid out as follows: Section 3.2 discusses the observations

and data processing. In ~3.3 we present the derived cluster properties, and comparisons










to previous works are detailed in ~3.4. Finally, in ~3.5 we summarize our results. We

note that these data and results were originally presented in Grocholski et al. 2006.

3.2 Data

3.2.1 Target Selection

We observed 28 prominent star clusters scattered across the face of the LMC,

in environments ranging from the dense central bar to the low-density regions near

the tidal radius (a 29th cluster was observed, but it appears to be too young to apply

the CaT method; see ~3.6.2). Our observations were aimed at clusters rich enough

and sufficiently large and diffuse to give us confidence in harvesting at least four

definite cluster members from which to derive the cluster metallicity. In order to obtain

leverage on the LMC age-metallicity relation, we included clusters from SWB class

IVB-VII, spanning the age range of clusters containing bright, well-populated RGBs

(Persson et al. 1983; Ferraro et al. 1995). Our sample was intentionally biased towards

those clusters with conflicting or uncertain previous abundance measurements, those

thought to lie near the edge of the age gap, and those whose radial velocities might

provide new insight into the dynamical history of the LMC-SMC system, based on

their location. Our targets and their positions, sizes, integrated V magnitudes, and SWB

types are listed in Table 3-1. A schematic of the LMC is presented in Fig. 4-4. Shown

are near-infrared isopleths from van der Marel (2001; solid ellipses), at semi-major

axis values of 1 1 5, 2 3 4 6 and 8 Prominent H I features (dashed lines;

Staveley-Smith et al. 2003) and the two largest centers of LMC star formation (30 Dor

and N11; open circles) are also plotted. Finally, the rotation center of intermediate-age

stars is denoted by the open square (van der Marel et al. 2002), and the H I rotation

center from Kim et al. (1998) is plotted as the open triangle. Our target clusters are

plotted with solid symbols, with the exception of NGC 1841, which lies farther south

than the area covered by this diagram.















30m

i



c






69"






13~


5 30mI


~ BJ~ -P


Figure 3-1:


Schematic diagram of the LMC showing the location of our target clusters
along with prominent features. Filled symbols represent the target clus-
ters, with symbol size directly related to V magnitude and shape denoting
SWB type, where the triangles, squares, and pentagons are type V, VI,
and VII, respectively. Note that NGC 1841 (declination ~ -84") is out-
side of the range of this plot and NGC 1861 (SWB type IVB) is marked
by a filled triangle. Near-infrared isopleths from van der Marel (2001)
are marked by solid lines while the dashed lines outline major H I fea-
tures (see Staveley-Smith et al. 2003. The H I rotation center (Kim et al.
1998) is marked with the open triangle, and the rotation center of the
intermediate-age stars (van der Marel et al. 2002) is shown by the open
square. Finally, the two largest H II regions are marked by open circles.









Pre-images of our target fields in the V and I bands were taken by ESO Paranal

staff in the fall of 2004, several months prior to our observing run. The pre-images

were processed within IRAF, and stars were identified and photometered using the

aperture photometry routines in DAOPHOT (Stetson 1987). Stars were cataloged using

the FIND routine in DAOPHOT and photometered with an aperture size of 3 pixels.

The V- and I-band data were matched to form colors. Red giant targets were chosen

based on the instrumental CMD, and each candidate was visually inspected to ensure

location within the cluster radius (judged by eye) and freedom from contamination by

very nearby bright neighbors. In each cluster we looked for maximum packing of the

S8"' long slits into the cluster area and for the best possible coverage of the magnitude

range from the horizontal branch/red clump (V M 19.2) to the tip of the RGB (V m

16.4). The positions of each target were defined on the astrometric system of the

FORS2 pre-images so that the slits could be centered as accurately as possible, and the

slit identifications were defined using the FORS Instrument Mask Simulator software

provided by ESO; the slit masks were cut on Paranal by the FORS2 team.

3.2.2 Acquisition

The spectroscopic observations were carried out with FORS2 in visitor mode at

the Antu (VLT-UT1) 8.2 m telescope at ESO's Paranal Observatory during the first half

of the nights of 21-24 December 2004; weather conditions were very clear and stable

during all four nights, with seeing typically 0."5-1"'0. We used the FORS2 spectrograph

in mask exchange unit (MXU) mode, with the 1028z+29 grism and OG590+32 order

blocking filter. The MXU slit mask configuration allows the placement of more slits

on the sky than the 19 movable slits provided in Multi-Object Spectrograph mode. We

used slits that were 1" wide and 8"' long (7"' in a few cases), and, as mentioned above,

targets were selected so as to maximize the number of likely cluster members observed;

typically 10 stars inside our estimated cluster radius were observed, with an additional









~ 20 stars outside of this radius that appeared to be LMC field red giants based on our

preimaging CMDs.

FORS2 uses a pair of 2k x 4k MIT Lincoln Laboratory CCDs, and the target

clusters were centered on the upper (master) CCD, which has a readout noise of 2.9

electrons, while the lower (secondary) CCD, with a readout noise of 3.15 electrons,

was used to observe field stars. The only exception to this was the Hodge 11-SL 869

field, where, with a rotation of the instrument, we were able to center Hodge 11 in

the master CCD and SL 869 in the secondary CCD. Both CCDs have an inverse gain

of 0.7e- ADU Pixels were binned 2 x2, yielding a plate scale of 0."25 pixel ,

and the resulting spectra cover 1750 A+, with a central wavelength of 8440 A+ and a

dispersion of ~0.85 A pixell (resolution of 2-3 A+). While the FORS2 field of view

is 6(8 across, it is limited to 4(8 of usable width in the dispersion direction in order to

keep important spectral features from falling off the ends of the CCD.

Each field was observed twice, with offsets of 2" between exposures, to ameliorate

the effects of cosmic rays, bad pixels, and sky fringing. The total exposure time in

each setup was either 2 x 300, 2 x 500 or 2 x 600 s. Both the readout time (26

s) and setup time per field (some 6-10 minutes) were very quick and allowed us to

obtain longer exposures than originally planned in many cases. For most of our targets

with short exposure times (300 s) we combined the spectra so as to improve the S/N.

However, with the longer exposures (500 and 600 s) we found that the S/N in a single

exposure was adequate, and cosmic rays and bad pixels were not a problem, so we

have used only one of the pair of exposures in our analysis. Column 8 of Table 3-1

gives the total exposure time that we have used in our analysis of each cluster.

Calibration exposures were taken in daytime under the FORS2 Instrument Team's

standard calibration plan. These comprise lamp flat-field exposures with two different

illumination configurations and He-Ne-Ar lamp exposures for each mask. Two lamp










settings are required for the flat fields because of parasitic light in the internal FORS2

calibration assembly.

In addition to the LMC clusters, we observed four Galactic star clusters (47

Tuc, M67, NGC 2298, and NGC 288), three of which are a subsample of the CaT

calibration clusters in Cole et al. (2004, hereafter, CO4). Since we used the same

instrument setup as CO4, we expected to use their CaT calibration, and these three

clusters were observed to serve as a check on the validity of that approach. Processing

of these three clusters shows that our results are identical to within the errors; thus,

we use the CaT calibration of CO4 rather than deriving our own CaT calibration

coeffici ents.

3.2.3 Processing

Image processing was performed with a variety of tasks in IRAF. The IRAF task

ccdproc was used to fit and subtract the overscan region, trim the images, fix bad pix-

els, and flat field each image with the appropriate dome flats. The flat-fielded images

were then corrected for distortions in order to facilitate extraction and dispersion cor-

rection of the spectra. The distortion correction is a two-step process, whereby first the

image of each slitlet is rectified to a constant range of y-pixel (spatial direction) values

on the CCD, and then the bright sky lines are traced along each slitlet and brought

perpendicular to the dispersion direction. The amount of the distortion is minimal near

the center of the field of view and increases toward the edges; in all cases it is fit with

a polynomial that is at most quadratic in y and linear in x. Although the distortion

corrections are small, they greatly reduce the residuals left after sky subtraction and

improve the precision and accuracy of the dispersion solution (see below).

