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Low Mass Star and Brown Dwarf Formation in the Orion B Molecular Cloud

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Firstandforemost,IwouldliketothankallofthemembersoftheUniversityofFloridaAstronomyDepartment.Inyourownway,eachofyouhavecontributedtomynishingthisdissertation.Specialthanksgotomyadviser,Dr.ElizabethLada,whowasaninspirationfromthebeginning,consistentlyprovidingmanyopportunitiestoexploreindependentresearchand,bestofall,gavemethefreedomtopursuemyownpathtothenish.Thankyoualsotomycommitteemembers(bothcurrentandformer)Drs.RichardElston,JonathanWilliams,SteveDetweiler,SteveEikenberry,andespeciallyAtaSarajediniwhoseofcedoorwasalwaysopenformetoaskeventhemostmundanephotometryquestions.Finally,thankyoutoAaronSteinhauer,AndreaStolte,NickRainesandtheothermembersoftheFLAMINGOSteam.WithoutthisamazinggroupofpeopleIwouldstillbereducingdataandclassiyingspectra.LargepinkbirdsandUFstarformationpostdocsareaspecialbreedindeed!MyfriendsandfellowstudentsintheAstronomyDepartmentkeptmesaneduringmytimeingraduateschool.IowemanythankstoSueLederer(myrsteverbigsister),LaurenshockedgasJonesandKarlHaisch(ofcematesandlocalISMgurus),DaveandJoOsip(tarotplayersextraordinaire),DavidDahari(boardandvideogamemaster),AaronGrocholski(mygainesvilleboyfriendandgeneralpartnerincrime),MargaretMoerchenandLaurenDavis(shoppingbuddies),AshleyEspy(sisterinkneeproblemscausedbyourrespectiveobsessions),AudraHernandez(fellowStarbucksaddictandfriendlyneighbor),LeahSimon(theotherastro-dancerwhorockclimbs),andtherealNODOUBT(a.k.a.thebestgraduatestudentofceEVER).IamalsodeeplygratefulfortheloveandsupportIreceivedfrommyfriendsinthedanceworld.JessicaMayhew,BlairLitaker,HeatherCollier,HeatherBanes,Stacey iv

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page ACKNOWLEDGMENTS ................................ iv LISTOFTABLES ................................... x LISTOFFIGURES ................................... xi ABSTRACT ....................................... xiv CHAPTER 1INTRODUCTION ................................ 1 1.1WhyStudyLowMassStarFormation? .................. 1 1.2TheCurrentParadigmforLowMassStarFormation ........... 2 1.2.1FormationofaSingleLowMassStar ............... 2 1.2.2StarFormationinClusters ..................... 4 1.2.3TheInitialMassFunction ..................... 4 1.2.4BrownDwarfs ........................... 5 1.3OrionB:AnIdealTestbedforStarFormationStudies .......... 7 1.4ANIRSpectroscopicStudyofYoungBrownDwarfsinOrionB .... 9 2NEAR-INFRAREDIMAGINGANDSPECTROSCOPYOFORIONB .... 12 2.1TheFLAMINGOSInstrument ...................... 12 2.2TheFLAMINGOSGMCSurvey ..................... 13 2.3FLAMINGOSImagingofOrionB .................... 13 2.3.1SurveyStrategyandObservations ................. 13 2.3.2LongLegs:TheDataReductionPipeline ............. 16 2.3.3Pinkpack:ThePhotometryandAstrometryPipeline ....... 19 2.3.4PositionalZeroPointCorrection ................. 25 2.3.5ImagingDataQualityandCompleteness ............. 28 2.4FLAMINGOSSpectraofOrionB .................... 34 2.4.1SpectroscopicSampleSelectionandMaskDesign ........ 34 2.4.2SpectroscopicObservations .................... 40 2.4.3DataReduction .......................... 43 2.4.4FinalSpectroscopicSample .................... 45 3SPECTRALCLASSIFICATIONOFYOUNGMSTARS ............ 47 3.1ClassicationStrategy ........................... 47 3.2FLAMINGOSLate-TypeSpectroscopicStandards ............ 47 vii

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49 3.4SurfaceGravity .............................. 54 4MSTARSANDBROWNDWARFSINNGC2024 ............... 56 4.1Introduction ................................ 56 4.2NewPhotometryforNGC2024 ...................... 57 4.3SpectroscopyofNGC2024 ........................ 59 4.3.1SampleandObservations ..................... 59 4.3.2Results ............................... 60 4.4TheHertzsprung-RussellDiagram .................... 65 4.4.1Extinction ............................. 66 4.4.2EffectiveTemperaturesandBolometricLuminosities ...... 68 4.4.3H-RDiagram ........................... 68 4.4.4MassesandAges ......................... 71 4.5PropertiesoftheLowMassClusterPopulation .............. 72 4.5.1ClusterMembership ........................ 72 4.5.2ClusterAge ............................ 73 4.5.3SpatialDistributionofSources .................. 75 4.5.4SubstellarDiskFrequency ..................... 77 4.5.5LowMassIMF .......................... 78 4.5.6TheRatioofBrownDwarfstoStars ............... 80 4.6Summary ................................. 82 5MSTARSANDBROWNDWARFSINNGC2068ANDNGC2071 ...... 84 5.1Introduction ................................ 84 5.2PhotometryofNGC2068andNGC2071 ................ 85 5.3SpectroscopyofNGC2068andNGC2071 ............... 87 5.3.1SampleSelection,Observations,andDataReduction ...... 87 5.3.2Results ............................... 90 5.4TheHertzsprung-RussellDiagram .................... 98 5.4.1Extinction,EffectiveTemperatures,andBolometricLuminosities 98 5.4.2H-RDiagramsforNGC2068andNGC2071 .......... 101 5.5LowMassPopulationsofNGC2068andNGC2071 .......... 102 5.5.1ClusterMembership ........................ 102 5.5.2ClusterAges ............................ 105 5.5.3SpatialDistributionofSources .................. 107 5.5.4InfraredExcessandSubstellarDiskFractions .......... 112 5.5.5InitialMassFunctionsforMStarsinNGC2068andNGC2071 112 5.5.6RatioofStarstoBrownDwarfsinNGC2068andNGC2071 .. 122 5.6Summary ................................. 125 viii

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............................ 127 6.1ModelsofBrownDwarfFormation .................... 127 6.2ObservationalConstraints ......................... 130 6.2.1SubstellarDiskFrequencies .................... 130 6.2.2InitialMassFunctions ....................... 132 6.2.3TheAbundanceofBrownDwarfs ................. 136 6.3OverallImplicationsforBrownDwarfFormation ............ 142 7THESTAR-FORMINGHISTORYOFORION ................. 149 8FUTUREWORK ................................. 154 APPENDIX AIMAGINGSURVEY:OBSERVINGLOGANDSURVEYSTATISTICS .... 156 BSPECTROSCOPICSURVEY:OBSERVINGLOG ............... 176 REFERENCES ..................................... 179 BIOGRAPHICALSKETCH .............................. 186 ix

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Table page 2RegionsTargetedforSpectroscopicObservation ............... 36 3YoungSpectralStandards ........................... 49 3SurfaceGravityStandards ........................... 54 4SlitMasksObservedinNGC2024 ...................... 59 4DataforClassiedSourcesinNGC2024 ................... 70 5DetailofSlitMasksObservedinNGC2068 ................. 87 5DetailofSlitMasksObservedinNGC2071 ................. 90 5DataforClassiedMStarsinNGC2068 ................... 99 5DataforClassiedMStarsinNGC2071 ................... 100 6AbundancesofBrownDwarfsinYoungStarFormingRegions ........ 137 6PhysicalPropertiesofYoungStarFormingRegions ............. 139 7AgesofStar-FormingRegionsinOrion .................... 151 AOrionBImagingObservingLog ........................ 157 AMeanPhotometricScatterbyField ...................... 173 ALuminosityFunctionPeaksbyField ...................... 174 BFLAMINGOS/OrionBSpectroscopicObservingLog ............ 178 x

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Figure page 1OpticalimageoftheOrionBregion ..................... 7 1COmapofOrionB .............................. 10 2KPNO2.1mFLAMINGOSFieldsObservedinOrionB ........... 14 2RawK-bandFLAMINGOSImage ...................... 16 2FinalImageReducedwithLongLegs ..................... 19 2PrecisionofPinkpackAstrometry ....................... 23 2SampleZeroPointCalculationHistograms .................. 24 2EffectofComaontheStellarPSF ...................... 26 2EffectofComaonStellarPhotometry .................... 27 2PhotometricZeroPointCorrection ...................... 28 2FinalPhotometricScatterbyBand ...................... 29 2TenSigmaDetectionLimitsfortheImagingSurvey ............. 31 2SurveyLuminosityFunctionsand90%CompletenessLimits ........ 33 2FieldsTargetedforSpectroscopicObservations ............... 35 2SampleSelectionDiagramsforSpectroscopicTargets ............ 38 2SampleMaskDesignedforN2071 ...................... 40 2ExampleofanAlignedMOSPlate ...................... 42 2ImagesofReducedSpectra .......................... 44 2Two-DimensionalImageofaSingleMOSSlitlet ............... 44 2FinalReducedSpectrum ........................... 45 3NIRspectraoflate-typeelddwarfs ..................... 48 3FLAMINGOSSpectralSequenceforYoungMStars ............ 50 3EffectofReddeningonMDwarfSpectra ................... 51 xi

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................... 52 3VisualClassicationofProgramSpectra ................... 53 3SurfaceGravityEffectsinNIRSpectraofMStars .............. 55 4OpticalImageofNGC2024 ......................... 57 4NIRPhotometryofClassiedSourcesinNGC2024 ............. 58 4Three-colorimageofNGC2024 ....................... 60 4NIRSpectraofMstarsinN2024 ....................... 62 4DistributionofAVforNGC2024SpectroscopicSample ........... 67 4H-RDiagramsforNGC2024 ......................... 69 4SpatialDistributionofClassiedSourcesinNGC2024 ........... 76 4UncorrectedandExtinction-LimitedKLFsforNGC2024 .......... 79 4MassFunctionforNGC2024 ......................... 81 5OpticalImageofNGC2068andNGC2071 ................. 84 5Color-MagnitudeandColor-ColorDiagramsforNGC2068 ......... 86 5Color-MagnitudeandColor-ColorDiagramsforNGC2071 ......... 86 5Three-ColorImageofNGC2068 ....................... 88 5Three-ColorImageofNGC2071 ....................... 89 5NIRSpectraofMstarsinN2068 ....................... 92 5NIRSpectraofMstarsinN2071 ....................... 96 5H-RDiagramsforNGC2068 ......................... 101 5H-RDiagramsforNGC2071 ......................... 102 5DistributionofAVforMStarsinNGC2068 ................. 104 5DistributionofAVforMStarsinNGC2071 ................. 105 5LocationofClassiedSourcesinNGC2068asaFunctionofAge ..... 108 5LocationofClassiedSourcesinNGC2068asaFunctionofMass ..... 109 5LocationofClassiedSourcesinNGC2071asaFunctionofAge ..... 110 5LocationofClassiedSourcesinNGC2071asaFunctionofMass ..... 111 xii

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. 113 5AverageKLFforOrionBControlFields ................... 115 5BackgroundSubtractedKLFforNGC2068 ................. 117 5BackgroundSubtractedKLFforNGC2071 ................. 118 5CompletenessofNGC2068Spectra ..................... 119 5CompletenessofNGC2071Spectra ..................... 120 5MassFunctionforMStarsinNGC2068 ................... 123 5MassFunctionforMStarsinNGC2071 ................... 124 6InitialMassFunctionsofYoungClusters ................... 133 6IMFPeaksvs.GasDensity .......................... 135 6Rssvs.GasDensity .............................. 144 6Rssvs.StellarDensity ............................. 145 6Rssvs.TotalMass ............................... 146 6Rssvs.SpectralType ............................. 147 6DistributionofBrownDwarfsinNGC2024RelativetotheHIIRegion ... 148 7MolecularCloudsinOrionandTheirRelationshiptoOriOB1 ....... 150 7HistogramofAgesforStar-FormingRegionsinOrion ............ 152 7Agevs.AngularDistancefromdOri ..................... 153 xiii

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especiallyimportantbecauseitdenesaclassofobjects(calledbrowndwarfs)whosemassesbridgethegapbetweenstars(M>0.08M)andplanets(M<0:012M).Unfortunately,unlikestarswhichareknowntoformfromthecollapseofmolecularcloudcores(x )andplanetswhicharethoughttoformviaaccretioninacircumstellardisk,theformationofbrowndwarfscurrentlyhasnowidelyacceptedexplanation.Thisdissertationcombinesphotometryandspectroscopyoflowmassstarsandbrowndwarfstoa)placeobservationalconstraintsonthepossiblemechanismsdrivingbrowndwarfformationandb)examinehowthebrowndwarfformationprocessrelatestothatoflowmassstars. Lada 2005 ).Thestarformationprocessconvertsthemolecularmaterialcontainedintheselargeandcoldcloudsintomuchsmallerandhotterhydrogen-burningstars. Shu 1977 ; Shuetal. 1987 ),neglectingtheinuencesofcloudrotationandmagneticelds,thestarformationprocessbeginswhenthemolecularcloudbeginstocollapseduetoitsinternalgravity.Asthecloudcollapses,thedensityincreasesnon-homogeneously,allowingthecloudtofragmentandformregionswithextremelyhighlocaldensities(n(H2)1045cm3).Ifthedensityishighenough,thesecloudcoresbecomegravitationallyunstableand

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collapseisothermallyuntiltheirincreaseddensitiescausethecorestobecomeopticallythicktotheirownradiation(e.g.radiationisabsorbedandactstoincreasethecentraltemperatureofthecores).Atthispointaprotostar(aquasi-staticstellarcore)isborn.Notethatclassicalstarformationtheorypredictsthatthereisaminimummassrequiredforthecollapseofadensecoreintoaprotostar( Jeans 1902 )andthatforatypicalstarformingcloudthismassisontheorderofasolarmass( Larson 1995 ).AsIwilldiscussinupcomingsections,thisisaproblemwhenconsideringtheformationofobjectswithmassessignicantlysmallerthan1M.Theprotostarissurroundedbyaninfallingenvelopeofgasanddust.Toconserveangularmomentum,theinfallingenvelopeformsadiskaroundtheembryoniccore.Theprotostarcontinuestogainmassthroughtheaccretionofmaterialfromthedisk.Atthispoint,protostellarluminositiesaredominatedbyanaccretionluminositywhicheffectivelydissipatesthegravitationalpotentialenergylostduringinfallandcollapse.Whenthemassoftheprotostarincreasessuchthatthecentraltemperaturereaches106K,deuteriumburningbegins,addinganewcomponenttotheprotostar'sluminosity.Attheonsetofdeuteriumburning,accretiontypicallyslowsdown,theremnantsoftheprotostellarenvelopedisperse,andtheprotostarbecomesvisible.Thisdeninesthestartofthepre-mainsequence(PMS)phaseofstellarevolution.Pre-mainsequencestarsburntheirprimordialdeuteriumratherquickly.Withoutsignicantaccretiontoreplenishtheirfuelstores,nuclearreactionsceaseandtheluminositybeginstodecrease.ThelackofradiationpressureinthecoreallowsgravitytotakeoverandthePMSstarwillagainbegintocontract.Duringthissecondepochofcontractionplanetformationisthoughttooccurwithintheremnantsofthecircumstellaraccretiondisk.PMSevolutionendswhenthegravitationalcontractionhascausedthecentraltemperaturetoincreaseto107K.Atthispoint,thenuclearhydrogenburningthatdenesastarbegins,thestar'sluminositystabilizes,andthestarissaidtobeonthemainsequence(MS).

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Ladaetal. 1991b ; Carpenter 2000 ; Lada&Lada 2003 ; Porrasetal. 2003 ).Ifwewishtotrulyunderstandthestarformationprocess,weneedtoexaminetheembeddedclusterpopulationofGMCs.Starclustersofanysortareimportantlaboratoriesforastrophysicalresearch.Theycontainastatisticallysignicant(N&50)numberofstarswithawiderangeofmassesinarelativelysmallvolumeofspace.Theirmembersshareacommonorigin,havingformedfromthesameparentGMC.Additionally,clusterstarsareatroughlyuniformdistanceswithsimilaragesandchemicalcompositions.Theembeddedphaseofacluster'sevolutiontypicallylastsaround3Myr( Lada 2005 ).Attheseages,eventhelowmassclusterpopulationisfairlybright(e.g.x and 1.2.4 )andmorereadilyobservablethanlaterinthecluster'slifetime.Further,embeddedclustersaretooyoungtohavelostsignicantnumbersofstarstostellarevolutionand/ordynamicalevaporation.Observationsofembeddedclustersthereforeprovidesnapshotsofstarsintheirnatalenvironments.Consequently,embeddedclustersareextremelyusefultoolsforstarformationstudies. Salpeter 1955 )anditisapowerfultoolusedtoconstrainformationandevolutiontheoryacrosstheentirespectrumofastrophysics.Inparticular,determiningtheshapeoftheIMF,includingthelocationsoftheturnoverandminimummasses,isvitaltoourunderstandingofthephysicalprocessesthatcontrolstarandplanetformation.However,inmostcasestheIMFisnotareadilyobservablequantity.N-bodysimulations

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showthatolderopenclusters(t100Myr)containamere20-30%ofthestarsintheoriginalembeddedclusters( Kroupaetal. 2001 ).TheseclustersrequirecorrectionsfordynamicalevolutionandmasslossinanyattempttodeterminetheirIMFs.Incontrast,theobservedmassdistributioninanembeddedclusterisitsIMF.TheIMFofmassivestarshasbeenstudiedinthismannerformanyyearsandisreasonablywellconstrained(see Massey 1998 ,forareview).TheIMFoflowmassstars,ontheotherhand,isfarlesscertaindueinparttotherelativelyrecent(withinthelast10years)additionofbrowndwarfs. Burrowsetal. 1997 ).Theidenticationofbrowndwarfsposesaninterestingobservationalchallenge,neatlyillustratedbythelongamountoftimethatelapsedbetweenthersttheoreticalmentionofblackdwarfsby Kumar ( 1963 )andtherstconrmeddetectionofsuchanobject(GL229B, Oppenheimeretal. 1995 ; Nakajimaetal. 1995 ).Thislongtimeintervalbetweentheoryandobservationwasdueinparttothelackofappropriatedetectortechnology.Themostmassivemiddle-agedbrowndwarfs(M=0.08M,age1Gyr)haveabsolutemagnitudesV19( Baraffeetal. 1998 ).Placingtheseobjectsatthedistanceoftheneareststar-formingregions(d100pc)yieldsapparentmagnitudesv24,wellbelowthelimitofstandardopticaldetectorsonmid-sizetelescopes.Atnear-infrared(NIR)wavelengths,however,browndwarfsaremuchbrighter.IntheKband(2.2m)ourexamplemiddle-agedbrowndwarfwouldhaveamagnitudeK15anda

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correspondingyoungbrowndwarfinthenearbystarformingregion(t1Myr)wouldbeblazingawayatK10,easilydetectableona1mclasstelescopewiththeinfrareddetectorsavailabletoday.Withtheadventofdeep,large-scalesurveyssuchastheDeepNear-InfraredSurvey(DENIS, Epchteinetal. 1994 ),the2MicronAllSkySurvey(2MASS, Skrutskieetal. 1997 ),andtheSloanDigitalSkySurvey(SDSS, Yorketal. 2000 ),theeldofbrowndwarfresearchhasundergonerapidexpansionoverthepastdecade,resultinginthedetectionofnumerouseldbrowndwarfsandthedenitionoftwonewspectralclasses(e.g. Delfosseetal. 1997 ; Kirkpatricketal. 1999 ; Leggettetal. 2000 ).Analysisoftheseobservationshasledtotheconclusionthatbrowndwarfsconstituteasignicantfractionofthestellarpopulationinthesolarneighborhoodandmaycompriseasmuchas15%ofthegalacticdiskmass( Reidetal. 1999 ; Chabrier 2002 ).Arobusttheoryofstarandplanetformationmustthereforetakebrowndwarfformationintoaccount.However,asmentionedearlierinthischapter,classicalstarformationtheoryhastroubleexplainingtheformationofobjectswithmassessignicantlysmallerthanasolarmass.Consequently,theoriginofbrowndwarfsisstillunclear.Dobrowndwarfsforminamannersimilartotheirstellarcounterpartsormoreakintotheirplanetarycousins?Whatmechanismdrivesbrowndwarfformationanddoesitdependonthestarformingenvironment?WhatistheshapeofthebrowndwarfIMF?Recentlymanytheoriesofbrowndwarfformationhavebeenproposed,includingturbulentfragmentationofamolecularcloud( Padoan&Nordlund 2002 ),ejectionofprotostellarembryos( Reipurth&Clarke 2001 ),protostellardiskcollisions( Linetal. 1998 ),andphoto-erosionofprestellarcores( Whitworth&Zinnecker 2004 ).Studiesofyoungbrowndwarfsintheirbirthenvironmentsareneededtodistinguishbetweenthesescenarios.

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Figure1. OpticalimageofOrionB:NorthisupandEastisleft.ThebrighteststarintheeldiszOri,theeasternmoststarinOrion'sBelt.NGC2024(theFlameNebula)liesjusteastofzOri.NGC2023isthebluereectionnebulasouthofNGC2024,withIC434andtheHorseheadNebulaslightlytothesouth-west.NGC2068(M78)andNGC2071areinthenortheastcorneroftheimage.Photo:W.H.Wang(IfA,U.Hawaii)

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identicationasadarkcloudby Lynds ( 1962 ).EarlystudiesdetectedanumberofHaemissionlinestarseastandnorthofOrion'sbelt( Haro&Moreno 1953 ; Herbig&Kuhi 1963 ),andtheregion'scolloquialnamesake,OrionB(thesecondstrongradiosourcedetectedintheOrionregion),wasfoundtobeacompactHIIregion(NGC2024)morethan50yearsago( Hepburnetal. 1954 ).Practically,OrionBlendsitselftothestudyofyoung,lowmassobjectsasitisoneofthenearestregionsofongoingstarformationandthenearestregionofmassivestarformation.Distanceestimatestothecloudrangefrom390-415pc( Anthony-Twarog 1982 )although Brownetal. ( 1994 )pointoutthattheremaybeagradientacrosstheregionwiththenearcloudedgeat320pcandthefaredgeat500pc.Assumingamediandistanceof400pc,theproximityofOrionBimpliesthatanunrededdened1MyrbrowndwarfwillhaveaKmagnitudeof13easilyobservablewithtoday'stelescopesandNIRinstruments.Inaddition,OrionBisconvenientlylocatedoutoftheplaneofthegalaxy(l=206,b=-15),reducingforegroundandbackgroundcontaminationbythegeneralgalacticstellarpopulation.OrionBcontainsmanyspectacularobjectsknownformorethanacenturytobothamateurandprofessionalastronomersalike:thereectionnebulaeNGC2068(M78),NGC2071,andNGC2023,theHIIregionsNGC2024(TheFlameNebula)andIC434,andthesmalldarkcloudoftheHorseheadNebula(B33)seeninsilhouette.AlloftheseregionscanbeseenintheopticalimageshowninFigure 1 .However,opticalimagesdonotshowthewholepicture.RecallthatthevisiblehallmarkofaGMCisabroad,darkswathofskywithlittletonoopticalstarlight.Inactuality,GMCsarenotdevoidofstarsatall.ItisthelargeconcentrationsofdustwithinaGMCthatabsorbandscatterthelightfrombackgroundstarscausingtheregiontoappeardark.InordertotrulyunlockthesecretsofstarformationcontainedinaGMC,longerwavelengthobservationsareneededtopenetratethedust.Radioobservations(l=2.6mm)oftheOrionBregionby Tuckeretal. ( 1973 )revealedalargecomplexofextendedCOemission(indicatingthepresenceofmolecular

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hydrogen)coincidingwiththeopticaldarkcloud.TheopticalnebulaedescribedabovewerefoundnottobesingularsourcesofCOemissionbutrathermaximalocatedwithinamuchlargerregionofmolecularlineemission.ThefullextentofOrionBwassubsequentlydelineatedbythehighersensitivityCOobservationsof Maddalenaetal. ( 1986 )whofoundthatthecloudsubtendsapproximately19deg2onthetheskyandcontainsnearly105Mofgas(Figure 1 ). Ladaetal. ( 1991a )carriedoutasystematicsearchfordensegas(n>104cm3)inOrionBusingtheJ(2-1)transitionofCSasatracerandfoundthatonlyasmallfraction(<19%)ofthetotalcloudmassislocatedinthedensecoresandisthusinvolvedinthestarformationprocess(recallx ).Inacompanion2.2msurvey, Ladaetal. ( 1991b )unexpectedlydiscoveredthat60-90%oftheyoungstellarpopulationresidesin3populousclusters(NGC2024,NGC2068,andNGC2071)embeddedinthemostmassivedensemolecularcores.Thisresultwasconrmedby Carpenter ( 2000 )usingdatafrom2MASS.However,thecompletenesslimitsofboththeLadasurvey(K<13.0)and2MASS(Ks14.0)weretoobrighttoprobeveryfarbelowthehydrogenburninglimit.ThisdissertationextendsthecurrentbodyofworkinOrionBbeyondthelimitsoftheprevioussurveys,specicallyfocusingonthecontributionofyoungbrowndwarfstotheoverallembeddedclusterpopulation. Hillenbrand&Carpenter 2000 ; Luhmanetal. 2000 ; Muenchetal. 2002 intheOrionNebulaClusterand Muenchetal. 2003 inIC348).However,stellarandsubstellarmassfunctionsderivedfromphotometryalonecanonlybestudiedinastatisticalsense.Individualmassestimatesremainambiguousduetouncertaintiesintheage,extinction,andmembershipstatusofanygivensource.AdeterminationofeffectivetemperaturesusingNIRspectroscopyalleviatessomeoftheseuncertaintiesand,withthehelpof

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Figure1. COmapofOrionBadaptedfrom Maddalenaetal. ( 1986 ).Thecloudsub-tends19squaredegreesontheskyandcontains105Mofmoleculargas( Maddalenaetal. 1986 ).Thethreemostprominentstarformingregionsareindicated. pre-mainsequencemodels,allowsformoreaccuratemassestimatesforeachindividualsource.

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Formydissertation,IamusingacombinationofNIRphotometryandspectroscopytocharacterizethebrowndwarfpopulationsinthedominantstarformingregionsinOrionB.Iwillusetheresultsofthisstudytoassesscurrenttheoriesofbrowndwarfformationinthecontextoftheobservations,ultimatelyexaminingtherelationshipbetweentheformationprocessesofbrowndwarfsandlowmassstars.Chapter2describesalldataacquisitionandreduction.Thisincludestheplanning,execution,andreductionofalarge-scaleNIRimagingsurveyofOrionB,theuseofthissurveytoselectthespectroscopicsample,andallspectroscopicobservationsandreductions.Chapter3describesourmethodsfortheclassicationofyoungMstarandbrowndwarfspectraandincludesadiscussionofsurfacegravity.Chapter4presentstheresultsofourstudyofNGC2024.Chapter5presentstheresultsforNGC2068andNGC2071.Chapter6examinestheseresultsandtheirimplicationforbrowndwarfformationinOrionB.Chapter7investigatesthestar-forminghistoryoftheregionandChapter8presentsmyconclusionsandplansforfuturework.

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Elston 1998 ).ConceivedbyRichardElstonandbuiltattheUniversityofFlorida,FLAMINGOSisacombinationofawide-eldNIRimagerandtheworld'srstfullycryogenicNIRmulti-objectspectrometer.FLAMINGOSemploysa20482048pixelHgCdTeHAWAII-2infraredarraywith18micronpixels.OntheKittPeakNationalObservatory(KPNO)4meter(4m)telescopethiscorrespondstoaplatescaleof0.31800/pixelanda10.8010.80eldofview.OntheKPNO2.1mtelescope,theplatescaleis0.60800/pixel,yieldingaeldofviewof20.5020.50.TheimagingltersincludestandardbroadbandJ(1.25m),H(1.65m),andK(2.2m)ltersroughlyequivalenttotheCaltech(CIT)system,aswellasaKs(2.12m)lter.Forspectralobservations,FLAMINGOSoffersbothaJHandHKgrismandltersetwhichprovidespectraacrosstwobands(JandHorHandK)simultaneously.Spectralresolutionsarelow(typicallyR=1000-1800dependingonthelterandgrismcombinationschosen).ThelargedetectorareaandwideeldofviewofFLAMINGOS,particularlyontheKPNO2.1m,makesFLAMINGOSaveryefcientinstrumentforimaginglargeregionsofthesky:anentiresquaredegreecanbeobservedinnineelds.Further,themulti-objectspectrometer(MOS)ofFLAMINGOSmakesitpossibletoobtainspectraof25-50objectssimultaneously.ThesequalitiesmakeFLAMINGOSanexcellentinstrumentfor 12

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large-scale,multi-dimensionalNIRsurveys.ThisdissertationisoneoftherstbroadapplicationsofFLAMINGOSasasurveyinstrument. 2.3.1SurveyStrategyandObservationsAsdiscussedinChapter 1 ,earlierstudiesofOrionBhaveshownthatthemajorityofstarformationinthecloudoccurswithinthedensecores( Ladaetal. 1991a b ; Carpenter 2000 ).TheFLAMINGOSsurveyregionwasthereforechosentocoincidewiththedensestgasasindicatedbytheCOmapof Maddalenaetal. ( 1986 ).Inaddition,thesurveyregionencompassesnearlytheentireCSsurveyof Ladaetal. ( 1991a )(Figure 2 ).Initialobservationsofthe50FLAMINGOSeldsshowninFigure 2 aswellasthreeoff-cloudcontroleldswereobtainedduringthe2001-2002winterobservingseasonontheKPNO2.1mtelescope.AlleldswereobservedintheJ,H,andKlters

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Figure2. COmapofOrionB( Maddalenaetal. 1986 )shownwiththeobservedFLAMINGOS/KPNO2.1msurveyelds.EachFLAMINGOSeldis200onasidetoallowforsomeoverlapfromeldtoeld.ThethickblacklineoutlinestheCSsurveyof Ladaetal. ( 1991a ).

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usinga9-pointditherpatterntoallowforthecreationandsubtractionofabackgroundskyframe.Theintegrationtimeateachpointwas35secondsinallbands,yieldingatotalof5minutesonsourceandatargeteddepthofJ=H=K18.Typicalseeingwas1:005-1:009asindicatedbythefullwidthathalfmaximum(FWHM)ofthestellarproles.Domeatsatallwavelengthsanddarkframeswerealsoobservedforcalibrationpurposes.Occasionally,duetopoorweatherconditions,instrument/telescopeproblems,orobservererror,theimagequalityand/ordepthwasmuchworsethanexpected(e.g.seeing2:000and/orJ=H=K18.0asindicatedbypreliminarydatareduction).Theseeldsweremarkedasre-takesandre-observedduringthewintersof2002-2003and2003-2004.Inaddition,duringthe2003-2004and2004-2005observingseasonsfourmoreoff-cloudcontroleldsand6gapeldsinthevicinityofNGC2024,NGC2068,andNGC2071wereobservedtoensurefullcoverageofthatregionofthemap.AcompleteobservinglogfortheOrionBeldscanbefoundinAppendix A .Overthecourseoftheentiresurvey,atotalof360individualOrionBimagesweretakencoveringnearly6squaredegreesofskyandyielding6gigabytesofdata.Itwasimpracticaltoreduceandphotometersuchalargeamountofdatabyhand;thus,twoautomatedprocessingpipelinesweredesignedtocompletethesetasks.Therstpipeline,dubbedLongLegsandauthoredbyCarlosRoman-Zuniga,takescareofpreliminaryprocessing,basicdatareduction,andconstructionofanalimage( Roman-Zuniga 2006 ).Thesecondpipeline,calledPinkpackandwrittenbythisauthor,incorporatessourceextraction,photometry,astrometry,andthecombinationofmulti-wavelengthdataintoasinglecatalog.Bothpipelineswereconstructedusingacombinationofpre-existingImageReductionandAnalysisFacility(IRAF)tasksandcustomCandFORTRANroutineswrittenbytheauthors.Eachpipelineistypicallyrunononenightofdata(300-500rawdataframes,16Mbeach)atatime.Thedetailedfunctionalityofthesepipelinesisdescribedinthefollowingsections.

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2 ).Inaddition,mostinfrareddetectorssufferfromvaryingsensitivityacross Figure2. RawKbandimageofanOrionBcontroleld(oribcf6)takenwithFLAMINGOSinNovember2004.Notethattherearehardlyanystarsvisi-ble.

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thearray,non-linearityinthedetectorresponse,andvariousdead,hot,orexcessivelynoisypixels.Alloftheseeffectsmustberemovedpriortoanystudyoftheastronomicaltarget.LongLegs( Roman-Zuniga 2006 )waswrittentoaccomplishthistaskforallFLAMINGOSimages.Asidefromitsabilitytocompletethedatareductionprocessforlargevolumesofdata,LongLegsisinnovativeinthatitincludesatwo-passskysubtractionroutinewhicheffectivelycreatesandremovesthelocalskybackgroundwhileleavingthesciencetargetuxesintact.TheremainderofthissectionprovidesthedetailsoftheLongLegsdatareductionprocess. 2 ,cleanamingostakescareofallpreliminarydataprocessingforanentirenightofdata,onesciencetargetatatime.Badpixels(pixelshavingcounts>60000or<-60000)areidentiedandsettohavevaluesof0.0.Imagesaregroupedaccordingtotype(e.g.object,at,dark)andthemeanandstandarddeviationarecalculated;frameshavingmeanvalueswhichdifferbymorethan2sigmafromthemeanofthegrouparerejected.Athird-degreepolynomialcorrectionisthenappliedtothegooddatatocorrecttheintegratedcountsineachpixelforthenon-linearresponseofthedetector.Darkframesaremediancombinedtocreateamasterdarkforthescienceimagesaswellasadarkfortheatelds.Amasterateldiscreatedbymediancombiningandnormalizingthedark-subtracteddomeats.Finally,abadpixelmask(BPM)iscreatedbyidentifyingallanomalouspixelsinthemasterdarkandthemasterat.

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andmediancombiningthe8adjacentdark-subtractedframes.Thislocalskyisthensubtractedfromthescienceimagestoremovetheskybackground.Theresultingdataaredividedbythemasterattoaccountforthepixel-to-pixelsensitivityvariationsinthedetector.Intheory,asinglepassthroughthereductionprocesswouldbesufcienttocreatesciencequalityimages.However,inmanycasestheobjectremovalduringskycreationisimperfectandlargenegativeresidualsareleftinthenalimages.Consequently,pass2ofcrunchamingosrecreatesthebackgroundskywiththeastronomicalobjectsmaskedout.TheIRAFsourcedetectionprogramdaondisrunonallframesinaditheredset,theframeoffsetsarecalculated,andapreliminarycombinedframeisconstructed.Allsourceswithuxeslargerthanauser-speciedsigmathresholdaremaskedoutfromtheindividualimagesandlocalskiesarere-createdfromthestarlessdataframes.ThesenewskyframesaresubtractedfromtheobjectimagesandusedtocreateanightskyatwhichissubequentlyusedtoattenthedataTheoutputofcrunchamingosisanimprovedsetofreducedimagesreadyfornalcombination. 2 ).Allnalimagesaretrimmedtobe40964096pixelsandarethenreadyforphotometry,astrometry,oranyotheranalysis.