Once distortion corrections were completed, the task pall (in the HYDRA

package) was used to define the sky background and extract the stellar spectra into one

dimension. The sky level was defined by performing a linear fit across the dispersion

direction to sky "windows" on each side of the star. This procedure presented few









difficulties, since the target stars were usually bright compared to the sky and the

seeing disks were small compared to the length of the slitlets. The only problems

arose when the star fell near the top or bottom of the slitlet; in these cases the sky

regions were chosen interactively, and we found for all of these spectra that the

resulting sky subtraction was indistinguishable from that of more centrally located

stars. While daily arc lamp exposures are available for dispersion-correcting the

spectra, telescope flexure during the night, along with small slit centering errors, makes

this a less desirable method for correcting the spectra. As such, more than 30 OH

night-sky emission lines (Osterbrock & Martel 1992) were used by the IRAF tasks

identify/, refspectra, and dispcor to calculate and apply the dispersion solution for each

spectrum, which was found to be ~ 0.85 A+ pixels with a characteristic rms scatter

of ~ 0.06 A+. For the short (300 s) exposure data, we processed both sets of images

for each pointing and combined the dispersion-corrected spectra using combine to

improve the S/Ns for these stars. In a few cases we found that averaging the stellar

spectra actually decreased the S/N; for these stars we chose to use the higher quality

of the two individual spectra in place of the averaged spectrum. All spectra were then

continuum-normalized by fitting a polynomial to the stellar continuum, excluding

strong absorption features (both telluric and stellar). For the final spectra, S/Ns are

typically 25-50 pixels with some stars as high as ~ 90 pixels and, in only a few

cases, as low as ~ 15 pixel Sample spectra showing the CaT region are presented in

Fig. 3-2.
3.2.4 Radial Velocities

Accurate radial velocities for our target stars are important for two reasons. First

and foremost, since a cluster's velocity dispersion is expected to be relatively small

compared to the surrounding field and its mean velocity quite possibly distinct from

the field, radial velocities are an excellent tool for determining cluster membership. In





[Fe/H] = -1.84







[Fe/H] = -1.31







[Fe/H] = -0.41


"1J""""~-


.Hodge 11 #9







.NGC 2019 #10







.IC 2146 #5



. ~Fe I F
~Co II


EEW = 5.57


EEW = 6.83


EEW = 9.24


Fe I Fe I


Fe


Co II
Co II


IIIIIIIIIIIIIIIIIII II


8400


8500


8600
Wavelength (Angstroms)


8700


8800


Figure 3-2:


Sample of spectra from RGB stars in our target clusters covering a range
in metallicities. The three CaT lines, along with some nearby Fe I lines,
are marked for reference; the change in CaT line strength with [Fe/H] is
readily visible. Calculated summed equivalent widths and metallicities for
each star are given.


addition, our equivalent-width measuring program uses radial velocities to derive the

expected CaT line centers.

Radial velocities for all target stars were determined through cross-correlation

with 30 template stars using the IRAF task fxcor (Tonry & Davis 1979), and we have

chosen to use template spectra from CO4. The template stars were observed as a part









of their CaT calibration program; thus, their sample offers a good match to the spectral

types of our target stars. In addition, their observations were made with a telescope and

instrument setup that is almost identical to ours. CO4 chose template stars for which

reliable published radial velocity measurements were available. Template velocities

came from the following sources: 11 stars from NGC 2298, NGC 1904, and NGC

4590 (Geisler et al. 1995); 8 stars from Berkeley 20 and Berkeley 39 (Friel et al.

2002); 2 stars from Melotte 66 (Friel & Janes 1993); 6 stars from M67 (Mathieu et al.

1986); and 3 stars from 47 Tuc (Mayor et al. 1983). In addition to calculating relative

radial velocities, fxcor uses information about the observatory location and the date

and time of the observations (once the ESO header has been appropriately reformatted)

to correct the derived velocities to the heliocentric reference frame. For a star's final

heliocentric radial velocity, we adopt the average value of each cross-correlation result.

We find good agreement among the template-derived velocities, with a typical standard

deviation of ~ 6 km sl for each star.

When the stellar image is significantly smaller than the slit width, systematic er-

rors due to imprecise alignment of the slit center and the stellar centroid can dominate

the error budget in the radial velocity measurements. With the grism and CCD used

here, an offset of 1 pixel across the 4 pixel wide slit would introduce an error in the

measured velocity of a 30 km s We follow the approach of Tolstoy et al. (2001)

in applying a correction to each measured radial velocity based on the individual slit

offsets; following CO4, we measure the slit offsets using acquisition (so-called through-

slit) images taken immediately prior to the spectroscopic measurement and estimate

a precision of m0.14 pixels on the measured offset value. This introduces an error

of + 4.2 km sl and, added in quadrature with the error resulting from the velocity

cross-correlations, gives an error of roughly 7.5 km s We adopt this as the error in

measuring the radial velocity of an individual star.









Table 3-2. CaT Line and Continuum Bandpasses
Feature Line Bandpass (A+) Blue Continuum (8+) Red Continuum (8+)

Ca II h8498 8490 8506 8474 8489 8521 8531
Ca II h8542 8532 8552 8521 8531 8555 8595
Ca II h8662 8653 8671 8626 8650 8695 8725


3.2.5 Equivalent Widths and Abundances

To measure the equivalent widths of the CaT lines, we have used a previously

written FORTRAN program (see CO4 for details). However, since this region of a

star's spectrum can be contaminated by weak metal lines and, in some cases, weak

molecular bands, measuring the true equivalent width of the CaT lines at all but

the highest spectral resolutions is impossible. Instead, we follow the method of

Armandroff & Zinn (1988) and define continuum bandpasses on either side of each

CaT feature. In this wavelength range, the continuum slope of a red giant star is

virtually flat; thus, the "pseudo-continuum" for each CaT line is easily defined by

a linear fit to the mean value in each pair of continuum windows. The "pseudo-

equivalent width" is then calculated by fitting the sum of a Gaussian and a Lorentzian,

required to have a common line center, to each CaT line with respect to the "pseudo-

continuum." For reference, the rest wavelengths of the line and continuum bandpasses,

as defined by Armandroff & Zinn (1988), are listed in Table 3-2. For many years it

has been known that even at the moderate spectral resolutions used here, a Gaussian

fit to the CaT lines is susceptible to loss of sensitivity at high metallicity because

the Gaussian fails to accurately measure the extremely broad wings of the lines (see

discussion in Rutledge et al. 1997b). We follow the procedure established in CO4

and add a Lorentzian profile to the Gaussian in order to recover sensitivity to the full

range of metallicities. Errors in the equivalent width measurements were estimated by

measuring the rms scatter of the data about the fits.









A number of previous authors have calibrated the relationship between the

strengths of the three CaT lines and stellar abundance using a variety of methods (see

Table 3 in Rutledge et al. 1997a). In all cases, a linear combination of the individual

line strengths was used to produce the summed equivalent width, CW, with weighting

and inclusion of lines (some authors dropped the weakest line, 8498 A+) varying based

on the quality of their data. Since the quality of our data is such that all three lines are

well measured, we adopt the same definition for CW as CO4,


CW EW8498 +tEW8542 +tEW8662. (3-1)


It is well known that TiO, which has a strong absorption band beginning near 8440

A+ (e.g., Cenarro et al. 2001), can affect the spectra of cool (~ MS or later), metal-rich

stars. This absorption feature, which depresses the "pseudo-continuum" around the

CaT lines and results in an underestimation of the measured equivalent widths, was

noted by OSSH in some of their LMC spectra. During processing, we checked each

spectrum for the appearance of this TiO absorption band and found no evidence that

TiO had affected any of our observations.

Both theoretical (Jarrgensen et al. 1992) and empirical (Cenarro et al. 2002)

studies have shown that effective temperature, surface gravity, and metallicity all

play significant roles in determining the CaT line strengths. However, it is well

established that for red giants of a given metallicity, there is a linear relationship

between a star's absolute magnitude and CW (Armandroff & Da Costa 1991), where

stars farther up the RGB have larger CW values. This is primarily due to the change

in surface gravity as a star moves along the RGB; stars near the bottom of the RGB

have smaller radii, thus larger surface gravities, which increases the H- opacity. Since

H- is the dominant opacity source in red giants, increasing the H- opacity depresses

the "pseudo-continuum", which in turn drives down the measured value for CW. To

remove the effects of luminosity on CW, similar to previous authors, we define a









reduced equivalent width, W', as


W' CW +t p(V VHB), (3-2)


where the introduction of the brightness of a cluster's horizontal branch (HB), VHB,

removes any dependence on cluster distance or reddening (see the thorough discussion

in Rutledge et al. 1997b). Due to the fact that a majority of our clusters are too

young and metal-rich to have a fully formed HB, we instead adopt the median value

of the core helium-burning red clump (RC) stars for these clusters (see ~3 for more

information). Values for p have been derived empirically by previous authors, with the

most robust determination being that of Rutledge et al. (1997b). Utilizing stars from 52

Galactic globular clusters, they found a metallicity-independent value of P = 0.64+0.02

A+ mag covering clusters in the range -2.1 g [Fe/H] g -0.6. Similarly, CO4 found

p = 0.66 + 0.03 for the globular clusters in their sample. However, when their open

clusters were included, the slope steepened to P = 0.73 + 0.04. This steepening of the

relationship between W' and V VHB with [Fe/H] is in qualitative agreement with the

theoretical results of Jorrgensen et al. (1992). Since our target clusters span an age and

metallicity range similar to the entire calibration cluster sample observed by CO4, for

p we have chosen to adopt their value of 0.73, which is based on both their open and

globular calibration clusters. To validate this approach, as mentioned in ~3.2.2, during

our science observations we observed a subsample of the calibration clusters used by

CO4 and found that, to within the errors, our measurements are identical to theirs, as is

expected, given that essentially the same instrument setup was used in both programs.

Before proceeding to the last step of the CaT calibration, we need to address

the issue of possible age effects on these calculations. As noted by previous authors

(e.g., Da Costa & Hatzidimitriou 1998; CO4; Koch et al. 2006), the age of a stellar

population affects the luminosity of core helium burning stars and may introduce

systematic errors in determining V VHB and, therefore, metallicities derived via









the CaT method. Experiments by CO4 and Koch et al. (2006) have shown that age

effects brought about by using an inappropriate VHB for any given RGB star will

typically cause errors in [Fe/H] on the order of + 0.05 dex, but these errors can,

in extreme cases, be as large as + 0.1 dex. One can avoid this type of uncertainty

by observing populous clusters, since this allows the correlation of a given RGB

star to a specific HB/RC, which is composed of stars of the appropriate age and,

therefore, has a well-defined mean magnitude. However, Da Costa & Hatzidimitriou

(1998) still had to address the issue of age effects for their sample of SMC clusters

due to the fact that many of their target clusters were considerably younger than the

Galactic globular clusters used in the CaT calibration of Da Costa & Armandroff

(1995); thus, they sought to correct for the difference in age between the target and

calibration clusters. Using adopted cluster ages, along with theoretical isochrones,

Da Costa & Hatzidimitriou (1998) estimated the change in VHB from the old to the

young populations, thereby creating age-corrected metallicities for their targets. Their

corrections were of the order of 0.05 dex, which is smaller than the precision of the

abundances. In contrast to Da Costa & Hatzidimitriou (1998), we have made no

attempt to calculate any age corrections for the following reason. We use the CaT

calibration of CO4, which is based on a sample of both globular and open clusters,

covering a wide range of ages and metallicities. With the inclusion of younger clusters,

the variation of VHB with age is built into the CaT calibration, specifically in Eq. 3-2,

and the steeper value for p than what has been found by authors only considering

globular clusters. Thus, age corrections are not required for our abundance data.

Finally, Rutledge et al. (1997a) showed that for MW globular clusters there is a

linear relationship between a cluster's reduced equivalent width and its metallicity on

the Carretta & Gratton (1997) abundance scale. CO4 extended this calibration to cover

a larger range of ages (2.5 Gyr g age g 13 Gyr) and metallicities (-2 g [Fe/H] g

-0.2) than previous authors, and, because their calibration is closer in parameter space









to our cluster sample, we adopt their relationship, where


[Fe/H] = (-2.966 + 0.032) +t (0.362 + 0.014)W'. (3-3)


We note that, while this calibration actually combines two metallicity scales

(Carretta & Gratton 1997 for the globular clusters and Friel et al. 2002 for the open

clusters), CO4 find no evidence of age effects on the calibration or any significant

deviation from a linear fit to suggest that these two populations are not ultimately on

the same [Fe/H] scale (see their Figure 4). Although some of our clusters are likely

younger than the 2.5 Gyr age limit established in the calibration of CO4, the CaT line

strengths for red giants of ~1 Gyr are not expected to deviate strongly from a simple

extrapolation of the fitting formula (based on the empirical fitting functions from

Cenarro et al. 2002 applied to isochrones published in Girardi et al. 2000), so we use

the above calibration for all of our clusters.

3.3 Analysis

As mentioned in ~3.2.5, knowledge of the relative brightness of each target star

and the cluster HB is imperative to the accurate calculation of W' and thus [Fe/H] for

each star. To determine V yHB we utilized the preimages necessary for creating the

slit masks used by FORS2. Small-aperture photometry was performed on these V- and

I-band images so as to allow us to create cluster CMDs below the core helium-burning

RC stars. For the younger clusters in our sample, yHB was measured as the median

magnitude of cluster RC stars. Cluster stars were isolated from the field by selecting

stars within the inner half of the apparent cluster radius. We then placed a standard-

sized box (0.8 mag in V and 0.2 mag in V I) around each cluster RC and used

only the stars within this box in our calculation of VH. Regarding clusters with bona

fide HBs, i. e. old clusters, we compared our instrumental photometry to published

photometry and calculated a rough zero point for our data, allowing the conversion

of published PHB values onto our instrumental system. Literature sources for the five










old clusters are as follows: NGC 1841, Alcaino et al. (1996); NGC 2019, Olsen et al.

(1998); NGC 2257 and Hodge 11, Johnson et al. (1999); and Reticulum, Walker

(1992a). Errors in VHB are taken as the standard error of the median for clusters in

which we measured the RC directly; for the HB in the old clusters we adopt 0.1 mag.

We note that although we have not calibrated our photometry onto a standard system,

the V-I color term for the FORS2 filter system is expected to be small (<0.02 mag),

thus having little effect on the relative brightnesses of our target stars over the small

range of colors covered by the RGB.

3.3.1 Cluster Membership

We use a combination of three criteria to isolate cluster members from field

stars. This process is identical for all clusters, so we illustrate the process using

Hodge 11. First, the cluster centers and radii are chosen by eye, based primarily on

the photometric catalog. As an example, Fig. 3-3 shows xy-positions for all stars

photometered in the Hodge 11 field, with large filled symbols denoting our target stars

and the large open circle representing the adopted cluster radius; target stars marked

in blue (see figure legends for a discussion of the color coding used in Figs. 3-3

through 3-5 and Fig. 3-7) are considered non-members due to their distance from the

cluster center. We note that stars outside of the cluster radius were observed so as to

define parameters for the LMC field, which aids in isolating cluster members. Next,

radial velocity versus distance is plotted in Fig. 3-4. Stars moving at the velocity of

Hodge 11 are easily identified due to their smaller velocity dispersion and lower mean

velocity than that of the field stars. Our velocity cut, denoted by the horizontal lines,

has been chosen to represent the expected observed velocity dispersion in each cluster.

To determine this, we have adopted an intrinsic cluster velocity dispersion of 5 km

s-1 and added this in quadrature with our adopted radial velocity error, 7.5 km s-1

which results in an expected dispersion of ~ 9 km s-1. Thus, we have rounded this

up and adopted a width of + 10 km s-1 for our radial velocity cut. The cluster radius























150UU I I I I


Hedge 11






...c~ ;~I
.~ 'i~...(i. .~....'.~':" ':i~'Z.
r v
:
I : '" :'::
'' .I ''r.: rrr .z ~i~ .
'..... Z"'i!,s::-5
\, r~'' i j~T~;~r
cr>
'7' 2~
'' *:' ;: .-rp: ~~C ~~L".
:'r;.i*G;
C :;
'1..;. i:i::. .... ~'..:' %~~:3 :~2e; iSl
"''"~'~' :~::Z4r: ~-~~
; ~fP~: ve.s-~n
:~::
... r. i;
..
''1?
'~' : (~C -
~..
: r".;::P
::.:~:
'~ 5~; .r--~~ .e
~ ~.;1.
.. .;.
....
"~'"
.: i. 5~
'' ''
r:: -
~i r:
:~.'..... ': ~. ~
:
:r.i; :" 2':'~ rl:
...~.:. ..
r. *
...