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Figure2. ThenalLongLegsproductfortheimageshowninFigure 2 .Thisimagehasbeenlinearized,dark-subtracted,sky-subtracted,at-elded,correctedforgeometricdistortion,combinedwithotherimagesintheditherset,re-sampledandtrimmedtohave40964096pixels.Notetheplethoraofstarsrevealedbythereductionprocess.

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photometryfordetectedobjects,solvefortheastrometricsolutionofeachimage,completepreliminarydataassessment,andcombinemulti-wavelengthdataintoasinglecatalog. Bertin&Arnouts 1996 )togenerateapreliminarysourcelistforthecurrentimage.ThislistispassedthroughadetectionlterwhichemploystheBPMtoremoveanyobjectsfallingonbadregionsofthechipwheretheirphotometrywillbecompromised.Thelteredsourcelististhenpassedthroughtwoadditionalroutineswhichremovesaturatedobjectsandselectstarssuitablefordeningtheimagepointspreadfunction(PSF).PSFstarsmustberelativelybright,isolated(nootherobjectswithin5FWHM),atleast100pixelsfromtheimageedges,beroughlyevenlydistributedacrosstheimageandaboveallexhibitatypicalstellarprole.Thus,objectsaggedbySExtractoraselongatedandobjectshavingaFWHMmorethan1.5timesthemeanorlessthan0.5timesthemeanareautomaticallyeliminatedfromthePSFstarlist.AftercreationofthenalsourcecatalogandPSFstarlist,pinkphotbeginsitsphotometryroutines.TheskybackgroundestimatedandsubtractedbyLongLegsistypicallyadecentrepresentationofthetruelocalsky,however,forverynebulousand/orcrowdedeldstheirwillstillbesomeelementofcontaminationinlargeaperturesbyuxfromnearbyneighborsornebulosity.Pinkphotcircumventsthisproblembyapplyinganaperturecorrection.AperturephotometryofthePSFstarsisobtainedusingtheIRAFphottaskovertwelveapertureswithradiirangingfrom0.5FWHMto3.25FWHM.Anaperturecorrectionfromapertures3-12(apertureradiifrom1-3.25FWHM)isthencalculatedfromthePSFstarphotometryusingtheIRAFtaskmkaple.Allsourcesinthe

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imagearephotometeredusingthesmallaperture(r=FWHM)andsubsequentlycorrectedouttothelargeraperture(r=3.25FWHM),yieldingcontaminant-freelargeaperturephotometryforallobjects.Notethatatthispointallphotometryisuncalibratedwiththezeropointsetat0.0.Oncethecorrectedaperturephotometryisobtained,pinkphotderivesPSFphotom-etryforallsourcesusingtwopassesthroughtheIRAFDAOPHOTroutines( Stetson 1987 ).Intherstpass,amodelPSFiscreatedfromthePSFstarsusingasecondordervariableMoffatfunctionwiththebparametersetto2.5.ThisPSFisttoeachobjectinthecompletesourcecatalogusingthelargeaperturephotometryasinput.Thettedstarsarethensubtractedfromtheoriginalimage,revealinganyclosecompanions.ThesecondpassrepeatstheinitialaperturephotometryandPSFttingprocessonthenewlydetectedsourcesusingtheaperturecorrectionandmodelPSFderivedinpassone.Thenetoutputfrompinkphotisacatalogcontainingthexandypixelcoordinates,thesmallapertureradiusinpixels,theuxinthataperture,thephotmagnitudeusingthesmallaperture,thelargeapertureradiusinpixels,theaperture-correctedmagnitudeanditsassociatederror,thePSFmagnitudeanditserror,anddetectionagsindicatingwhetheranobjectispotentiallysaturatedand/orelongated.Itshouldbenotedthatwhiletheaperturephotometryisincludedforcompletenessanditspotentialusefulnessindeterminingthemagnitudesofnon-stellarsources,allsurveymagnitudesdiscussedfromthispointforwardarederivedfromPSFphotometry,duetoitsimprovedabilitytohandleextremelycrowdedornebulouseldsandvariationsinstellarPSFsacrossanimage(refertox forfurtherdiscussionofthiseffect).

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standardsphericalgeometryitisthereforepossibletoderiveatransformationbetweenthetwo.Thistransformation,knowninastronomicaltermsastheplatesolution,denestheimageworldcoordinatesystem(WCS),linkingeachimagepixeltoaspeciclocationonthecelestialsphere.PinkastromemploysthepixelandcelestialcoordinatesofknownobjectsintheeldtoderiveaplatesolutionandWCSforeachimage,subsequentlyapplyingthesolutionandderivingastronomicalcoordinatesforallpinkphotdetections.NotethatpinkastromusesthestandardequatorialcoordinatesofRightAscension(R.A.)andDeclination(Dec.)todescribeobjectpositionsonthecelestialsphere.TheepochofallequatorialcoordinatesisJ2000.Pinkastrombeginsbyobtainingthecoordinatesoftheimagecenter(R.A.andDec.)fromtheimageheader.Pinkastromthencallsouttoanonlinecatalogrepositoryanddownloadstheequatorialcoordinatesandmagnitudesofallsourceswithina15arcminuteradiusofthispoint.HeretheuserhastheoptionofemployingthemostrecentU.S.NavalObservatoryallskycatalog(USNOB1.0)orthe2MicronAllSkySurvey(2MASS).ForthepurposesoftheFLAMINGOSGMCsurvey,wehavefoundthatthe2MASScatalogconsistentlyyieldsahighernumberofmatchespereld(likelybecauseitisalsoaninfraredsurvey),providingabettersampledgridofpointsfromwhichtocalculatethesolution.Consequently,allplatesolutionsandcelestialcoordinatesquotedinthisdissertationwerederivedusing2MASS.Oncepinkastromhasacatalogofcelestialcoordinatescorrespondingtoobjectsintheeld,thesecoordinatesareconvertedtoroughxandypixelpositionsusingestimatedWCSinformationprovidedintheimageheaderandmatchedtoobjectsinthepinkphotcatalogusingtheIRAFtaskxyxymatch.TheIRAFtaskccmapisthenusedtocalculatearoughplatesolutionusingasecondorderpolynomialttothematchedlist.Thedownloaded2MASScatalogandtheFLAMINGOScatalogaresubsequentlyrematchedwiththeIRAFtaskccxymatchusingthetransformationgivenbytheroughplatesolution.Analfourthorderpolynomicalsolutioniscalculatedfromthere-matchedlists.

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Thecatalogsarematchedathirdtimeusingthenewplatesolution.Atthispoint,thenalnumberofmatchesandtheastrometricresidualsarechecked,andiftheyaresatisfactory(nmatch>50,residuals<0:0025)theimageWCSisupdatedandallpinkphotpixelpositionsareconvertedtoJ2000R.A.andDec.Theoutputfrompinkastromisanewcatalogcontainingbothpixelandcelestialcoordinatesforeachobject,adatabaselecontainingtheplatesolution,andaloglecontainingthepreciseplatescaleinarcsecondsperpixel,imagerotationindegrees,andrmsvaluesoftheastrometryinarcsecondsforallimagesintheinputlist.AsindicatedbyFigure 2 ,typicalrmsvaluesforthenalpositionsare.0:0025. Figure2. Thedeviations(D)inarcsecondsofpinkastromcoordinatesfrom2MASSpositionsfor863objectsintheimageshowninFigure 2 .BoththemeandeviationsandthermsvaluesforR.A.andDec.arelessthan0:0025.

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Thetaskbeginsbyassessingwhetheramatched2MASS/FLAMINGOScatalogexists.Ifnot,sourcematchingisaccomplishedusingccxymatchinthesamemannerdescribedabove.Thematchedcatalogisthenpassedthroughalteringroutinewhichremovesstarsoutsidetheacceptablemagnituderangeforzeropointcalculation.StarsusedinthezeropointcalculationmustbefainterthantheFLAMINGOSsaturationlimit,conservativelyestimatedatJ=H=K=11.0,andbrighterthanJ=15.5,H=15.0,andK=14.0.Thefaintlimitsweresettobeslightlybrighterthanthe2MASScompletenesslimits(J'15:8,H'15:1,andK'14:3, Carpenter 2000 ).Onceanappropriatesamplehasbeenidentied,thedifferencesbetweentheFLAMINGOSand2MASSmagnitudesarecalculatedforeachobjectinthelist.massmatchthencreatesahistogramofthesedifferencesandtakesthecentroid.BecausetheFLAMINGOSphotometrywasinitiallyderivedusingazeropointsetto0.0,thecentroidofthisdifferencehistogramisthephotometriczeropoint(Figure 2 ). Figure2. Sampledifferencehistograms(D=M2MASS-MFLMN)usedbymassmatchtodeterminethezeropoint.ThesedataarefromtheJHKimagesetoforib-07,takenon2001Dec18.Thedashedlineineachhistogramisthelocationofthecentroid,thecorrespondingzeropointsarelabelledatopeachplot. Aftercalculatingthezeropoint,massmatchusesthisvaluetocorrectthephotom-etry,updatingthepinkastromcatalogsothatitnowcontainscalibratedphotometry.Inaddition,massmatchalsogeneratesanumberofdataassessmenttools.Theseinclude:a

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calibratedluminosityfunction,magnitudecomparisonles,andmagnitudescatterdata.Thesetoolswillbediscussedfurtherinthequalityassessmentsectionbelow(x ). 2 ,presentingdatatakenin2001December)whilethestrongesteffectsareseenindatafromthe2004-2005observingseason(e.g.Figure 2 ,presentingdatatakenin2004November).

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AperturephotometrycannotaccountforvariationsinstellarPSFsacrossanimage.PSFphotometryasderivedfrompinkphotdoesattempttocompensateforsomevariationinthePSF,howeverthemodelusedisasmoothfunctionwhichcannothandletheamountofPSFdegradationpresentintheFLAMINGOSimagesaffectedbythedeteriorationofthelenscoating.Inparticular,sourcesinthesoutheastcornerofrecent(2004-present)FLAMINGOSimageshavewide,coma-shapedhaloscontainingasignicantportionofthestellaruxes(Figure 2 ). Figure2. (a)SoutheastcornerofthereducedK-bandimageofcontroleld6(orib-cf6-k1)takeninNovemberof2004.(b)Contourplotoftwoofthestellarproles.ThePSFsareclearlyelongatedandexhibitstrongcomashapedhaloswhichincreaseinsizetowardstheedgeoftheimage. ThemodelPSFsusedinthepinkphotttingroutinescannotaccountfortheuxinthesehalos.Consequently,thettedmagnitudesofobjectsintheaffectedregionsaresystematicallytoofaintcomparedtotheir2MASScounterparts,withtheseverityofthiseffectdependentonradialdistancefromtheopticalcenter(Figure 2 a).Inaddition,ifenoughsourcesareaffectedthisproblemhastheadditionalconse-quenceofskewingthecentralzeropointfortheimage.Inotherwords,iftheobservedmagnitudesaretoofaint,thevalueofD(2MASS-FLAMINGOSmagnitude)willbe

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Figure2. (a)Radialdependenceofthephotometricdeviationfrom2MASSfororib-cf6-k1dataafterarstpasszeropointhasbeenapplied.(b)SkewedzeropointcausedbytheopticaldistortionsshowninFigure 2 .Thedashedlinein(b)isthecentroidedzeropointcalculatedbymassmatch.Thedottedlineinboth(a)and(b)isthecorrectzeropointfortheimage. smallerthanitshouldbeandthecentroidedzeropointwillinturnbetoobrightsincetheoffsetdistributionisnowasymmetricwithabrighttail(Figure 2 b).TheseeffectswouldappeartohavecatastrophicimplicationsforpipelinedFLAMINGOSphotometry,however,asitturnsout,theyarecorrectable.AndreaStoltehasdevisedaroutinewhichbothadjuststheskewedzeropointandcorrectsthemagnitudesaffectedbythelostux.Theroutineoperatesbyttinga6thorderLegendrepolynomicaltothedeviationbetweencalibratedFLAMINGOSand2MASSmagnitudesasafunctionofpositiononthedetector(Figure 2 a).Basedonthet,wethenapplythispolynomicalcorrectiononebandatatimetothezeropointtoobtainnal,correctcalibratedphotometry(Figure 2 b).Thedetailsofcalculatingandapplyingthetcanbefoundin Roman-Zuniga ( 2006 ).Thepolynomialzeropointcorrectionwasappliedtoallimagingdata.Theresultsofthiscorrectionincludeareductionintheoverallphotometricscatter(seex foradiscussiononhowthisvalueiscalculated)andassurancethatthenalphotometryisin

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Figure2. (a)UncorrectedK-bandphotometryforOriBcontroleld6shownwiththe6thorderpolynomialttothedata.(b)CorrectedphotometryforthesameimageshownwiththenewmedianDKandthestandarddeviationofthisvalue. agreementwith2MASStowithin0.1magnitudesforatypicalimage.NotethatsincethePSFdistortioneffectsworsenedwithtime,theamountofimprovementinthephotometryisalsotimedependent:atypicalimprovementinphotometricscatterwas0.01-0.02magnitudesfordatatakenpriortofall2003and0.03-0.04magnitudesforeldsobservedNovember2003-present.Fortunately,themajorityoftheOrionBsurveydataweretakenduringthe2001-2002observingseason,onlyrequiringasmallcorrection. 2.3.5.1AccuracyofthePhotometryOncethezeropointcorrectionwasapplied,photometricqualitywasassessedusingthedifferencebetweenthenalFLAMINGOSphotometryandthe2MASSdatabase.The1srmsvaluesofthedeviationfrom2MASSwerecalculatedin0.5magbinsandplottedasafunctionoftheFLAMINGOSmagnitudeineachband(Figure 2 ).TheoverallphotometricscatterforeachimagewasderivedbytakingthemeanofallrmsvaluesinthemagnituderangefromtheFLAMINGOSsaturationmagnitudetotheconservative2MASScompletenessestimatesstatedabove.ThenalscattervaluesforeacheldcanbefoundinTable A

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Figure2. Samplescatterplotsfordatatakenduringthe2001-2002observingseason(orib-08,observed2001Dec20)afterapplicationofthezeropointcorrec-tion.Theleft-handpanelsplotthedeviationbetween2MASSandFLAMIN-GOSmagnitudesasafunctionofFLAMINGOSmagnitude.Thedottedlineindicatesthefaintlimitforttingthepolynomialzeropointcorrection.Theright-handpanelsshowthecalculatedrmsvaluesasafunctionofmagnitude.Dottedlinesareplottedatlimitsof0.05and0.10magnitudes.Thedashedlineisthemeanofthermsvaluesdowntothetlimit(thenumericalvalueisindicatedatthetopoftheplot).

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ThemedianscattervaluesfortheentireOrionBsurvey(excludingextremelynebulousframes)aresJ=0.05,sH=0.04,andsK=0.05.Consequently,inregionswithlittletononebularemission,weestimatethebulkofourphotometryisaccuratetowithin0.05magnitudes.Forimageswithlargeamountsofnebula(e.g.orib-01,orib-14,orib-34,andorib-37whicharecenteredonNGC2024,NGC2023,NGC2068,andNGC2071respectively)thescatterwithrespectto2MASSismuchlarger(&0.1magnitudes)thanthatexpectedfrompurephotometricnoise.Asimilareffecthasbeennotedbyotherauthorsstudyingyoungclusterswithsignicantnebularemission(e.g. Muenchetal. 2003 inIC348and Muenchetal. 2002 and Slesnicketal. 2004 intheTrapezium)andisusuallyattributedtothelargesizeofthe2MASSpixels(2.000),intrinsicvariabilityofyoungobjects,andvariationsinaperturesizecoupledwiththestrongnebularbackground.Finally,itshouldalsobenotedthatfordatatakenin2001-2002thescatterwithrespectto2MASSisalsolarger(0.1mag)forobjectsontheedgeofdetectorwherethedatarapidlydegradeduetoadelaminationoftheengineeringarray. Newberry ( 1991 ).Ifwewishtondthe10sdetectionlimit,wesimplyneedtolocatethemagnitudeatwhichtheerrorsagreewiththevalueofslimataSNRof10:s10'0:109.Figure 2 showsthemedianphotometricerror(estimatedin0.2mag-nitudebins)asafunctionofmagnitudeineachbandfortheentireFLAMINGOS/Orion

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Figure2. ErrorplotsconstructingusingdatafromtheentireFLAMINGOS/OrionBimagingsurvey.Ineachband,themedianerrors(calculatedin0.2magni-tudebins)areplottedasafunctionofmagnitude(solidline).Errorbarsarethermsineachbin.Thedottedlinesareplottedatducialvaluesof0.05and0.10magnitudes.Thehorizontaldot-dashedlinesindicatethelimitingerrorfor10smagnitudesandtheverticaldot-dashedlinesshowthelocationofthe10slimitsforthissurvey.

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Bimagingsurvey.The10sdetectionlimits(wheretheerrorfunctioncrossesthelimitingvalue)indicatedbythegureareJ=18.9,H=17.9,andK=17.6. 2 )andwritesthepeakofthisfunctiontotheimageheaders.TheLFpeaksforallsurveyimagescanbefoundinTable A .Iestimatethe90%completenesslimitsofthesurveytobethemedianofthesevaluesforeachband:Jlim=18.5,Hlim=17.75,andKlim=17.5,whicharecertainlybelowthespectroscopiclimits(x ).

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Figure2.

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1 ,infraredphotometryaloneisnotsufcienttoestimatemassesforindividualobjects.Rather,bycombiningphotometrywithNIRspectrawecanderiveeffectivetemperaturesandstellarluminosities,ultimatelyusingtheoreticalPMSevolutionarymodelstoidentifyyoungbrowndwarfs.InthissectionIdiscussthedesignofthespectroscopicobservingprogram,thedetailsofthespectroscopicobservations,andourdatareductiontechniques. 2 showstheareasoriginallychosen(indicatedbyboxesrepresentingthe100100eldofviewofFLAMINGOSonthe4m)andtheirrelationshiptothedensegas.Centercoordinatesforeacheldaswellasthecorresponding2.1mimagingsurveyregionandanyindicationsofknownstarformationactivityarelistedinTable 2

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Figure2. RegionsoftheOrionBcloudtargetedforspectroscopicobservationsshownwiththeCScontoursof Ladaetal. ( 1991a ).(RecallthatFigure 2 showstheoutlineofthisCSmapinreferencetotheFLAMINGOSimagingsurveyandtheCOmapof Maddalenaetal. ( 1986 ).)Boxesare4mFLAMINGOSelds(10'10')andcrossesindicatethe5sCSclumpsidentiedby Ladaetal. ( 1991a ).Notethatthespectroscopicsurveypredominantlytargetstheregionsofdensegasafliatedwithknownstarformation,however,afewregionsdevoidofdensegasarealsotargeted.

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Table2. RegionsTargetedforSpectroscopicObservation Ladaetal. ( 1991a ).

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thisstudywereplacedonmasksdesignedforobservationontheKPNO4mtelescope,makinguseoftheincreasedsensitivity(1hourlimitingmagnitude,K15.0)tomaximizethenumberofbrowndwarfspectraobtained.However,selectedbrightsourcesweretargetedforobservationonthe2.1mtelescope(1hourlimitingmagnitude,K13.0). 2 ,left).Duringthe2003-2004observingseasonprioritywasgiventoinfraredexcess(IRX)sources(acommonindicatorofyouth,e.g.Chapter 4 ,x )havingmagnitudesbrighterthanK=16.5.Inthiscase,IRXtargetswereidentiedusingJHvsHKcolor-colordiagrams(Figure 2 ,right).Ineithercase,onceatargetlistwascompiledwiththeavailableIRXsourcesorbrowndwarfcandidates,asecondarylistwascreatedcontainingallavailableobjectshavingKmagnitudes12.0
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Figure2. SampleselectiondiagramsforspectroscopictargetsinNGC2071:a)Color-magnitudediagram(CMD)forallsourcesdowntoK<16.5.Theleftmostdottedlineisthemainsequencefrom Bessell&Brett ( 1988 ).Theshorterdottedlineisanabbreviated1Myrisochroneof D'Antona&Mazzitelli ( 1997 ).Thedot-dashedlinesarereddeningvectorsusingtheextinctionlawof Cohenetal. ( 1981 )placedat0.08and0.02M.Selectedbrowndwarfcandidatesareshownassolidreddots.b)Color-colordiagram(CC)forallsourcesdowntoK<16.5.Thesolidlinesarethegiantcolorsof Bessell&Brett ( 1988 )coupledwithacombinationof Bessell&Brett ( 1988 )dwarfcolorsforspectraltypesdowntoK7and Leggett ( 1992 ), Leggettetal. ( 1996 ),and Dahnetal. ( 2002 )forspectraltypesfromM0toM6(RefertoCh. 4.4.1 foranexplanationofdwarfcolorchoice).Thedot-dashedlinesarethereddeningvectorsof Cohenetal. ( 1981 )andthedashedlineistheclassicalTTaurilocusof Meyeretal. ( 1997 ).SelectedIRXsourcesareshowninred. sinceafewoftheobservedobjectsmaybeextremelyyoungMstarsandstatisticsofthebrightersourceswillaidinassessingouroverallcompletenessintheregion.All2.1mtargetswereselectedsolelyonthebasisofamagnitudelimit.Sourcesselectedduringthe2003-2004observingseasonhadmagnitudesK<13.0andsourcesselectedduringthe2004-2005observingseasonhadH<14.0.

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catalogandanassociatedimageaswellasauserspeciedeldcenter(theMOSeldofviewonFLAMINGOSis1/3ofthefullimagingeld)andplacesasmanyslitsaspossibleintheMOSelddowntoaspeciedmagnitudelimit.Itisuptotheusertoad-justtheselectionstoensurethatthedesiredsourcesaretargetedandalsotokeeptrackofsourceswhichhavebeenplacedonpreviousmasks.Inaddition,theusermustalsoselectatleastthreebrightstarsdistributedacrosstheeldtobeusedassetupstars.Thesestarsarerequiredtoalignthemaskatthetelescope-withoutthesetupstars,itisimpossibletocenterthesciencetargetsintheirslits.Finally,itisimportanttonotethatforMaskdesigntorunproperly,theimageusedtodesignthemasksmusthavetheplatescaleandeldofviewcorrespondingtotheFLAMINGOSeldatthetelescopewheretheMOSobserva-tionswillbetaken.The2.1msurveyimagesandcatalogsthusrequiredresamplingandtrimmingpriortotheiruseinmaskdesign-thiswasaccomplishedusingtheIRAFtaskgeotran.Inaddition,inafewcases(particularlyforthe2001-2002observingseason)selecteldswerepre-imagedwithFLAMINGOSatthe4m.Theseimageswerealsorunthroughthepipelinesandtheoutputcouldthenbedirectlyusedformaskdesign.AllslitmasksfortheOrionBspectroscopicsurveyweredesignedasfollows:rstamaximumnumberofthepriorityobjectsdiscussedabovewereplacedonthemask.Oncethiswascompleted,anyavailablemagnitude-limitedllerobjectswereaddedtomaximizethetotalyieldfromthemulti-objectobservations.Inallcases,thetotalnumberofobjectstargetedoneachmaskwaslimitedbythespatialdistributionofsourcesintheregionsbeingtargeted.Typicallywewereabletoplace20-30objectspermaskforeldstargetingthedensestportionsoftheclustersand10-20objectspermaskinthenon-clusteredregions.Figure 2 showstheregionleoutputbyMaskdesignindicatingthelocationoftheslitsandacquisitionstarsforasamplemask(n2071a2)targetingtheeastern(lowstellardensity)regionofNGC2071.

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Figure2. SlitmasksdesignedfortheFLAMINGOSGMCsurveywerefabricatedusinganon-sitelaserintheUniversityofFlorida'sInfraredInstrumentationlabandeithersentortakenbytheobserverstothetelescopes.

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bothtelescopes,however,therewereafewdifferences,thusIhavechosentodiscussthemseparately.AcompleteobservinglogforthemasksobservedonbothtelescopescanbefoundinTable B inAppendix B B as4mmaskswereobservedontheKPNO4mtelescopeduringfourconsecutivewinterobservingseasonsfromJanuary2003throughDecember2005usingthefollowingprocedure:First,theeldinquestionwasimagedusingashortexposuretime(typically10-30s)intheKorKslter.Iftheresultantimagewassuitablycenteredwiththesetupstarsvisible,aguidestarwasacquired,andtheMOSmaskwasputintoplace.Atthispointtheeldwasre-imaged(withtheslitmaskin)tocheckmaskalignment.Aperfectlyalignedeldwillhavestarscenteredinallofthealignmentboxesandsomeoftheslits(Figure 2 ).Ontheotherhand,ifthealignmentstarswerenotcenteredintheirboxes,theIRAFtaskxboxwasusedtodeterminesmallcorrectionaloffsetstothetelescopeposition.Theseoffsetswereapplied,theeldwasreimaged,andthisprocesswasrepeateduntilacceptablemaskalignmentwasachieved.AllspectraweretakenusingtheJHlter(0.9-1.8m)coupledwiththeJHgrism,providingcompletespectralcoverageinboththeJandHbandssimultaneously.Theslitwidthinallcaseswas3pixelsor0:0095onthe4m,yieldingaspectralresolution,R1300.Foreachmask,300sor600sexposuresweretakeninsetsoffour,ditheringbetweentwopositionsonthechipinastandardABBApatterntoallowforbackgroundskysubtraction.TheseparationbetweenpositionsAandBwas400alongthelongdimensionoftheslitlets.Totalexposuretimesforeachmasktypicallyrangedfrom40-80minutesandaredetailedinTable B .Oncethescienceexposureswerecompleted,internalquartzlampateldsandHeNeArarclampspectraweretakenforcalibrationpurposes.Shortexposure,longslitspectraofanearbyGdwarfatsimilarairmasswerealsotakenimmediatelyfollowingthescienceobservationstocorrectfortelluricabsorption.

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Figure2. ImageofaproperlyalignedMOSplate.Theplateshownisn2071a2(c.f.Figure 2 )withNorthupandEastleft.Notethatallofthesetupboxescontainreasonablycenteredstars.

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SpectraofthebrightertargetswereobtainedwithFLAMINGOSontheKPNO2.1mtelescopeduringthe2003-2004and2004-2005winterobservingseasons.Aswiththe4mspectra,allsourceswereobservedusingthecombinationoftheJHlterandtheJHgrism.Individual300sexposuresweretakeninsetsoffour,yieldingtypicaltotalintegrationtimesof1hour(seeTable B forpreciseexposuretimespermask).Allslitswereagain3pixelswide,whichonthe2.1mcorrespondstoawidthof1:0082andresultsinaspectralresolutionofR1300.Flateldsweretakenusinganilluminatedwhitescreenmountedontheinsideofthetelescopedome.Duetothelackofinternalarclampsatthe2.1m,nodesignatedwavelengthcalibrationframeswereobtained-wavelengthcalibrationwouldbedeterminedusingtheatmosphericOHemissionlinesintrinsictoallNIRspectra.Finally,aswiththe4mdataacquisition,anearbyGstarwasobservedtocorrectfortelluricabsorption. 2 ).

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Figure2. Reduced(butnotextracted)spectraforn2071a2(left)andthetelluricstan-dardHD23050(right)bothobservedon2004Nov26.Thenegativeregionsarebyproductsofthepairwisesubtraction Oncebasicreductionswerecomplete,thetwo-dimensionalimageswereconvertedtoone-dimensionalspectraasfollows:Thelongslit(e.g.standardstar)spectrawereextractedimmediately,usingtheIRAFtaskapalltoidentifyandtracetheapertureandextracttheux.Thesameaperturetracewasthenemployedtoextractarclampspectra(4mdata)orbackgroundskyspectra(2.1mdata)forwavelengthcalibration.Thepositionsofknownlinesinthesespectrawereidentiedandusedtoderivethedispersioncorrection(toconvertpixelnumberstowavelengths).ThecorrectionwassubsequentlyappliedusingtheIRAFtaskdispcor.Extractionofthemulti-slitdatawasaccomplishedbyrstcuttingatwo-dimensionalimageofeachslitletfromthenalcombinedimagedescribedabove(Figure 2 ). Figure2. Two-dimensionalimageofasinglereducedMOSslitletpriortoextractionoftheone-dimensionalspectrum.Theimageshownisn2071a2 06-thesixthslitletonn2071a2.

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Eachspectrumwasthentracedandextractedseparatelyusingapall.Thedispersionsolutionsforthemulti-slitdatawerederivedusingbackgroundOHemissionlineslocaltoeachslitlet.Oncethewavelengthcalibrationwasapplied,targetspectraweredividedbythetelluricstandardtocorrectforatmosphericabsorption.Featuresintroducedbythisdivisionwereremovedbymultiplyingtheresultantspectrabythesolarspectrum,therebyyieldingthenalproductreadyforspectralclassication(Figure 2 ). Figure2. Thenalreducedandcalibratedspectrumforn2071a2 06. B ,onlyelds2(NGC2071),4(NGC2068),6(densecore),8(densecore),9(densecore),10(NGC2024),and13(nodensegas)wereobserved.Inpartthiswasachoicemadeaftertherstobservingseasontoincreasetheoverallnumberofsourcesobserved(andhencethestatisticalsignicance)intheclusterregions.Unfortunately,intheend,thelimitedoff-clusterdatatakenprovedtobe

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insufcientforanalysis,largelyduetolowsignal-to-noisevalues(SNR<20ascomparedtoSNR30-100fortheclusterspectra)reducingtheoverallnumberofextractableand/orclassiablesourcesintheseregions.ReasonsforthesepoorSNRvaluesincludeweather,largeextinctioninthecores,andpoorplatealignment.Consequently,intheremainingchapters,IfocussolelyontheimagingandspectraofthethreeOrionBclusters:NGC2024,NGC2068,andNGC2071.

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Cushingetal. 2005 ; McLeanetal. 2003 ; Reidetal. 2001 ; Leggettetal. 2001 1996 ; Jonesetal. 1994 ).Themostprominentfeaturesinlow-moderateresolutionNIRspectraoftheselate-typestarsarethenarrowatomiclinesofAlI,NaI,KI,andMgIandbroadabsorptionbandsduetosteam(H20),ironhydride(FeH),titaniumoxide(TiO),andcarbondioxide(CO).Thestrengthofthesefeaturesisstronglydependentonspectraltype(e.g.themolecularabsorptioninFigure 3 .ThewaterabsorptionbandsareidealforclassifyingMdwarfs(e.g. Wilkingetal. 1999 ; Reidetal. 2001 ; Jonesetal. 2002 ; Slesnicketal. 2004 )astheyproduceaverydistinctcontinummshapewhichbecomesevenmorepronouncedforlaterspectraltypes,evenatverylowspectralresolutions(Figures 3 and 3 ).However,steamabsorptionissignicantlystrongerintheNIRspectraofyoungobjectsthaninelddwarfsofthesameopticalspectraltype(e.g.x and Lucasetal. 2001 ; McGovernetal. 2004 ).Consequently,ifweuseelddwarfstandardstotypeouryoungclustermembers,thederivedspectraltypeswillbesystematicallytoolate.Rather,wemustuseopticallyclassiedyoungobjectstomakeanaccuratecomparison. 47

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Figure3. Asampleoflowresolutionnear-infraredspectra(R600)forlate-typeelddwarfsadaptedfrom Leggettetal. ( 2001 ).Notetheincreaseinwaterabsorptionforlaterspectraltypes. spectralstandards,weturnedtothenearbyPerseusmolecularcloudandtheyoungclusterIC348.IC348isapartiallyembeddedyoungclusterwhichhasbeenextremelywellstudiedbymultipleauthors(e.g. Luhmanetal. 2003b ; Muenchetal. 2003 andrefererencestherin).Therelativelylowextinctionintheregionhasallowedforanumberofknownmemberswithestablishedopticalclassications.Further,theyouthofthecluster(meanage2Myr, Herbig 1998 )impliesthatspectraltemplatestarstakenfromIC348membershiplistswillbesimilarinageandsurfacegravitytoourOrionBprogramobjects(seex forfurtherdiscussionofsurfacegravityeffects).Table 3 liststheidentications,positions,andopticalspectraltypesoftheIC348standardsastakenfrom Luhmanetal. ( 2003b ).

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Table3. YoungSpectralStandards IDR.A.Dec.OpticalSpectralType I348-05203:44:43.53+32:07:43.0M1I348-12203:44:33.22+32:15:29.1M2.25I348-20703:44:30.30+32:07:42.6M3.5I348-09503:44:21.91+32:12:11.6M4I348-26603:44:18.26+32:07:32.5M4.75I348-23003:44:35.52+32:08:04.5M5.25I348-29803:44:38.88+32:06:36.4M6I348-32903:44:15.58+32:09:21.9M7.5I348-40503:44:21.15+32:06:16.6M8I348-60303:44:33.42+32:10:31.4M8.5KPNO-Tau404:27:28.01+26:12:05.3M9.5 Note.SpectraltypesfortheIC348objectsarethespectraltypesadoptedby Luhmanetal. ( 2003b )andfoundintable2ofthatwork.ThespectraltypeforKPNO-Tau4wasdeterminedby Bricenoetal. ( 2002 ). FLAMINGOSspectraofthesestandardswereobtainedonthenightsof2004October04and05ontheKPNO4mtelescopeandreducedaccordingtotheproceduresdescribedinChapter2aspartofaparallelsurveytoclassifynewmembersofIC348 Luhmanetal. ( 2005a ,hereafterL05).Inaddition,wealsoobtainedaspectrumoftheyoungTaurusmemberKPNO-Tau4toprovideaverylatetypetemplate(M9.5, Bricenoetal. 2002 ).TheentireFLAMINGOSMstarstandardsequencecanbeseeninFigure 3 4 ).Consequently,theresultantspectrawillbehighlyreddened,alteringboththecontinuumslopeandthedepthofanyabsorptionfeatures.ThisisaproblemfortraditionalMstarclassicationschemeswhichemployspectralindicesbasedon

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Figure3. FLAMINGOSMstarspectralsequencecompiledusingyoung(t1-2Myr)membersofIC348andTaurus( Luhmanetal. 2003b ; Bricenoetal. 2002 ).Prominentfeaturesintheselowresolutionspectraareindicated.NotethatobjectswithspectraltypesearlierthanM6aredisplayedatR=500andob-jectslaterthanM6areshownwithR=200forthereasonsdiscussedbelow(x ).Thecentralregionsofthespectraareblockedoutfordisplaypurposesbecauseinmostcasesthesignaltonoiseintheseregionsisverylowduetotheoverwhelmingtelluricabsorption. continuumratiostodeterminespectraltype.Inthesecases,theextinctiontowardsanobjectisusuallyderivedphotometricallyandthespectrumisthendereddenedbythatamount.Thespectralindexissubsequentlydeterminedfromthedereddenedspectrum,implyingthatthenalclassicationisdependentonthecalculatedextinction(seeFigure 3 ).Thisdependenceofspectralclassicationonknowingtheactualexctinctiontoanobjectcanbedangerous.AswewillshowinChapter 4 ,photometricallyderivedAVval-uescansometimesbeoffbyasmuchas3-4magnitudes,whichwillmostcertainlyalterthederivedspectraltype.Forexample, Slesnicketal. ( 2004 )foundthatanuncorrectedspectrumwithAV=5magnitudeswasenoughtocauseonetomis-classifyanobjectby2spectraltypesusingaspectralindexbasedonthedepthoftheJ-bandwaterfeature.

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Figure3. ReddenedanddereddenedFLAMINGOSspectraofanM6.25objectinNGC2068.TheJ-bandH2Oabsorptionfeatureandnearbycontinuumpeakareindicatedonbothspectra.Itisclearthatthecontinuum/lineratioforthisabsorptionfeaturewilldependontheappliedreddeningcorrection. Followingtheworkof L05 ,wehaveelectedtouseavisualmethodtoclassifyourspectra.Generally,thismethodinvolvesapseudo-dereddeningprocedurewhichdereddensthespectrabyarbitraryamountsuntilallobjectshaveuniformcontinuumslopes.ThesedereddenedspectraarethencomparedtothespectraoftheIC348/TaurusstandardsshowninFigure 3 .Themajoradvantagetothismethodisthatitislargelyindependentofthetrueline-of-sightextinctiontowardsanobjectandtheeffectnotedby Slesnicketal. ( 2004 )anddiscussedinthecaptiontoFigure 3 doesnotapply.Whetherornotthereddeningvaluedeterminedvisuallyisaccurate,becauseallspectraandstandardshavethesameJ-banduxvalues(seebelow)wecanusetheRELATIVEdepthsofthewaterfeaturetoclassifyobjectswithoutworryingabouttheaccuracyofphotometricallydeterminedreddeningvalues.Putanotherway,thevisualmethodofclassicationweusehereisarobustwaytodeterminespectraltypeswithoutrequiringatrueextinctioncorrection.Thedetailsofthisvisualmethodareasfollows: 1. Allreducedspectra(includingthestandards)aresmoothedtoaresolutionR500andnormalizedsuchthatthepeakuxintheH-band,locatedat1.68mhasa

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valueof1.0.Notethatbecauseweareusingavisualclassicationprocessbasedonbroadabsorptionproleswearenotlosinganycriticalinformationbyloweringtheresolutionrather,theincreaseinS/Nmakesthecontinuumshapeeasiertodiscern,facillitatingspectralclassication. 2. Theslopesofthestandardspectraareadjusted(ifnecessary)usingtheIRAFdereddentaskuntiltheyhaveuniformJ-banduxvalues(ux'1.21)at1.32m(cf.Figure 3 ).Thepointsat1.32and1.68mwerechosenbecausetheyrepresenttheregionsleastaffectedbythestellarabsorptionfeaturesandarethereforeclosesttothetruecontinuumlevels. Figure3. Exampleofthevisualdereddeningprocess.Objectsaredereddenedandover-plottedwiththestandardspectrauntiltheiruxvaluesat1.32mmatchesthestandardtemplateux,irrespectiveofspectraltype. 3. TheIRAFdereddentaskisthenusedtovisuallyreddenordereddentheprogramspectrauntiltheuxvalueat1.32mmatchestheuxofthestandardtemplates(Figure 3 ).Notethatalthoughanominalvalueofextinctionisgeneratedviathisprocess,itisnotnecessarilyreectiveofthetrueextinctiontowardprogramobjectsasthisdereddeningissimplyanariticialmechanismtoensurethatallobjectshavearelativelyuniformappearance.

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Figure3. Exampleofthevisualclassicationprocess.Inthiscase,theJ-bandfall-offofthebluespectrum(M5.25)isshallowerthantheOrionobjectwhilethesamewaterabsorptionfeatureintheredspectrum(M7.5)istoodeep.ThesameeffectcanbeseenintheH-bandintheregionfrom1.5-1.7m.Conse-quently,thetruespectraltypeofthisobjectmustbesomewhereinbetween.(Intheend,afterplottingthisobjectwiththeM6standardandagainstsimi-larlyclassiedOrionspectra,weestimateitsspectraltypetobeM6.250.75subclasses.) 4. Aspectraltypeforeachobjectisdeterminedbyvisuallycomparingtheprogramspectratotheindividualstandardsuntilabesttmatchisdetermined.ParticularattentionispaidtotheslopeanddepthoftheJ-bandfall-offat1.35m,thestrongabsorptionfeaturesintheH-bandoneithersideof1.68m,andifpresent,thestrengthoftheMgIline(Figure 3 ).Iftheobjectappearstobelate-type(M6),thespectrumissmoothedtoR200tofurtherincreasetheS/N,makingtheexactspectraltypemoreobvious.Finally,onceallobjectshavebeenassignedarst-passspectraltype,classicationsarene-tunedbyplacingthespectrainorderoftheirMsubclassesandadjustingthesequencetoensurethatthestrengthofthewaterabsorptionmonotonicallyincreaseswithspectraltype.Weestimatethismethodofspectralclassicationtobequiterobust,withtypicalerrorsinspectraltypeof0.5-1subclasses.

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Table3. SurfaceGravityStandards Gorlovaetal. 2003 ; McGovernetal. 2004 ).Inparticular,certainnarrowabsorptionfeaturesaswellasthebroadwaterabsorptionbandsdiscussedaboveareespeciallysensitivetosurfacegravityeffects.Thissensitivityprovidesanaturalmethodfordistinguishingyoungsources(whichhaveintermediatesurfacegravities)fromrelativelyhighsurfacegravityelddwarfsorlowgravitybackgroundgiants.InordertoevaluatethesurfacegravityoftheOrionsourcesandultimatelyassistwithmembershipassessment(seeChapters 4 and 5 ),IhaveassembledasmallselectionofsurfacegravitystandardsobservedwithFLAMINGOS.ThesourcesandobservinglogarelistedinTable 3 andthereducedspectraareshowninFigure 3 .Figure 3 illustratestheprogressionofthestrongestgravitysensitivefeaturesvisibleinourdatathebroadwaterabsorptionfeaturesinboththeJandHbandsandanarrowpotassiumdoubletat1.243/1.252masafunctionofbothsurfacegravityandspectraltype.FortheM3objectsboththeJandH-bandH2Oinducedfall-offsaresteepestfortheyoungstar.ThiseffectbecomesmoredramaticfortheM6andM9objectswheretheelddwarfcontinuumproleshavebroadH-bandplateausversusadistincttriangularshapefortheyoungobjects.(Thesameeffectwasalsonotedby Lucasetal. ( 2001 )and L05 ).LookingattheM6andM9.5objects,theeldstarshave

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Figure3. FLAMINGOSspectraoftheyoungobjectsI348-207(M3.5),I348-298(M6),andKPNO-Tau4(M9.5),shownwithspectraoftheelddwarfsGL388(M3V),GJ1111(M6.5V),LHS2065(M9.5V)andtheMGiantsHD39045(M3III)andHD196610(M6III).ThemostprominentgravitysensitivefeaturesatR500arelabeled.ThespectraofbothgiantsappeartohaveamuchhigherH-bandlinefrequencythantheyoungobjectsorelddwarfs.Inaddition,waterabsorptioncausestheyoungerobjectstohaveamuchmoretriangularH-bandshapewhichcanbeusedtodistinguisheldstarsfromyoungclustermembers. astrongpotassiumdoubletwhichisweakorabsentinthelowergravityatmospheresoftheyoungstarsandgiant.Finally,itisalsoapparentthatthetwogiantshaveaattercontinuumproleandamuchhigherfrequencyofH-bandabsorptionlines.Consultationoftheliterature(e.g.thelowresolutioninfraredspectrallibrariesof Lancon&Rocca-Volmerange ( 1992 ))conrmsthatthisisahallmarkofgiantstarsandislikelycausedbyovertonesofCOandOHaswellasblendedmolecularlinesonlyvisibleatverylowsurfacegravities.ThesetrendsinspectralshapeandlinefrequencywillbeusedinupcomingchapterstoassessthesurfacegravityofeachOrionspectrum.AsinTable 3 ,sourceswillbeassignedadesignationoflow(indicatingalowsurfacegravityyoungobject),high(indicatingaelddwarf),andveryloworgiant(indicatingaverylowgravitygiant).Theresultsofthisgravityassessmentwillthenbeusedtoassistinestablishingthemembershipstatusforeachobject.

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Anthony-Twarog 1982 ).Opticalimagesshowaspectacular,ame-shapednebulawithadarkdustlaneatthecenterobscuringtheheartoftheclusterandmostofthestarsintheregion(Figure 4 ).Thecenteroftheopticalnebulaisabrightradiosourcewhichexhibitsbothradiocontinuumemissionandrecombinationlines,indicatingthepresenceofamassivestar(spectraltypeO9)responsiblefortheionizingradiation(x Kruegeletal. 1982 ; Barnesetal. 1989 ).Duetothelargeamountofdustintheregion,thisionizingsourcewasonlyrecentlyidentiedtobethelateOtoearlyBstarIRS2b( Biketal. 2003 ).ThefullextentoftheclusterassiciatedwiththeHIIregionwasrevealedbytheK-bandimagingsurveyof Ladaetal. ( 1991b )whodetected300sourcesdowntoK<14.0.Multi-wavelengthinfraredphotometricstudiesoftheclustershowthatmajorityofdetectedobjectsexhibitnear-infraredexcessemissionindicativeofhotcircumstellarmaterial( Comeronetal. 1996 ; Haischetal. 2000 ).Theproximity,extremeyouth,and 56

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Figure4. OpticalimageofNGC2024(TheFlameNebula)takenfromthedigitalskysurvey.Northisup,Eastisleft,andtheeldofviewis200200. indicatorsofactivestarformationinNGC2024combinetomakethisanidealregiontostudytheyoung,lowmasspopulation. 2 ,J,H,andK-bandimagesofNGC2024wereobtainedon2001November19usingFLAMINGOSontheKPNO4mtelescope.Thedataweretakenusinga16-pointditherpatternwithindividualexposuretimesof60sforJandHand30sforK,yieldingtotalexposuretimesof16minutesinJandHand8minutesinK.Typicalseeingatallwavelengthswas1:001-1:002FWHMandthe10sdetectionlimitsareJ=19.4,H=18.8,andK=17.8.Theseimageswerereducedandassessedusingthepipelinesandroutinesdescribedinx and 2.3.3.1 .TheJHKphotometryfor400sourceshavingcolorerrors<0.1

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Figure4. Color-magnitude(left)andcolor-color(right)diagramsforallclassiedobjectsinNGC2024.IntheCMD,theleftmostdottedlineisthemainse-quencefrom Bessell&Brett ( 1988 )andtotherightisthe1Myrisochroneof D'Antona&Mazzitelli ( 1997 ).Thedot-dashedlinesarereddeningvec-torsusingtheextinctionlawof Cohenetal. ( 1981 )placedat3,0.08,and0.02M.Inthecolor-colordiagram,thedottedlinesarethegiantcolorsof Bessell&Brett ( 1988 )combinedwith Bessell&Brett ( 1988 )dwarfcolorsforspectraltypesdowntoK7and Leggett ( 1992 ), Leggettetal. ( 1996 ),and Dahnetal. ( 2002 )dwarfcolorsforspectraltypesfromM0toM6.Thedot-dashedlinesarethereddeningvectorsof Cohenetal. ( 1981 )andthedashedlineistheclassicalTTaurilocusof Meyeretal. ( 1997 ).Opencirclesaregeneralphotometriccatalogsources,lledcirclesarespectroscopictargets,andtheredtrianglesrepresentthenalclassiedsample. magnitudesisshowninFigure 4 .Inregionswithlittletononebularemission,thephotometricaccuracy(asindicatedbyacomparisonto2MASS)isestimatedtobe0.03magnitudes.Forregionswithlargeamountsofnebula,asinthecenterofNGC2024,thescatterwithrespectto2MASSwasmuchlarger(0.15magnitudes)thanthatexpectedfrompurelyphotometricnoise.Reasonsforthisdifferencearediscussedinx .Thescatterwithrespectto2MASSwasalsolargerforobjectsontheedgeofdetectorwherethedatarapidlydegradeduetoadelaminationoftheengineeringarray.Forobjectsinthisregion(generallynon-nebular),the2.1mphotometryfromtheimagingsurveywasused.ThesurveyeldcorrespondingtoNGC2024isorib-01.Giventhatthesourcesrequiring

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Table4. SlitMasksObservedinNGC2024 A 4.3.1SampleandObservationsThespectroscopicsampleforNGC2024wasselectedaccordingtotheguidelinesdescribedinChapter 2 .Intheend,148sourcesweretargetedforobservationon6differ-entslitmasks.ThetargetingbreakdowncanbefoundinTable 4 andthephotometryisshownbylledcirclesinFigure 4 .ThespatialdistributionofthesetargetscanbeseeninFigure 4 .FLAMINGOSobservationsofthe4mslitmasksweretakenonthenightsof2003January19,2003December06,2003December10and2004December01.Spectrafromthe2.1mslitmaskwereobtainedwithFLAMINGOSonthenightof2003November29.Thefulldetailsoftheobservingproceduresatbothtelescopescanbefoundinx .ThespecicintegrationtimesbymaskcanbefoundinAppendix B .Allspectroscopic

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Figure4. Three-colorimageofNGC2024takenwithFLAMINGOSontheKPNO4mtelescope.Northisup,Eastistotheleft,andtheeldisapproximately10'onaside.Circledobjectsareall4mspectroscopictargetsandrectanglesenclosethe2.1mtargets. datawerereducedusingtheproceduresdetailedinx andclassiedaccordingtothemethodsdevelopedinChapter 3 .Totalsbymaskofthenumberofsourcestargeted,extracted,andclassiedarelistedinTable 4 4.3.2.1SpectralClassicationThenalclassicationsyielded65uniqueobjectsfromthe4msamplewithidentiableMtypespectra(rangingfromM1to>M8)and2sourceswithspectral

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typesearlierthanM0.Inaddition,4duplicatesourceswerealsoextractedwhichwhenindependentlyclassiedyieldedspectraltypesinagreementwiththeoriginalsourcetowithin0.25subclasses.Ofthe404mextractedobjectswhichwerenotclassed,14werellertargetswithKmagnitudes>15.0whicharetypicallytoofainttoclassifywithourcurrentexposuretimes.TheremainingunclassiedsourceswhilebrightatK,weretypicallyhighlyreddenedobjectswithpoorsignaltonoiseintheJandHbandsafterdereddening.The2.1msampleyielded4newMtypeobjects,6sourceswithspectraltypesearlierthanM0,andoneduplicateclassication(whichagreedwiththe4msourcetowithin0.5subclasses).AllclassiedspectraareshowninFigure 4 alongwithselectedobjectsfromtheFLAMINGOSMstarstandardsequence(x ).Objectswithspectraltypes
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Figure4. NIRspectraofallclassiedMstarsinNGC2024(labeledwithbothspectraltypeandID)shownwiththeIC348opticallyclassiedyoungstandards(labeledwithspectraltypeonly).Prominentspectralfeaturesareidentiedatthetop.Objectshavingspectraltypes
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Figure4. continued

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Figure4. continueds

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intheweakeningorveilingofbothnarrowandbroad-bandspectrallines.Iftheamountofveilingissignicant,itcanaffectspectralclassicationcausinganobjecttoappearearlierthanitstruespectraltype.InthissectionIattempttoquantifytheeffectofveilingonourclassicationprocessbyvisuallyinspectingoursampleandexaminingitsinfraredexcess(IRX)properties.Visualinspectionofthespectrayieldedoneobject(source5,M1.75)withobviouslyweakMgIabsorptionlikelycausedbyveiling.Inaddition,excludingthepossiblegiants(seeabove),27outof67objectsor40%9%ofourclassiedMstarsampleexhibitanIRXasdeterminedviaacomparisonofeachobject'sexpectedintrinsicHKcolor(inferredfromspectraltype)withitsdereddenedobservedHKcolor.(Thereaderisreferredtox foranexplanationofintrinsiccolorchoiceanddereddeningmethods.)WhatfractionofIRXsourcescanbeexpectedtohavesignicantveiling?ToanswerthisquestionIhavecalculatedrk,theK-bandveilingindexforeachsource. 4 ,onlysource38(rk=0.62)exhibitsanamountofveilingnearrk=0.6,themedianvalueforClassicalTTauristars( Meyeretal. 1997 ).Consequently,Inotethepotentialbiastowardsanearlierspectraltypeforthisobject. Meyeretal. 1997 ).

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thenplacethesedataontheHertzprung-Russell(H-R)diagramandusetheoreticalPMSevolutionarymodelstoinfermassesandagesforthesample. 1997 ; Luhmanetal. 2003b ).AsIdonothavereliableopticalphotometryforNGC2024,Ielectedtousethebluestinfraredbandstoderiveextinctionestimatesforeachsource.ExtinctionestimatesweredeterminedbycomparingourobservedJHcolorswiththeempiricallydeterminedintrinsicMdwarfcolorsof Leggett ( 1992 ); Leggettetal. ( 1996 )and Dahnetal. ( 2002 )andthenconvertingthecolorexcesstoanAVmeasurementusingthereddeninglawof Cohenetal. ( 1981 ).Thechoiceofboththeintrinsiccolorsandthereddeninglawwasbasedprimarilyonphotometricsystem.FLAMINGOSlterscloselyapproximatetheCITsystem( Elstonetal. ( 2003 )andtheFLAMINGOSwebpages)thusIoptedforareddeninglawandintrinsiccolorsetderivedinthesamesystem.IoptedagainstusingtheoreticalPMScolorssinceatyoungagesthesearehighlydependentonmodelinputphysics.FortheonesourcelackingJ-bandphotometry,anextinctionestimatewasderivedusingHKcolors.ItshouldbenotedthatforcomparisonIalsoestimatedvisualextinctionsforallsourcesusingHKintrinsiccolors

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andbydereddeningobjectstoamodelisochroneinbothJ=JHandH=HKcolormagnitudediagrams.ThesemethodsyieldedAVvalueswhichdeviatedfromtheJHintrinsiccolorestimatesbyasmuchas1-2magnitudesforJ=JHand3-4magnitudesforH=HK.Effectsofthisdeviationwillbediscussedinxx and 4.5.2 Figure4. DistributionofAVfortheNGC2024spectroscopicsample.AVvalueswerederivedbycomparingobservedJHcolorswiththeintrinsicJHcolorsof Leggett ( 1992 ); Leggettetal. ( 1996 ); Dahnetal. ( 2002 ). Figure 4 showsthedistributionofvisualextinctionsderivedfromJHintrinsiccolors.Valuesrangefrom1-30visualmagnitudeswithameanAVof10.7magnitudes.

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Thisisingoodagreementwiththesurveyof Haischetal. ( 2000 ),whondarangeofAVfromroughly0-30visualmagnitudesandameanAVof10.4. andapplyingadistancemodulusof8.09( Anthony-Twarog 1982 ).Bolometricmagnitudesandluminositieswerederivedusingthebolometriccorrectionsof Leggett ( 1992 ); Leggettetal. ( 1996 )and Dahnetal. ( 2002 )astheywereobservationallydeterminedusingCITphotometry.WhileJ-bandistypicallythepreferredwavelengthforderivingbolometriclumi-nositiesascontaminatingexcesseffectsareminimized( Luhman 1999 ,seealsox ),IhaveelectedtousetheK-bandsinceluminositiesderivedfromKmagnitudesarefarlesssensitivetoerrorsindereddening.Asdiscussedabove,photometricallyderivedextinctionvaluescanhaveerrorsaslargeas3-4magnitudes.AchangeinAVof3magnitudescorrespondstonearlyamagnitudeofuncertaintyindereddenedJ-bandmagnitudesbutyieldsamuchsmallerDK(<0.3mag).AlthoughKmagnitudesaremoresensitivetoexcessemissionfromawarmcircumstellardisk,thiseffectissmallinlog-Luminosityspace(averageDlogL0.06dex).EvenwhencombinedwiththeAVuncertainty,thenetuncertaintyinK-derivedbolometricluminosities(0.17dex)remainssmallerthanthecorrespondinguncertaintyusingJ-bandtoderivebolometricluminosities(0.32dex). 4 showsH-RdiagramsfortheclassiedsourcesinNGC2024alongwiththePMSevolutionarymodelsof D'Antona&Mazzitelli ( 1997 )and Baraffeetal. ( 1998 ).Thetriangularpointsrepresent2.1mclassicationsandsourceswithdiamondswere

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Figure4. H-RDiagramsforNGC2024shownwiththepre-mainsequencemodelsof D'Antona&Mazzitelli ( 1997 )(left)and Baraffeetal. ( 1998 )(right).Thediamondsrepresentpointswith4mspectraandthetrianglesaresourcesclassiedwith2.1mspectra.Asterixesarepotentialbackgroundgiants.Rep-resentativeerrorbarsforanM5objectareshown.Thesolidlineaccountsforerrorsinderivedspectraltype,distancemodulus,andphotometryandthedashedlineincorporatesanadditionalerrorof3magnitudesofvisualextinction(seex ). classiedusing4mspectra.Thetwoasterixesrepresentthepossiblebackgroundgiants(seex ).IndividualobjectdataaretabulatedinTable 4 .TwotypicalerrorbarsforanM5dwarfareshowninthelowerleftcorner.Thesolidlinewasderivedbyclassicallypropagatingthemeasurederrorsinthephotometry,spectraltype(0.75subclasses),anddistancemodulus.Inthiscase,theerrorinluminosityisdominatedbyerrorinthedistancetoNGC2024.Thedashedlineincorporatesanadditionalerrorof3magnitudes(correspondingto0.2dex)inthereddeningestimate(refertox )whichdominatestheerrorbarandleadstoalargeruncertaintyinthecalculatedluminosity.

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Table4. DataforClassiedSourcesinNGC2024 0112.0611.0710.30
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Table4Continued 5216.3014.7313.706.50low9.000.083.461-1.2150.065316.5414.8613.718.25low9.120.103.419-1.2740.035418.0515.3613.754.25low18.860.113.509-0.8030.2455...15.8813.767.00low29.11-0.143.449-0.5310.045616.4014.7713.766.75low9.520.023.455-1.2280.055715.9114.6413.828.50low5.360.013.412-1.4610.025817.2415.3214.007.50low12.910.073.437-1.2280.035916.1314.8614.027.50low7.00-0.023.437-1.4490.036017.8615.6414.194.00giant14.640.243.514-1.1230.246116.4015.0314.212.75low6.660.163.539-1.3730.376216.4415.0214.247.25low7.98-0.123.443-1.4930.046317.5615.6314.374.75low11.980.193.499-1.3160.166417.4415.6014.425.00giant11.230.153.494-1.3710.146516.6815.3114.506.25low7.27-0.013.466-1.5880.076617.1015.5414.617.75low8.85-0.063.431-1.6260.036717.7416.1115.058.00low8.700.053.425-1.8160.036813.74b12.2511.595.00clow8.05-0.143.494-0.3990.156913.8312.4811.756.75clow6.98-0.093.455-0.5160.057013.8812.7212.042.50clow4.680.163.544-0.5680.477115.5413.4312.077.25clow14.250.033.443-0.3990.04 D'Antona&Mazzitelli ( 1997 ,hereafterDM97)and Baraffeetal. ( 1998 ,hereafterBCAH98).Theprimarydifferencesbetweenthesetwomodelsaretheirtreatmentofconvection(mixing-lengththeoryforBCAH98andfullspectrumturbulenceforDM97)andtheassumptionofgreyatmospheresinDM97vs.non-greyinBCAH98.BCAH98andreferencesthereinarguethatthegreyatmosphereapproximationisinappropriateforstarswhoseeffectivetemperaturesfall

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below4500-5000Kasmoleculespresentintheatmosphereswillintroducestrongnon-greyeffects.Thereissomeevidencesupportingthisclaimasboth Whiteetal. ( 1999 )and Luhmanetal. ( 2003b )usedempiricalisochronesdeninedwithlowmassmembersofIC348andTaurusandtheyoungquadrouplesystemGGTAUtoshowthattheBCAH98modelsagreebetterwithobservationalconstraints.Consequently,whileIpresentH-Rdiagramsusingbothsetsoftracks,fortheremainingdiscussionwewillfocusprimarilyonresultsderivedfromtheBCAH98models.MassandageestimateswerederivedfromtheBCAH98modelsbyinterpolatingbetweentheisochronesandmasstracksshowninFigure 4 .Sourcesfallingabovetheyoungestisochrone(1Myr)wereassumedtohaveanage<1Myrandweredroppeddowntothe1Myrisochronealongalineofconstanteffectivetemperaturetoderiveamassestimate.Inthismanner,Iderivedmassesspanningarangefrom0.02to0.72M(with23objectsfallingbelow0.08M)andagesrangingfrom<1to30Myr. 4.5.1ClusterMembershipPriortodrawinganyconclusionsregardingtheageofNGC2024oritssubstellarpopulation,itisnecessarytoevaluatethemembershipstatusofsourcesinoursample.Intheabsenceofpropermotiondata,wemustrelyonotherdiagnosticstodeterminewhetherobjectsarebonadeclustermembersorforegroundorbackgroundsourcesprojectedontheclusterarea.Thediscussionofsurfacegravityeffectsinx rulesoutforegroundorbackgrounddwarfcontaminationinourspectroscopicsampleastherearenopotassiumlinespresentinourspectra.Inaddition,NGC2024isdeeplyembeddedinacoreofdensegas( Ladaetal. 1991a 1997 )whichwillobscurebackgroundeldstars,limitingthenumberofeldcontaminantsinthephotometricsample.Theaveragecolumndensityofhydrogenina0.6pcclumpcenteredonNGC2024hasbeenestimatedfromC18OemissiontobeN(H2)=4.61022cm2( Aoyamaetal. 2001 ).Giventhatamolecularhydrogencolumndensityof1021cm2correspondsto1magnitudeof

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visualextinction( Bohlinetal. 1978 ),backgroundsourcesinthisregionwillbeviewedthrough46magnitudesofvisualextinction,or4.1magnitudesofK-bandextinction.ThespectroscopicsampleincludessourcesdowntoK'15.BackgroundobjectscontaminatingthissampleareseenthroughthecloudandthuswillhaveunreddenedmagnitudesK11.Lookingatthedistributionofsourcesinanoff-cloudFLAMINGOScontroleldaswellasasimilarareafromthe2MASSdatabase,Iestimatethattherearenomorethan5backgroundsourceswithK11.Asthisisarelativelyinsignicantcontributiontothetotalphotometricluminosityfunction,Iconcludethatabackgroundcorrectionisunnecessary.Idonotethepossibilityofgiantcontaminationfortwospectroscopicsources(60and64)whichdisplayenhancedabsorptionintheH-band,thustheseobjectsareexcludedfromfurtheranalysis. 4 donotfallalongasingleisochronebutrathershowascatterinagerangingfrom1Myrto30Myr,irrespectiveofthePMSmodelsused.Thistypeofwidthintheevolutionarysequenceofyoungclustersiscommonandisusuallyattributedtoavarietyofeffectsincluding:realagedifferencesbetweensources,errorsinluminosityderivedfromuncertaintiesinthederivedreddening,photometricuncertainty(theseeffectsarerepresentedbytheerrorbarsinthegure),aswellasdistancevariationsbetweensources,variabilityduetoaccretionandrotationofyoungobjects,andunresolvedbinaries.Themedianageoftheentiresamplehowevershouldberepresentativeofthemedianageoftheclusterpopulationinthemassrangedetectedhere.UsingthemodelsofBCAH98themajorityofsourcesfallabovethe1Myrisochrone.Consequently,themedianageoftheclustercanonlybeconstrainedto<1Myr.However,theDM97modelsextendtoyoungeragesthanthoseofBCAH98.EventhoughIhaveelectedtoplacemoreweightonresultsderivedwiththeBCAH98models(seex ),theDM97modelsprovideuswithadditionalinformationontheage

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oftheclusterpopulationaswellasameanstocompareourresultswithprevioussurveysofNGC2024andotherregionswhereauthorshaveusedtheDM97modelstoderiveanage.UsingtheDM97modelswederiveamedianageof0.5Myr.Ifwefactorinerrorsinthedistancemodulus(8.090.17, Anthony-Twarog 1982 ),thisleadstoapossibleagerangeof0.4-0.6Myr.Includinga3magnitudeshiftinreddening(refertox )yieldsalargerrangeof0.2-0.9Myr.AlloftheseresultsremainconsistentwiththeagederivedfromtheBCAH98models,placingNGC2024at<1Myr.Ourderivedageof0.5MyrforNGC2024isingoodagreementwithagesfoundbyprevioussurveysofNGC2024.Both Meyer ( 1996 ,hereafterM96)and Alietal. ( 1998 )usedinfraredphotometryandspectroscopywiththemodelsof D'Antona&Mazzitelli ( 1997 )toderivemeanagesof0.3and0.5Myrrespectively.Further,M96usedadistancemodulusof8.36magnitudes.Increasingthedistancetotheclusteractstoincreasethederivedbolometricluminosityofsources,makingobjectsappearyounger.Indeed,usingthelargerdistancemoduluswiththemodelsofDM97,Ideriveamedianageof0.3MyrwhichisinexcellentagreementwiththeresultsofM96.AfewsourcesinourH-Rdiagramsappeartohaveageswhichdeviatesignicantlyfromthemedian.UsingeithersetofPMSmodelsthereisasmall,lowerluminositypopulationwithinferredages>3Myr.Inordertoattributetheselowluminositiestogeneralscattercausedbyphotometricerrorsanduncertaintiesintroducedbyvariability(generallynomorethan0.2magatK),derivedreddening(0.3mag,seeabove),anddistancemodulus(0.2mag),theseeffectswouldhavetocombinetoproduceatleasta1-2magnitudeshiftatK.Ontheotherhand,ithasbeennotedbymultipleauthors(e.g. Luhmanetal. 2003b ; Slesnicketal. 2004 ; Wilkingetal. 2004 )thatacircumstellardiskcanacttooccultthecentralsource,resultinginanunderestimateoftheobject'sluminosityandthusanoverestimateoftheobject'sage.Ihaveexaminedtheinfraredexcesspropertiesofthesubsampleinquestionand,irrespectiveofthemodelisochrones

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used,allbutoneoftheobjectswithaninferredage>3Myrhaveexcessux,indicatingthepresenceofcircumstellarmaterial.LookingatFigure 4 ,itwouldappearthatobjectagemaybeslightlymassdependentwiththelessmassivesourcesappearingyounger.Toquantifythistrend,Ihavedividedoursampleintotwopopulations:objectswithmasseslowerthanthemedianmassandobjectswithmasseshigherthanthemedianmass(Mmedian'0.15M).UsingtheBCAH98modelstheredoesnotseemtobeanagedifferencebetweenthelowandhighmasssamplesasbothhavemedianages<1Myr.However,themedianagesindicatedbytheDM97modelsaresomewhatdifferentfromonepopulationtotheother.Thelowmasssamplehasanageof0.3Myrandthehighermasssamplehasanageof0.9Myr.Thereareanumberofpossibleexplanationsforthiseffect.First,thetrendmaybeanartifactarisingfromuncertaintiesintheevolutionarymodelsatveryyoungagesandlowmasses.NotwosetsofPMStrackslookalikeinthebrowndwarfregimethusitisadistinctpossibilitythattheapparentagesegregationiscausedbyaproblemwiththetracks.Ontheotherhand,theobservedmassdependencecouldbeaselectioneffectcausedbytheintrinsicfaintnessoftheoldersubstellarpopulationaccordingtotheBCAH98modelseventhehighestmassbrowndwarfswillbeundetectablebyoursurveybythetimetheyreachagesof2-3Myrandthislimitbecomesyoungerforlowermassobjects.Finally,itisalsopossiblethatthiseffectisreal.Ifso,thismaybeevidenceforsequentialformationasafunctionofmasswherelowermassobjectsformlaterintheevolutionarysequenceofayoungcluster.Unfortunately,ourdataarenotsensitiveenoughtodistinguishbetweenthesepossibilitiesdeeperspectroscopicobservationsareneeded. 4 presentsthespatialdistributionofallsourcesclassiedusingFLAMIN-GOSspectra.OpencirclesareobjectswithM>0.08M,lledtriangleshavemassesM<0.08M,andasterixesarethepossiblegiants.ThestaratthecenteroftheclusterrepresentsIRS2b,thelikelyionizingsourcefortheregion(seebelow).Itcanbeseen

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Figure4. SpatialdistributionofbothstellarandsubstellarobjectsinNGC2024.TheopencirclesrepresentobjectswithmassesM>0.08M,starsareobjectswithM<0.08M,asterixesarepossiblebackgroundgiants,andthelargedotrepresentsIRS2. fromthisgurethatthesubstellarobjectsarenotlocalizedtooneregionbutratherappeartobedistributedsimilarlytothestellarmassobjectsclassiedhere.Itshouldbenotedthatthedearthofclassiedobjects(eitherstellarorsubstellar)inthecenteroftheclusterisaselectioneffectcausedbythehighextinctioninthisregionblockingmuchoftheJandH-bandux.

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,thepresenceofaninfraredexcessiscommonlytakentobeanindicatorofthermalemissionfromacircumstellardisk.Disksaroundbrowndwarfsareofparticularinterestbecausetheirpresenceorabsencehasimplicationsforthelikelihoodofplanetformation(planetsformwithincircumstellardustdisks)andtheformationmechanismofbrowndwarfs(accretiondisksplayanimportantroleinthestarformationprocess).CombiningourspectralclassicationswithHKintrinsiccolorswend40%9%ofsourcesinourtotalsamplehaveanHKcolorexcess.Thismethodofselectingexcesssourceshasbeenshownby Liuetal. ( 2003 )tobemoresensitivetosmallIRexcesses(asopposedtothetraditionalJHKcolor-colordiagrams)andisthuswellsuitedforinvestigatingthediskpropertiesofbrowndwarfs,whichareexpectedtohavesmallerexcessesthantheirstellarcounterparts.Approximatelyonethirdoftheexcesssourcesdetectedusingthecolor-spectraltypeanalysishavemasseswhichplacethembelowthehydrogen-burninglimit(spectraltypesM6).ThisyieldsasubstellarHKexcessfractionforNGC2024of9/23or39%15%,wherequotederrorsarederivedfromPoissonstatistics.Substellarexcessfractionshavebeencompiledforanumberofotherregions.Forexample, Muenchetal. ( 2001 )usedJHKcolor-colordiagramstoexamineasetofphotometricallyselectedbrowndwarfsintheTrapeziumclusterandfoundasubstellarexcessfractionof65%15%.Inafollow-upL0studyofthesameregion, Ladaetal. ( 2004 )ndaKL0excessfractionof5220%fortheirspectroscopicallyselectedbrowndwarfsampleand67%usingaJHKLcolor-coloranalysisforthelargerphotometricsample.Morerecently, Luhmanetal. ( 2005b )usedtheSpitzerSpaceTelescopetoobtainmid-infraredphotometryforlowmassmembersoftheIC348andChamaeleonIclusters,ndingthat42%13%ofbrowndwarfsinIC348and50%17%ofbrowndwarfsinChamaeleonexhibitexcessemission,consistentwithourresultforNGC2024.