1000 1


500 1


--Snn


. ~I .


. I I .


500


1000
x position (pixels)


1500


2000


Figure 3-3:


The xy positions of our target stars (large filled symbols) in the Hodge 11
field. The adopted cluster radius is marked by the large open circle, and
stars outside of this radius are considered non members. The color coding

of symbols in Figs. 3-3 through 3-5 and Fig. 3-7 is as follows: blue
points represent non members that are outside the cluster radius; teal and
green symbols represent non members that were cut because of discrepant
radial velocities and metallicities, respectively; and, finally, red symbols
denote cluster members.









































- -
I
I

I
I


""I""I""I
I
I
I
350 Hodge 11


-


~300
E
Y
v
,X
o
o
a, 250

,u
L
c
o
o
o
o
I 200


1-


100
Distance (orcsec)


150


Figure 3-4:


Radial velocities for our spectroscopic targets in Hodge 11, plotted as a
function of distance from the cluster center. The horizontal lines represent
our velocity cut and have a width of +10 km s The cluster radius is
shown by the vertical line, and the color coding of symbols is discussed
in Fig. 3-3. The error bars represent the random error in determining the
radial velocity for each star, where we have added in quadrature the slit
centering and cross-correlation errors.


(Fig. 3-4, vertical line) is marked for reference. Finally, Fig. 3-5 shows metallicity as

a function of distance for the stars in Hodge 11, with horizontal lines representing the

[Fe/H] cut that has been applied to these data. For the stars in six of our clusters we

have processed both sets of spectra and compared the two [Fe/H] measurements so as


I liii~iilll I!lalr'i.

































--1.5-






--2.0-






Figure 3-5:


50 100 150
Distance (orcsec)


Hodge 11 target star metallicities. Metallicities are plotted as a func-
tion of distance for all target stars in Hodge 11. The [Fe/H] cut of +0.20
dex is denoted by the horizontal lines. For this old, metal-poor cluster,
the field ([Fe/H] ~ -0.5) is easily distinguished from the cluster (red
symbols). We note that the color coding is the same as in Fig. 3-3. The
plotted error bars represent the random error in calculating [Fe/H], where
we have propagated the error in measuring the equivalent widths through
our calculations.


to directly determine the metallicity error for each star. Based on these data we find

G[~Fe/H] M 0.15 dex, which we adopt as the random error in [Fe/H] for each star. We

have rounded this up to + 0.20 dex for use as the metallicity cut shown in Fig. 3-5.















Hodge 11
[Fe/H] = -1.84























1 0 -1 -2 -3 -4
V-VHB (mog)


Figure 3-6: CW vs. V yHB for Hodge 11; only stars considered to be cluster mem-
bers are plotted. The dashed line is an isoabundance line at the mean
metallicity of the cluster, [Fe/H] = -1.84, and has a slope P = 0.73.


Red symbols denote stars that have made all three cuts and are therefore considered

to be cluster members. Since we had no a priori membership information, up to this

point we have used a value for yHB that was derived from the entire field, rather than

just the cluster. Thus, we have recalculated W' (and [Fe/H]) using the appropriate

cluster yHB value. In Fig. 3-6 we present the traditional CW versus V yHB plot for

cluster members, with the dashed line representing the mean metallicity of Hodge















g *.
Hodge 11.















-~ ~ ~ ~ ~~ 1. 10-.5000. .

v-i (inst
Figure 3-7 CMD for he entireHodge 11 ield, wit taret tr akda ecie
in~~~~~~~~ Fi.33 lse ebr leaogteRBadAB

11.Th CM i Fi. -7 hos al tar potmetre inth Hoge11 ied; lute
members~~~~~~~~~~~~~~~L= (rdsmos i nte G n smttcgan rnh(G) iue

3-1 thoug 3-8 peset te custr mmbe seecionposo h eann lses
inth fllwngfoma: ah lute s plt ve wofiurs wthth frt igrei
eac par(d ubrdfgue)soigtex-pstoso h age tr uprlf
panl) heicnti aia eoit n FeH esu itnc rmth lsercne
(upr ih adlwe efrspciel) ndfnalLWvrusV- H oralclse










members (lower right). The second figure for each cluster (even numbers) shows the

CMD for all stars in each pointing (cluster and field stars), with the spectroscopic

targets marked, using the color coding as discussed in Fig. 3-3. We note that these

figures are located at the end of this chapter.

In Table 3-6, also located at the end of this chapter, for all stars determined

to be members of the observed LMC clusters, we list the following information:

stellar identification number, right ascension and declination (as determined from the

preimages), heliocentric radial velocity and its associated error, V VHB, and CW,

along with the error in measuring CW. Although we do not discuss the field stars,

for completeness, in Table 3-7 we present our measured values for all field stars. In

this table, "primary" refers to all stars that fell on the same FORS2 chip as the target

cluster, but were found to be non-members of the cluster and "secondary" denotes the

stars that were observed on the non-cluster array. We note that Hodge 11 and SL 869

were observed in the same pointing, with Hodge 11 on the primary array and SL 869

on the secondary, hence the lack of secondary fields listed for either cluster.

3.3.2 Cluster Properties

Cluster properties derived from our data are presented in Table 3-3, with the

number of cluster stars given in column 2, the mean heliocentric radial velocities

and mean metallicities in columns 3 and 5, and their respective standard error of the

mean values in columns 4 and 6. For the clusters SL 4, SL 41, SL 396, Hodge 3, SL

663, and SL 869, we report the first spectroscopically derived metallicity and radial

velocity values based on individual stars within these clusters. In addition, NGC 1718

and NGC 2193 have no previously reported spectroscopic [Fe/H] values; however,

OSSH derived velocities for these two clusters. Of these eight clusters, NGC 1718

occupies a particularly interesting area of parameter space, as it is the most metal-poor

of our intermediate-age clusters, with a metallicity comparable to that of ESO 121 (see












Table 3-3. Derived LMC Cluster Properties
Cluster n Stars RV ofi [Fe H] ~Ielq
Name (lon s-1) (km s-1) (dex) (dex)
SL 4 5 227.1 3.6 -0.51 0.06
Reticulum 13 247.5 1.5 -1.57 0.03
NGC 1651 9 228.2 2.3 -0. 53 0.03
NGC 1652 7 275.7 1.3 -0.46 0.04
NGC 1841 16 210.3 0.9 -2.02 0.02
SL 41 6 229.3 1.3 -0.44 0.03
SL 61 8 221.9 2.0 -0.35 0.04
NGC 1718 3 278.4 2.2 -0. 80 0.03
NGC 1751 6 245.4 2.1 -0.44 0.05
NGC 1846 17 235.2 0.9 -0.49 0.03
NGC 1861
SL 396 5 225.2 1.1 -0.39 0.05
NGC 1942 8 293.7 2.3 -0. 50 0.04
NGC 2019 5 280.6 2.3 -1.31 0.05
Hodge 4 7 310.8 1.9 -0.55 0.06
Hodge 3 7 277.4 0.8 -0.32 0.05
IC 2146 18 226.3 0.6 -0.41 0.02
SL 663 8 301.4 1.5 -0. 54 0.05
NGC 2121 12 232.5 1.2 -0. 50 0.03
NGC 2173 6 237.4 0.7 -0.42 0.03
NGC 2155 7 309.1 1.6 -0. 50 0.05
NGC 2162 5 322.6 3.5 -0.46 0.07
NGC 2203 9 245.5 1.4 -0.41 0.03
NGC 2193 5 291.2 2.0 -0.49 0.05
NGC 2213 6 242.7 1.2 -0. 52 0.04
Hodge 11 12 245.1 1.0 -1.84 0.04
SL 869 3 258.4 2.1 -0.40 0.04
NGC 2231 9 277.6 1.4 -0. 52 0.03
NGC 2257 16 301.6 0.8 -1.59 0.02


discussion in ~3.6.1). As mentioned previously, we have not derived values for NGC

1861, since it appears to be younger than 1 Gyr (see ~3.6.3).