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Basedontheaboveresults(placingmoreemphasisonstudieswithspectroscopicinformation),wecanconcludethat40-50%ofbrowndwarfsaresurroundedbycir-cum(sub)stellardisks.Notethoughinmanycasesthequotedsubstellarexcessfractionsaredeemedlowerlimitstothetruesubstellardiskfraction(e.g. Ladaetal. 2004 ; Luh-manetal. 2005b ).ThisisalsotrueforNGC2024.Diskmodelingby Liuetal. ( 2003 )showsthatthemaximumexpectedK-bandexcessforadiskwithnoinnerholeis0.42magnitudesforanM6dwarfand0.31magnitudesforanobjectclassiedasM9.How-ever,theL0observationsof Liuetal. ( 2003 )aremoreconsistentwithdiskshavinganinnerholeRin(23)R.TheK-bandexcessfortheseobjectswouldbeverysmallorundetectableusingtheHKanalysisIpresenthere.ThechoiceofintrinsiccolorsmayalsoleadtoanunderestimateofthesubstellardiskfractioninNGC2024.Ihaveusedanintrisiccolorsetderivedfromobservationsofelddwarfs.Ourtargetsarepre-mainsequenceobjectswhichhavelowersurfacegravitiesthanelddwarfs(e.g.x )andthusbluerHKintrinsiccolorsforagivenspectraltype(refertothelowsurfacegravitygiantsequenceplottedinFigure 4 ascomparedtothedwarfsequenceplottedinthesamegure).TheassumptionoftheredderdwarfcolorswillprecludeobjectswithaK-bandexcesssimilartoorsmallerthanthedifferencebetweenPMSanddwarfcolorsfrombeingcountedasexcesssources.CombiningthiseffectwiththefactthatHKexcessisapoorindicatorofdiskemissionforsubstellarobjects(seeabove),IconcludethetruesubstellardiskfractionforNGC2024maybesignicantlyhigherthan39%.Thisyieldsfurtherweighttotheideathatthemajorityofbrowndwarfsformthroughadiskaccretionprocesssimilartotheirstellarcounterparts. 4 showstheuncorrectedK-bandluminosityfunction(KLF)forthephotometricsamplewiththeKLFofthenalclassiedspectroscopic

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sample.Withoutplacinganylimitsonthedata,itcanbeseenthatourspectroscopic Figure4. surveyistypicallyonly10-20%completeinthemagnituderangefromK=10.5-15.0.Asdiscussedinx ,correctingforbackgroundeldstarswillhavelittleeffect.Rather,muchofourincompletenessiscausedbyhighreddeningwithinthemolecularclouditself(x ).ImposinganextinctionlimitonthedatayieldsahighercompletenessfractionandgivesamorecontrolledsamplefromwhichanIMFcanbeconstructed.Theright-handpanelofFigure 4 showstheK-bandluminosityfunctionsforallsourceshavingAV15inboththephotometriccatalogandthenalsampleofclassiedobjects.Disregardingthebinsoneitherend(astheycontainonlyoneobjecteach),itnowappearsthatthespectroscopicKLFisagoodrepresentationofthetotalphotometricKLFinthemagnituderange11.25
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Icorrectedforthisincompletenessbyaddingsourcestoeachdecientmagnitudebinaccordingtotheobjectmassdistributioninthatbin.Figure 4 showsthespectroscopicallyderivedmassfunctionforNGC2024.ThesolidlineisthemassfunctionforallobjectswithspectraltypesM0,excludingthetwopossiblegiants(x ).ErrorbarsarederivedfromPoissonstatistics.ThedashedlineshowstheIMFforthesamesamplecorrectedfortheincompletemagnitudebinsdowntoK=14.75.Weestimatethatforourextinction-limitedsample,thiscorrespondstoamasscompletenesslimitof0.04M.Themassfunctionrisestoapeakat0.2Mbeforedecliningacrossthestellar/substellarboundary.Thereisanapparentsecondarypeakaround0.03MalthoughtheerrorbarsarealsoconsistentwitharelativelyatIMFinthisregime.TheimplicationsofthismassfunctionwillbediscussedinChapter 6 .ItshouldbenotedthattheexactshapeofthesubstellarIMFissomewhatdependentonthechoiceofbincentersandsizes.Forabinwidthof0.3dex,shiftingthebincentersinincrementsof0.05dexshiftsthelocationofboththeprimaryandsecondarypeaksthrougharangeofmassesfrom0.25-0.1Mand0.03-0.04Mrespectively.Additionally,insomecasesthesecondarypeakdisappearsandthesubstellarIMFbecomesat.Decreasingthebinwidthby30%emphasizesthesecondarypeak,however,theerrorsremainconsistentwithaatIMF.Increasingthebinwidthsby30%eitherpreservesthesecondarypeak,attensthesubstellarmassfunction,orcausesittodeclinethroughoutthebrowndwarfregimedependingonthechoiceofbincenters. Bricenoetal. ( 2002 )denetheratioofthenumbersofstellarandsubstellarobjectsas

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Figure4. MassfunctionforallclassiedobjectsinNGC2024whosespectraindicatethattheyareclustermemberswithspectraltypesM0.ThesolidlineistheuncorrectedrawmassfunctionshownwithPoissonerrorbarsandthedashedlinehasbeencorrectedformagnitudeincompletenessintherangefrom11.25
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Inourcompleteness-corrected,extinction-limitedmassfunctionforNGC2024thereare45objectswithmasses0:02M0:08M.Inaddition,thereare36sourcesinourphotometriccatalogwithKmagnitudesbrighterthanthebrightlimitofourmassfunction(K=11.25,x ).Sincetheyoungest(andthusbrightest)objectclassiedassubstellarhasaKmagnitudeof11.75withthemajorityofbrowndwarfsfallingbelowK=12.75itisreasonabletoinferthatallofthesebrightphotometricsourceshavemassesgreaterthan0.08M.Finally,Iinclude9sourcesfromthe2MASScatalogwithmagnitudesbrighterthantheFLAMINGOSsaturationlimitwhicharealsoexpectedtobefarmoremassivethanthesubstellarlimit.ThisyieldsavalueofRss=45/148or0.300.05assumingPoissonerrors.AcomparisonoftheRssforNGC2024withthatofotherlowmassstarformingregionscanbefoundinChapter 6 D'Antona&Mazzitelli ( 1997 ).Thisvalueisconsistentwithamedianage<1Myrasderivedfromthemodelsof Baraffeetal. ( 1998 ).Estimatedmassesrangefrom0.02Mto0.72Musingthe Baraffeetal. ( 1998 )models,with23ofthe67objectsfallingbelowthestellar/substellarboundary.Thespatialdistributionofsourcesindicatesthatthebrowndwarfsappeartobeevenlydistributedrelativetotheirstellarcounterpartsandthirty

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ninepercentoftheclassiedbrowndwarfsappeartohaveaninfraredexcess,possiblyindicativeofthermalemissionfromawarmdisk.Usinganextinctionlimitedsubsampleofthespectroscopicsources(AV<15.0),IconstructedthelowmassIMFfortheregion.TheIMFforNGC2024peaksat0.2Mandthendeclinesintothebrowndwarfregime.Thereisapossiblesecondarypeakaround0.035M.Finally,theratioofstellartosubstellarobjectsinNGC2024isRss=0.300.05.

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5 )andhavelongbeenknowntobeareasofactivestarformation.EarlyspectroscopicandphotometricstudiesdetectedanumberofHaemissionlinestarsandconrmedthe Figure5. OpticalImageofNGC2068andNGC2071fromtheDigitalSkySurvey.Northisupandeastisleft. 84

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presenceofapopulationofveryyoung(t0.1Myr)stars(e.g. Herbig&Kuhi 1963 ; Strometal. 1975 ).Morerecentworkintheregionincludesthemolecularlinemapsof Maddalenaetal. ( 1986 )inCOand Ladaetal. ( 1991a )and Ladaetal. ( 1997 )inCS,the2.2msurveyof Ladaetal. ( 1991b ),anddustcontinuummappingat850mby Mitchelletal. ( 2001 )and Johnstoneetal. ( 2001 ).However,todate,therehavebeennodetailedstudiesofthelowmassclusterpopulations.Thischapterpresentsanin-depthstudyoftheMstarpopulationofNGC2068andNGC2071,includingnewH-Rdiagrams,ages,andmassfunctionsforbothclusters. 2 ,x )onthenightsof2002January13and2002December31.Alldataweretakenusingasinglepassofthestandard9-pointditherpatterndescribedinx ,withtheexceptionoftheH-bandforNGC2068whichwasobservedtwicetoaccountforbadreads.Theexposuretimeateachpointwas35seconds,yieldingatotalexposureof8minutesfortheN2068H-bandditherset;theotherimagesetsallhadtotalsof5minutesonsource.TheseeingforNGC2068was1:007-1:009FWHMandtheseeingforNGC2071was1:007-1:008FWHM.AllimagingdatawerereducedandphotometeredusingtheFLAMINGOSdatareductionandanalysispipelinesdescribedinChapter 2 .Theresultantzeropoint-correctedcatalogsfortheentire2.1meldofview(200200)yielded800sourcesinN2068and700sourcesinN2071withPSF-ttingcolorerrorslessthan0.1magnitudes.ThisphotometryisshowninFigures 5 and 5 .Photometricqualitywasassessedinthestandardmannerforthesurvey(x ).Themeanphotometricscatterwithrespectto2MASSinallbandsis0.06-0.07magnitudes(e.g.Table A ).The10sdetectionlimitsfortheclusterdataareJ=19.3,H=18.8,andK=17.8andthephotometriccatalogluminosityfunctionsforbothclustersturnoverat

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Figure5. Color-magnitude(left)andcolor-color(right)diagramsforallobjectsinNGC2068withcolorerrors<0.1magnitudes.SymboldenitionsarethesameasinFigure 4 ,withopencirclesrepresentingthegeneralclusterpopulation,lledcirclesrepresentingthespectroscopictargets,andlledredtrianglesaretheclassiedsourcesinNGC2068. orbeyond19.0,18.25,and17.5magnitudesforJ,H,andKrespectively(Table A ).Usingtheseturnoversasrepresentativecompletenesslimits(x ),itisclearthatthe Figure5. SameasFigure 5 butforNGC2071.

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Table5. DetailofSlitMasksObservedinNGC2068 ). 5.3.1SampleSelection,Observations,andDataReductionThespectroscopicsamplesforNGC2068andNGC2071wereselectedfromtheabovephotometryaccordingtotheguidelinesdescribedinChapter 2 .InNGC2068,atotalof141sourcesweretargeted,with80sourcesonfour4mslitmasksandtheremaining61objectsonthree2.1mslitmasks.InNGC2071,wetargetedatotalof234sources:176objectsonnine4mmasksand58objectsonthree2.1mmasks.ThetargetingbreakdownbyMOSplatecanbefoundinTables 5 and 5 ;thephotometryoftargetedsourcesisshownbylledcirclesinFigures 5 and 5 ,whilethespatialdistributionofthesetargetscanbeseeninFigures 5 and 5 .SpectraoftheNGC20684mtargetswereobtainedusingFLAMINGOSonthenightsof2003January15and2004December01-03.SpectraoftheNGC20682.1mtargetsweretakenon2003November30and2004December14.InNGC2071,spectraofthe4mtargetswereobtainedon2003December11,2004January06-07,2004November26,2004December03,2005December17,and2005December20/21.The

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Figure5. Three-colorcombinedimageofNGC2068.ThedataweretakenwithFLAMINGOSontheKPNO2.1mtelescope.Northisup,Eastistotheleft,andtheeldisapproximately20'onaside.Objectsenclosedinpurplecirclesarethe4mspectroscopictargetsandbluerectanglesenclosethe2.1mtargets.

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Figure5. SameasFigure 5 butforNGC2071

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Table5. DetailofSlitMasksObservedinNGC2071 .ThespecicintegrationtimesbymaskcanbefoundinAppendix B ,Table B .Allspectroscopicdatawerereducedusingtheproceduresdetailedinx andclassiedaccordingtothemethodsdevelopedinChapter 3 .Totalsbymaskofthenumberofsourcestargeted,extracted,andclassiedarelistedinTables 5 and 5 5.3.2.1SpectralClassicationFigures 5 and 5 showthenalsetsofMstarspectrainNGC2068andNGC2071respectively.InNGC2068,37uniqueMstarswereclassiedfromthe4msampleand9uniqueMstarswereclassiedfromthe2.1msample,yieldingatotalof46classiedMstars.InNGC2071,Iclassied47uniqueMstarsfromthe4mspectraand7uniqueMstarsfromthe2.1mspectraresultinginatotalof54classiedMstars.In

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addition,Ialsoextractedatotalof22duplicatesources.Whenindependentlyclassied(withoutaprioriknowledgeoftheirduplicity),thespectraltypesfor12oftheseobjectsmatchedtheoriginalsourceexactlyortowithin0.25subclasses.Spectraltypesforallbutone(locatedontheedgeofaplate)oftheremaining10duplicateswerewithin1subclassoftheoriginalobject,thus,Iamcondentthatallspectraltypesareaccuratewithintheerrorsquoted(typically1subclassorless).Consideringthe48extracted4mobjectswhichwerenotclassed,14werellertargetswithKmagnitudes>15.0whichwehavelearnedaretypicallytoofainttoclassifywithourcurrentexposuretimes.TheremainingunclassiedsourceswhilebrightatK,weretypicallyhighlyreddenedobjectswithpoorsignaltonoiseintheJandHbandsafterdereddening.Targetswhichwerenotextractableweresimplytoofainttoachieveanaccurateaperturetrace. ).Ihavesearchedforthesefeaturesinallofourspectraandusetheirrelativestrengthstoassessthegravityofeachsource.ResultsofthisassessmentarenotedinTables 5.4.1 and 5.4.1 .Adesignationoflowindicatesasourceisalowsurfacegravityyoungobject,highindicatesaelddwarf,andgiantisaverylowgravitygiant.AnellipsisinthegravitycolumnindicatesthatIwasunabletocompletethegravityassessment,eitherbecausetheJ-bandsignal-to-noisewastoolowtodistinguishthepotassiumlinesorbecausethespectraltypewastooearlyandthehighgravityindicatorswerenotstrongenoughtobedetected.However,foralluncertaincasestheH-bandwassufcienttoruleoutbackgroundgiants.

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Figure5. NIRspectraofallclassiedMstarsinNGC2068(labeledwithbothspectraltypeandID)shownwiththeIC348opticallyclassiedyoungstandards(labeledwithspectraltypeonly).Prominentspectralfeaturesareidentiedatthetop.AswithNGC2024,objectshavingspectraltypes
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Figure5. continued

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Figure5. NIRspectraofMstarsinNGC2071.

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Figure5. NIRspectraofMstarsinNGC2071continued.

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Figure5. continued

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ObjectsofparticularinterestinNGC2068includen2068-35whichexhibitsrelativelystrongKIabsorptionaswellasanH-bandplateauconsistentwithaelddwarf,andn2068-41andn2068-43whichbothexhibitalargenumberofH-bandlinesandarelikelybackgroundgiantsalthoughthesespectraarequitenoisyduetothefaintnessoftheobjects.Sourcen2068-32alsoshowssomeenhancedabsorptionfeaturesbuttheH-bandproleremainstriangular.Sourcesn2068-01,05,11,and16allexhibitweakpotassiumlineswhilesimultaneouslyshowingsignsofyouth(Paband/ortriangularproles).InNGC2071,enhancedH-bandabsorptionimpliesthatn2071-48andn2071-54arelikelybackgroundgiants.Relativetoallotherobjects,Sourcesn2071-46andn2071-49showsignicantpotassiumabsorption,however,theirKIlinesarenotquiteasstrongasexpectedforatrueelddwarf.Inaddition,theirH-bandprolesremainsomewhattriangular.Ithereforeclassifytheseobjectsashavingmediumsurfacegravitytheymaybeforegrounddwarfsorsimplybeoldermembersofthecluster.Finally,InotethatalthoughitisclassiedasalowsurfacegravityobjectbasedonitsH-bandprole,n2071-14doesalsoexhibitweakKIabsorption. L05 ).Inaddition,n2071-01,n2071-09,n2071-11,n2071-13,n2071-31,andn2071-40aswellasn2068-01allshowasignicantemissionfeatureat1.283m.ThisfeatureisattributedtoPaschenBeta(Pab)emissionandisindicativeofongoingdiskaccretionintheseobjects.ObjectswithPabemissionarenotedinTables 5.4.1 and 5.4.1

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4 toderiveextinctionestimates,effectivetemperaturesandbolometricluminositiesforthe46classiedMstarsinNGC2068andthe54classiedMstarsinNGC2071.IthenplacetheseobjectsontheH-Rdiagramandusepre-mainsequenceevolutionarymodelstoinfermassesandagesforalllikelyclustermembers. 4 ,extinctionstowardseachsourcewereestimatedusingtheintrinsicdwarfcolorsof Leggett ( 1992 ), Leggettetal. ( 1996 ),and Dahnetal. ( 2002 )tocalculateE(JH)andsubsequentlyconvertingthiscolorexcesstoAVwiththereddeninglawof Cohenetal. ( 1981 ).Spectraltypeswereconvertedtoeffectivetemperaturesusingalinearttothetemperaturescalepresentedin Luhmanetal. ( 2003b ).BolometricluminositieswerederivedbycombiningdereddenedK-bandmagnitudeswithadistancemodulusof8.0( Anthony-Twarog 1982 )andthebolometriccorrectionsof Leggett ( 1992 ), Leggettetal. ( 1996 ),and Dahnetal. ( 2002 ).ThesequantitiesaretabulatedforeachobjectinTables 5.4.1 and 5.4.1

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Table5. DataforClassiedMStarsinNGC2068 0112.4811.2910.555.002.50low3.540.010.59weakKI,Pabemission0212.2711.2010.874.183.25low3.53-0.180.440313.2311.8811.026.168.00low3.43-0.330.030414.0412.2411.1410.553.00low3.53-0.050.490512.8811.9111.413.003.00low3.53-0.430.45weakKI0612.9311.9611.513.897.25low3.44-0.580.040713.2312.2011.534.057.00low3.45-0.570.040813.1112.0911.643.453.00low3.53-0.500.420913.4712.2411.645.685.00low3.49-0.490.151013.3812.2611.714.825.50low3.48-0.570.121113.0912.1611.753.147.00low3.45-0.690.04weakKI1213.1612.2211.793.273.50low3.52-0.590.361313.9812.4611.837.821.50...3.56-0.370.681414.2812.7811.898.456.25low3.47-0.530.071513.3112.4511.942.455.50low3.48-0.740.121614.9913.0212.1112.093.00low3.53-0.380.46weakKI1714.6413.0912.378.362.75low3.54-0.610.431813.8912.9112.473.363.25low3.53-0.850.341914.4013.2512.604.824.50low3.50-0.890.222014.3013.1712.684.734.00low3.51-0.910.262115.3513.6612.709.865.00low3.49-0.760.152214.9013.4712.717.434.75low3.50-0.850.202314.3313.2912.743.894.75low3.50-0.990.202414.0613.1812.752.505.00low3.49-1.050.172513.7913.1712.970.001.25...3.57-1.100.582617.3614.7713.0818.055.00...3.49-0.620.152716.5014.6313.1611.505.00low3.49-0.890.162814.7813.8713.243.055.75low3.48-1.250.102916.0214.2613.2410.826.25low3.47-0.990.073014.7813.8413.253.366.25low3.47-1.260.073116.1414.5213.269.407.75low3.43-1.100.033216.8414.5413.2715.274.50...3.50-0.780.223315.4514.2613.505.455.50...3.48-1.260.123414.9514.1213.623.007.50low3.44-1.460.033514.5113.8213.630.552.75high3.54-1.40...KIpresent3616.7214.7113.6612.271.75...3.56-0.950.563716.6414.8913.8010.616.75low3.46-1.240.053815.6014.6314.044.277.50low3.44-1.590.033916.4315.2914.475.827.50low3.44-1.700.034016.8015.2714.487.911.50...3.56-1.430.504116.2315.1314.744.092.25giant3.55-1.69...4216.2215.3414.772.706.75...3.46-1.910.054317.5116.0415.497.552.50giant3.54-1.88...4417.4216.2815.554.188.50low3.41-2.230.024517.1716.2315.662.368.50low3.41-2.340.024618.1816.8015.856.099.00low3.40-2.300.02

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Table5. DataforClassiedMStarsinNGC2071 01a10.239.028.325.001.50...3.5620.9320.77Pabemission,veiling0212.7911.1510.539.503.25low3.5290.1470.440312.0910.9510.564.953.75low3.519-0.0430.340412.9211.2910.579.003.00...3.5340.1280.490512.2911.0810.645.454.00low3.514-0.0650.290612.3511.1010.705.552.50...3.544-0.0360.590711.9810.9910.723.142.50low3.544-0.1300.590813.6011.8110.8111.003.50low3.5240.0840.390915.1412.5210.9418.553.50low3.5240.3030.39Pabemission1013.0311.7011.206.273.00...3.534-0.2220.491114.5712.3711.2814.273.25...3.5290.0300.44multipleemissionlines1213.4711.9711.327.641.00...3.571-0.1570.821314.0212.4811.358.272.75...3.539-0.2080.54multipleemissionlines,veiling1412.8811.9211.463.094.50low3.504-0.4950.20weakKI1513.5412.2811.526.025.25low3.488-0.4390.131613.1212.0011.574.773.75low3.519-0.4530.341713.8612.4711.797.004.50low3.504-0.4860.201813.3112.3211.813.414.25low3.509-0.6150.241913.7012.4911.915.364.50low3.504-0.5930.202013.4512.3611.944.324.25low3.509-0.6350.242115.2813.2012.0913.364.00low3.514-0.3600.292214.0312.7812.235.823.25low3.529-0.6610.362318.7914.6112.2532.182.50...3.5440.3020.592414.5213.1012.417.555.50low3.483-0.7490.122515.9313.7712.4613.328.75low3.406-0.6750.022616.0513.8012.5015.917.50low3.437-0.5520.032714.2113.0412.565.093.25low3.529-0.8190.342815.5613.8112.6410.092.75...3.539-0.6550.442915.2613.6812.669.005.50low3.483-0.7960.123015.1213.4612.679.823.50low3.524-0.7030.333115.7613.9812.7010.362.25...3.548-0.6570.51Pabemission3214.5413.4012.724.865.00low3.494-0.9520.173316.2114.0112.7514.706.75low3.455-0.6700.053414.6613.4312.775.614.75low3.499-0.9370.203514.4913.4712.923.915.50low3.483-1.0840.123614.6513.5012.934.955.00low3.494-1.0330.173717.1114.6212.9717.094.00...3.514-0.5780.303816.0814.1613.0711.825.00low3.494-0.8370.153917.3914.7413.1418.595.00...3.494-0.6260.154016.2214.4713.2510.555.50low3.483-0.9770.12Pabemission4116.2714.2913.4612.434.75low3.499-0.9670.204215.4814.3113.695.415.75low3.478-1.3460.104315.0314.2213.702.437.25low3.443-1.5080.044416.1114.4113.729.451.00...3.571-1.0510.624516.6614.9013.7510.916.00low3.472-1.1810.094615.0114.4214.200.005.00medium3.494-1.719...KI4716.5915.2214.367.053.75...3.519-1.4870.244817.2115.6014.478.732.25giant3.548-1.421...4916.6815.4814.695.344.75medium3.499-1.715...KI5017.3515.9915.136.188.50low3.412-1.9880.025117.1516.0915.473.458.50low3.412-2.2220.025217.8916.9016.152.888.00low3.425-2.4980.035318.2717.1616.373.649.00low3.400-2.5990.025417.3516.6516.450.553.00giant3.534-2.529... J054707+001931forthisobject.bSources46and49havesignicantKIabsorption,however,theyalsoshowtriangularH-bandprolesthuswehestitatetoclas-sifythemaselddwarfs-ratherweclassifythemasintermediatesurfacegravityobjects.

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5 and 5 .AswithNGC2024,Ihaveelectedtodisplaytwosetsofdiagramsforeachcluster:thediagramsontheleftineachgureareshownwiththeevolutionarymodelsof DM97 ,diagramsontherightareshownwiththemodelsof BCAH98 and Chabrieretal. ( 2000 ,hereafterknowncollectivelyastheLyonmodels).Inallcases,diamondsareobjectsclassiedusing4mspectra,trianglesareobjectsclassiedfrom2.1mspectra,asterixesaresourcesidentiedaslikelybackgroundgiantsandsoliddotsarepotentialforegrounddwarfs.ArepresentativeerrorbarforanM5objectisshowninthelowerleftcorner. Figure5. H-RdiagramsforNGC2068shownwiththepre-mainsequencemodelsof DM97 (left)andtheLyonmodels(right).Thediamondsareobjectsclassiedusing4mspectraandthetrianglesaresourcesclassiedfrom2.1mspec-tra.Asterixesrepresentbackgroundgiantsandthesoliddotisaforegrounddwarf.RepresentativeerrorbarsforanM5objectareshown.Thesolidlineaccountsforerrorsinderivedspectraltype,distancemodulus,andphotom-etryandthedashedlineincorporatesanadditionaluncertaintyinAVof3magnitude(x ). Massandageestimatesarederivedforalllikelyclustermembers(seex )byinterpolatingbetweentheisochronesandmasstracksplottedinFigures 5 and 5 .IthasbeenshownthattheLyongroupmodelsareinbetteragreementwithobservational

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Figure5. H-RdiagramsforNGC2071.SymboldenitionsarepredominantlythesameasinFigure 5 ,however,inthiscase,thesoliddotsrepresentthemediumsurfacegravityobjects(x ) constraints(Chapter 4 andreferencestherein),thus,whileIpresentdiagramsusingtheDM97models,fortheremainingdiscussionIwillfocusprimarilyonresultsobtainedusingtheLyontracks.IntheH-Rdiagramsforbothclusters,manyobjectsfallabovetheyoungestisochrone(1Myr).Theseobjectsareassumedtohaveanage<1Myrandarethendroppeddowntothe1Myrisochronealongalineofconstanteffectivetemperaturetoderiveamassestimate.Forallotherobjects,theiractualpositionsontheH-Rdiagramareusedtoderiveageandmassestimates.InthismannerIderivedmassesspanningarangefrom0.02to0.68M(with16objectsfallingbelowtheHBL)andagesrangingfrom<1to100MyrforNGC2068.MassesderivedforobjectsinNGC2071rangefrom0.02to0.82M(with8objectsfallingbelowtheHBL)withagesrangingfrom<1to35Myr. 5.5.1ClusterMembershipClassiedstarsinbothclustersmustfallintooneofthreecategories:foregroundsources,clustermembers,orbackgroundstars.Ultimately,inordertoinvestigatethe

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propertiesofthestarformingclusterpopulations,wemusteliminatetheobjectswhichdonotappeartobetypicalmembersofeitherNGC2068orNGC2071.Inordertodistinguishbetweenclustermembersandnon-members,Ihaveemployedacombinationofspectroscopicandphotometrictechniques.First,usingthesurfacegravityassessmentinx ,objectsin 5.4.1 and 5.4.1 withgravitydesignationsoflowaretakentobeclustermembers.Inaddition,sourceswithsignicantPabemissionarealsoassumedtobemembers.Sourceswithgravitydesignationsofhighorgiantareassumedtobeforegroundorbackgroundsourcesandexcludedfromfurtheranalysis.Although,thetwoobjectsinNGC2071withgravitydesignationsofmediummaybeyoungsources,uponexaminingtheirpositionintheH-RdiagramsinFigure 5 ,itisclearthattheyaresegregatedfromthebulkoftheclusterpopulation.Consequently,Ihavechosentoremovethemfromthegeneralmembershipsample.Thereare8sourcesinNGC2068and9sourcesinNGC2071withnospectroscopicindicatorsofmembership.Inthesecasesweturnedtothephotometricallyderivedextinctionmeasurements(x )todeterminemembershipstatus.Figures 5 and 5 showthedistributionofvisualextinctionsfortheclassiedMstarsineachcluster.ThedistributionforNGC2068peaksatAV'4.5withameanvalueof6.1magnitudes;forNGC2071thepeakisslightlyhigheratAV'5.5withanadditionalpopulationhavingevenlargerextinctionsasindicatedbythemeanvalueof8.3magnitudes.Indeed,lookingatthehistogramofAVforNGC2071,thereisasecondarypeakaroundAV'10.Atthedistanceoftheclusters(d400pc, Anthony-Twarog 1982 ),onlyobjectswithAV<1canbeforegroundsourcesasthelineofsightextinctiontothemolecularcloudis<1.Thisqualicationrulesoutallbutoneofthe17objectsunderconsideration-sourcen2068-25hasanAVof0.0,makingalikelycandidateforaforegroundobject.Indeed,lookingattheH-Rdiagrams,n2068-25liesalongthesameisochroneasthespectroscopicelddwarfn2068-35thusIexcludeitfromfurtheranalysis.

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Figure5. DistributionofAVforMStarsinNGC2068.AllextinctionvalueswerederivedphotometricallyusingthemethodsdescribedinChapter 4 ,x Theremaining15sourcesmusteitherbeclustermembersorbackgroundobjects.Sourceswithextinctions>1andapositionabovethemainsequenceintheH-Rdiagramcannotbebackgrounddwarfssincedwarfluminositiesasseenthroughthecloudwouldplacethembelowthemainsequence.Theonlyremainingpossibilityotherthanclustermembershipisthatthesesourcesarebackgroundgiants.However,asdiscussedinx Iamcondentthatalloftheobjectswithincompletegravityassessmentsarenotgiants,thereforetheymustbeclustermembers.Thisyieldsanalsampleof42likelymembersinNGC2068and50likelymembersinNGC2071.Finally,whenlookingatthepositionsoftheclassiedsourcesinthecolor-magnitudeandcolor-color(Figure 5 and 5 ),thereare2objectsineachclusterwhichappeartobeonthemainsequenceratherthanthe1MyrisochroneintheCMDs

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Figure5. DistributionofAVforMStarsinNGC2071. andlieontheunreddeneddwarf/giantsequenceinthecolor-colordiagrams.Inallcases,theseareobjectswhichIhavealreadyexcludedfromthemembershipsamples:n2068-25,n2068-35,andn2071-46arelikelyforegrounddwarfsandn2071-54wasidentiedasagiant.Basedontheseresults,Iamcondentthatallobjectsremaininginthenalsampleareclustermembers.

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UsingtheLyongroupmodels(theright-handdiagramsinFigures 5 and 5 ),themedianageoftheclassiedmembersinbothclustersis<1Myr.AlthoughtheLyonmodelsarethepreferredPMStracksforderivingagesandmasses(x ,theseresultsdonotallowforacomparisonofyoungclusteragessincetheagesofNGC2024,NGC2068,andNGC2071areallconstrainedtothesameupperlimit.Consequently,asinChapter 4 IusetheDM97modelstoderiveadetailedageestimateforallclustermemberswithintheboundariesoftheisochrones.ThemedianageofNGC2068computedfromtheDM97modelsis0.9Myr.Theerroronthisvalueasindicatedbyacomputationofthestandarderrorofthemedianyieldsapossiblerangeofagesfrom0.6-1.2Myr.ForNGC2071,themedianageoftheclassiedmembersis0.4Myrandtherangeofpossibleagesindicatedbythestandarderrorofthemedianis0.3-0.6Myr.NotethatthestandarderrorsofthemedianwerecomputedinlogspaceastheisochronesarelogarithmicontheH-Rdiagram.TheDM97agesforbothclustersareconsistentwiththe<1MyrestimatederivedfromtheLyonmodels.Inotherwords,bothNGC2068andNGC2071appeartobeveryyoung.Inaddition,whileitisonlya1sigmaeffect,itispossiblethatN2071isyoungerthanNGC2068by0.5Myr.Thereissomeconcernthattheapparentyouthofthebrowndwarfpopulationinbothclustersisaselectioneffectcausedbyourinabilitytodetectedolder,lowmassobjects(e.g.Chapter 4 ,x )andthattheseobjectssubsequentlyskewthemedianageestimatestowardsyoungervalues.Consequently,IhavealsoderivedthemedianageforNGC2068andNGC2071excludingobjectswithmassesM<0.08M.ThemedianageofthestellarpopulationinNGC2068remainsat0.9Myrwithaslightlylargererrorrangefrom0.6-1.3Myr.ThemedianageofstellarsourcesinNGC2071issurprisinglyyoungerthanthepreviousestimate,havingavalueof0.3Myrwith1suncertaintyrangeof0.2-0.5Myr.Myr.Furtherdiscussionoftheseresultsandcomparisonswithotheryoungclusters,includingNGC2024,canbefoundinChapter 6

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5 through 5 presentthespatialdistributionofallclassiedMstarsinNGC2068andNGC2071.SourcepositionsasafunctionofageareshowninFigures 5 and 5 .Intheseplots,opencirclesaresourceswithages>1Myr,lledtrianglesaresourceswithage<1Myrandthecrossesareobjectsidentiedinx .Inaddition,thedottedlineinFigure 5 representsthe1001004mFLAMINGOSeldcenteredonNGC2068.Thefoursourceswhichfalloutsidethisboundarywereclassiedfrom2.1mspectra.ThissubdivisionisnotnecessaryforNGC2071becauseallclassied2.1msourcesfellwithintheboundariesofthe4meld.SourcepositionsasafunctionofmassareshowninFigures 5 and 5 .Inthesegures,opencirclesareobjectswithM>0.08M,starsareobjectswithM<0.08M,andthecrossesarethenon-members.ThedottedregioninFigure 5 isthe4melddescribedabove.NotethatallagesandmassesforthisanalysisarederivedfromtheLyontracks(recallthattheseagesremainconsistentwiththoseofDM97).InNGC2068,thedistributionofyoungsourcesinthesouthernpartoftheeldappearstobeslightlyskewedtowardsthewesternpartofthecluster.However,forthemoreextendedclusterregiontheyoungsourcesappeartoberandomlydistributedwithrespecttotherestoftheclassiedpopulation.ThisisincontrasttoNGC2071wheretheyoungpopulationisextendedacrosstheentireregion,withaslightelongationalongthenorth-southdirection.Inaddition,itisinterestingtonotethatinNGC2071manyoftheyoungsourceshaveextinctionsinexcessof10magnitudes.ThisisnotthecaseforNGC2068.Lookingatsourcepositionsasafunctionofmass,itwouldappearthatinNGC2068thelocationofthebrowndwarfsroughlyfollowsthedistributionoftheyoungestsources,withthebrowndwarfsclusteredtowardsthewest.ThiseffectcanalsobeseenintheH-RdiagramforNGC2068(Figure 5 ,right)wherethereisonlyoneobjectclassiedasabrowndwarfwithanage>1Myr.InNGC2071,ontheotherhand,therearefarfewer

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Figure5. LocationofclassiedsourcesinNGC2068asafunctionofage.Opencirclesareallsourceswithages>1Myr,lledtrianglesaresourceswithages<1Myr,andthecrossesarethesourcesidentiedinx asnon-members.TheregionenclosedbythedottedlinerepresentstheFLAMIN-GOS4meldcenteredonNGC2068.

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Figure5. LocationofclassiedsourcesinNGC2068asafunctionofmass.Opencir-clesareallsourceswithmassesM>0.08M,starsaresourceswithmassesM<0.08M,andcrossesrepresentthenon-members.ThedottedlineistheFLAMINGOS4meldcenteredonNGC2068.

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Figure5. LocationofclassiedsourcesinNGC2071asafunctionofage.SymboldenitionsarethesameasinFigure 5

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Figure5. LocationofclassiedsourcesinNGC2071asafunctionofmass.SymboldenitionsarethesameasinFigure 5

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browndwarfsandthosethatarepresentareroughlyevenlydistributedthroughouttheregion(althoughagain,mostofthebrowndwarfsdohaveages<1Myr). 4 ,thepresenceofexcessuxinthenearinfrarediscommonlytakentobeanindicatorofthermalemissionbywarmdustinacircumstellardisk.InordertoassessthethelikelihoodofdisksaroundsourcesinNGC2068andNGC2071,IhavecalculatedtheinfraredexcessfractionsforbothclustersinthesamemannerasforNGC2024(e.g.viaacomparisonofeachclustermember'sexpectedintrinsicHKcolorwithitsdereddenedobservedHKcolor).InNGC2068,21outof42sourcesexhibitsomelevelofexcess,yieldinganoverallIRXfractionof50%13%.InNGC2071,20outof50clustermembersexhibitaninfraredexcess,yieldinganIRXfractionof40%11%.Consideringjustthesubstellarpopulationofeachregion,8out16browndwarfsinNGC2068exhibitanHKexcessand2outof8browndwarfsinNGC2071exhibitanHKexcess,yieldingsubstellarIRXfractionsof50%22%and25%20%forNGC2068andNGC2071respectively.NotethatallquotederrorsarederivedfromPoissonstatistics.Forcomparison,theIRXfractionscalculatedinChapter 4 forNGC2024were40%9%forthegeneralclusterpopulationand39%15%forthebrowndwarfs.TheimplicationsofthesediskfractionswillbediscussedfurtherinChapter 6

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4 ,priortodeterminingtheIMFofayoungclusteritisimportanttoensurethatthecontributingsourcesarerepresentativeoftheoverallclusterpopulation.Figure 5 showsboththespectroscopicandphotometricK-bandluminosityfunctionsforthe4mFOVinNGC2068andNGC2071.WhileIamcondentthatallspectroscopicsourcesinthecurrentsamplesareinfactclustermembers,itisclearfrombothplotsthatneitherspectroscopicsampleisacompletecensusofitsparentcluster.Thus,aswithNGC2024,acompletenesscorrectionfromthephotometryisrequired.However,thereisacriticaldifferencebetweenthephotometricKLFspresentedhereandtheKLFofNGC2024(Figure 4 )-namelythesignicanceofthestellarbackground. Figure5. UncorrectedK-BandLuminosityFunctionsforNGC2068andNGC2071,shownwiththeKLFsofspectroscopicallyclassiedMstars.BothKLFsareshownfortheboundariesofthe4meldonly.Inbothclustersthenum-bercountsinthephotometryshootupbetweenK=14-15,indicatingthepresenceofalargebackgroundpopulation. Lookingattheleft-handpanelofFigure 4 ,therawphotometricluminosityfunctiondoesnotbegintorisesteeply(indicatingthepresenceofbackgroundsources)

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untilK'16.Onceanextinctionlimithasbeenapplied(Figure 4 ,right)thebackgroundcontributionisfurtherminimizedandeffectivelynegligibledowntothespectroscopiclimitofK'15.0.Ontheotherhand,thephotometricluminosityfunctionsinFigure 5 bothbegintoriseinthevicinityofK=14.0-14.5,nearlytwomagnitudesbrighterthaninNGC2024.ThiseffectcanbeexplainedbythelowerdensityofcloudmaterialintheregionssurroundingNGC2068andNGC2071(e.g. Ladaetal. 1991a ; Aoyamaetal. 2001 )allowingthelightfrombackgroundstarstoshinethrough.ThereducedamountofextinctioninNGC2068andNGC2071coupledwithsmallerbutdeepersamplesmeansthatasimpleapplicationofanextinctionlimitwillnotsufcetoremovethebackgroundcontribution.Rather,thispopulationmustbeaccountedforpriortoestimationofthespectroscopiccompletenesslimitstoavoidover-correction.RecallthataspartoftheFLAMINGOS/OrionBimagingsurvey,datawereobtainedforanumberofoff-cloudcontrolelds.ThepurposeofthesedataistoaccountforthespecicdistributionofbackgroundeldstarsatthegalacticlatitudeofOrionB.Figure 517 showstheaverageK-bandluminosityfunctionforcontrolelds1-4(thickhistogram),scaledtothe4meldsizeof10arcminutes.Forcomparison,theKLFfora2MASSeldcenteredonthecoordinatesofcontroleld1isalsoshown(thinhistogram)andthenumbercountsagreetowithin1sdowntoK=15.0,where2MASSphotometrybecomesveryuncertainandincomplete.ThedistributionofbackgroundsourcesestimatedfromthecontroleldscantheoreticallybesubtractedfromtherawKLFsshowninFigure 5 toyieldnewKLFscontainingonlythestatisticalcontributionfromclustermembers.However,withoutaccountingforthereddeningwithinthemolecularclouditself,directsubtractionofanunreddenedbackgrounddistributionwillresultinanover-correction.GiventhatthecloudmaterialinOrionBactsasascreeninfrontofthebackground,beforesubtractingthebackgroundKLFfromtheclusterluminosityfunctions,thisKLFmustbereddenedbytheamountofextinctionpresentinthespeciedregionofthecloud.ThemeanAV

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Figure5. AverageKLFforOrionBcontrolelds1-4(thickline),scaledto10ar-cminutes,shownwiththeKLFforthe2MASSeldcenteredoncontroleld1(thinline).ErrorbarsinbothcasesarePoissonerrorspropagatedtoaccountfortheaveragingandscalingofthehistograms. forthespectroscopicsamplesinNGC2068andNGC2071are6.1and8.3magnitudesrespectively.However,recallthatallclassiedspectraarerelativelybrightinJandH.Consequently,thereddestsourcesineachclusterarepreferentiallyexcludedfromthemeanAVcalculations,implyingthatthesevaluesarenotnecessarilyrepresentativeofthetrueline-of-sightextinctionthroughthecloud.Amoreappropriateextinctionestimateisameanvaluederivedfromlongerwavelengthphotometryofallobjectsintheeld,therebyallowingtheentirerangeofAVtobesampled.IhaverecalculatedtheextinctionvaluesforeachclusterusingtheHandKphotome-tryfromtheimagingsurveycoupledwiththeNICEtechniquefordeterminingextinctions( Ladaetal. 1994 ).ThebasicideaofNICEisverysimilartothemethodsusedearlierin

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thisthesis(e.g.x andx ),whereextinctionswerecalculatedusingthedifferencebetweenobservedandintrinsicJHcolorstodeterminethecolorexcessandthenconvertingtoAVusingareddeninglaw.ThemajordifferenceisthatNICEisapurelyphotometrictechnique,thusratherthanusingintrinsiccolorsbasedonspectraltype,theintrinsicJHorHKcolorsaredrawnfromthecontroleldphotometry.UsingtheNICEtechniqueandincorporatingtheHandKphotometryforallpossiblesourcesinthe10arcminuteeldscenteredoneachclusteryieldsnewmeanextinctionestimatesforNGC2068andNGC2071of9.7and11.3magnitudes,respectively.Whilethesevaluesarestillsingleestimatesforapatchyregioninamolecularcloud,theyarepreferabletoanestimatederivedsolelyfromJandHphotometry.Figures 5 and 5 showboththereddenedcontroleldhistograms(reddenedbythemeanextinctionsderivedabove)andthenalbackground-subtractedluminosityfunctionsalongwiththeKLFsofthespectroscopicallyclassiedMstars.Errorbarsshownarethecountingerrorsinbothhistograms.Lookingatthebackground-subtractedKLFsitisclearthatthecompletenessofbothspectroscopicsurveysismuchimproved,particularlyatthefaintend. 4 thatthiscorrectioniscalculatedbyexaminingthedifferencebetweenthephotometricandspectroscopicKLFs,addingsourcestotheincompletemagnitudebinsaccordingtotheobjectmassdistributioninthosebins.Cautionmustbeused,however,becauseifcertainbinsinthespectroscopicKLFarenotwellpopulated,itispossibletoinadvertantlyover-correctforthemassesrepresentedinthosebinsandunder-correctforthemassesnotpresent.Tominimizethisproblem,IhaveincludedanadditionalsampleofsourcesinthespectroscopicluminosityfunctionwhicharepartofthebrightstarsurveyofOrionB(Chapter 2 ,x Hernandez 2006 ).

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Figure5. BackgroundsubtractedKLFforNGC2068.Thetoppanelshowstheuncor-rectedluminosityfunctionforthecluster(solidline)alongwithbackgrounddistributionfromFigure 5 (dashedline),reddenedby9.7magnitudesofAV.Thebottompanelshowsthebackground-subtractedphotometrywithPoissonerrorbars(solidline)andtheKLFofthe42classiedmembersofNGC2068(shadedhistogram).

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Figure5. BackgroundsubtractedKLFforNGC2071.Thetoppanelshowstheuncor-rectedluminosityfunctionforthecluster(solidline)alongwithbackgrounddistributionfromFigure 5 (dashedline),reddenedby11.3magnitudesofAV.Thebottompanelshowsthebackground-subtractedphotometrywithPoissonerrorbars(solidline)andtheKLFofthe50classiedmembersofNGC2071(shadedhistogram).

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Althoughtheseobjectshavenotyetbeenclassied,thespectrahavebeenvisuallyexaminedbymultipleauthorsandassessedtobeearlierthanM.Inaddition,thebrightmagnitudesofthesesources(thevastmajorityhaveK<12.0)makesitstatisticallyunlikelyforthemtobebackgroundobjects(c.f.thetoppanelsofFigures 5 5 ),thustheyarelikelyclustermembers.Therefore,incorporatingthesesourcesintothespectroscopicKLFswillincreasetheoverallcompletenessfractions(therebyreducingthesizeofthecorrectionneeded)withoutchangingthestatisticsoftheMstarsoraffectingtheshapeoftheMstarmassfunction. Figure5. CompletenessofNGC2068Spectra.Thesolidlineisthebackground-subtractedKLFandtheshadedhistogramisthesumofthe24earlyand42Mstarspectra. Figures 5 and 5 showthebackground-subtractedluminosityfunctionsforeachclusteralongwiththenewlycombinedspectroscopicKLFs.InNGC2068,thereare38classiedmembersand31early-typespectrainthemagnituderange8.010.5containMstars.InNGC2071,thereare50classiedmembersand26early-type

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spectrainthemagnituderange8.010.5containMstars.Thus,tocreatetheMstarIMFforeachcluster,IneedonlycorrecttheNGC2068KLFintherangefrom11.0
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5 and 5 showthespectroscopicallyderivedmassfunctionsforNGC2068andNGC2071,respectively.Inbothcases,thesolidlineistheIMFforallclassiedmembersofeachcluster,shownwithPoissonerrors.Thedashedlinesrepresenttheincompleteness-correctedmassfunctionsusingthesmallmagnituderangesspeciedabove.Thedottedlines,shownforcomparison,arethemassfunctionscorrectedovertheentiremagnituderangeofthespectroscopicsamples.ThemassfunctionforNGC2068peaksinthebincenteredonLog(M/M)=-0.5,correspondingtoamassrangeof0.2-0.5M.TheIMFappearstoslowlydeclineintothebrowndwarfregimealthoughtheerrorsarealsoconsistentwithaatIMFthroughouttheentireMstarrange.Neitherincompletenesscorrectionchangestheoverallshapeofthemassfunctionsignicantly,noristhereasignicantdifferencebetweenthetwocorrectionsuntilthemassrangebelow0.03Misreached.Atthispoint,thelargercompletenesscorrection(magnituderangefrom9.5
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(dottedline),thedecitoflowmassobjectsinNGC2071ascomparedtoNGC2068remainssignicant.SimilartotheearlierdiscussionoftheNGC2024massfunction(x ),itshouldbenotedherethattheexactshapesofthemassfunctionsforNGC2068andNGC2071arealsosomewhatdependentonthechoiceofbincentersandsizes.Consquently,aspointedoutinChapter 4 itremainstruethatthemorerobustchoiceforinvestigatingtheimportanceofbrowndwarfsinaregionistherelativenumbersofstarsandbrowndwarfsineachcluster. 4 ,x ,theratioofbrowndwarfstostars(Rss)isanextremelyusefultoolforquantifyingtheIMFasitisindependentofthedetailedstructureandexactshapeofclustermassfunctions.Inthesmallrangeincompleteness-correctedmassfunctionforNGC2068(dashedlineinFigure 5 )thereare16objectswithmasses0:02M0:08M.Addinginthestatisticsfortheearly-typesource(whichareconrmedearlierthanMandthusnotbrowndwarfs)addsanadditional30objectswithmassesM>0:08M.Finally,thereare7sourcesinthebackground-subtractedphotometricKLFwithmagnitudesbrighterthanK=11.0whichwerenotincludedinthecompletenesscorrectedmassfunction.Incorporatingthesesourcesyieldsayieldingatotalof69stellarsources.(RecallthattherearenosubstellarobjectswithKmagnitudesbrighterthan12.0,thusitissafetoassumethatthe7photometricsourcesareallstars.)ThisresultsinanRssforNGC2068of16/69or0.230.06.InNGC2071,theincompleteness-correctedmassfunctioncontains9browndwarfsand50stellarsources.Includingtheearly-typestarsaddsanadditional20objectswithM>0:08M.Finally,thereare4sourceswithmagnitudesK<11.0inthephotometricKLFandoneadditionalsourcewhichwasnotintheFLAMINGOScatalogduetoitsextremebrightness(K=6.33,courtesyof2MASS)butisquiteobviousinthe4mimage,

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Figure5. MassfunctionforMstarsinNGC2068.Thesolidlineistheuncorrected,purelyspectroscopicmassfunctionforallclassiedmembersofNGC2068.Thedashedlineisthespectroscopicmassfunctioncorrectedformagnitudeincompletenessintherangefrom11.0
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Figure5. MassfunctionforMstarsinNGC2071.Thesolidlineistheuncorrected,purelyspectroscopicmassfunctionforallclassiedmembersofNGC2071.Thedashedlineisthespectroscopicmassfunctioncorrectedformagnitudeincompletenessintherangefrom11.0
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bringingthestellartotalto75.ThisyieldsanRssforNGC2071of9/75or0.120.04.NotethatneitherRssderivedhereincludesacorrectionforincompletebinsbelowK=15.0(NGC2068)orK=14.5(NGC2071),asitishighlyuncertainwhethermissedsourcesinthesebinsareclustermembersorbackgroundobjects(inkeepingwiththediscussionintheprevioussection.TheimplicationsofbothpossibilitiesalongwithacomparisonofRssvaluesforotheryoungclusterscanbefoundinthenextchapter. D'Antona&Mazzitelli ( 1997 )themedianageofMstarsinNGC2068is0.9Myrwitharangefrom0.6-1.2Myr,asindicatedbythestandarderrorofthemedian;themedianageforNGC2071is0.4Myrwitharangefrom0.3-0.6Myr.Thesevaluesareconsistentwithmedianages<1Myrasderivedfromthemodelsof Chabrieretal. ( 2000 ).ThephotometrywasusedtocompileK-bandluminosityfunctionsforbothclusters.TheseKLFswerecorrectedforbackgroundsourcesandthenusedtoassessthecomplete-nessofthenalmembershipsamplesineachcluster.Incorporatingthebrightspectra

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of Hernandez ( 2006 ),thenalincompletenesscorrectionsweresmall,withmanyofthebinshavingcompletenessfractionsof90-100%.Theresultantmassfunctionsbothrisetobroadpeaksaround0.2-0.5M,however,theIMFforNGC2068remainsatthroughoutthesubstellarregimewhilethatofNGC2071exhibitsarelativebrowndwarfdefecit.ThecorrespondingvaluesofRss,theratioofbrowndwarfstostars,illustratethemagnitudeofthisdefecit,withanRssof0.230.06forNGC2068andanRssof0.120.04forNGC2071.Theimplicationsoftheseresultswillbediscussedinthenextchapter.

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GmH23=24 3pr01=2;(6.1)whereTisthecloudtemperature,isthemeanmolecularweight,mH2isthemassofahydrogenmolecule,r0isthegasdensity(whichequalsnH2mH2),andkandGaretheBoltzmannandgravitationalconstants( Jeans 1902 ; Spitzer 1978 ).Foratypicalstar-formingmolecularcloud,thetemperatureandnumberdensity(nH2)areontheorderof10Kand105cm3,respectively,resultinginanaverageJeansmassof'2.3M.Thisclearlycontradictstherelativelylargenumbersoflowmassstarsandbrowndwarfsobserved,illustratingtheproblemwithclassicaltheory.Inanefforttosurmountthisobstacle,manyauthorshaverecentlyproposednewtheoriesoflowmassstarandbrowndwarfformation.Broadly,theyincludefourmainmechanisms:turbulentfragmentationofmolecularcloudcores,ejectionofprotostellarembryosfromunstablemultiplesystems,thecollisionofprotostellardisks,andphotoerosionofpre-stellarcores.Fortheremainderofthissection,Isummarizeeachofthesemodelsinturn.Inthefollowing 127

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sectionsIexaminethemodelpredictionsincontextoftheobservationalwork,ultimatelyplacingconstraintsonthemostlikelymethodsofbrowndwarfformation.TurbulentFragmentation:Theminimummassdiscussedaboveresultsfromlineardensityperturbationsinamolecularcloudoffsettingthebalancebetweengravityandthermalpressure,thuscausingthecollapseofacloudcore( Jeans 1902 ).However,thegasdensityandvelocitystructureofstar-formingcloudsarehighlynonlinear(recall,forexample,theclumpystructureofOrionBasseenintheCSmapof Ladaetal. ( 1991a )showninFigure 2 )duetothepresenceofsupersonicturbulence(e.g. Padoan&Nordlund 2002 ,andreferencestherein).Theturbulentowhasextremelyhighkineticenergy,leadingtoalargenetworkofshockedgasthroughoutthecloud.Theseshockscompressthecloudmaterial,creatingregionswithveryhighlocaldensities.Ifthedensityofagivenregionislargerthanitslocalcriticaldensityitwillcollapse,irrespectiveoftheaverageJeansmassofthecloud.Consequently,densecoresofanysizecanforminaturbulentcloud. Padoan&Nordlund ( 2004 )investigatedthepossibilitythatbrowndwarfsareformeddirectlyviaturbulentfragmentation.Theycomputedbothanupperlimittothebrowndwarfmassfraction(assumingbrowndwarfsonlyarisefromturbulentfragmentation)andtheanalyticalmassdistributionofcollapsingcoresinaturbulentcloud,varyingboththegasdensityandthesoundspeed(whichisdensity-dependent).Theiranalysisshowsthatforhighervaluesofgasdensityandsoundspeed,thecoredistributionpeakstowardslowermassesandtherelativecontributionofbrowndwarfstotheIMFincreases.Embryo-Ejection:Firstproposedby Reipurth&Clarke ( 2001 ),theembryo-ejectionmodelforbrowndwarfformationpostulatesthatstarsareborninunstablemultiplesystemsofsmallprotostellarembryos,allgainingmassfromasharedreservoirofgas.Browndwarfsareformedwhenaccretionisprematurelyterminatedduetodynamicalejectionfromthesystem.

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Bateetal. ( 2002 )and Bate&Bonnell ( 2005 )haveruntwoidenticalhydrodynamicalsimulationsofthecollapseandfragmentationofturbulentmolecularclouds,differingonlyinthedensityofthetwoclouds.Intheirmodels,threequartersofthebrowndwarfsformwithingravitationallyunstablecircumstellardisks.Thediskfragmentsinteractandthosedestinedtobecomebrowndwarfsareejectedwithin104-105Myr.Theremainingbrowndwarfsinitiallyformfromisolatedfragmentsofdensemoleculargasbutquicklyfallintounstablemultiplesystemsandaresubsequentlyejectedbeforetheycanaccretetostellarmass.Comparingtheresultsofthetwocalculations,theiraccretion/ejectionmodelinaturbulentcloudpredictsthatthemainvariationoftheIMFindifferentstarformingenvironmentsisthelocationofthepeakmass.Inaddition,theyndthatthedensercloudproducesahigherproportionofbrowndwarfs.Finally,largedisks(radii10AU)havedifcultysurvivingtheejectionprocess,withafrequencynohigherthan5percent. Kroupa&Bouvier ( 2003 )havealsomodelledtheformationofbrowndwarfsviaembryo-ejectionusingN-bodysimulations.Theirmodelpredictsthattheejectionsoccurwithinafewsystemdynamicaltimes(t1:6104yr)andtheresultantdistributionofejectionvelocitiescontainsahighvelocitytailforbrowndwarfs.Inaddition,largedisksarenotexpectedtosurvivetheejectionprocess.Finally,theypredictthatassumingthattheproductionofbrowndwarfsissolelyduetoembryo-ejectionandthatthisejectionoccursinthesamemannerfordifferentenvironments,thetotalnumberbrowndwarfsperstarinagivenstarformingregionshouldbeinvariate.DiskCollisions:Athirdproposedmechanismistheformationofbrowndwarfsinducedbythecollisionofmassiveprotostellardisks(e.g. Linetal. 1998 ).Inthisscenario,tidalinteractionsbetweentwoprotostarswithmassivecircumstellardisksresultintheformationofatidaltail.Thistailbecomesgravitationallyunstableandcollapsestoformasubstellarobjectwhichissubsequentlyejectedfromthesystemasthetaildispersesonatimescaleof5103yraftertheoriginalcollision.Browndwarfsformedinthis

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mannerareexpectedtohavesignicantejectionvelocitiesandnotexpectedtomaintaincircumstellardisks.Photoerosion:Thefourthcommonlydiscussedprocessfortheformationoflowmassstarsandbrowndwarfsisthephotoerosionofprestellarcoresbyradiationfrommassive(OB)stars( Hesteretal. 1996 ; Whitworth&Zinnecker 2004 ).Inthiscase,adense,stellarmasscoreisoverrunbyanHIIregion.TheambientLymancontinuumradiationdrivesanionziationfrontintothecore,erodingitfromtheoutsideandapplyinganexternalpressurewhichcausesthecoretocollapse.Thenalmassoftheobjectisthendeterminedbyacompetitionbetweenaccretionontothecentralprotostarandtheerosionofmaterialattheboundary. Kroupa&Bouvier ( 2003 )and Whitworth&Zinnecker ( 2004 )haveexaminedanalyticalmodelsofbrowndwarfformationviaphotoerosionofprestellarcoresandbothauthorsndthatphotoevaporationofaccretionenvelopescansignicantlyaffecttheIMFnearandbelowthesubstellarboundaryintheimmediatevicinityofionizingstars(e.g.withintheHIIregion).Inaddition, Whitworth&Zinnecker ( 2004 )notethattheeffectivenessofthisprocessimpliesthatanyintermediate-massprotostarsintheregionmusthavebeenwellontheirwaytoformationbeforetheOBstarsturnedon.Neithermodelmakesanypredictionsregardingthesurvivalofcircumstellardisks,however,logicdictatesthatthesameenvironmentwhichishostiletoprotostellaraccretionenvelopeswillalsoacttoerodediskmaterial. 6.2.1SubstellarDiskFrequenciesAspartoftheefforttocharacterizethelowmasspopulationsofNGC2024,NGC2068,andNGC2071,IhaveusedtheHKcolorsandspectraltypesofallclassiedbrowndwarfstoassesstheamountofinfraredexcess(IRX)presentineachobject.(RememberthatthepresenceofanIRXisconventionallytakenasanindicatorofemissionfromawarmcircumstellardisk.)InthismannerIderiveasubstellardisk

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fractionforNGC2024of39%15%(Chapter 4 ,x ),andsubstellardiskfractionsof40%11%and25%20%forNGC2068andNGC2071,respectively(Chapter 5 ,x ).ReferringtothediscussionofbrowndwarfdisksinChapter 4 ,althoughtheerrorsarelarge,thesubstellarexcessfractionsforallthreeOrionBclustersareconsistentwiththeexcessfractionsfoundforotheryoungstarformingregionssuchastheTrapezium,ChamaeleonI,andIC348.Inaddition,recentmodellingofspectralenergydistributionsconstructedfromSpitzerdatainIC348showthat42%ofbrowndwarfsinthatregionaresurroundedbyopticallythick,primordialdisks( Ladaetal. 2006 ),whichisagainconsistentwiththeinferreddiskfractionsderivedinthisthesis.Finally,thesubstellardiskfractionsfoundhereareingoodagreementwithrecentworkontheaccretionpropertiesofbrowndwarfs.SincetherstdenitivedetectionofactiveaccretioninasubstellarobjectinTaurus( Muzerolleetal. 2000 ),manybrowndwarfshavebeenfoundtopossessaccretiondisks(e.g. Jayawardhanaetal. 2003 ; Nattaetal. 2004 ; Muzerolleetal. 2005 ; Allersetal. 2006 ).Further,in1-3MyroldregionssuchasrOph,Taurus,ChameleonI,andIC348,thetypicalfractionofsubstellaraccretorsis30-60%( Mohantyetal. 2005 ),consistentwithmyconclusionthat40-50%ofbrowndwarfsintheOrionBclustersarelikelysurroundedbycircum(sub)stellardisks(seethediscussionaboveandthatinChapter 4 ).Thedetectionofasignicantdiskfractionforyoungbrowndwarfsplacesanimportantconstraintontheirformationmechanism.RecallfromChapter 1 thathydrogen-burningstarsgainmuchoftheirmassthroughtheaccretionofmaterialfromacircum-stellardisk.Thepresenceofaccretiondisksaroundsubstellarobjectsaswellimpliesacertainlevelofuniformityintheoverallformationprocessandprovidesasignicantchallengefortheembryo-ejectionanddiskcollisionmodelsofformation,neitherofwhichareexpectedtocreatebrowndwarfswithsignicantdisks.Indeed,thesimulationsof Bateetal. ( 2002 ); Bate&Bonnell ( 2005 )showamere5%frequencyofejected

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browndwarfshavingdiskswithradii>10AU.However,theseresultsareconsistentwithmodelsofturbulentfragmentationwherelowmassobjectsareabletomaintainsignicantdisks. 4 and 5 IhavederivedtheMstarinitialmassfunctions(IMFs)forNGC2024,NGC2068,andNGC2071(left-handpanelsofFigure 6 ).Allthreemassfunctionsrisetosimilarpeaksinthevicinityof0.2-0.3M,however,theirbehaviorbelowthestellar/substellarboundaryisquitedifferent;themassfunctionforNGC2024dropsatthebrowndwarflimitbutexhibitsamodestsecondarypeakat'0.03MwhilethemassfunctionforNGC2068beginsaslowdeclinearound0.2Mwhichextendsbeyondthecompletenesslimitof0.035M.ThemassfunctionforNGC2071showsasharpdropatthebrowndwarflimitandcontainsonlyminimallypopulatedbinsbeyondthispoint.LowmassIMFshavebeenconstructedforanumberofotheryoungstarformingregionsandinsomecasestheirbehaviorisquitesimilartotheIMFspresentedhere.Theright-handpanelsofFigure 6 showthemostrecentspectroscopicallyderivedmassfunctionsforthe1MyrTrapeziumclusterintheOrionAmolecularcloud(adaptedfrom Slesnicketal. ( 2004 )),the2MyrIC348clusterinPerseus(adaptedfrom Luhmanetal. ( 2003b )),andthe1-2MyrTaurusstarformingregion(adaptedfrom Luhman ( 2004 )).NotethatbothIC348andtheTrapeziumareexamplesoftheclusteredmodeofstarformation,similartotheOrionBclustersandcharacterizedbyrelativelyhighstellarandgasdensities.Incomparison,Taurusisanexampleoftheisolatedordistributedmodeofstarformation,occuringinanumberoflowdensitystellaraggregateswidelyspreadthroughouttheTaurus-Aurigamolecularcloud.TheTrapeziumIMFrisestoamaximumat'0.2Mbeforedecliningatthehydrogen-burninglimit(HBL),ingoodagreementwiththepeaksoftheOrionBIMFsandconsistentwithadditionalphotometricandspectroscopicstudiesoftheTrapezium( Hillenbrand&Carpenter 2000 ; Luhman

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Figure6. Initialmassfunctionsofyoungclusters.Theleft-handpanelsshowthecompleteness-correctedmassfunctionsderivedinChapters 4 and 5 forNGC2024,NGC2068,andNGC2071.Theright-handpanelsshowtheIMFsfortheONC/Trapeziumregion(adaptedfrom Slesnicketal. ( 2004 ),IC348(adaptedfrom Luhmanetal. ( 2003b ),andtheTaurusstarformingregion(adaptedfrom Luhman ( 2004 )).

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2000 ; Muenchetal. 2002 ; Lucasetal. 2005 ).TheIC348IMFpeaksataslightlylowervalue('0.1M)thantheOrionclustersbeforefallingoffrelativelysharplyattheHBL( Luhmanetal. 2003b ; Muenchetal. 2003 ),similartothesharpdropseenintheNGC2071IMF.Ontheotherhand,themassfunctionderivedforTaurusrisesmoresharplytoasignicantlyhighermaximumof0.8-0.9Mandexhibitsamuchslowerdeclineintothesubstellarregime( Bricenoetal. 2002 ; Luhmanetal. 2003a ; Luhman 2004 ). Bricenoetal. ( 2002 )and Luhmanetal. ( 2003b )offertheexplanationthatthedifferencesbetweenthepeakmassoftheTaurusIMFandtheIMFsoftheTrapeziumandIC348mayreectadisparityamongthelocalJeansmasses.Theresultsfromthisdissertationsupportthistheoryasdothemodelsofturbulentfragmentationdescribedinx .Simulationsby Padoan&Nordlund ( 2002 2004 )and Bate&Bonnell ( 2005 )ndthatwhilethemassdistributionforstarformingcoresappearstobeindependentofenvironmentformasseslargerthanasolarmass,theshapeofthemassfunctionforsubsolarmassesisdependentonthegasdensityandthelevelofturbulenceinthecloud.Specically,forhighergasdensities(andthusincreasedturbulence),theIMFshouldpeakatlowermasses.InFigure 6 IplotthedistributionofIMFpeakmassesshowninFigure 6 againstthecolumndensityofhydrogendetectedineachregion(Table 6 ).WiththeexceptionofIC348,theoveralltrendisinagreementwiththemodelpredictions,leadingfurtherweighttotheideathatbrowndwarfsformviaturbulentfragmentation.TheobservedvariationsinthesubstellarIMFaremorechallengingtoexplain.Both Muenchetal. ( 2002 )and Slesnicketal. ( 2004 )citeapossiblesecondarypeakbelowtheHBLintheTrapeziumIMF,similartotheresultfoundinthisthesisforNGC2024.However,thelocationsoftheirrespectivepeaksdifferbothfromeachotherandthesecondarypeakIdetectforNGC2024(0.05MforSlesnicketal.,0.025MforMuenchetal.,ascomparedto0.035MforNGC2024).Ifthesepeaksarerealfeaturesinthemassfunctions,theymayindicateabreakinformationmechanismforlowmassobjects( Muenchetal. 2002 ).Ontheotherhand,theymaybeartifactsintroduced

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Figure6. IMFpeaksversusgasdensityformassfunctionsshowninFigure 6 .Errorbarsaresimplyreectiveofthebinwidthsusedineachregion.Referencesforthecolumndensityofhydrogen(shownonthex-axis)canbefoundinTable 6

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bythemass-luminosityrelationforbrowndwarfsandnotatruereectionofthemassfunctionitself( Muenchetal. 2003 ).Inaddition,asdiscussedinx andx ,theexactshapeofthesubstellarmassfunctioncanbeinuencedbybinning,particularlywhendealingwithsmallnumberstatistics(asinNGC2071forexample).Consequently,accurateinferencesregardingthesignicanceofthesubstellarpopulationsintheseclustersisbetterlefttoamorerobustquantitysuchastheratioofbrowndwarfstostars,discussedbelow. 4 thatinordertofacillitatequantitativecomparisonsbetweenthelowmassIMFsofdifferentregions, Bricenoetal. ( 2002 )denedthequantityRsstobetheratioofbrowndwarfs(0.02M=M0.08)tostars(0.08M=M10).Inthesamevein,IhavecomputedtheRssfortheOrionBclustersstudiedinthisthesis:Rss=0.300.05forNGC2024,Rss=0.230.06forNGC2068,andRss=0.120.04forNGC2071.ThevaluesofNGC2024andNGC2071clearlydiffersignicantly,withfarfewerbrowndwarfsinNGC2071thaninNGC2024.TheRssforNGC2068fallsinbetween;atrendthatisreectedbytheshapesofallthreesubstellarmassfunctionsshownintheleft-handpanelsofFigure 6 .Theratioofbrowndwarfstostarshasbeencalculatedforthestarformingre-gionsdiscussedintheprevioussection,alsowithavarietyofresults.IntheTrapez-ium, Slesnicketal. ( 2004 )ndRss=0.20,whichisslightlylowerthanthevalueofRss=0.260.04foundby Luhman ( 2000 ). Slesnicketal. ( 2004 )attributethisvariationtothedifferencebetweenspectroscopicandphotometricIMFs,however,IcontendthatthedifferentRssvaluesarelikelycausedbytheuseofdifferentevolutionarymodels. Slesnicketal. ( 2004 )employedthepre-mainsequencemodelsof D'Antona&Mazzitelli ( 1997 )toderivemasseswhereas Luhman ( 2000 )employedthemodelisochronesoftheLyongroup( Baraffeetal. 1998 and Chabrieretal. 2000 ).IhaverecomputedtheTrapeziumRssusingthedataof Slesnicketal. ( 2004 )withtheLyonmodelsandndahighervalueof

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Table6. AbundancesofBrownDwarfsinYoungStarFormingRegions RegionRss=N(0:020:08M) Taurus.........0.170.041,2IC348.........0.120.033NGC2071......0.120.044NGC2068......0.230.064NGC2024......0.300.054Trapezium......0.300.054,5 References.(1) Luhman ( 2006 );(2) Guieuetal. ( 2006 );(3) Luhmanetal. ( 2003b );(4)thiswork;(5) Slesnicketal. ( 2004 ). 4 5 ,and 5 ,itisclearthathadIchosentoemploytheDM97modelsIwouldhavecountedfewerbrowndwarfs,leadingtolowervaluesofRssforeachcluster.Theseresultsbringupaninterestingpoint-namelythatexactvaluesofRssareclearlydependentonthechoiceofevolutionarymodels.Thus,ifwewishtoexaminetherelativeproportionsofstarsandbrowndwarfsfromregiontoregion,itisimportanttocompareresultsderivedfromthesamesetofPMStracks.Toremainconsistentintheensuingdiscussion,fromthispointforwardIwilldiscussonlythosevaluesderivedusingtheLyonmodels.Table 6 liststhemostrecentspectroscopicallyderivedRssvaluesforthethreeOrionBclusters,theTrapezium,IC348,andTaurus.NotethattheRssfortheTrapeziumistherecalculationdescribedabove,usingthedatafrom Slesnicketal. ( 2004 )withtheLyongroupmodels.Inaddition,thevaluepresentedforTauruswarrantssomediscussion.Originally,intheirstudiesoftheTaurusaggregates, Bricenoetal. ( 2002 )and Luhmanetal. ( 2003b )calculatedaslightlylowervalueof0.140.04fortheRss.AssurveysofTaurushavebeenexpandedbeyondtheaggregateradii,thecorrespondingratioofbrowndwarfstostarshasalsoincreased. Luhman ( 2004 )combinedopticalimaging

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andspectroscopywithinfraredphotometryfrom2MASStondRss=0.180.04)fora12.4deg2area.Morerecently, Guieuetal. ( 2006 ,hereafterG06)completedanopticalsurveycovering28deg2,nding12newbrowndwarfsintheregionandanupdatedRssof0.230.05.However,shortlythereafter, Luhman ( 2006 ,hereafterL06)completedasearchforbrowndwarfsintheentire225deg2areaofTaurusandfoundthat6ofthenewbrowndwarfsfoundbyGuieuetal.infacthadspectraltypesearlierthanM6andthuswerenotsubstellar. L06 recalculatedtheRssof G06 withtheupdatedclassicationsandderivedalowervalueofRss=0.170.04;thisisthevaluepresentedinTable 6 .Itisimportanttonotethat L06 doesnotattempttocalculateRssforhisentiresurvey,citingsignicantincompletenessinthespectraltyperangefromM2-M6.Further,henotesthathisrevisionofthe G06 resultmaystillnotbeanaccuraterepresentationoftheTauruspopulationduetosignicantincompletenessatM4-M6(33%complete)andM6(63%complete).Thus,itismostlikelythatanRssof0.17forTaurusisanupperlimittothetruevalue.LookingatTable 6 ,theabundancesofbrowndwarfsinIC348andNGC2071areconsistentwitheachotherandsignicantlylowerthanthoseofNGC2024ortheTrapezium.Inlightofthecommentsabove,itisalsoprobablethattheRssforTaurusisconsistentwiththelowvaluesofNGC2071andIC348ratherthanthehigherRssofNGC2024andtheTrapezium.ItisunclearwheretoplacetheRssforNGC2068.However,ascanbeseenfromFigure 5 ,thelargestamountofincompletenessinthesurveyofNGC2068occursinthemagnituderange11.095%complete.Thus,asforTaurus,itislikelythatthecurrentcalculationofRssforNGC2068isanoverestimateduetoincompletenessinthespectroscopicsamplejustabovetheHBL.