3.3.2.1 Metallicities


Positions on the sky for each cluster are shown in Fig. 3-8, along with the

metallicity bin into which each cluster falls, represented by the color of the plotting


symbol. For two of the higher metallicity bins (orange and green symbols), the bin

size is roughly twice the standard error in [Fe/H], so it is possible that cluster errors

could "move" clusters between these and adjacent bins. The adopted center of the


LMC (oc = Sh27nz36s, 6 = -69 52'l2"'; van der Marel et al. 2002) is marked by

the filled square, and the dashed oval represents the 2" near-infrared isopleth from

van der Marel (2001), which roughly outlines the location of the LMC bar. Conversion

from right ascension and declination to Cartesian coordinates was performed using

a zenithal equidistant projection (e.g., van der Marel & Cioni 2001, their equations










1-4); for reference, lines of right ascension and declination are marked with dotted

lines. In Figs. 3-9 and 3-10 we further explore the metallicity-position relationship

for LMC clusters by plotting metallicity as a function of deprojected position angle

and radial distance (in kiloparsecs), respectively. We have corrected for projection

effects by adopting 34P7 as the inclination and 122P5 for the position angle of the

line of nodes of the LMC (van der Marel & Cioni 2001). In this rotated coordinate

system, a cluster with a position angle of zero lies along the line of nodes, and angles

increase counterclockwise; for reference, NGC 2019 has a position angle of ~ 8".

Radial distances were converted from angular separation to kiloparsecs by assuming

an LMC distance of (m -240 = 18.5 (~ 50 kpc); at this distance, l' is ~ 870 pc.

Combined, these three figures illustrate that, similar to what was found by OSSH (and

Geisler et al. 2003), there is no [Fe/H] gradient in terms of either position angle or

radial distance for the higher metallicity clusters in our sample. While we cannot make

strong comments on the metal-poor clusters due to our small sample size, it is well

known that a number of metal-poor clusters ([Fe/H] g -1.5) exist in the inner portions

of the LMC (e.g., OSSH), suggesting that neither the Population I nor the Population II

clusters exhibit a metallicity gradient.

In Fig. 3-10 we have over plotted both the MW open cluster metallicity gradient

from Friel et al. (2002, dashed line) and the M33 gradient from Tiede et al. (2004,

solid line). Neither of these disk abundance gradients resembles what we see among

the LMC clusters. The question of how to interpret this difference takes us to the

work of Zaritsky et al. (1994). They studied the H II region oxygen abundances in

39 disk galaxies. Their data suggest that disk abundance gradients are ubiquitous in

spiral galaxies. However, the presence of a classical bar in the galaxy one that

extends over a significant fraction of the disk length tends to weaken the gradient.

This observation seems to find support in the appearance of Fig. 3-10. In the case of

the LMC, the presence of a strong bar component may have diluted the metallicity






59












6" 20" 6" 00" 5" 40" 5" 20" 5" 00" 4" 40"

-0 I : I '...'. '... :' ...'....'...l...'...i... .'...I...'.. .' ....;I : '
]..cllien. -58.