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Table6. PhysicalPropertiesofYoungStarFormingRegions RegionN(H2)aStellarDensitybClusterMasscEarliestSpectralTyped(cm2)(stars/pc3)(M) Taurus0.81013B9IC3482.0159160B5NGC20711.513060B2.5NGC20683.0130110B2NGC20244.6537180O9Trapezium>7.61616413O7 Aoyamaetal. 2001 (NGC2024,NGC2068,NGC2071), Hatchelletal. 2005 (IC348), Wilsonetal. 1999 (Trapez-ium),and Onishietal. 1998 (Taurus).bMassesoftheyoungclustersaretakenfrom Lada&Lada ( 2003 ).Thecorrespondingrefer-encesforTaurusis Gomezetal. ( 1993 ).cStellardensitiesfortheyoungclusterswerecalculatedbytakingtheclustercenterslistedin Lada&Lada ( 2003 )andusingFLAMINGOSphotometryofeachregiontocountthenumberofsourcesina0.5pcradiusdowntoK15.0.ThestellardensityoftheTaurusaggregateswastakenfrom Gomezetal. ( 1993 ).dThespectraltypeofthemostmassivestarineachregion,takenfrom Biketal. 2003 (NGC2024), Strometal. 1975 (NGC2068,NGC2071), Luhmanetal. 2003b (IC348), vanAltenaetal. 1988 (Trapezium),and Strom&Strom 1994 (Taurus). 6 presentsvariousphysicalpropertiesfortheregionsdiscussedabove,includinggascolumndensity,localstellardensity,totalmass,andthespectraltypeoftheearliest(i.e.mostmassive)starineachregion.Figures 6 to 6 showtherelationshipofeachoftheseparameterstotheRss.Itshouldbenotedthatwhilethegascolumndensity,totalmass,andspectraltypesforeachregionweretakendirectlyfromtheliterature,thestellardensitiesfortheyoungclusterswerecalculatedinaself-consistentmannerusingphotometryfromtheFLAMINGOSGMCsurvey.Clustercenterswere

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obtainedfrom Lada&Lada ( 2003 )andthenumberofsourcestoK15.0containedwithina0.5pcradiusofthecenterweretalliedandusedtocomputethevolumedensity.Inthisway,itisassuredthattherelativestellardensitiesofeachregionarecorrect.Figure 6 presentstherelationshipbetweenthehydrogencolumndensityandtheRss.Generally,theregionswithlowergasdensitieshavefewerbrowndwarfs,consistentwiththepredictionsoftheturbulentfragmentationmodels( Padoan&Nordlund 2004 ; Bate&Bonnell 2005 ).However,notallpointstthetrend.IfwetakethedatumforTaurusatfacevalue,IC348andNGC2071appeartohaveadefecitofbrowndwarfscomparedtoTaurusandNGC2024hasanexcess.EvenifwetakeintoaccounttherelativelylargeerrorbarsandconsidertheTaurusandNGC2068valuesasupperlimits,browndwarfformationarisingsolelyfromturbulentfragmentationcannotquiteexplainwhytheabundancesofbrowndwarfsinNGC2024andtheTrapeziumappeartobethesame,giventhelargevariationintheirrespectivecoredensities.Figure 6 showstheRssasafunctionofthestellardensityinstars/pc3.ThetrendisnotasclearinthisgureascomparedtoFigure 6 ,however,ifwetaketheTaurusandNGC2068pointsasupperlimits,theplotmayindicatethatthereisastellardensitythresholdabovewhichyoungclusterseitherproducemorebrowndwarfsorretainthemasejectedembryoscaughtinthelargergravitationalpotentialofthedenserclusters,assuggestedby Kroupa&Bouvier ( 2003 ).Intherstcase,ifmorebrowndwarfsareactuallyproduced,itmaybethatthedependenceoftheRssonstellardensityissimplyareectionoftherelationshipbetweenstellarandgasdensities(e.g. Ladaetal. 1991a )andbrowndwarfformationisagaintiedtotheJeansmassofeachregion.Inthesecondcase,iftheeffectisduetotheretentionofalargernumberofbrowndwarfsindenserregions,wemightthenexpecttoseeadependenceoftheRssonthetotalmassofeachregion.Figures 6 showstheRssasafunctionthetotalcluster(oraggregate)massinsolarmasses.Theredoesnotappeartobeasignicantcorrelation.Ifembryo-ejectionisthedominantmodeofbrowndwarfformationandifthemodelbehavessimilarlyin

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eachregion(e.g. Kroupa&Bouvier 2003 ),wemightexpecttoseelargerbrowndwarfabundancesinthemoremassiveclustersastheirlargergravitationalpotentialswouldmakeitharderfortheejectedobjectstoescape,however,thisdoesn'tseemtobethecase.Figure 6 showstheRssasafunctionofthespectraltypeoftheearlieststarineachregion.Aswiththeplotforgasdensity,thereappearstobeageneraltrendinthedata:theregionscontainingmassivestarshavelargernumbersofbrowndwarfs.IfweassumethatthetrueRssforTaurusandNGC2068(thetworegionsdepictedasupperlimits)aresmallerthandepicted,itisalsopossiblethatthecorrelationisastepfunction.Inthiscase,theregionswithOstarsaredifferentiatedfromthosewithout.ThisisconsistentwiththephotoerosionmodelsofbrowndwarfformationwheresubstellarobjectsformwhentheiraccretionenvelopesareprematurelyerodedbythestrongionizingradiationofmassivestarsinanHIIregion.ThereissomeadditionalevidencethatphotoionizationofprestellarcoresisalikelyexplanationfortheexcessofbrowndwarfsintheTrapeziumandNGC2024.OfthestarformingregionsdiscussedinthisChapter,onlyNGC2024andtheTrapeziumareafliatedwithHIIregions.IntheTrapezium,theOrionNebulaisexcitedbythe5Trapeziumstarsthemselves,whichrangeinspectraltypefrom07V-B0.5V.Diskaccretionratesinthevicinityoftheseobjectshavebeenfoundtobelowerthanaverage( Robbertoetal. 2004 ).TheionizingsourcefortheHIIregionassociatedwithNGC2024(theFlameNebula)hasrecentlybeenidentiedasIRS2b,anO8V-B2Vstarlocatedinthecoreofthecluster(cf.Figure 4 )andatthecenteroftheradiocontinuumradiationeld( Biketal. 2003 ). Biketal. ( 2003 )notethatIRS2bismorelikelytobelateOthanearlyB.Inaddition,NGC2024alsocontainsIRS2,acandidateearlyBstar( Lenorzeretal. 2004 )andsevencompactdustcondensationsthatarelikelymassiveprotostars( Mezgeretal. 1988 ).Finally,recentanalysisofarchivalChandradataby Ezoeetal. ( 2006 )hasuncovereddiffuseX-rayemissionemanatingfromtheregion.ThecenteroftheX-rayemissioncoincideswiththeninesourcesdescribedabove;theauthorsconcludethat

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theenergysourcefortheemissionislikelyshocksgeneratedbytheaccumulatedstellarwindsofyoungmassivesources.Ifphotoevaporationofaccretionenvelopesisthedominantmodeofsubstellarformation,wemayexpecttoobserveahigherdensityofbrowndwarfsinregionsclosesttotheionizingsource(s).Figure 6 showsthespatialdistributionofbrowndwarfsinNGC2024withtheradiocontinuumcontoursof Barnesetal. ( 1989 )outliningtheHIIregion.Whileitistruethatthemajorityofthebrowndwarfs(15/23)dofallwithintheboundariesoftheionizationfront,theregionalsoencompassessmostofourstellarsources.Unfortunatelythecurrentsurveydoesnothavethestatisticstoconclusivelytestthistheory,thusIdeferfurtherdiscussiontofutureworkandsimplynotethatphotoevaporationofprotostellarcoresremainsaviableoptionforbrowndwarfformationinregionswithmassivestars.

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wheretheregionswithhighergasdensitieshavelowercharacteristicmasses.Numericalsimulationsofthefragmentationofaturbulentmolecularcloudsupportthisconjecture,withthecharacteristicmassbeingsensitivelydependentonthelocalgasdensityandthesoundspeed.

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Figure6. 6 .NotethatthevaluefortheTrapeziumisalowerlimit.

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Figure6. 6

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Figure6. 6 .NotethatthemassshownforTaurusisthetypicalmassofoneoftheaggregates.

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Figure6. 6

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Figure6. DistributionofbrowndwarfsinNGC2024relativetotheHIIregion.Opencirclesarethestarsandlledtrianglesarethebrowndwarfs.Thepurplecon-toursshowthe1667MHzradiocontinuumemissiondetectedby Barnesetal. ( 1989 ).Theresolutionoftheradiomapis400.

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4 and 5 IhaveusedFLAMINGOSphotometryandspectroscopytoderivemedianagesforthelowmasspopulationsofthreeyoungclustersinOrionB. Steinhaueretal. ( 2006 )havealsoemployedFLAMINGOSdatatoderiveamedianagefortheTrapeziumclusterinexactlythesamemannerasIhavedonehere.Inthisnalchapter,IcombinethesedatawithpublishedworkontheOrionOBassociationtoexaminethestar-forminghistoryoftheregion.TheOrionOBassociation,OriOB1,haslongbeenknowntobealargeregionthatisactivelyformingmassivestars. Blaauw ( 1964 )identiedfourmajorsubgroupsintheareawhichdiffervastlyinbothageandamountofambientgasanddust.OriOB1aand1barelargelylocatedawayfromthemolecularcloudswithnoambientmolecularmaterialandwereestimatedby( Blaauw 1991 )tobeolder,withagesrangingfrom7-10Myr.OriOB1cisintheforegroundoftheOrionAcloudwithanoriginalageestimate(byBlaauw)of3Myr.OriOB1distheyoungTrapeziumcluster,stillembeddedinOrionAandestimatedbymanyauthorstobe<1Myr.Figure 7 showsthespatialrelationshipoftheOrionAandBmolecularclouds(indicatedbytheCOcontoursof Maddalenaetal. 1986 )totheassociationOriOB1anditssubgroups1a,1b,1candtheTrapezium.Inhisreview, Blaauw ( 1991 )postulatesthatthestarformationintheregionbeganwithOriOB1aandproceededinasequentialfashion,with1band1cresponsiblefortriggeringstarformationeventsintheOrionBandAclouds,respectively.InowexaminethishypothesisinlightofthenewagesdeterminedforthelowmasspopulationsofboththeOrionembeddedclustersandtheOBassociationsubgroups.Table 7 liststhenewlyderivedagesfortheclustersstudiedinthisdissertationalongwithagesfortheTrapeziumandsubgroups1a,1b,and1c.Asmentionedabove, 149

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Figure7. MolecularcloudsinOrionandtheirrelationshiptotheOrionOB1associa-tionanditssubgroups.Figurefrom Blaauw ( 1991 ). theageoftheTrapeziumisalsoderivedfromastudyoflowmassstarsintheregion( Steinhaueretal. 2006 ).Theagesofsubgroups1aand1bwereobtainedthroughthespectroscopicidenticationof200newlowmass(0:1M
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Table7. AgesofStar-FormingRegionsinOrion NGC202485.42750-1.896110.50.11NGC206886.674170.105830.90.31NGC207186.791670.321940.40.21Trapezium83.81833-5.389720.20.12OriOB1c83.86667-5.268894.62.03OriOB1b84.38333-1.671115.01.04OriOB1a81.641670.441948.51.54 References.(1)Thiswork;(2) Steinhaueretal. ( 2006 );(3) Brownetal. ( 1994 );(4) Bricenoetal. ( 2005 ). 7 showsahistogramofagesfortheregionsdiscussedabove.Basedonthisdiagram,itwouldappearthattherehavebeenthreedistinctepisodesofstarformationinOrion.Theoldestregionissubgroup1aat8.5Myr,followedbysubgroups1band1cat5Myrandnallytheyoungclustersembeddedintheclouds,whichallhaveages<1Myr.Itisinterestingtonote(1)thatallstarformationinthecloudsappearstobeoccuringsimultaneouslyand(2)thatwhileallfouroftheyoungestregionsremaindeeplyenshroudedintheirnatalclouds,theintermediate-agedsubgroupsarepartiallytomostlyremovedfromthecloudmaterialandOriOB1ashowsnoevidenceforanyremainingmoleculargas.Thisimpliesthatcloudlifetimesmustbelessthan8-10Myr,withmostofthemolecularmaterialdispersingonevensmallertimescales(t.5Myr).Finally,comparingFigures 7 and 7 itdoesappearthatstarformationinOrionisproceedingradiallyawayfromsubgroup1a.ThemostmassivestarinOB1aisdOri,anO9.5giantestimatedtobe45M( Brownetal. 1994 ).Figure 7 showstheageofeachregionasafunctionoftheangulardistance(indegrees)fromdOri,calculatedusingthecoordinateslistedinTable 7 .WiththeexceptionofOB1c(whoseagewasdeterminedinafardifferentmannerthanallotheragesunderdiscussion),theredoesseemtobeacorrelationbetweenageandpositionrelativetodOri.Notethatthisresultshouldbe

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takenastentativesincethisanalysisdoesnotincorporatethephysicaldistancetoeachregionastherearelargeuncertaintiesinthesemeasurements.Inconclusion,theobservedtrendinageasafunctionofpositionandthetrimodaldistributionofagesfortheregionisinagreementwithBlaauw'sinitialhypothesisthatstarformationinOrionisproceedingsequentially.ThetriggerfortherststarformationeventremainsunknownbutislikelyoneormoresupernovaexplosionsintheinitialOBassociation. Figure7. Histogramofagesforarangeofstar-formingregionsinOrion.Theexactages,errors,andreferencesfortheseregionscanbefoundinTable 7

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Figure7. Ageversusangulardistance(indegrees)fromdOriforstar-formingregionsinOrion.

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1. YoungbrowndwarfsareprevalentthroughoutOrionB. 2. Signicantnumbersofbrowndwarfs(40-50%)inOrionBarelikelysurroundedbycircumsubstellardisks. 3. ThemassfunctionsoftheOrionBclustersaresimilartoeachotherabovethehydrogen-burninglimitbutvarysignicantlyatsubstellarmasses.Inaddition,theIMFpeaksappeartobesomewhatdependentonthelocalgasdensity. 4. Theratioofbrowndwarfstostarsisnotinvariantbutappearstodependoncertainphysicalpropertiesofthestar-formingenvironment,includingthelocalgasdensity,stellardensity,andthepresenceorabsenceofhotmassivestars.Theseresultsindicatethatbrowndwarfformationisnotuniversalbutinsteadisde-pendentonthelocalstar-formingconditions.Themodelswhichbestttheobservationsarethoseinvolvingtheturbulentfragmentationoflowmasscloudcores,thephotoero-sionofaprestellarcorebyionizingradiationfromamassivestar,oracombinationof 154

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both.Thedatadonotfavortheformationofsignicantnumbersofbrowndwarfsviaembryo-ejectionorprotostellardiskcollisions.TheOrionBdatapresentedherehavehelpedplacepowerfulcontraintsonthepossiblemechanismsoflowmassstarandbrowndwarfformation.However,thereexiststhepotentialformanymorein-depthstudieswhichmaystrengthentheconclusions.Forexample,acommonsourceofuncertaintyinboththemassfunctionsandRssvaluespresentedhereoccursinthecorrectionforincompleteness.Followingtheprecedentof Luhmanetal. ( 2003b )inIC348and Luhman ( 2006 )inTaurus,onepossiblesolutionistoobtainaspectrumofeverysourceinthephotometricKLF.Thiswouldremovetheneedforacompletenesscorrection,therebyreducingtheuncertaintyinthenalIMF.Inaddition,increasingthesamplesizewillallowforsmallerbinsintheIMF,potentiallyrevealingadditionalstructurebelowthestellar/substellarboundary.Anotherlimitationtothedataisthelackofspatialcompletenessineachcluster.Asdiscussedinthepreviouschapter,radialsearchesforbrowndwarfsinthevicinityofHIIregionsmayprovefruitfulindeterminingtheimportanceofphotoerosionasamechanismforbrowndwarfformation.AdditionalconstraintsforthismodelmayalsobeprovidedbyastudyoftheaccretionratesoflowmasssourcesinNGC2024,similartotheworkdoneby Robbertoetal. ( 2004 )intheTrapezium.Finally,allspectrapresentedhereareinyoungclusters.Inordertofurtherinves-tigatetheeffectofenvironmentonthebrowndwarfIMF,itwouldbeinterestingtoexaminethedistributedbrowndwarfpopulationinOrionB.AsystematicspectroscopicstudyofMstarsinthecoreregionsof Ladaetal. ( 1991a ),thesmallclusterNGC2023,andthegeneralcloudpopulationwillprovidenewdatapointsandpossiblyrevealaddi-tionalfunctionaldependenciesoftheRssandcharacteristicmass,allowingustobetterunderstandthemysteriousbrowndwarfformationprocess.

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2 .Allinformationpresentedhereisforthenalselectionofeldsonly;dataarenotpresentedforallobservationsofeveryeld.Table A containstheobservinglogfortheeldschosenfornalphotometry.Table A presentsthenalphotometricscattermeasurementsbybandforeacheld,afterapplicationofthezeropointcorrection.Table A presentstheluminosityfunctionpeaksbybandforeacheld.Notethatthesevaluesaretakenasour90%completenesslimits. 156

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TableA. OrionBImagingObservingLog 0105:41:31.99-01:54:16.02001Dec18H3593151.2271.930105:41:31.99-01:54:16.02001Dec18J3593151.2601.980105:41:31.99-01:54:16.02001Dec18K3582801.2471.910205:40:11.99-01:54:16.02001Dec18H3572451.2201.770205:40:11.99-01:54:16.02001Dec18J3593151.2421.860205:40:11.99-01:54:16.02001Dec18K3582801.2801.690305:40:11.99-01:34:16.02001Dec18H3572451.2091.790305:40:11.99-01:34:16.02001Dec18J3593151.2211.920305:40:11.99-01:34:16.02001Dec18K3582801.3021.660405:41:31.99-01:34:16.02001Dec18H3593151.2012.020405:41:31.99-01:34:16.02001Dec18J3593151.2141.990405:41:31.99-01:34:16.02001Dec18K3593151.3201.630505:42:51.99-01:34:16.02001Dec18H3593151.1992.00

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TableAContinued 0505:42:51.99-01:34:16.02001Dec18J3593151.2062.030505:42:51.99-01:34:16.02001Dec18K3593151.3431.670605:44:11.99-01:34:16.02001Dec18H3593151.2002.040605:44:11.99-01:34:16.02001Dec18J3593151.2032.010605:44:11.99-01:34:16.02001Dec18K3593151.3701.660705:45:31.99-01:34:16.02001Dec18H3582801.4491.880705:45:31.99-01:34:16.02001Dec18J3593151.4931.800705:45:31.99-01:34:16.02001Dec18K3582801.4101.620805:46:51.99-01:34:16.02001Dec20H3593151.2381.850805:46:51.99-01:34:16.02001Dec20J3572451.2541.850805:46:51.99-01:34:16.02001Dec20K3593151.2441.730905:42:51.99-01:54:16.02001Dec20H3593151.2371.810905:42:49.99-01:53:46.02001Dec20J3582801.2831.80

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TableAContinued 0905:42:51.99-01:54:16.02001Dec20K3593151.2291.811005:44:11.99-01:54:16.02001Dec20H3572451.2231.901005:44:12.10-01:54:16.02001Dec20J3593151.3071.771005:44:11.99-01:54:16.02001Dec20K3593151.2211.861105:45:31.99-01:54:16.02001Dec20H3593151.2151.861105:45:31.99-01:54:16.02001Dec20J3593151.3271.771105:45:31.99-01:54:16.02001Dec20K3582801.2151.831205:46:51.99-01:54:16.02001Dec20H3582801.2091.921205:46:51.99-01:54:16.02001Dec20J3593151.3501.701205:46:51.99-01:54:16.02001Dec20K3582801.2101.971305:40:11.99-02:14:16.02001Dec20H3593151.2131.851305:40:11.99-02:14:16.02001Dec20J3593151.4121.751305:40:11.99-02:14:16.02001Dec20K3593151.2091.83

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TableAContinued 1405:41:31.99-02:14:16.02001Dec20H3593151.2091.971405:41:31.99-02:14:16.02001Dec20J3593151.4441.801405:41:31.99-02:14:16.02001Dec20K3562101.2081.951505:42:51.99-02:14:16.02002Feb01H3593151.2132.051505:42:51.99-02:14:16.02002Feb01J3593151.3421.961505:42:51.99-02:14:16.02002Feb01K3593151.2092.031605:44:11.99-02:14:16.02002Feb01H3593151.2191.991605:44:11.99-02:14:16.02002Feb01J3593151.3251.971605:44:11.99-02:14:16.02002Feb01K3593151.2092.011705:45:31.99-02:14:16.02002Jan05H3572451.3381.701705:45:31.99-02:14:16.02002Jan07J3582801.2631.811705:45:31.99-02:14:16.02002Jan05K3551751.2242.161805:46:51.99-02:14:16.02002Jan05H3593151.3121.81

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TableAContinued 1805:46:51.99-02:14:16.02002Jan07J3593151.2511.751805:46:51.99-02:14:16.02002Jan05K3593151.2192.291905:40:11.99-02:34:16.02002Jan05H3572451.3121.781905:40:11.99-02:34:16.02002Jan07J3572451.2361.831905:40:11.99-02:34:16.02002Jan05K3593151.2142.502005:41:31.99-02:34:16.02001Dec24H3593151.4262.412005:41:31.99-02:34:16.02002Jan05H3551751.2222.232005:41:31.99-02:34:16.02001Dec24J3593151.4672.542005:41:31.99-02:34:16.02001Dec24K3593151.3272.172105:42:51.99-02:34:16.02001Dec24H3593151.3852.232105:42:47.99-02:35:16.02001Dec24J3562101.5122.362105:42:51.99-02:34:16.02001Dec24K3593151.3522.282205:44:11.99-02:34:16.02002Jan07H3593151.3261.93

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TableAContinued 2205:44:11.99-02:34:16.02002Jan07J3582801.2171.752205:44:11.99-02:34:16.02002Jan07K3593151.2901.742205:44:11.99-02:34:16.02002Jan15K4093601.2401.962305:45:31.99-02:34:16.02002Jan07H3593151.3461.982305:45:31.99-02:34:16.02002Jan07J3593151.2141.812305:45:31.99-02:34:16.02002Jan07K3593151.2551.862405:46:51.99-02:34:16.02002Jan12H3572451.3552.222405:46:51.99-02:34:16.02002Jan07J3593151.2131.702405:46:51.99-02:34:16.02002Jan07K3593151.2361.892505:40:11.99-01:14:16.02002Feb01H3593151.4212.192505:40:11.99-01:14:16.02002Feb01J3582801.4852.212505:40:11.99-01:14:16.02002Jan07K3593151.2141.932605:41:31.99-01:14:16.02002Jan13H3572451.2551.73

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TableAContinued 2605:41:31.99-01:14:16.02002Jan13J3593151.2721.732605:41:31.99-01:14:16.02002Jan13K3593151.2331.582705:42:51.99-01:14:16.02002Jan13H3593151.2391.722705:42:51.99-01:14:16.02002Jan13J3562101.2881.692705:42:51.99-01:14:16.02002Jan13K3593151.2231.592805:44:11.99-01:14:16.02002Jan13H3593151.2251.692805:44:07.99-01:13:16.02002Jan13J3572451.3121.722805:44:11.99-01:14:16.02002Jan13K3593151.2141.632905:45:31.99-01:14:16.02002Jan20H3593151.4272.052905:45:31.99-01:14:16.02002Jan20J3593151.4681.902905:45:31.99-01:14:16.02002Jan20K3593151.2391.793005:46:51.99-01:14:16.02002Jan20H3593151.3882.503005:46:51.99-01:14:16.02002Jan20J3593151.5061.88

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TableAContinued 3005:46:51.99-01:14:16.02002Jan20K3593151.2511.753105:45:15.00+00:07:48.02002Jan13H3593151.1951.793105:45:15.00+00:07:48.02002Jan13J3593151.3081.603105:45:15.00+00:07:48.02002Jan13K3593151.1891.733205:45:50.00-00:13:48.02002Jan20H3593151.3441.853205:45:50.00-00:13:48.02002Jan20J3593151.5391.873205:45:50.00-00:13:48.02002Jan20K3593151.2551.763305:47:10.00-00:13:48.02002Jan20H3593151.3132.263305:47:10.00-00:13:48.02002Jan20J3593151.5831.913305:47:10.00-00:13:48.02002Jan20K35144901.2711.693405:46:35.00+00:07:48.02002Jan13H35144901.1831.813405:46:35.00+00:07:48.02002Jan13J3593151.3341.663405:46:35.00+00:07:48.02002Jan13K3593151.1831.88

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TableAContinued 3505:47:55.00+00:07:48.02002Jan21H3593151.2741.763505:47:55.00+00:07:48.02002Jan21J3582801.4101.883505:47:51.00+00:08:48.02002Jan21K3582801.1821.723605:47:55.00+00:27:48.02002Jan21H3572451.2512.093605:47:55.00+00:27:48.02002Jan21J3593151.4412.003605:47:55.00+00:27:48.02002Jan21K3593151.1751.753705:46:35.00+00:27:48.02002Jan13H3593151.1752.043705:46:35.00+00:27:48.02002Jan13J3593151.3681.773705:46:35.00+00:27:48.02002Jan13K3593151.1742.023805:45:15.00+00:27:48.02002Jan21H3593151.2241.843805:45:15.00+00:27:48.02002Jan21J3572451.5432.093805:45:15.00+00:27:48.02002Jan21K3572451.1731.753905:45:11.00+00:46:48.02002Jan21H3562101.2111.82

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TableAContinued 3905:45:15.00+00:47:48.02002Jan21J3593151.5812.113905:45:15.00+00:47:48.02002Jan21K3593151.1701.764005:46:35.00+00:47:48.02002Jan21H3593151.1941.654005:46:35.00+00:47:48.02002Jan21J3593151.6322.124005:46:35.00+00:47:48.02002Jan21K3593151.1731.724105:47:55.00+00:47:48.02002Jan21H3593151.1841.814105:47:55.00+00:47:48.02002Jan21J3593151.6872.284105:47:55.00+00:47:48.02002Jan21K3593151.1771.714205:47:08.00-00:33:18.02002Jan19H3582801.2212.084205:47:10.00-00:33:48.02002Jan19J3593151.2681.844205:47:10.00-00:33:48.02002Jan19K35103501.2621.844305:45:50.00-00:33:48.02002Jan19H35144901.2122.024305:45:50.00-00:33:48.02002Jan19J3562101.2922.40

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TableAContinued 4305:45:50.00-00:33:48.02002Jan19K3593151.2221.874405:45:50.00-00:53:48.02002Jan19H3593151.2081.934405:45:50.00-00:53:48.02002Jan19J3593151.3141.914405:45:50.00-00:53:48.02002Jan19K3593151.2121.894505:47:10.00-00:53:48.02002Jan19H3593151.1982.024505:47:10.00-00:53:48.02002Jan19J3593151.3371.814505:47:10.00-00:53:48.02002Jan19K3593151.2042.035605:43:53.99-03:14:16.02002Jan19H3593151.2281.965605:43:53.99-03:14:16.02002Jan19J3593151.4211.885605:43:53.99-03:14:16.02002Jan19K3593151.2282.065705:43:53.99-03:34:16.02002Jan19H3593151.2301.885705:43:53.99-03:34:16.02002Jan19J3593151.4631.935705:43:53.99-03:34:16.02002Jan19K3572451.2281.90

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TableAContinued 5805:45:13.99-03:34:16.02002Jan19H3593151.6512.165805:45:13.99-03:34:16.02002Jan19J3593151.4972.005805:45:13.99-03:34:16.02002Jan19K3582801.8622.485905:45:13.99-03:54:16.02002Jan19H3593151.6042.115905:45:13.99-03:54:16.02002Jan19J3593151.5472.045905:45:13.99-03:54:16.02002Jan19K3572451.7922.34n207105:47:08.00+00:19:55.02002Dec31H3593151.1761.76n207105:47:08.00+00:19:55.02002Dec31J3593151.1801.73n207105:47:08.00+00:19:55.02002Dec31K3593151.1741.82gap105:47:15.00-00:03:48.02003Nov20H3593151.1821.71gap105:47:15.00-00:03:48.02003Nov20J3593151.4411.78gap105:47:15.00-00:03:48.02003Nov20K3593151.3501.75gap205:45:55.00-00:03:48.02003Nov20H3593151.1791.69

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TableAContinued gap205:45:55.00-00:03:48.02003Nov20J3593151.3621.74gap205:45:55.00-00:03:48.02003Nov20K3593151.4121.72gap305:45:55.00+00:17:48.02003Nov20H3593151.1761.77gap305:45:55.00+00:17:48.02003Nov20J3593151.3091.72gap305:45:55.00+00:17:48.02003Nov20K3593151.4461.71gap405:42:12.00-01:44:16.02003Nov20H3593151.2061.69gap405:42:12.00-01:44:16.02003Nov20J3593151.3041.70gap405:42:12.00-01:44:16.02003Nov20K3593151.5531.71gap505:40:52.00-01:44:16.02003Nov20H3593151.2162.23gap505:40:52.00-01:44:16.02003Nov20J3593151.2561.72gap505:40:52.00-01:44:16.02003Nov20K3593151.6271.78cf105:58:00.00-04:56:00.02004Jan27H3593151.5851.87cf105:58:00.00-04:56:00.02004Jan27J3593151.4292.01

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TableAContinued cf105:58:00.00-04:56:00.02004Jan27K3593151.3971.89cf205:48:00.00-04:56:00.02004Jan27H3593151.4871.99cf205:48:00.00-04:56:00.02004Jan27J3593151.4222.16cf205:48:00.00-04:56:00.02004Jan27K3593151.3461.93cf305:41:13.00-03:06:00.02003Nov20H3593151.2061.58cf305:41:13.00-03:06:00.02003Nov20J3593151.4681.75cf305:41:13.00-03:06:00.02003Nov21K3593151.1691.93cf405:41:16.00-02:36:00.02003Nov20H3593151.2381.57cf405:41:16.00-02:36:00.02003Nov20J3593151.4231.72cf405:41:16.00-02:36:00.02003Nov21K3593151.1641.84cf505:58:00.00-02:00:00.02004Dec16H35134551.2361.71cf505:58:00.00-02:00:00.02004Dec16J35134551.2691.72cf505:58:00.00-02:00:00.02004Nov20K3593151.5431.73

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TableAContinued cf605:58:00.00-02:30:00.02004Dec16H35134551.2201.74cf605:58:00.00-02:30:00.02004Dec16J35134551.2871.75cf605:58:00.00-02:30:00.02004Nov20K3593151.5011.75cf705:41:00.00-03:36:00.02004Dec16H35134551.1531.76cf705:41:00.00-03:36:00.02004Dec16J35134551.2751.76cf705:41:00.00-03:36:00.02004Nov20K3593151.4601.76

PAGE 187

TableA. MeanPhotometricScatterbyField FieldsJsHsKComments 010.100.090.11NGC2024020.050.050.04030.050.030.04040.050.050.05050.050.050.05060.050.040.05070.040.040.05080.050.040.04090.040.040.06100.060.050.04110.050.050.04120.040.050.05130.050.040.04140.050.070.06NGC2023150.060.050.06160.060.060.07170.040.040.04180.040.040.11190.050.040.08200.060.040.04210.050.070.05220.040.050.04230.050.040.05240.050.040.04250.040.040.03260.050.050.04270.060.040.04280.060.050.05290.050.040.05300.040.040.04310.060.040.04320.040.040.05330.050.040.04340.070.070.06NGC2068350.040.040.05360.050.050.06370.060.060.07NGC2071

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TableAContinued FieldsJsHsKComments 380.040.040.03390.040.040.04400.040.040.04410.040.040.06420.050.040.04430.060.040.04440.040.050.04450.050.040.04560.040.040.04570.060.040.04580.040.040.05590.040.040.05n20710.100.080.08NGC2071g10.100.090.08NGC2024g20.080.080.10NGC2024g30.100.090.08NGC2024g40.110.090.12NGC2024g50.100.070.08

PAGE 189

TableA. LuminosityFunctionPeaksbyField FieldJLFpeakHLFpeakKLFpeak

PAGE 190

TableAContinued FieldJLFpeakHLFpeakKLFpeak

PAGE 191

2 ,x 176

PAGE 192

TableB. FLAMINGOS/OrionBSpectroscopicObservingLog RegionaMaskIDTelescopeObs.DatetexpNexpTotaltexpTargetSelectionCriteria 02(NGC2071)n2071f212.1m2003Nov28300s103000sKmagnituden2071f222.1m2003Dec01300s123600sKmagnitudeoc34mf1a4m2003Dec11300s226600sIRXsourcesoc44mf114m2003Dec11300s82400sIRXsourcesoc34mf2a4m2004Jan06300s247200sIRXsourcesoc44mf314m2004Jan07300s226600sIRXsourcesn2071a24m2004Nov26300s123600sHmagnituden2071a44m2004Dec03300s164800sHmagnituden2071a212.1m2004Dec20300s123600sHmagnituden2071a64m2005Dec17300s144200sHmagnituden2071a34m2005Dec20300s164800sHmagnituden2071a54m2005Dec21300s82400sHmagnitude04(NGC2068)ob4nm14m2003Jan15600s42400sBDcandidates300s82400sn2068f112.1m2003Nov30300s123600sKmagnituden2068f212.1m2003Nov30300s123600sKmagnituden2068a14m2004Dec01300s82400sHmagnituden2068a24m2004Dec02300s123600sHmagnituden2068a44m2004Dec03300s123600sHmagnituden2068a312.1m2004Dec14300s123600sHmagnitude06(LBS23)ob6bd14m2003Jan23600s84800sBDcandidatesob64m114m2004Jan03300s123600sIRXsourcesob64m214m2004Jan05300s113300sIRXsources08(LBS29,30)ob84m114m2004Jan06300s123600sIRXsources09(LBS31,32)ob94m114m2004Jan03300s61800sIRXsources

PAGE 193

TableBContinued RegionaMaskIDTelescopeObs.DatetexpNexpTotaltexpTargetSelectionCriteria 10(NGC2024)n2024bd14m2003Jan19600s84800sBDcandidatesn2024f312.1m2003Nov29300s144200sKmagnitudeoc24mf114m2003Dec06300s82400sIRXsourcesoc24mf214m2003Dec10300s133900sIRXsoucesn2024b24m2004Dec01300s103000sHmagnituden2024b34m2004Dec01300s103000sHmagnitude13ob13off14m2003Jan23300s82400sBDcandidates 2 andTable 2

PAGE 194

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JoannaLisaLevinewasbornonDecember3,1975inLeuven,Belgium.Afterspendingtherstfewmonthsofherlifeinthiswonderfulcountry,plantingtheseedsforalifelongappreciationofwafes,chocolate,andfruityLambicbeer,sherelocatedwithherparentstoNewYork.LivinginBrooklynforthreeyearstaughtherthatsheshouldalwaysrootforallNewYorksportsteams(atleastifshewantedDadtobeinagoodmood).JoannathenmovedtoPortChester,NYwheresheacquiredayoungersisterandnallytoWorcester,Massachusettswheresheacquiredayoungerbrotherandherownroom.InWorcester,atage6,Joannatookherrstballetclassandimmediatelyfellinlove.Atage9,JoannalookedthroughherrsttelescopeandaftercatchingaglimpseofJupiter,decidedthatitwouldbemorefuntogosleepinthecar.ShedidhoweverspendmanyhourswatchingStarTrekwithherfather,slowlyfosteringaninterestinthemysteriesofspace.Afterspendingthenext7yearsatthePerformingArtsSchoolofWorcesterandWalnutHillSchoolfortheArts,pursuingherdreamtobecomeaballerina,akneeinjurytemporarilysidelinedouryounghero.Ratherthandancingduringthesummerof1992,JoannawentofftoAstronomyCampinTucson,AZwhere,afterherrsttasteofrealastronomy,shedecidedtobecomeanastronomer.AftergraduationfromWalnutHill,JoannaattendedcollegeattheUniversityofMassachusettsatAmherst.Shebrieyentertainedtheideaofdouble-majoringinphysicsanddancebutuponndingthattheuniversityinconvenientlyscheduledphysicslabsduringballetclass,Joannasettledonsolelystudyingphysics.Althoughthelargeredfailingmarksoneverysingleoneofherfreshmanyearphysicsexamscouldhavebeenadeterrent,Joannastubbornlypersevered.Hereffortswererewardedduringherjunior 186

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yearwhenJoannanallyreceivedherrstAonaphysicsexamandwasawardedthespring1996PhysicsBookAward-alargehandbookofChemistryandPhysicswhichhasbeencollectingdusteversince.Duringthesummerof1996JoannatraveledtoFlagstaff,AZandspent10weeksworkingwithDr.StephenTeglerontheprimoridalsurfacesofKuiperBeltobjects.Thisresearchstimulatedherinterestintheformationofsolarsystems(oursandothers)andtaughthermorethansheeverwantedtoknowaboutphotometry(knowledgethatwillhauntherforever).Finally,attheendofthesummer,SteveencouragedJoannatoapplytotheUniversityofFloridaandforthatshewillalwaysbegrateful.JoannagraduatedfromUMASSinMay1997withaB.S.inPhysicsandmovedtosunnyGainesville,FL.ThereshebeganworkingwithElizabethLadawhotaughtherallaboutstarformationandthetruemeaningofthephrasestatisticallysignicant.Afteranunmentionablenumberofyears(longenoughfortheGatorstowinaNationalChampionshipinbasketball),Joannadefendedherdissertationandisnowponderingwhattodonext.HertimeatFloridawaspriceless-shemettheloveofherlife,TimothyB.Spahr,andmademanyclosefriendsalongtheway.Mostimportantly,duringthetailendofhergraduatecareerJoannafoundherwaybacktodance,spendingmuchtimedancingwithAloraHaynes,AlbertoAlonso,SoniaCalero-Alonso,andtheDanceTheatreofSantaFeCommunityCollege.Joannaisnowhopingtondajobthatwillallowhertobalancethescienceofdancewiththeartofscience.Inthemeantime,ifyouarereadingthisandhiring,feelfreetocontacther.