10 -

so -N2162 N194~2 -- -2

5 N2 5 ... ....9 .--------4H dg 4-*
5 -NS..--N2193N2155 SL663 j -64*

~~~~~~~N1846/ N1718';""--. ._g
N2231 Hodae3*
iv I'~~';"'~V"-...........19.N652 -...
..- SHd6g9e f: ,N2019 \\ N1751 .: -8
E 0 .. --- 5--- ... to ..e f,i 465.1.
g ~ N2213 N2\9 2'. -70"
---- .....'... 6 ......SL41. Sk.
.:: *....~. .. .
0 ...------."'~ '~""~'f6::--- 1 2T46"..'::'.
-5 .--N2203 SL61 .',
:- : *, -74.-6



*~[Fe/j~~~.t-l] +64 ... "
-10 9 0.406 ~[Fe/H]+ -0.}.0 .............-
0.50 >[Fes/lj3'] ~0.6D ',
p.,so [Fe/H3'' -1. 0 .. ......... :..
**il~ $ .. :, : : N1841 ... -800


-6 -4 -2 0 2 4 6
X (degrees from center)


Figure 3-8: Positions on the sky and derived metallicities for our target clusters.
Metallicity bins are given in the lower left corner of the plot. The adopted
LMC center is marked with the filled square, and the dashed line roughly
outlines the bar. See ~3.3.2 for a detailed discussion.






60











0.0






-0.5 e *






-1.0 --






-1.5--






-2.0- -



O 100 200 300
Deprojected Position Angle (degrees)


Figure 3-9: Cluster metallcity vs. position angle. We plot the metallicities of our
target clusters as a function of deprojected position angle, where we have
used the LMC geometry of van der Marel & Cioni (2001) to correct for
projection effects. This plot illustrates that there is no apparent relation
between position angle and metallicity in the LMC. The error bar shown
in the lower left corner of the plot illustrates the average random error in
[F e/H] .

















0.0






-0.5






,-1.0-






-1.5-






-2.0-







Figure 3-10:


5 10 15
Deprojected Distance from Center (kpc)


Cluster metallicity vs. radial distance. Cluster metallicities are plotted as
a function of deprojected distance (in kpc) from the center of the LMC.
We have assumed a distance of (m M~1o = 18.5. Overplotted are the
metallicity gradients observed in the MW open clusters (dashed line;
Friel et al. 2002) and M33 (solid line; Tiede et al. 2004), which help
to further illustrate that the LMC's cluster system lacks the metallicity
gradient typically seen in spiral galaxies. This flattened gradient is likely
caused by the presence of the central bar (Zaritsky et al. 1994). As in
Fig. 3-9, the average random error is illustrated by the error bar on the
lower left.










gradient originally present in the star clusters, leading to a cluster population that
is well mixed. We note that this result is also consistent with the conclusion of

Pagel et al. (1978), who found little evidence for a gradient in oxygen abundance based

on a survey of H II regions within 4 kpc of the LMC center. The Pagel result, that

dlog(O/H)/dR = -0.03 + 0.02 dex kpc parallels our non-detection of a gradient in

cluster metallicities.

3.3.2.2 Kinematics

To characterize the rotation of their clusters, Schommer et al. (1992) fit an

equation of the form


V(6) = if ,, [tan(6 8o) seci]" +ti }-o. + Em (3-4)


to their radial velocity data using a least-squares technique to derive the systemic

velocity (Vms), the amplitude of the rotation velocity (y ,), and the orientation of the

line of nodes (8o); they adopted an inclination of 27". Their best-fit parameters give

a rotation amplitude and dispersion consistent with the LMC clusters having disk-like

kinematics, with no indications of the existence of a pressure supported halo. We note

that, due to the non-circularity of the LMC, 8o in Eq. 3-4 is not the true orientation of

the line of nodes (the intersection of the plane of the sky and the plane of the LMC),

but rather it marks the line of maximum velocity gradient (van der Marel & Cioni

2001). More recently, van der Marel et al. (2002) used velocities of 1041 carbon

stars to study kinematics in the LMC. Similarly, they found that these stars exhibit a

disk-like rotation with V/o = 2.9 + 0.9, suggesting that these stars reside in a disk that

is slightly thicker than the MW thick disk (V/o a 3.9).

In Fig. 3-11 we have plotted galactocentric radial velocity versus position

angle on the sky for our sample, along with velocity data for all clusters listed in

Schommer et al. (1992). To be consistent with the approach of Schommer et al.

(1992), we have adopted the galactocentric velocity corrections calculated by









Feitzinger & Weiss (1979). Additionally, for this figure only, we have adopted

their LMC center (oc = Sh20nz40s, 6 = -69 14'10"; J2000.0) for use in calculating the

position angles of our clusters. We have used the standard astronomical convention in

which north has a position angle of zero and angles increase to the east; NGC 1942 has

a position angle of ~ 4" in this coordinate system. Data from Schommer et al. (1992)

are plotted as open circles, and our data are plotted as filled stars for the clusters with

previously unpublished velocities and filled circles for the remainder of our clusters;

overplotted on this figure (dashed line) is the rotation curve solution number 3 from

Schommer et al. (1992). For the clusters in common between these two data sets,

we find excellent agreement, with a mean offset of 0. 15 km s where our velocities

are faster than those of Schommer et al. (1992). Additionally, the derived velocities

for the six "new" clusters show that their motions are consistent with the findings of

Schommer et al. (1992) in that the LMC cluster system exhibits disk-like kinematics

that are very similar to the H I disk and has no obvious signature of a stellar halo.

3.4 Comparison with Previous Work

As mentioned in 93.1, OSSH and Suntzeff et al. (1992) have provided the only

previous large-scale, spectroscopic [Fe/H] calculations for clusters in the LMC. Similar

to our work, they utilized the CaT lines as a proxy for measuring Fe abundance

directly, but with two important differences: they used the absolute magnitude of their

stars, based on the spectral intensity at 8600 A+, as a surface gravity estimator instead

of V VHB, and their [Fe/H] calibration was based largely on the Zinn & West (1984)

metallicity system, with the addition of two open clusters that have metallicities derived

from various spectrophotometric indices (see their Table 7). This introduced two

systematic offsets that make it inappropriate to directly compare the OSSH values to

our work and other recently measured CaT abundances: first, the use ofM8S600 creates

a dependence on the relative distances of the calibrating clusters and the LMC, and the

globular cluster distance scale has been much revised in the post-Hipparcos era (Reid




















100 200 300
Position Angle (degrees)


1001--


o d
u
c ~-
~o,


8
q


50 |-


O ,'
\ o oO
o_ h"
O h 0"0 \ \e ~,3)



Figure 3-11:


Cluster radial velocity vs. position angle. Galactocentric radial velocities
as a function of position angle on the sky are plotted for the clusters in
our sample (filled symbols) as well as those from Schommer et al. (1992,
open circles). The six clusters in our sample with no previous velocity
determinations are plotted as filled stars and all others in our sample are
filled circles. Rotation curve solution number 3 from Schommer et al.
(1992) is overplotted as the dashed line, showing that both data sets are
consistent with circular rotation. We note that we have not plotted a rep-
resentative error bar since our plotting symbols are roughly the same size
as the average random velocity error.










1999). Second, it has been shown (e.g., Rutledge et al. 1997a) that the Zinn & West

scale is non-linear compared to the more recent Carretta & Gratton (1997) scale based

on high-resolution spectra of globular cluster red giants. To put the work of OSSH on

the Carretta & Gratton system, Cole et al. (2005) perform a non-linear least-squares fit

to calibration clusters in common with their work and that of OSSH. They find that

one can estimate the abundance of OSSH clusters on the metallicity system we have

used via the following conversion:


[Fe/H] a -0.212 +t 0.498 [Fe/H]OSSH 0. 128 [Fe/H/OSS.(35


This equation approximates the metallicity that OSSH would have derived from their

spectroscopic data and calibration procedure but with updated metallicities for their

calibration clusters; it does not attempt to account for any other differences in the

treatment of the data.

In columns 3 and 4 of Table 3-4 we list [Fe/H] for clusters in OSSH and

Suntzeff et al. (1992) in common with our target clusters, where column 3 gives

their published values and in column 4 we have converted their numbers onto our

metallicity system using Eq. 3-5. The number of stars used by OSSH in calculating

final cluster metallicities is given in parentheses in column 4, and our derived metallic-

ities are given in column 2 for reference. In Fig. 3-12 we plot the difference between

our metallicities and their converted [Fe/H] values as a function of our metallicities.

OSSH give their [Fe/H] errors for an individual star as 0.2 dex; therefore, deviations

between these data sets as large as + 0.2 are not unexpected, suggesting that these

results are in relative agreement, with no offset. We note, however, that even with the

use of Eq. 3-5, it is very difficult to directly compare the derived cluster abundances

because of the differences in target selection and calibration strategy.

While a direct comparison of [Fe/H] values is difficult, we can readily compare

the metallicity distributions of these two data sets. As such, in Fig. 3-13 we have








































































converted onto our system




d onto our systent using


Table 3-4. Publish


ed LMC Cluster Metallicities

[Fe H]a [Fe H]b [Fe H]
CaT CaT High-Res.


Cluster
Name

SL 4
Reticulum
NGC 1651
NGC 1652
NGC 1841
SL 41
SL 61
NGC 1718
NGC 1751
NGC 1846
SL 396
NGC 1942
NGC 2019
Hodge 4
Hodge 3
IC 2146
SL 663
NGC 2121
NGC 2173
NGC 2155
NGC 2162
NGC 2203
NGC 2193
NGC 2213
Hodge 11
SL 869
NGC 2231
NGC 2257


[Fe H]
(This Work)

-0.51
-1.57
-0.53
-0.46
-2.02
-0.44
-0.35
-0.80
-0.44
-0.49
-0.39
-0.50
-1.31
-0.55
-0.32
-0.41
-0.54
-0.50
-0.42
-0.50
-0.46
-0.41