Permanent Link: http://ufdc.ufl.edu/UFE0017504/00001

Material Information

Title: Low Mass Star and Brown Dwarf Formation in the Orion B Molecular Cloud
Physical Description: Mixed Material
Copyright Date: 2008

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Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
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Permanent Link: http://ufdc.ufl.edu/UFE0017504/00001

Material Information

Title: Low Mass Star and Brown Dwarf Formation in the Orion B Molecular 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: UFE0017504:00001


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LOW MASS STAR AND BROWN DWARF FORMATION IN THE ORION B
MOLECULAR CLOUD















By
JOANNA LISA LEVINE


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


2006

































Copyright 2006

by
Joanna Lisa Levine

















This work is lovingly dedicated to:


Oma and Opa, Grandma and Grandpa, Mom, Dad, Sara, Jonathan, and Tim:

"Call it a clan2, call it a network, call it a tribe, call it a family. TThalvrlr' 1 you call it,

whoever you are, you need one. "

-Jane Howard







Alora, Alberto, Sonia, and everyone else along the way who has encouraged me to dance

while writing this dissertation:

"Great dancers~ddd~~~~ddd~~~ddd are not great because of their technique, they are great because of

their passion. "

-Mar tha Graham







The memory of Richard J. Elston (1961-2004):

"If you pretend you 're playing a computer game, data reduction can be really fun! "

-Richard Elston















ACKNOWLEDGMENTS

First and foremost, I would like to thank all of the members of the University of

Florida Astronomy Department. In your own way, each of you have contributed to my

finishing this dissertation. Special thanks go to my adviser, Dr. Elizabeth Lada, who was

an inspiration from the beginning, consistently providing many opportunities to explore

independent research and, best of all, gave me the freedom to pursue my own path to

the finish. Thank you also to my committee members (both current and former) Drs.

Richard Elston, Jonathan Williams, Steve Detweiler, Steve Eikenberry, and especially

Ata Saraj edini whose onfce door was always open for me to ask even the most mundane

photometry questions. Finally, thank you to Aaron Steinhauer, Andrea Stolte, Nick

Raines and the other members of the FLAMINGOS team. Without this amazing group

of people I would still be reducing data and classifying spectra. Large pink birds and UF

star formation postdocs are a special breed indeed!

My friends and fellow students in the Astronomy Department kept me sane during

my time in graduate school. I owe many thanks to Sue Lederer (my first ever big sister),

Lauren "shocked gas" Jones and Karl Haisch (officemates and local ISM gurus), Dave

and Jo Osip (tarot players extraordinaire), David Dahari (board and video game master),

Aaron Grocholski (my "gainesville boyfriend" and general partner in crime), Margaret

Moerchen and Lauren Davis (shopping buddies), Ashley Espy (sister in knee problems

caused by our respective obsessions), Audra Hernandez (fellow Starbucks addict and

friendly neighbor), Leah Simon (the other astro-dancer who rock climbs), and the real NO

DOUBT (a.k.a. the best graduate student onfce EVER).

I am also deeply grateful for the love and support I received from my friends in

the dance world. Jessica Mayhew, Blair Litaker, Heather Collier, Heather Banes, Stacey










Readout, and the other members of the Dance Theatre of Santa Fe Community College

(DTSF) accepted me as one of their own, keeping me young and happy and reminding

me what it means to dance. Rachel Everett kept me company, both in and out of ballet

class, proving to be a true kindred spirit who understands the beauty of a good friendship

and the agony of dancing in pointe shoes past age 30. Tari Kendall always provided

encouragement, even when I was taking up her office space, and Sonia Calero-Alonso

always uplifted me with her smile. Alberto Alonso, Brian Brooks, Janis Brenner, and

Peter Kalivas gave the gift of choreography, making me a part of their legacy and

allowing me to share my art with the world for that I will be eternally in their debt.

There are a few additional, extraordinary people who deserve special mention. My

officemate of five years, Carlos Roman-Zuiiga, played many roles including collaborator,

comedian, philosopher, counselor, coffee brewer (or re-brewer), life coach, and Sith lord.

However, first and foremost, he is, was, and always will be a great friend. I could not have

chosen a better person with whom to share graduate school.

Doug Ratay and Catherine Garland redefined the meaning of friendship. Even

though I was the first to start and the last to finish, they stood by and commiserated and/or

cheered me on every step of the way. It's not every day you find friends who are willing

to listen to a rough draft of your dissertation over the phone, one paragraph at a time.

Alora Haynes, co-director of DTSF, provided me with a place to go when I had

nowhere else. Her kind nature and encouraging words both in and out of the studio were

a constant source of inspiration and motivation as well as a strong reminder that there

was in fact a light at the end of the tunnel. By taking me into the DTSF community she

afforded me the opportunity to reawaken my passion and recapture the joy of dancing. As

a result, my final years in graduate school were the most fulfilling. There are not enough

stars in the sky to express my gratitude.

Finally, I could not have finished this dissertation without the love, encouragement,

and support of my family. Mom always listened and tried her best to understand graduate










student life. If that didn't work (or even if it did), I could always count on a little bit

of home (whether it was fall maple leaves, fresh apples, blueberry muffins, or Belgian

waffles) to arrive in the mail. Dad always reminded me that science should be fun; his

enj oyment of all things astronomical is an ongoing inspiration. I thank Sara for showing

me that the world is not a scary place and for always making me laugh. I thank Jonathan

for always being my voice of reason and for keeping me company in the world of science.

Last but by no means least, I will be forever grateful to my fiance~, Tim Spahr, for

supporting me wholeheartedly from day one in a way that no one else in the world ever

could. His endless love, unwavering support, and above all, extreme patience during my

tenure in graduate school made him a true prince charming.

Many people have been with me on this j ourney and I would not have had it any

other way.


















TABLE OF CONTENTS


page


ACKNOWLEDGMENTS .

LIST OF TABLES

LIST OF FIGURES .

ABSTRACT. .

CHAPTER

1 INTRODUCTION

1.1 Why Study Low Mass Star Formation?
1.2 The Current Paradigm for Low Mass Star Formation .
1.2.1 Formation of a Single Low Mass Star .
1.2.2 Star Formation in Clusters .
1.2.3 The Initial Mass Function .
1.2.4 Brown Dwarfs .
1.3 Orion B: An Ideal Testbed for Star Formation Studies
1.4 A NIR Spectroscopic Study of Young Brown Dwarfs in Orion B

2 NEAR-INFRARED IMAGING AND SPECTROSCOPY OF ORION B


2.1 The FLAMINGOS Instrument ....
2.2 The FLAMINGOS GMC Survey . .
2.3 FLAMINGOS Imaging of Orion B .....
2.3.1 Survey Strategy and Observations . .
2.3.2 LongLegs: The Data Reduction Pipeline . .
2.3.3 Pinkpack: The Photometry and Astrometry Pipeline .
2.3.4 Positional Zero Point Correction ....
2.3.5 Imaging Data Quality and Completeness . .
2.4 FLAMINGOS Spectra of Orion B .........
2.4.1 Spectroscopic Sample Selection and Mask Design .
2.4.2 Spectroscopic Observations . .
2.4.3 Data Reduction ....
2.4.4 Final Spectroscopic Sample .....

3 SPECTRAL CLASSIFICATION OF YOUNG M STARS . .

3.1 Classification Strategy .
3.2 FLAMINGOS Late-Type Spectroscopic Standards .


. 12
. 13
. 13
. 13
. 16
. 19
. 25
. 28
. 34
. 34
.. .. 40
. 43
. 45

. 47










3.3 A Reddening-Independent Procedure for Classifying Late-Type Spectra


3.4 Surface Gravity


. . 54


4 M STARS AND BROWN DWARFS INNGC 2024

4.1 Introduction
4.2 New Photometry for NGC 2024 .
4.3 Spectroscopy of NGC 2024 .
4.3.1 Sample and Observations
4.3.2 Results .
4.4 The Hertzsprung-Russell Diagram
4.4.1 Extinction
4.4.2 Effective Temperatures and Bolometric Luminosities
4.4.3 H-R Diagram
4.4.4 Masses and Ages
4.5 Properties of the Low Mass Cluster Population .
4.5.1 Cluster Membership .
4.5.2 Cluster Age
4.5.3 Spatial Distribution of Sources
4.5.4 Sub stellar Disk Frequency .
4.5.5 Low Mass IMF
4.5.6 The Ratio of Brown Dwarfs to Stars
4.6 Summary

5 M STARS AND BROWN DWARFS IN NGC 2068 AND NGC 2071 .

5.1 Introduction
5.2 Photometry of NGC 2068 and NGC 2071
5.3 Spectroscopy of NGC 2068 and NGC 2071
5.3.1 Sample Selection, Observations, and Data Reduction
5.3.2 Results .
5.4 The Hertzsprung-Russell Diagram
5.4.1 Extinction, Effective Temperatures, and Bolometric Luminosities
5.4.2 H-R Diagrams for NGC 2068 and NGC 2071
5.5 Low Mass Populations of NGC 2068 and NGC 2071
5.5.1 Cluster Membership .
5.5.2 Cluster Ages.
5.5.3 Spatial Distribution of Sources
5.5.4 Infrared Excess and Substellar Disk Fractions ..........
5.5.5 Initial Mass Functions for M Stars in NGC 2068 and NGC 2071
5.5.6 Ratio of Stars to Brown Dwarfs in NGC 2068 and NGC 2071 ..
5.6 Summary .........


112
112
122
125












6 LOW MASS STARS IN YOUNG CLUSTERS: IMPLICATIONS FOR BROWN
DWARF FORMATION . . 127


6.1 Models of Brown Dwarf Formation ....
6.2 Observational Constraints .....
6.2.1 Substellar Disk Frequencies .....
6.2.2 Initial Mass Functions ......
6.2.3 The Abundance of Brown Dwarfs ....
6.3 Overall Implications for Brown Dwarf Formation


.... . 17
130
130
132
136
. . 142


7 THE STAR-FORMINGHISTORY OF ORION . . 149

8 FUTURE WORK . . 154

APPENDIX


A IMAGING SURVEY: OBSERVING LOG AND SURVEY STATISTICS .

B SPECTROSCOPIC SURVEY: OBSERVING LOG ......

REFERENCES ......

BIOGRAPHICAL SKETCH ......


. 156

. 176

. 179

. 186




















LIST OF TABLES


Table

2-1


3-1


3-

4-1


4-

5-1


5-


5-2

5- L


6-1


6-

7-1


A-

A-


A-


. 36


.. .. 49


. 54

. 59


. 70

. 87


. 90

. 99


S. . 100


. . 137


S. . 139

S. . 151


S. . 157

S. . 173


S. . 174


Regions Targeted for Spectroscopic Observation . .


Young Spectral Standards ....

Surface Gravity Standards .....

Slit Masks Observed in NGC 2024 ....


Data for Classified Sources in NGC 2024 . .

Detail of Slit Masks Observed in NGC 2068 ....


Detail of Slit Masks Observed in NGC 2071 ....


Data for Classified M Stars in NGC 2068 ....

Data for Classified M Stars in NGC 2071 ....


Abundances of Brown Dwarfs in Young Star Forming Regions .


Physical Properties of Young Star Forming Regions ....

Ages of Star-Forming Regions in Orion . .

Orion B Imaging Observing Log . .

Mean Photometric Scatter by Field ....


Luminosity Function Peaks by Field . .


B-1 FLAMINGOS/Orion B Spectroscopic Observing Log



























. 10

. . 14

. 16

. 19

. 23

.. .. 24

.. .. 26

.. .. 27

. 28

.. .. 29

. 31

. . 33

. 35

. 38

.. .. 40

.. .. 42

.. .. 44

.. .. 44

. 45

. 48

. 50

S51


1 Optical image of the Orion B region ....

2 CO map of OrionB ....

1 KPNO 2.1Im FLAMINTGOS Fields Observed in Orion B .


2 Raw K-band FLAMINGOS Image . ...

3 Final Image Reduced with LongLegs . ..

-4 Precision of Pinkpack Astrometry . ...

5 Sample Zero Point Calculation Histograms . .

-6 Effect of Coma on the Stellar PSF ....


7 Effect of Coma on Stellar Photometry ....

-8 Photometric Zero Point Correction ....


9 Final Photometric Scatter by Band . ...

10 Ten Sigma Detection Limits for the Imaging Survey . .

11 Survey Luminosity Functions and 90% Completeness Limits

12 Fields Targeted for Spectroscopic Observations ....

13 Sample Selection Diagrams for Spectroscopic Targets .

14 Sample Mask Designed for N2071 ....

15 Example of an Aligned MOS Plate . .

16 Imagesof Reduced Spectra . .

17 Two-Dimensional Image of a Single MOS Slitlet . .

18 Final Reduced Spectrum ....

1 NIR spectra oflate-type field dwarfs . .

2 FLAMINGOS Spectral Sequence for Young M Stars ....

3 Effect of Reddening on MDwarf Spectra . .


LIST OF FIGURES


e


Figurl

1-
1-


1-
2-


2-
2-


2-
2-


2-
2-


2-
2-


2-
2-


2-
2-


2-
2-


2-
2-


2-
3-

3-












Visual Dereddening of Program Spectra ....

Visual Classification of Program Spectra . .

Surface Gravity Effects in NIR Spectra of M Stars . .


Optical Image ofNGC 2024 ....

NIR Photometry of Classified Sources in NGC 2024 .

Three-color image ofNGC 2024 ....

NIR Spectra of M stars in N2024 .....

Distribution of Av for NGC 2024 Spectroscopic Sample .

H-R Diagrams for NGC 2024 . .


Spatial Distribution of Classified Sources in NGC 2024 ..

Uncorrected and Extinction-Limited KLFs for NGC 2024 .

Mass Function for NGC 2024 . .


Optical Image of NGC 2068 and NGC 2071 ....

Color-Magnitude and Color-Color Diagrams for NGC 2068

Color-Magnitude and Color-Color Diagrams for NGC 2071


. .

. .


. 52

. 53

. 55

. 57

. 58

. 60

.. .. 62

. . 67

. 69

. . 76

. . 79

. 81

. 84

. . 86

. . 86

. 88

. 89

.. .. 92

. 96

101

102

104

105


5-4 Three-Color Image ofNGC 2068 . .

5-5 Three-Color Image ofNGC 2071 .....

5-6 NIR Spectra of M stars in N2068 .....

5-7 NIR Spectra of M stars in N2071 ....

5-8 H-R Diagrams for NGC 2068 . .

5-9 H-R Diagrams for NGC 2071 .....

5-10 Distribution of Av for M Stars in NGC 2068 .

5-1 1 Distribution of Av for M Stars in NGC 2071 ..

5-12 Location of Classified Sources in NGC 2068 as ~

5-13 Location of Classified Sources in NGC 2068 as ~

5-14 Location of Classified Sources in NGC 2071 as ~

5-15 Location of Classified Sources in NGC 2071 as ~


a Function of Age ..

a Function of Mass ..


a Function of Age ..

a Function of Mass ..


.. 108

. 109

. 110

. 111











5-16 Uncorrected K-Band Luminosity Functions for NGC 2068 and NGC 2071 113

5-17 Average KLF for Orion B Control Fields . . 115

5-18 Background Subtracted KLF for NGC 2068 . . 117

5-19 Background Subtracted KLF for NGC 2071 . . 118

5-20 Completeness ofNGC 2068 Spectra . . 119

5-21 Completeness ofNGC 2071 Spectra . . 120

5-22 Mass Function for M Stars in NGC 2068 . . 123

5-23 Mass Function for M Stars in NGC 2071 . . 124

6-1 Initial Mass Functions of Young Clusters . . 133

6-2 IMF Peaks vs. Gas Density . . 135

6-3 Rssvs. Gas Density . ...... ... .. 144

6-4 Rssvs. Stellar Density . ..... .. .. 145

6-5 Rssvs. Total Mass . ..... .. .. 146

6-6 Rssvs. Spectral Type . ...... .. .. 147


Distribution of Brown Dwarfs in NGC 2024 Relative to the HII Region

Molecular Clouds in Orion and Their Relationship to Ori OB1 ....

Histogram of Ages for Star-Forming Regions in Orion ..... ..

Age vs. Angular Distance from 8 Ori .......


.. 148

. 150

. 152

. 153















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 Philosophy

LOW MASS STAR AND BROWN DWARF FORMATION IN THE ORION B
MOLECULAR CLOUD

By

Joanna Lisa Levine

December 2006

Chair: Elizabeth A. Lada
Major Department: Astronomy

I present an extensive near-infrared imaging and spectroscopic survey of young, low

mass obj ects in the Orion B molecular cloud. Results of this survey are used to investigate

the shape of the low mass initial mass function (IMF) and examine the stellar and

substellar populations of three young clusters in Orion B, ultimately placing observational

constraints on models of brown dwarf formation.

Classical star formation theory predicts that the minimum mass required for the

birth of a star is roughly one solar mass. However, studies of Galactic field stars have

revealed many smaller obj ects, including significant populations of sub-solar mass stars

(M~0.2-0.3Meo) and brown dwarfs (M<0.08Meo). The origin of these objects remains

an unsolved problem in modern astrophysics. Using FLAMINGOS on the Kitt Peak

National Observatory 2. 1 and 4 meter telescopes, I have completed a new J, H, and

K-band imaging survey of ~6 square degrees of Orion B and compiled a new library of

~200 JH spectra of M stars in the young clusters NGC 2024, NGC 2068, and NGC 2071.

I combine the photometry and spectroscopy to construct Hertzsprung-Russell diagrams,

inferring masses and ages for cluster members using pre-main sequence evolutionary










models. Median ages, substellar disk frequencies, IMFs, and the abundance of brown

dwarfs (Rss) are determined and the spatial distribution of M stars is discussed.

The results show the IMF peaks for the Orion B clusters (Mpeak 0.2-0.3Me)

are consistent with each other but different from isolated star forming regions such as

Taurus. There is also evidence for a dependence of the peak mass on local gas density. A

significant fraction of brown dwarfs are shown to have an infrared excess, indicative of

circumsubstellar disks. Finally, I find that the Rss is not universal but varies from region to

region. After examining the dependence of Rs on local physical properties, I find no clear

trend with cluster mass, but some dependence on stellar and gas density and the spectral

type of the most massive star in each region. I conclude that the outcome of the brown

dwarf formation process appears to be dependent on the local star-forming environment.















CHAPTER 1
INTRODUCTION

1.1 Why Study Low Mass Star Formation?

Stars are the basic units of structure in our universe. Individual stars are home to

planetary systems, providing life-sustaining energy and heat. The disk of our galaxy is

comprised of expanding populations of stellar clusters which play a key role in galactic

structure and dynamics. The death of massive stars enriches the interstellar medium

(ISM) with heavy elements thus stellar evolution is a direct influence on the chemical

evolution of galaxies. Finally, the net abundance of stars determines the mass-to-light

ratio of galaxies, affecting dynamics and evolution on a universal scale. Clearly, if

astronomers are to understand how the universe and its components formed and evolved,

we must begin by quantifying the life histories of stars.

During the past century astronomers have made great progress towards this goal

by developing detailed theories of stellar structure and evolution. However, a complete

picture of the star formation process still eludes us. For example, there is currently no

single theory which can predict the observed distribution of stellar masses at star birth

(otherwise known as the initial mass function or IMF). In addition, while the birth and

evolution of isolated, solar-type stars is reasonably well constrained, there are large gaps

in our understanding of the physical processes governing star formation at the high and

low mass ends of the mass spectrum. Further, it is unclear how these processes at either

end are affected by the local star-forming environments.

The goal of this dissertation is to investigate the low mass star formation process.

(In this context, the phrase "low mass star" refers to obj ects with masses less than

one solar mass, M~<1 Me0.) In particular, I seek to determine the shape of the initial

mass function below the hydrogen-burning limit (HBL, M~<0.08 Me0). This regime is










especially important because it defines a class of objects (called brown a~varfs) whose

masses bridge the gap between stars (M> 0.08 M..) and planets (M~< 0.012M..).

Unfortunately, unlike stars which are known to form from the collapse of molecular cloud

cores ( 1.2. 1) and planets which are thought to form via accretion in a circumstellar

disk, the formation of brown dwarfs currently has no widely accepted explanation. This

dissertation combines photometry and spectroscopy of low mass stars and brown dwarfs

to a) place observational constraints on the possible mechanisms driving brown dwarf

formation and b) examine how the brown dwarf formation process relates to that of low

mass stars.

1.2 The Current Paradigm for Low Mass Star Formation

When looking at the Milky Way on a clear moonless night, the sharp-eyed observer

will notice that the bright band of stars extending across the sky is interrupted by a

number of dark patches through which no visible starlight pentrates. These regions,

known as Giant Molecular Clouds (GMCs), are the sites of star and planet formation in

our galaxy.

GMCs are composed primarily of very cold (T~ 10-50 K) molecular hydrogen

gas (H2) and interstellar dust. Their sizes range from 20-100 pc and their total masses

range from 104-106 M0,, yielding mean densities of 50-100 cm3 (Lada, 2005). The

star formation process converts the molecular material contained in these large and cold

clouds into much smaller and hotter hydrogen-burning stars.

1.2.1 Formation of a Single Low Mass Star

According to the theory developed by Shu and collaborators (Shu, 1977; Shu et al.,

1987), neglecting the influences of cloud rotation and magnetic fields, the star formation

process begins when the molecular cloud begins to collapse due to its internal gravity.

As the cloud collapses, the density increases non-homogeneously, allowing the cloud

to fragment and form regions with extremely high local densities (n(H2)~ 104-5 cm ).

If the density is high enough, these cloud cores become gravitationally unstable and










collapse isothermally until their increased densities cause the cores to become optically

thick to their own radiation (e.g. radiation is absorbed and acts to increase the central

temperature of the cores). At this point a protostar (a quasi-static stellar core) is born.

Note that classical star formation theory predicts that there is a minimum mass required

for the collapse of a dense core into a protostar (Jeans, 1902) and that for a typical star

forming cloud this mass is on the order of a solar mass (Larson, 1995). As I will discuss

in upcoming sections, this is a problem when considering the formation of obj ects with

masses significantly smaller than 1 Me-

The protostar is surrounded by an infalling envelope of gas and dust. To conserve

angular momentum, the infalling envelope forms a disk around the embryonic core.

The protostar continues to gain mass through the accretion of material from the disk.

At this point, protostellar luminosities are dominated by an accretion luminosity which

effectively dissipates the gravitational potential energy lost during infall and collapse.

When the mass of the protostar increases such that the central temperature reaches 106K,

deuterium burning begins, adding a new component to the protostar's luminosity. At

the onset of deuterium burning, accretion typically slows down, the remnants of the

protostellar envelope disperse, and the protostar becomes visible. This definines the start

of the pre-main sequence (PMS) phase of stellar evolution.

Pre-main sequence stars bum their primordial deuterium rather quickly. Without

significant accretion to replenish their fuel stores, nuclear reactions cease and the

luminosity begins to decrease. The lack of radiation pressure in the core allows gravity

to take over and the PMS star will again begin to contract. During this second epoch of

contraction planet formation is thought to occur within the remnants of the circumstellar

accretion disk. PMS evolution ends when the gravitational contraction has caused the

central temperature to increase to 10 K. At this point, the nuclear hydrogen burning that

defines a star begins, the star's luminosity stabilizes, and the star is said to be on the main

sequence (MS).










1.2.2 Star Formation in Clusters

The scenario above describes the formation of a single, isolated low mass star.

However, we have learned that most stars are not born in isolation but rather form

in dense clusters embedded in the largest and most massive cores of GMCs (Lada

et al., 1991b; Carpenter, 2000; Lada & Lada, 2003; Porras et al., 2003). If we wish to

truly understand the star formation process, we need to examine the embedded cluster

population of GMCs.

Star clusters of any sort are important laboratories for astrophysical research.

They contain a statistically significant (N E2 50) number of stars with a wide range of

masses in a relatively small volume of space. Their members share a common origin,

having formed from the same parent GMC. Additionally, cluster stars are at roughly

uniform distances with similar ages and chemical compositions. The embedded phase

of a cluster's evolution typically lasts around 3 Myr (Lada, 2005). At these ages, even

the low mass cluster population is fairly bright (e.g. 01.2. 1 and 1.2.4) and more readily

observable than later in the cluster's lifetime. Further, embedded clusters are too young to

have lost significant numbers of stars to stellar evolution and/or dynamical evaporation.

Observations of embedded clusters therefore provide snapshots of stars in their natal

environments. Consequently, embedded clusters are extremely useful tools for star

formation studies.

1.2.3 The Initial Mass Function

A good example of the power of embedded clusters is the role they play in deter-

mining the initial mass function (IMF). The IMF is defined as the distribution of stellar

masses at star birth (Salpeter, 1955) and it is a powerful tool used to constrain formation

and evolution theory across the entire spectrum of astrophysics. In particular, determining

the shape of the IMF, including the locations of the turnover and minimum masses, is

vital to our understanding of the physical processes that control star and planet formation.

However, in most cases the IMF is not a readily observable quantity. N-body simulations










show that older open clusters (z ~ 100 Myr) contain a mere 20-30% of the stars in the

original embedded clusters (Kroupa et al., 2001). These clusters require corrections for

dynamical evolution and mass loss in any attempt to determine their IMFs. In contrast,

the observed mass distribution in an embedded cluster is its IMF. The IMF of massive

stars has been studied in this manner for many years and is reasonably well constrained

(see Massey, 1998, for a review). The IMF of low mass stars, on the other hand, is far less

certain due in part to the relatively recent (within the last ~-10 years) addition of brown

dwarfs.

1.2.4 Brown Dwarfs

Brown dwarfs (BDs) are low mass (M< 0.08 M.), low luminosity objects charac-

terized by their inability to sustain nuclear hydrogen burning. Rather, they shine brightly

for the first few million years of their lives via the transformation of gravitational poten-

tial energy into heat. However, while full-fledged stars achieve core temperatures high

enough to ignite hydrogen, the core temperatures of brown dwarfs remain below the

HBL (Tcore a 107K). Without a stable internal energy source, brown dwarfs spend the

remainder of their lives cooling and fading from view (Burrows et al., 1997).

The identification of brown dwarfs poses an interesting observational challenge,

neatly illustrated by the long amount of time that elapsed between the first theoretical

mention of "black dwarfs" by Kumar (1963) and the first confirmed detection of such

an object (GL 229B, Oppenheimer et al., 1995; Nakajima et al., 1995). This long time

interval between theory and observation was due in part to the lack of appropriate

detector technology. The most massive middle-aged brown dwarfs (M=-0.08M.,, age~ 1

Gyr) have absolute magnitudes V ~19 (Baraffe et al., 1998). Placing these objects at

the distance of the nearest star-forming regions (d~100 pc) yields apparent magnitudes

v ~24, well below the limit of standard optical detectors on mid-size telescopes. At near-

infrared (NIR) wavelengths, however, brown dwarfs are much brighter. In the K-band

(2.2 pum) our example middle-aged brown dwarf would have a magnitude K ~ 15 and a










corresponding young brown dwarf in the nearby star forming region (z~1 Myr) would

be blazing away at K ~-10, easily detectable on a 1 m class telescope with the infrared

detectors available today.

With the advent of deep, large-scale surveys such as the Deep Near-Infrared Survey

(DENIS, Epchtein et al., 1994), the 2 Micron All Sky Survey (2MASS, Skrutskie et al.,

1997), and the Sloan Digital Sky Survey (SDSS, York et al., 2000), the field of brown

dwarf research has undergone rapid expansion over the past decade, resulting in the

detection of numerous field brown dwarfs and the definition of two new spectral classes

(e.g. Delfosse et al., 1997; Kirkpatrick et al., 1999; Leggett et al., 2000). Analysis of

these observations has led to the conclusion that brown dwarfs constitute a significant

fraction of the stellar population in the solar neighborhood and may comprise as much as

15% of the galactic disk mass (Reid et al., 1999; Chabrier, 2002).

A robust theory of star and planet formation must therefore take brown dwarf

formation into account. However, as mentioned earlier in this chapter, classical star

formation theory has trouble explaining the formation of obj ects with masses significantly

smaller than a solar mass. Consequently, the origin of brown dwarfs is still unclear.

Do brown dwarfs form in a manner similar to their stellar counterparts or more akin

to their planetary cousins? What mechanism drives brown dwarf formation and does

it depend on the star forming environment? What is the shape of the brown dwarf

IMF? Recently many theories of brown dwarf formation have been proposed, including

turbulent fragmentation of a molecular cloud (Padoan & Nordlund, 2002), ej section of

protostellar embryos (Reipurth & Clarke, 2001), protostellar disk collisions (Lin et al.,

1998), and photo-erosion of prestellar cores (Whitworth & Zinnecker, 2004). Studies of

young brown dwarfs in their birth environments are needed to distinguish between these

scenarios.














































Figure 1-1. Optical image of Orion B: North is up and East is left. The brightest star in
the field is 5 Ori, the easternmost star in Orion's Belt. NGC 2024 (the Flame
Nebula) lies just east of 5 Ori. NGC 2023 is the blue reflection nebula south
of NGC 2024, with IC 434 and the Horsehead Nebula slightly to the south-
west. NGC 2068 (M78) and NGC 2071 are in the northeast corner of the
image. Photo: W. H. Wang (IfA, U. Hawaii)

1.3 Orion B: An Ideal Testbed for Star Formation Studies

This dissertation is a study of young brown dwarfs and low mass stars in the natal

environments of the Orion B molecular cloud. Historically, Orion B (or Lynds 1630)

has long been known to be an extensive locale of active star formation, even prior to its









identification as a dark cloud by Lynds (1962). Early studies detected a number ofH oc

emission line stars east and north of Orion's belt (Haro & Moreno, 1953; Herbig & Kuhi,

1963), and the region's colloquial namesake, Orion "B" (the second strong radio source

detected in the Orion region), was found to be a compact H II region (NGC 2024) more

than 50 years ago (Hepburn et al., 1954). Practically, Orion B lends itself to the study of

young, low mass objects as it is one of the nearest regions of ongoing star formation and

the nearest region of massive star formation. Distance estimates to the cloud range from

390-415 pc (Anthony-Twarog, 1982) although Brown et al. (1994) point out that there

may be a gradient across the region with the near cloud edge at 320 pc and the far edge at

500 pc. Assuming a median distance of 400pc, the proximity of Orion B implies that an

unrededdened 1 Myr brown dwarf will have a K magnitude of ~13 easily observable

with today's telescopes and NIR instruments. In addition, Orion B is conveniently located

out of the plane of the galaxy (l=206", b=-15"), reducing foreground and background

contamination by the general galactic stellar population.

Orion B contains many spectacular obj ects known for more than a century to both

amateur and professional astronomers alike: the reflection nebulae NGC 2068 (M78),

NGC 2071, and NGC 2023, the H II regions NGC 2024 (The Flame Nebula) and IC 434,

and the small dark cloud of the Horsehead Nebula (B33) seen in silhouette. All of these

regions can be seen in the optical image shown in Figure 1-1. However, optical images

do not show the whole picture. Recall that the visible hallmark of a GMC is a broad, dark

swath of sky with little to no optical starlight. In actuality, GMCs are not devoid of stars

at all. It is the large concentrations of dust within a GMC that absorb and scatter the light

from background stars causing the region to appear dark. In order to truly unlock the

secrets of star formation contained in a GMC, longer wavelength observations are needed

to penetrate the dust.

Radio observations (h=2.6 mm) of the Orion B region by Tucker et al. (1973)

revealed a large complex of extended CO emission (indicating the presence of molecular









hydrogen) coinciding with the optical dark cloud. The optical nebulae described above

were found not to be singular sources of CO emission but rather maxima located

within a much larger region of molecular line emission. The full extent of Orion B was

subsequently delineated by the higher sensitivity CO observations of Maddalena et al.

(1986) who found that the cloud subtends approximately 19 deg2 on the the sky and

contains nearly 105 M.0 of gas (Figure 1-2).

Lada et al. (1991a) carried out a systematic search for dense gas (n>104cm ) in

Orion B using the J(2-1) transition of CS as a tracer and found that only a small fraction

(< 19%) of the total cloud mass is located in the dense cores and is thus involved in

the star formation process (recall ~1.2. 1). In a companion 2.2 pum survey, Lada et al.