-0.49
-0.52
-1.84
-0.40
-0.52
-1.59


-1.44"(9)
-0.41 (0.5)
-0.46 (2)
-1.83"(8)

-0.49 (1)

-0.31 (0.5)
-0.62 (1)

-0.14 (1)
-1.53 (1)
-0.29 (1)

-0.43 (2)

-0.56 (1.5)
-0.34 (1)
-0.52 (2.5)
-0.33 (2)
-0.51 (2)

-0.22 (1)
-1.78 (2)

-0.60 (1.5)


-2.07e









1.24f(3)














2.13f(2)


-1.86e


aFront OSSH, unless otherwise noted.

bFront OSSH, unless otherwise noted,
using Eq. 3-5.
CFront Suntzeff et al. (1992).

dFront Suntzeff et al. (1992), converted
Eq. 3-5.
eFroni Hill (2004)

fFront Johnson et al. (2006)

















"I'"'"I'"'"I'"'"I'"'"I'


-






*
-


**

**
*
**
---------------------------*----------

*
*

'S

-

*
*

*
-


.I....I....I....I....I.


0.4 1-


0.2 1-


0.01-


-0.2 1-


-0.4 1-


-2.0


-1.5


-1.0
[Fe/H] (This work)


-0.5


Figure 3-12:


Metallicity comparison with OSSH. We compare derived metallicities for
clusters in common between our study and that of OSSH. We note that
[Fe/H] values from OSSH were converted onto the metallicity scale we
have used via Eq. 3-5. This comparison shows that, to within the errors,
there is relatively good agreement between our results and those of OSSH
(see ~3.4 for more details).


plotted the metallicity distribution of OSSH's raw data (top panel), converted [Fe/H]

values (middle panel), and our results (bottom panel). The dark shaded histogram

shows only the 20 clusters in common between the three panels, while the lighter

histogram plots all the clusters in each sample. From this figure it is clear that both










the raw and converted OSSH samples show an extended distribution of intermediate-

metallicity clusters, whereas our cluster sample exhibits a very tight distribution. For

the 20 clusters in common, we find a mean [Fe/H] = -0.47 with 0- = 0.06, while

the converted OSSH metallicities give [Fe/H] = -0.42 +0. 14. Our tight metallicity

distribution, with a lack of higher metallicity clusters ([Fe/H] X -0.30), is an important

feature of our data for the following reason. Chemical evolution models suggest that

metallicity is a rough estimator of age, in that younger stellar populations should be

more metal-rich than older populations, since there has been more time to process

material and enrich the interstellar medium. Thus, intermediate-age clusters should

be more metal-poor than younger stellar populations in the LMC. However, some

intermediate-age clusters in the sample of OSSH appeared to be more metal-rich than

much younger stellar populations in the LMC, which would indicate the presence

of a large spread of metallicities at any given age. In Table 3-5 we give the mean

metallicity and spread of our entire sample of intermediate-age clusters and all clusters

in OSSH with converted metallicities above -1.0 dex, along with published results for

a sample of younger stellar populations (e.g., B dwarfs, Rolleston et al. 2002; Cepheid

variables, Luck et al. 1998; young red giants, Smith et al. 2002) and intermediate age

RGB field stars in the LMC bar (Cole et al. 2005). This table shows that, as we would

expect from chemical enrichment models, the intermediate-age clusters are slightly

more metal-poor than the younger populations in the LMC. Thus, the much tighter

metallicity distribution seen in our clusters is in excellent agreement with the expected

chemical enrichment pattern in the LMC and alleviates the problem created by the high

metallicity tail of intermediate-age clusters in the OSSH results. In addition, Table

3-5 shows that our intermediate-age clusters have a mean metallicity and distribution

similar to that of the metal-rich component of the bar field studied by Cole et al.

(2005). The similarity between these two populations is in good agreement with the

models of Bekki et al. (2004), in which the formation of the LMC bar and the restart










of cluster formation (the end of the age gap) are both a result of the same very close

encounter with the SMC.

Finally, in Table 3-4 we have also included [Fe/H] values derived from high-

resolution spectra for NGC 1841 and NGC 2257 from Hill (2004) and NGC 2019

and Hodge 11 from Johnson et al. (2006). For the two clusters from Johnson et al.

(2006), we list [Fe/H] values that are the average of their metallicities determined from

Fe I and Fe II lines, and the number of stars observed in each cluster is given. Two

clusters, NGC 1841 and NGC 2019, show good agreement between our metallicities,

calculated from the CaT lines, and metallicities derived from fitting to high-resolution

spectra. In contrast, Hodge 11 and NGC 2257 show a roughly 0.3 dex offset between

these methods in the sense that our values are more metal-rich than the results from

high-resolution spectra. Similarly, a preliminary result for ESO 121, which is more

metal-rich than the aforementioned clusters, suggests an offset in the same direction,

where the CaT method gives a [Fe/H] value higher than what is measured with high-

resolution spectra (A. A. Cole, private communication). It has been suggested that

variations in [Ca/Fe] between calibrating clusters in the MW and target clusters in

the LMC may cause a breakdown in the utility of CaT lines as a metallicity indicator.

However, abundances based on high-resolution spectra show that [Ca/Fe] is typically

lower for LMC cluster giants than for MW giants of the same [Fe/H], which is in the

opposite direction of what is needed to explain the difference between CaT and high-

resolution results. We also note that, for low-metallicity stars, previous authors have

shown that metallicities derived from high-resolution spectra can vary considerably (0.3

dex is not uncommon), depending on which ionization stages, what temperature scale,

and what model atmospheres are being used (e.g., Johnson et al. 2006; Kraft & Ivans

2003).


















.. Row [Fe/H] from OSSH










..Converted [Fe/H] from OSSH


: I I I------


10 |-


~cci


[Fe/H] from this paper


0.0


10


-2.0


-1,5


-1.0
[Fe/H]


-0.5


Figure 3-13:


Metallicity distribution of LMC clusters as determined by OSSH and
this paper. Published values from OSSH are given in the top panel, while
the middle panel shows their values converted onto our metallicity scale
using Eq. 3-5; in the bottom panel we have plotted our results. In all
three panels, the dark shaded region shows the distribution for the 20
clusters in common between OSSH and this paper, while the light shaded
region shows the entire cluster sample from each study. Our results in-
dicate that the LMC's intermediate-age cluster metallicity distribution is
actually much tighter than suggested by the results of OSSH.


mmi m









Table 3-5. Metallicities of Young and Intermediate-Age Stellar Populations
Population Age Estimate [Fe/H] G[Fe/H] Reference
(Myr)
B dwarfs <20 -0.31 0.04 Rolleston et al. (2002)
Cepheid variables 10-60 -0.34 0.15 Luck et al. (1998)
Young RGB stars 200-1000 -0.45 0.10 Smith et al. (2002)
Intermediate-age clusters 1000 -3000 -0.48 0.09 This paper
Intermediate-age clusters 1000 -3000 -0.48 0.17 OSSH
Bar RGB stars, metal-rich 1000-5000 -0.37 0.15 Cole et al. (2005)
Bar RGB stars, metal-poor X5000 -1.08 0.47 Cole et al. (2005)


3.5 Summary

As discussed in 93.1, determining abundances for populous clusters within

the LMC is an important step in understanding the history of this satellite galaxy.

Accurate [Fe/H] values help to break the age-metallicity degeneracy that arises when

trying to fit theoretical isochrones to cluster CMDs, which allows the unequivocal

determination of cluster ages, thereby providing a clear picture of the LMC's cluster

age-metallicity relation. These clusters also serve to fill a region of the age-metallicity

plane that is void of MW clusters; this makes the LMC cluster system an important

testbed for a variety of stellar population models. Additionally, in a previous paper

Grocholski & Sarajedini 2002, we showed that knowledge of a cluster's age and

metallicity is essential to predicting the K-band luminosity of the RC for use as a

standard candle. In a future work we will use the metallicities derived herein to

determine distances to individual populous LMC clusters, which will allow us to

compare the cluster distribution to the LMC geometry calculated from field stars (e.g.,

van der Marel & Cioni 2001).

In this chapter we have presented the results of our spectroscopic study of the

near-infrared Ca II triplet lines in individual RGB stars in 28 populous LMC clusters.

Utilizing the multi-object spectrograph, FORS2, on the VLT, we have been able to

determine membership and calculate metallicities and radial velocities for, on average,

eight stars per cluster, with small random errors (1.6 km sl in velocity and 0.04 dex










in [Fe/H]). The number of cluster members observed, combined with the updated

CaT calibration of CO4 (they extended the calibration to younger and more metal rich

clusters than previous work), has allowed us to improve on the work of OSSH, which

is the only previous large scale spectroscopic study of individual cluster stars within the

LMC. The main results of our paper are as follows:

1. We report the first spectroscopically derived metallicities and radial velocities

for the following clusters: SL 4, SL 41, SL 396, SL 663, SL 869, and Hodge 3. In

addition, NGC 1718 and NGC 2193 have no previously reported spectroscopic [Fe/H]

values.

2. NGC 1718 is the only cluster in our sample that falls into the range -1.3 <

[Fe/H] I -0.6. This metallicity region corresponds to the well known 3-13 Gyr "age

gap," within which there is only one cluster, ESO 121. However, unlike ESO 121, the

CMD of NGC 1718 suggests an age (~ 2 Gyr) much younger than the age gap; we

use archival Hubble Space Telescope (HST) Wide Field Planetary Camera (WFPC2)

photometry to investigate this point in the ~3.6.1. This age makes NGC 1718 one of

the most metal-poor intermediate-age clusters in the LMC.

3. The intermediate-age clusters in our sample show a very tight distribution,

with a mean metallicity of -0.48 dex (0 = 0.09) and no clusters with metallicities

approaching solar. While this is in contrast to previous cluster results, it suggests that

the formation history of the bar (mean [Fe/H] = -0.37, o = 0. 15; Cole et al. 2005)

is very similar to that of the clusters. This agrees well with the theoretical work of

Bekki et al. (2004), which indicates that a close encounter between the LMC and SMC

caused not only the restart of cluster formation in the LMC but the generation of the

central bar as well.

4. Similar to previous work, we find no evidence for the existence of a metallicity

gradient in the LMC cluster system. This is in stark contrast to the stellar populations

of both the MW and M33, which show that metallicity decreases as galactocentric










distance increases; the LMC's stellar bar is likely responsible for the well-mixed cluster

sy stem.

5. We find that our derived cluster velocities, including the six "new" clusters, are

in good agreement with the results of Schommer et al. (1992) in that the LMC cluster

system exhibits disk-like rotation with no clusters appearing to have halo kinematics.

6. Comparing our results for four clusters to [Fe/H] values recently derived

through high-resolution spectra, we find that two of the four clusters are in good

agreement, while the other two have [Fe/H] values derived via the CaT method that are

~ 0.3 dex more metal-rich than what is found from high-resolution spectra; a similar

effect is seen in preliminary results for an additional two LMC clusters. The source of

this difference is unclear, and it is not immediately explained by variations in [Ca/Fe]

between the CaT calibration clusters in the MW and the LMC target clusters. Further

high-resolution studies, especially covering the LMC's intermediate-age clusters, are

needed to fully address this issue.

3.6 Notes on Individual Clusters

3.6.1 NGC 1718

While only three of the stars observed in NGC 1718 appear to be cluster mem-

bers, these stars are, on average, 0.3 dex more metal-poor than all but one of the other

stars observed in this field. As mentioned in ~3.3.2, this causes NGC 1718 to occupy

an interesting position in the LMC's age-metallicity relation; its metallicity is com-

parable to that of ESO 121, which seems to be the only cluster residing in the LMC

having an age between ~ 3 and 13 Gyr (Da Costa 2002). The cluster CMD resulting

from our aperture photometry is not well populated around the MSTO, so we have

used archival HS77WFPC2 data (GO-5475) to create a cluster CMD reaching below the

MSTO. The images were reduced using the procedure outlined by Sarajedini (1998).

In summary, all detected stars on the Planetary Camera CCD were photometered in the

F450W and F555W filters using a small aperture. These were then corrected to a 0"'5



















18-








20-








22-










Figure 3-14:


0.0 0.5 1.0 1.5


B-V


Cluster CMD for NGC 1718, based on aperture photometry of archival
HST/WFPC2 images. We overplot isochrones of 1.3, 2.0, and 2.5 Gyr
(top to bottom) from Girardi et al. (2002) that have a metallicity (~ -0.7
dex) similar to the value we have derived for this cluster (-0.8 dex).
Although this cluster has a metallicity similar to that of ESO 121, the
isochrones suggest an age of ~ 2.0 Gyr for this cluster, leaving ESO 121
as the only known LMC cluster with an age between approximately 3
and 13 Gyr.










radius, adjusted for the exposure time, and transformed to the standard system using

the equations from Holtzman et al. (1995). In Fig. 3-14 we present the CMD of NGC

1718 with isochrones from Girardi et al. (2002) overplotted; the isochrones have [Fe/H]

S-0.7, close to our measured cluster value of -0.8 dex, and ages ranging from 1.3 to

2.5 Gyr. This figure suggests that NGC 1718 has an age of roughly 2.0 Gyr, making

it an intermediate-age cluster and leaving ESO 121 as still the only cluster known to

occupy the LMC's cluster age gap. However, the existence of an intermediate-age

cluster at this low metallicity is intriguing, as it indicates that some pockets of unen-

riched material must have remained intact even though most of the gas that formed the

intermediate-age clusters was well mixed.

3.6.2 NGC 1846

Given the sloped appearance of the RC and the width of the RGB (see Fig. 3-28),

NGC 1846 is suffering from differential reddening, making it difficult to accurately

measure the true location of the cluster RC, as well as V VHB for target stars.

To address this problem, we make no adjustments to the instrumental magnitudes,

but we measure the median magnitude of the entire differentially reddened RC,

effectively measuring the RC at the mean reddening of the cluster. Since the amount of

extinction suffered by the RGB stars should be scattered about the mean reddening, this

approach smooths over the differential reddening, allowing us to accurately measure

the cluster metallicity. We note that this method increases the scatter in [Fe/H] for

cluster members; as such, we have relaxed the metallicity cut in our member selection

method to include all stars moving at the radial velocity of the cluster. For reference,

if V VHB for any given star is off by & 0.2 mag (we estimate that the differential

reddening is 0.4 mag in V), the effect on [Fe/H] for that star is roughly & 0.05.

3.6.3 NGC 1861

This cluster is listed as SWB type IVB, suggesting an age range of 0.4-0.8 Gyr

(Bica et al. 1996), which is roughly the age at which the RC first forms (~ 0.5 Gyr;









Girardi & Salaris 2001). Plotting a CMD of stars within the apparent cluster radius

reveals what appears to be a fairly young MSTO in addition to no obvious cluster RC

or RGB. Therefore, we assume that NGC 1861 is a young cluster and all observed

RGB stars are actually part of an older field population.





















I l l
IC 2146





50 100 150
Distance (arcsec)




[Fe/H] = -0.41 *
















1 0 -1 -2 -3
V-VHB (rnog)


-t -i 4- -1 -













..I .. II .I
50 100 150
Distance (arcsec)


I


1000



500



0


300


250 -


500 1000 1500
x position (pixels)


-0.5


CI
"8
E
o
L
CI
V1
(r
C
Se

o
o


I

Y


-2.0


Figure 3-15:


IC 2146 cluster member selection. In this figure we illustrate our clus-
ter member selection process for IC 2146, using a combination of a
star's distance from the cluster center (upper left), radial velocity (upper

right), and metallicity (lower left) to separate field stars (blue, teal, and
green points) from cluster members (red points; see text for a complete
discussion of the color code). The lower right panel plots the summed

equivalent width as a function of V VHB for all stars considered to be
cluster members; the dashed line is an isoabundance line at the mean
metallicity of the cluster.






























































IC 2146 cluster and field CMD. Shown is the instrumental CMD for the
entire IC 2146 field (cluster and surrounding field stars), with the target
stars marked as described in the text; cluster members (red points) lie
along the RGB, AGB or in the RC.





IC 2146



.*

-~ -



.:. e.











I I .' I .


1 2


14t


IsC


181


-1.5


-1.0


0.5


-0.5
v-i (inst)


Figure 3-16:

























1 I I I
NGC 1 651




8 i.



~.il.


350 I


SI

I I

300


250


20 40 60 80 100 120
Distance (arcsec)




[Fe/H] = -0.53

+,












1 0 -1 -2 -3
V-VHB (rnog)


1000



500



0


500 1000 1500
x position (pixels)


-0.5


C
E 8




CI


-2.0


20 40 60 80 100 120
Distance (arcsec)


Figure 3-17: NGC 1651 cluster member selection. Same as Fig. 3-15 except the plots
shown are for NGC 1651.





























NGC 1651 **








--
.. .. ..

*I Y 9*
..* **
. .- ,


1 21


IsC


-1.5




Figure 3-18:


-1.0


-0.5
v-i (inst)


0.5


NGC 1651 cluster and field CMD. Same as Fig. 3-16 except the CMD
shown is for the entire NGC 1651 field.



























NGC 1652


"', i

.;


50 100 150
Distance (arcsec)




[Fe/H] = -0.46
-,













1 0 -1 -2 -3
V-VHB (rnog)


I







~.. I ....I.
I0 10 5
Disac (rsc


1000



500



0


500 1000 1500
x position (pixels)


-0.5


S8
E
E
6
CI
V 4


I


-2.0


Figure 3-19: NGC 1652 cluster member selection. Same as Fig. 3-15 except the plots
shown are for NGC 1652.



























1 1 1 1 l i l l l i l l l i l l l i l

NGC 1652


e.
.

-
-: '

?*









-





.... *, .

.s ..* *. *

... ''..*.-'.e..'I..'....."I'..... ''


1 2


1 6


-1.5




Figure 3-20:


-1.0


-0.5
v-i (inst)


0.5


NGC 1652 cluster and field CMD. Same as Fig.
shown is for the entire NGC 1652 field.


3-16 except the CMD



























NGC 1718








*4 .






500 1000 1500
x position (pixels)






j I i












20 40 60 80 100 120 140
Distance (arcsec)


20 40 60 80 100 120 140
Distance (arcsec)




[Fe/H] = -0.80


,











1 0 -1 -2 -3
V-VHB (rnog)


1000



500



0


- -t -


-
.


al$)i;-t


i


I
1
I


I
I
I


-0.5


E 8


C
6e


4


L .


-2.0


Figure 3-21: NGC 1718 cluster member selection. Same as Fig. 3-15 except the plots
shown are for NGC 1718.







84
















10 I I I


NGC 1718







.*:: ;* *,*,~ g



to~, .- :.. *g
.c 14 -" : -. --.. -








..' ~ ~ ** n 5r q:'. .:. ** .
18 ~ ~ ~ ~ ~ ~ : ...I I I . .


-1.5t -10- .
v-i 2' (inst)

Fiue32:NG 78cute n il CMD.. Same as Fi. 31 xeph M
shw sfr the' enie G 11fed



























NGC 1751


.:LI.:j I
.~-;1;-91; ~;rrr;~
'~;-- :
...
;r
- ... ~-.,
. r S~C~;.-*
~ .~..
:s..Y;i.~*~: ~; -"~ iL:.::
'L'- '-
c "~' I.
;- .1~ i


20 40 60 80 100 120 140
Distance (arcsec)




[Fe/H] = -0.44
,**













1 0 -1 -2 -3
V-VHB (rnog)


1000





o0


I :




(I w


3001-


500 1000 1500
x position (pixels)


-0.5


C
E 8




CI


-2.0


20 40 60 80 100 120 140
Distance (arcsec)


Figure 3-23: NGC 1751 cluster member selection. Same as Fig. 3-15 except the plots
shown are for NGC 1751.























10 **


NGC 1751












16 -~ ..



18 *,- '"
-1.5~ -10-05
av-i

Figre -24 NG 171 custr ad feld
shon i fo th enireNGC


0.0 0.5 1.0
(inst)

CMD. Same as Fig. 3-16 except the CMD
1751 field.