(1991Ib) unexpectedly discovered that 60-90% of the young stellar population resides

in 3 populous clusters (NGC 2024, NGC 2068, and NGC 2071) embedded in the most

massive dense molecular cores. This result was confirmed by Carpenter (2000) using data

from 2MASS. However, the completeness limits of both the Lada survey (K <13.0) and

2MASS (K, ~14.0) were too bright to probe very far below the hydrogen burning limit.

This dissertation extends the current body of work in Orion B beyond the limits of the

previous surveys, specifically focusing on the contribution of young brown dwarfs to the

overall embedded cluster population.

1.4 A NIR Spectroscopic Study of Young Brown Dwarfs in Orion B

Many recent studies of the low mass populations in young clusters have detected

significant numbers of brown dwarfs using deep near-infrared (NIR) photometry (e.g.

Hillenbrand & Carpenter, 2000; Luhman et al., 2000; Muench et al., 2002 in the Orion

Nebula Cluster and Muench et al. 2003 in IC 348). However, stellar and substellar

mass functions derived from photometry alone can only be studied in a statistical sense.

Individual mass estimates remain ambiguous due to uncertainties in the age, extinction,

and membership status of any given source. A determination of effective temperatures

using NIR spectroscopy alleviates some of these uncertainties and, with the help of









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For my dissertation, I am using a combination of NIR photometry and spectroscopy

to characterize the brown dwarf populations in the dominant star forming regions in Orion

B. I will use the results of this study to assess current theories of brown dwarf formation

in the context of the observations, ultimately examining the relationship between the

formation processes of brown dwarfs and low mass stars. Chapter 2 describes all data

acquisition and reduction. This includes the planning, execution, and reduction of a large-

scale NIR imaging survey of Orion B, the use of this survey to select the spectroscopic

sample, and all spectroscopic observations and reductions. Chapter 3 describes our

methods for the classification of young M star and brown dwarf spectra and includes a

discussion of surface gravity. Chapter 4 presents the results of our study of NGC 2024.

Chapter 5 presents the results for NGC 2068 and NGC 2071. Chapter 6 examines these

results and their implication for brown dwarf formation in Orion B. Chapter 7 investigates

the star-forming history of the region and Chapter 8 presents my conclusions and plans

for future work.















CHAPTER 2
NEAR-INFRARED IMAGINTG AND SPECTROSCOPY OF ORION B

2.1 FLAMINGOS: The FLoridA Multi-object near-INfrared Grism Observational
Spectrometer

All imaging and spectroscopic data included in this work were taken using

FLAMINGOS, the Florida Multi-Object Imaging Near-Infrared Grism Observational

Spectrometer (Elston, 1998). Conceived by Richard Elston and built at the University of

Florida, FLAMINGOS is a combination of a wide-field NIR imager and the world's first

fully cryogenic NIR multi-object spectrometer.

FLAMINGOS employs a 2048 x 2048 pixel HgCdTe HAWAII-2 infrared array

with 18 micron pixels. On the Kitt Peak National Observatory (KPNO) 4 meter (4 m)

telescope this corresponds to a plate scale of 0.3 18"/'pixel and a 10. 8'x 10.8'field of view.

On the KPNO 2. 1 m telescope, the plate scale is 0.608"/'pixel, yielding a field of view

of 20.5' x 20.5'. The imaging filters include standard broadband J (1.25 pum), H (1.65

pum), and K (2.2 pum) filters roughly equivalent to the Caltech (CIT) system, as well as

a Ks (2. 12 pum) filter. For spectral observations, FLAMINGOS offers both a JH and

HK grism and filter set which provide spectra across two bands (J and H or H and K)

simultaneously. Spectral resolutions are low (typically R=1000-1800 depending on the

filter and grism combinations chosen).

The large detector area and wide field of view of FLAMINGOS, particularly on the

KPNO 2.1m, makes FLAMINGOS a very efficient instrument for imaging large regions

of the sky: an entire square degree can be observed in nine fields. Further, the multi-

obj ect spectrometer (MOS) of FLAMINGOS makes it possible to obtain spectra of 25-50

obj ects simultaneously. These qualities make FLAMINGOS an excellent instrument for










large-scale, multi-dimensional NIR surveys. This dissertation is one of the first broad

applications of FLAMINGOS as a survey instrument.

2.2 Surveying the Stellar and Substellar Content of GMCs with FLAMINGOS

In the year 2000, the National Optical Astronomy Observatories (NOAO) approved

the long-term survey proj ect Toward' a Complete Near-Infa~red Spectroscopic and

Imaging Survey of Giant M~olecular Clouds (NOAO Survey Program 2000B-0028,

P.I.: E. A. Lada). The main goals of the survey were to use FLAMINGOS imaging and

spectroscopy to determine the number and spatial extent of young stars in six of the

nearest GMCs and to measure their ages and masses. When data acquisition, reduction,

and analysis for all clouds are complete, the data will also be used to study the IMF

of each region and to address issues such as the variations of star formation efficiency

and rate from cloud to cloud, the formation of clusters and sequential star formation.

The clouds selected for study were Orion B, Perseus, Rosette, and Monoceros. The

contribution of my dissertation is a broad imaging survey of Orion B and an in-depth

analysis combining both imaging and spectroscopy of the most active star forming

regions in the cloud.

2.3 FLAMINGOS Imaging of Orion B

2.3.1 Survey Strategy and Observations

As discussed in Chapter 1, earlier studies of Orion B have shown that the maj ority of

star formation in the cloud occurs within the dense cores (Lada et al., 1991a,b; Carpenter,

2000). The FLAMINGOS survey region was therefore chosen to coincide with the

densest gas as indicated by the CO map of Maddalena et al. (1986). In addition, the

survey region encompasses nearly the entire CS survey of Lada et al. (1991la) (Figure

2-1).

Initial observations of the 50 FLAMINGOS fields shown in Figure 2-1 as well

as three off-cloud control fields were obtained during the 2001-2002 winter observing

season on the KPNO 2. 1 m telescope. All fields were observed in the J, H, and K filters










ORION B MOLECULAR CLOUD.


Y I Y [ U


b~ _21


-4P


+05 50 27.9
right arcension (2000)


+05 38 27.9


+06 02 27.9


Figure 2-1. CO map of Orion B (Maddalena et al., 1986) shown with the observed
FLAMINTGOS/KPNO 2.1m survey fields. Each FLAMINTGOS field is 20'
on a side to allow for some overlap from field to field. The thick black line
outlines the CS survey of Lada et al. (1991a).


*r BEAM~










using a 9-point dither pattern to allow for the creation and subtraction of a background

sky frame. The integration time at each point was 35 seconds in all bands, yielding a

total of ~5 minutes on source and a targeted depth ofJ = H = K > 18. Typical seeing

was 11'5-11'9 as indicated by the full width at half maximum (FWHM) of the stellar

profiles. Dome flats at all wavelengths and dark frames were also observed for calibration

purposes.

Occasionally, due to poor weather conditions, instrument/telescope problems, or

observer error, the image quality and/or depth was much worse than expected (e.g. seeing

> 21'0 and/or J = H = K
were marked as re-takes and re-observed during the winters of 2002-2003 and 2003-2004.

In addition, during the 2003-2004 and 2004-2005 observing seasons four more off-cloud

control fields and 6 "gap" fields in the vicinity of NGC 2024, NGC 2068, and NGC 2071

were observed to ensure full coverage of that region of the map. A complete observing

log for the Orion B fields can be found in Appendix A.

Over the course of the entire survey, a total of ~360 individual Orion B images were

taken covering nearly 6 square degrees of sky and yielding ~6 gigabytes of data. It was

impractical to reduce and photometer such a large amount of data by hand; thus, two

automated processing pipelines were designed to complete these tasks. The first pipeline,

dubbed LongLegs and authored by Carlos Roman-Zufiiga, takes care of preliminary

processing, basic data reduction, and construction of a final image (Roman-Zufiiga,

2006). The second pipeline, called Pinkpack and written by this author, incorporates

source extraction, photometry, astrometry, and the combination of multi-wavelength data

into a single catalog. Both pipelines were constructed using a combination of pre-existing

Image Reduction and Analysis Facility (IRAF) tasks and custom C and FORTRAN

routines written by the authors. Each pipeline is typically run on one night of data (300-

500 raw data frames, 16Mb each) at a time. The detailed functionality of these pipelines

is described in the following sections.










2.3.2 LongLegs: The Data Reduction Pipeline

A standard infrared image does not just contain flux from the desired targets. Rather,

background emission from the atmosphere, telescope, and instrument all contribute

to the total flux incident on the detector. In the NIR, the sky background dominates,

often rendering even the brightest science targets nearly invisible in the raw data frames

(Figure 2-2). In addition, most infrared detectors suffer from varying sensitivity across


Figure 2-2. Raw K-band image of an Orion B control field (oribcf6) taken with
FLAMINGOS in November 2004. Note that there are hardly any stars visi-
ble.










the array, non-linearity in the detector response, and various dead, hot, or excessively

noisy pixels. All of these effects must be removed prior to any study of the astronomical

target. LongLegs (Roman-Zufiiga, 2006) was written to accomplish this task for all

FLAMINGOS images. Aside from its ability to complete the data reduction process

for large volumes of data, LongLegs is innovative in that it includes a two-pass sky

subtraction routine which effectively creates and removes the local sky background while

leaving the science target fluxes intact. The remainder of this section provides the details

of the LongLegs data reduction process.

2.3.2.1 Cleanflamingos

The first step in the LongLegs pipeline is cleanflamingos. Beginning with raw data

frames such as the one shown in Figure 2-2, cleanflamingos takes care of all preliminary

data processing for an entire night of data, one science target at a time. Bad pixels (pixels

having counts >60000 or <-60000) are identified and set to have values of 0.0. Images

are grouped according to type (e.g. obj ect, flat, dark) and the mean and standard deviation

are calculated; frames having mean values which differ by more than 2 sigma from the

mean of the group are rejected. A third-degree polynomial correction is then applied to

the good data to correct the integrated counts in each pixel for the non-linear response of

the detector. Dark frames are median combined to create a master dark for the science

images as well as a dark for the flat fields. A master flat field is created by median

combining and normalizing the dark-subtracted dome flats. Finally, a bad pixel mask

(BPM) is created by identifying all anomalous pixels in the master dark and the master

flat.

2.3.2.2 Crunchflamingos

The main data reduction engine in Longlegs is crunchflamingos, which completes

two separate passes through the reduction process. Pass 1 begins by subtracting the

master dark from all science frames, removing the dark current and bias voltage from

the images. A local sky frame is created for each individual science image by scaling










and median combining the 8 adj acent dark-subtracted frames. This local sky is then

subtracted from the science images to remove the sky background. The resulting data

are divided by the master flat to account for the pixel-to-pixel sensitivity variations in the

detector.

In theory, a single pass through the reduction process would be sufficient to create

science quality images. However, in many cases the obj ect removal during sky creation

is imperfect and large negative residuals are left in the final images. Consequently, pass

2 of cru nchfla m ingos recreates the background sky with the astronomical obj ects masked

out. The IRAF source detection program daofind is run on all frames in a dithered set,

the frame offsets are calculated, and a preliminary combined frame is constructed. All

sources with fluxes larger than a user-specified sigma threshold are masked out from the

individual images and local skies are re-created from the "starless" data frames. These

new sky frames are subtracted from the obj ect images and used to create a night sky

flat which is subsequently used to flatten the data The output of cru nchfla m ingos is an

improved set of reduced images ready for final combination.

2.3.2.3 Smoothflamingos

The last task in the LongLegs pipeline is smoothflamingos. Smoothflamingos

creates the final, science-grade images according to the following steps: First, a 6th order

Chebyshev polynomical correction is applied to all flat-fielded data frames and BPMs

using the IRAF task geotran to correct for geometric distortion (caused by imperfections

in the FLAMINGOS optical system). Note that the exact coefficients of the polynomial

depend on the observing season since the correction was redetermined using images of

a 20 x 20 pinhole grid array each time the array or filter positions were disturbed. Once

all data have been geotranned, the frame offsets for each dither set are recalculated, the

pixels are resampled to half of their original size, and the dither set is average combined

to create a final image (Figure 2-3). All final images are trimmed to be 4096 x 4096 pixels

and are then ready for photometry, astrometry, or any other analysis.














































Figure 2-3.


The final LongLegs product for the image shown in Figure 2-2. This image
has been linearized, dark-subtracted, sky-subtracted, flat-fielded, corrected
for geometric distortion, combined with other images in the dither set, re-
sampled and trimmed to have 4096 x 4096 pixels. Note the plethora of stars
revealed by the reduction process.


2.3.3 Pinkpack: The Photometry and Astrometry Pipeline

Pinkpack, written and tested by this author in 2001 and updated in 2002 and 2003,

is the FLAMINGOS photometry and astrometry pipeline. Once a night of data has been

fully reduced using LongLegs, Pinkpack tasks can be used to generate image statistics,

detect all sources in the images, obtain calibrated aperture and point spread function










photometry for detected obj ects, solve for the astrometric solution of each image,

complete preliminary data assessment, and combine multi-wavelength data into a single

catalog.

2.3.3.1 Pinkphot

The first maj or task in the Pinkpack pipeline is pi nkphot, the FLAMINGOS

photometry task. Pinkphot detects sources and derives both aperture and profile fitting

photometry on an image-by-image basis for as many images as the user chooses to place

in the input list. The task begins by running the source extraction program SExtractor

(Bertin & Arnouts, 1996) to generate a preliminary source list for the current image. This

list is passed through a detection filter which employs the BPM to remove any obj ects

falling on bad regions of the chip where their photometry will be compromised. The

filtered source list is then passed through two additional routines which remove saturated

obj ects and select stars suitable for defining the image point spread function (PSF). PSF

stars must be relatively bright, isolated (no other obj ects within 5 FWHM), at least 100

pixels from the image edges, be roughly evenly distributed across the image and above

all exhibit a typical stellar profile. Thus, obj ects flagged by SExtractor as elongated and

obj ects having a FWHM more than 1.5 times the mean or less than 0.5 times the mean are

automatically eliminated from the PSF star list.

After creation of the final source catalog and PSF star list, pin kphot begins its

photometry routines. The sky background estimated and subtracted by LongLegs is

typically a decent representation of the true local sky, however, for very nebulous and/or

crowded fields their will still be some element of contamination in large apertures by flux

from nearby neighbors or nebulosity. Pinkphot circumvents this problem by applying an

aperture correction. Aperture photometry of the PSF stars is obtained using the IRAF

phot task over twelve apertures with radii ranging from 0.5FWHM to 3.25FWHM.

An aperture correction from apertures 3-12 (aperture radii from 1-3.25FWHM) is then

calculated from the PSF star photometry using the IRAF task mkapfile. All sources in the










image are photometered using the small aperture (r=FWHM) and subsequently corrected

out to the larger aperture (r=3.25FWHM), yielding contaminant-free large aperture

photometry for all obj ects. Note that at this point all photometry is uncalibrated with the

zero point set at 0.0.

Once the corrected aperture photometry is obtained, pinkphot derives PSF photom-

etry for all sources using two passes through the IRAF DAOPHOT routines (Stetson,

1987). In the first pass, a model PSF is created from the PSF stars using a second order

variable Moffat function with the P parameter set to 2.5. This PSF is fit to each obj ect in

the complete source catalog using the large aperture photometry as input. The fitted stars

are then subtracted from the original image, revealing any close companions. The second

pass repeats the initial aperture photometry and PSF fitting process on the newly detected

sources using the aperture correction and model PSF derived in pass one.

The net output from pinkphot is a catalog containing the x and y pixel coordinates,

the small aperture radius in pixels, the flux in that aperture, the phot magnitude using

the small aperture, the large aperture radius in pixels, the aperture-corrected magnitude

and its associated error, the PSF magnitude and its error, and detection flags indicating

whether an object is potentially saturated and/or elongated. It should be noted that while

the aperture photometry is included for completeness and its potential usefulness in

determining the magnitudes of non-stellar sources, all survey magnitudes discussed from

this point forward are derived from PSF photometry, due to its improved ability to handle

extremely crowded or nebulous fields and variations in stellar PSFs across an image (refer

to $2.3.4 for further discussion of this effect).

2.3.3.2 Pinkastrom

At this point in the pipeline, obj ects are identified only by their x and y positions

on the detector. These coordinates, while useful when looking at the image itself, yield

no information regarding an obj ect's true position in the sky. However, each image is a

two-dimensional projection of a portion of the three-dimensional celestial sphere. Using









standard spherical geometry it is therefore possible to derive a transformation between

the two. This transformation, known in astronomical terms as the plate solution, defines

the image world coordinate system (WCS), linking each image pixel to a specific location

on the celestial sphere. Pinkastrom employs the pixel and celestial coordinates of known

obj ects in the field to derive a plate solution and WCS for each image, subsequently

applying the solution and deriving astronomical coordinates for all pinkphot detections.

Note that pin kastrom uses the standard equatorial coordinates of Right Ascension (R.A.)

and Declination (Dec.) to describe obj ect positions on the celestial sphere. The epoch of

all equatorial coordinates is J2000.

Pin kastrom begins by obtaining the coordinates of the image center (R.A. and

Dec.) from the image header. Pin kastrom then calls out to an online catalog repository

and downloads the equatorial coordinates and magnitudes of all sources within a 15

arcminute radius of this point. Here the user has the option of employing the most recent

U. S. Naval Observatory all sky catalog (USNO B1.0) or the 2 Micron All Sky Survey

(2MASS). For the purposes of the FLAMINGOS GMC survey, we have found that the

2MASS catalog consistently yields a higher number of matches per field (likely because

it is also an infrared survey), providing a better sampled grid of points from which to

calculate the solution. Consequently, all plate solutions and celestial coordinates quoted

in this dissertation were derived using 2MASS. Once pin kastrom has a catalog of celestial

coordinates corresponding to obj ects in the field, these coordinates are converted to

rough x and y pixel positions using estimated WCS information provided in the image

header and matched to obj ects in the pin kphot catalog using the IRAF task xyxymatch.

The IRAF task ccmap is then used to calculate a rough plate solution using a second

order polynomial fit to the matched list. The downloaded 2MASS catalog and the

FLAMINGOS catalog are subsequently rematched with the IRAF task ccxymatch using

the transformation given by the rough plate solution. A final fourth order polynomical

solution is calculated from the re-matched lists.










The catalogs are matched a third time using the new plate solution. At this point,

the final number of matches and the astrometric residuals are checked, and if they are

satisfactory (nnzatch >50, residuals < Of'25) the image WCS is updated and all pinkphot

pixel positions are converted to J2000 R.A. and Dec. The output from pinkastrom is a

new catalog containing both pixel and celestial coordinates for each obj ect, a database file

containing the plate solution, and a logfile containing the precise plate scale in arcseconds

per pixel, image rotation in degrees, and rms values of the astrometry in arcseconds for

all images in the input list. As indicated by Figure 2-4, typical rms values for the final

positions are <0!'25.
1 I i 1 1 lI "l

mean A = 0.138" mean A = 0.129"
rms =0.136" rms = 0.129"
0.75 -- 0.75 --







dI I I~ I I' I I'



89.6 89.5 89.4 -2.6 -2.5 -2.4
R.A. (J2000) Dec. (J2000)

Figure 2-4. The deviations (A) in arcseconds of pin kastrom coordinates from 2MASS
positions for 863 obj ects in the image shown in Figure 2-3. Both the mean
deviations and the rms values for R.A. and Dec. are less than Of'25.


2.3.3.3 Massmatch

The third maj or task in the Pinkpack pipeline is massmatch. As indicated by its

name, massmatch matches FLAMINGOS detections to their 2MASS counterparts (if

they haven't already been matched in the previous step) and calculates a photometric

zero point for each FLAMINGOS image using 2MASS photometry. Note that this task

is wavelength-dependent and must be run separately for the J, H, and K band images in

each night.










The task begins by assessing whether a matched 2MASS/FLAMINGOS catalog

exists. If not, source matching is accomplished using ccxymatch in the same manner

described above. The matched catalog is then passed through a filtering routine which

removes stars outside the acceptable magnitude range for zero point calculation. Stars

used in the zero point calculation must be fainter than the FLAMINGOS saturation limit,

conservatively estimated at J = H = K =1 1.0, and brighter than J=15.5, H=15.0, and

K=14.0. The faint limits were set to be slightly brighter than the 2MASS completeness

limits (J~- 15.8, H ~ 15.1, and K ~ 14.3, Carpenter, 2000). Once an appropriate sample

has been identified, the differences between the FLAMINGOS and 2MASS magnitudes

are calculated for each obj ect in the list. massmatch then creates a histogram of these

differences and takes the centroid. Because the FLAMINGOS photometry was initially

derived using a zero point set to 0.0, the centroid of this difference histogram is the

photometric zero point (Figure 2-5).

Jzpt = 20.604 Hzpt = 21.123 Kzpt = 20.657
80 80 80

60 I -1 60 -1 60-


S40- 1 40-~ I~ 40-

20 201 -11 1 -1 20 -

O 0" 0
20.2 20.4 20.6 20.8 21 20.8 21 21.2 21.4 21.6 20.2 20.4 20.6 20.8 21 21.2
aJ aH AK

Figure 2-5. Sample difference histograms (A=M2nn4SS MFLMNn) used by massmatch to
determine the zero point. These data are from the JHK image set of orib-07,
taken on 2001 Dec 18. The dashed line in each histogram is the location of
the centroid, the corresponding zero points are labelled atop each plot.


After calculating the zero point, massmatch uses this value to correct the photom-

etry, updating the pin kastrom catalog so that it now contains calibrated photometry. In

addition, massmatch also generates a number of data assessment tools. These include: a










calibrated luminosity function, magnitude comparison files, and magnitude scatter data.

These tools will be discussed further in the quality assessment section below ( 2.3.5).

2.3.3.4 Jhkmatch

Jhkmatch is the final step in the Pinkpack pipeline. The task uses the plate solutions

determined with pinkastrom coupled with the IRAF matching task ccxymatch to merge

the calibrated data from individual images into a single, multi-wavelength catalog for

each field. In addition, jhkmatch uses the celestial coordinates of each object to assign

a unique catalog identifier. The output from jhkmatch is a single catalog for each field

containing obj ect ids, celestial coordinates (J2000), pixel coordinates, JHK magnitudes

and errors derived from aperture photometry, JHK magnitudes and errors derived from

PSF photometry, and the informational data flags discussed above.

2.3.4 Positional Zero Point Correction

The presence of small misalignments and/or fabrication errors in the FLAMINGOS

optical system does cause some noticeable PSF degradation in our images, particularly at

the field edges. This PSF variation is present in survey data from all epochs. In addition,

during the summer of 2005, it was noticed that stars located in the upper right quadrant

of FLAMINGOS had pinkphot magnitudes which were systematically fainter than their

2MASS counterparts by ~0.5 magnitudes. A comparison of fiat fields taken in 2004 and

2005 revealed a small but significant flux loss (~5%) in this portion of the chip for the

more recent data. A working theory by members of the FLAMINGOS team attributes

the flux loss to a deterioration of the coating on the "field lens" (which is effectively the

entrance window to the camera dewar). Because the stripping of the lens coating is highly

structured, the net result is that the PSFs in affected region vary rapidly with position.

This effect worsened over time: data taken in the 2001/2002 observing season are only

minimally affected (e.g. Figure 2-5, presenting data taken in 2001 December) while the

strongest effects are seen in data from the 2004-2005 observing season (e.g. Figure 2-7,

presenting data taken in 2004 November).










Aperture photometry cannot account for variations in stellar PSFs across an image.

PSF photometry as derived from pinkphot does attempt to compensate for some variation

in the PSF, however the model used is a smooth function which cannot handle the amount

of PSF degradation present in the FLAMINGOS images affected by the deterioration of

the lens coating. In particular, sources in the southeast corner of recent (2004-present)

FLAMINGOS images have wide, coma-shaped halos containing a significant portion of

the stellar fluxes (Figure 2-6).



















Figure 2-6. (a) Southeast corner of the reduced K-band image of control field 6 (orib-
cf6-kl) taken in November of 2004. (b) Contour plot of two of the stellar
profiles. The PSFs are clearly elongated and exhibit strong coma shaped
halos which increase in size towards the edge of the image.


The model PSFs used in the pinkphot fitting routines cannot account for the flux

in these halos. Consequently, the fitted magnitudes of obj ects in the affected regions are

systematically too faint compared to their 2MASS counterparts, with the severity of this

effect dependent on radial distance from the optical center (Figure 2-7a).

In addition, if enough sources are affected this problem has the additional conse-

quence of skewing the central zero point for the image. In other words, if the observed

magnitudes are too faint, the value of A (2MASS FLAMINGOS magnitude) will be










1 , , ,6 0






-0.5-

-1 40 t
O 100 200 300 0.4 20.6 20. 21 1.2 21.
Radial-----;- Ditnc rom Opt-ical Ceter
Figure; 2-7 (a) Raildpnec ftepooetrcdvainfo M Sorb






ainl both()ande (b)m Oisa the retzero on o h i age


smaller thani shoul bfer an frth cenrode zero point will ien turnbed toobrih sinced the




offset distribution is now asymmetric with a bright tail (Figure 2-7b).

These effects would appear to have catastrophic implications for pipelined

FLAMINGOS photometry, however, as it turns out, they are correctable. Andrea

Stolte has devised a routine which both adjusts the skewed zero point and corrects the

magnitudes affected by the lost flux. The routine operates by fitting a 6th order Legendre

polynomical to the deviation between calibrated FLAMINGOS and 2MASS magnitudes

as a function of position on the detector (Figure 2-8a). Based on the fit, we then apply

this polynomical correction one band at a time to the zero point to obtain final, correct

calibrated photometry (Figure 2-8b). The details of calculating and applying the fit can

be found in Roman-Zufiiga (2006).

The polynomial zero point correction was applied to all imaging data. The results

of this correction include a reduction in the overall photometric scatter (see ~2.3.5 for a

discussion on how this value is calculated) and assurance that the final photometry is in













0.5 -- 0.5




O, 100 20 00010 00 30
R {xc-3000 ye20)R(c300 e-00

Figue28 a norce -adpooer o OriB onro fl 6r shown, withh
6t ore poyoma fi to th daa (b) Corete phtmer forl the same~a-









magniudes for. data U etaen prior d to t r fall 2003 an 0.3-.4anitudes ford fils observedth

November g 20-rsent Fortuntey the e mdajority of the Orion Bsrve dvatao wer taken


duriemng wthe 2001200 observing0. seaso, nly requrin a smpiall correc Ntion. ine






2.3.5 Imaging Data Quality and Completeness

2.3.5.1 Accuracy of the Photometry

Once the zero point correction was applied, photometric quality was assessed using

the difference between the final FLAMINGOS photometry and the 2MASS database. The

1 0- rms values of the deviation from 2MASS were calculated in 0.5 mag bins and plotted

as a function of the FLAMINGOS magnitude in each band (Figure 2-9). The overall

photometric scatter for each image was derived by taking the mean of all rms values in

the magnitude range from the FLAMINGOS saturation magnitude to the conservative

2MASS completeness estimates stated above. The final scatter values for each field can

be found in Table A-2.












1.0 I I 0.25
Il.0II0+-0.155 fit limit : 011>-1,n = 1:.047
0.20-

0.5 -5







0.00
-1 0
11: 12 14 16 18 11: 12 14 16 18
J FLFFLF


-0.005+-0.051: flt lImit in =014
0.20-

0.5* -.





Ki I ~0.00


-1.0
10 11 12 13 14 15 16 17 10 11 12 13 14 15 16 17
H,,, HEL11
1.01 """"""'"" """' "" 0.25
-CI III7+-0.046 fit 1imit : Giln-, = 0 043
0.20-







-1.0 ..'



1011 12 13 14 15 16 10 11 12 13 14 15 16
FLHN, FLHN



Figure 2-9. Sample scatter plots for data taken during the 2001-2002 observing season
(orib-08, observed 2001 Dec 20) after application of the zero point correc-
tion. The left-hand panels plot the deviation between 2MASS and FLAMIN-
GOS magnitudes as a function of FLAMINGOS magnitude. The dotted line
indicates the faint limit for fitting the polynomial zero point correction. The

right-hand panels show the calculated rms values as a function of magnitude.
Dotted lines are plotted at limits of 0.05 and 0.10 magnitudes. The dashed
line is the mean of the rms values down to the fit limit (the numerical value is
indicated at the top of the plot).









The median scatter values for the entire Orion B survey (excluding extremely

nebulous frames) are a;=0.05, oH=0.04, and GK=0.05. Consequently, in regions with

little to no nebular emission, we estimate the bulk of our photometry is accurate to

within 0.05 magnitudes. For images with large amounts of nebula (e.g. orib-01, orib-

14, orib-34, and orib-37 which are centered on NGC 2024, NGC 2023, NGC 2068,

and NGC 2071 respectively) the scatter with respect to 2MASS is much larger (~20. 1

magnitudes) than that expected from pure photometric noise. A similar effect has been

noted by other authors studying young clusters with significant nebular emission (e.g.

Muench et al., 2003 in IC 348 and Muench et al., 2002 and Slesnick et al., 2004 in the

Trapezium) and is usually attributed to the large size of the 2MASS pixels (~2.0"),

intrinsic variability of young obj ects, and variations in aperture size coupled with the

strong nebular background. Finally, it should also be noted that for data taken in 2001-

2002 the scatter with respect to 2MASS is also larger (~0. 1 mag) for obj ects on the edge

of detector where the data rapidly degrade due to a delamination of the engineering array.

2.3.5.2 Sensitivity Limits

The sensitivity limits of the survey were derived through an analysis of the pho-

tometric errors, following procedures similar to those employed by 2MASS. Simply

put, there is a theoretical limit to the photometric precision at every signal to noise ratio

(SNR) which takes into account the contribution from all internal noise sources in a

digital, sky-subtracted image. This limit (oins) is given by the equation


clin; ~ 1.0857(S/N) (2.1)


derived fully by Newberry (1991). If we wish to find the 100 detection limit, we simply

need to locate the magnitude at which the errors agree with the value of clin, at a SNR of

10: olo ~~ 0.109. Figure 2-10 shows the median photometric error (estimated in 0.2 mag-

nitude bins) as a function of magnitude in each band for the entire FLAMINGOS/Orion






















6 8 10 12 14 16 18 2C
SMagn tude













6 8 10 12 14 16 18 2C
Hi Magn tude








-I


0.10


0.05


0.1b


C]0.10
O
S0.05


0.00





0.1b


C~0.10


=- 0.05


0.00


6 8 10 12 14
K Magnitude


16 18 20


iFigure 2-10. Error plots constructing using data from the entire ii ii s. iGO ii:S/O)rion B
imaging survey. In each band, the median errors (calculated in 0.2 magni-
tude bins) are plotted as a function of magnitude (solid line). Error bars are
the rmns in each bin. The dotted lines are C.T..U .-1 : at fiducial values of 0.05
and 0. 10 magnitudes. The horizontal dot-dashed lines :::- i: .:i the limiting
error for 100 magnitudes and the: vertical dot-dashed lines showv the: location
of the 100 limits for this survey.










B imaging survey. The 100- detection limits (where the error function crosses the limiting

value) indicated by the figure are J=18.9, H=17.9, and K=17.6.

2.3.5.3 Completeness

For the purpose of this dissertation, the immediate function of the imaging survey

is to provide a photometric catalog deep enough to select spectroscopic targets down

to and below the HBL (K ~-13.0 at the distance of Orion B), subsequently using this

photometry to place spectroscopically observed and classified obj ects on the Hertzprung-

Russell diagram. Consequently, I am only concerned with ensuring that the photometric

completeness limits are deeper than the magnitude limits for spectroscopic observations

(H=15.0 on the KPNO 4 m telescope, H=13.0 on the KPNO 2. 1 m telescope).

Completeness limits at the 90% level are canonically estimated using the turnover

magnitude of a field's luminosity function. As mentioned above, massmatch outputs

calibrated luminosity functions (LFs) for each image (e.g. Figure 2-11) and writes the

peak of this function to the image headers. The LF peaks for all survey images can be

found in Table A-3. I estimate the 90% completeness limits of the survey to be the

median of these values for each band: Jliz=18.5, H~iin=17.75, and Kinz=1 7. 5, which are

certainly below the spectroscopic limits (02.4.1).


















Orimi B Imaging Survey Data


8000


6000


a4000


20800


0







2104


88000


0 000


48000


28000


0


10 15 20
J maagnitude


10 15 20
H magnitude


peak of J band at 18.5 naag


peak of H band at 17.75 mag


peak of K band at 17.5 mag


All possible sources.


da 104


5000


0


10 15 20
K magnitude


Figure 2-11. J,H, and K-band luminosity functions for the entire combined Orion B
imaging survey. The dashed lines indicate the peak of each LF as well as the
estimated 90% completeness limits for each band.









2.4 FLAMINGOS Spectra of Orion B

As mentioned in Chapter 1, infrared photometry alone is not sufficient to estimate

masses for individual objects. Rather, by combining photometry with NIR spectra we can

derive effective temperatures and stellar luminosities, ultimately using theoretical PMS

evolutionary models to identify young brown dwarfs. In this section I discuss the design

of the spectroscopic observing program, the details of the spectroscopic observations, and

our data reduction techniques.

2.4.1 Spectroscopic Sample Selection and Mask Design

The FLAMINGOS/Orion B imaging survey contains more than 100,000 individual

sources. Clearly, even with FLAMINGOS' multi-object capabilities it would be im-

practical to obtain a spectrum for every source. Rather, a more realistic course of action

is to define an intelligent subsample suitable for MOS observations on the KPNO 4 m

and/or 2. 1 m telescopes which includes both candidate brown dwarfs and young stars

representative of the general population in Orion B.

The first step in the sample selection process was to identify broad regions of interest

in the cloud. Recall that the stated goal of this proj ect is to characterize the brown dwarf

populations in the most active areas of star formation in Orion B. Thus, the bulk of the

regions chosen for spectroscopic observation are either the locations of known clusters

or dense cores, with a few off-cluster/off-core regions added for comparison. Figure

2-12 shows the areas originally chosen (indicated by boxes representing the 10' x 10'field

of view of FLAMINGOS on the 4 m) and their relationship to the dense gas. Center

coordinates for each field as well as the corresponding 2. 1 m imaging survey region and

any indications of known star formation activity are listed in Table 2-1.










Orion B CS Ma


- J2000 Coordinates


I I I I
5,8 578 .7 5.9 .6




Right Aseso hus




Fiure21.Rgoso h ro lu agtdfrsetocpcosrain hw
wit th CS cnor fLd ta.(91) Rcl htFgr hw
th ouln fti Smp nrfrnet h LA IG Siaigsre

an thel CO1 map of Madaen eta.(96. oe r LMNO











Figue 212.regions of dene gasn afflatd wigth known spetarosoi form action, oevr feow



regions deodof dense gas a rflae wt alsow tag ted. ratohoee,










Table 2-1. Regions Targeted for Spectroscopic Observation
ID R.A. (J2000) Dec.(J2000) Imaging Field Commentsa

105:47:51.54 +00:20:43.3 orib-36 LBS 1,2,5
2 05:47:07.54 +00:22:43.3 orib-37 NGC 2071
3 05:46:19.54 +00:27:43.3 orib-37 LBS 13-15,18,20,22
4 05:46:47.54 +00 02 43.3 orib-34 NGC 2068
5 05:46:23.54 +00:05:43.3 orib-32/33 LBS 10
6 05:46:07.54 -00:08:16.7 orib-32 LBS 23
7 05:43:19.54 -01:16:16.7 orib-27 LBS 26-28
8 05:42:35.54 -01:16:16.7 orib-28 LBS 29,30
9 05:42:23.54 -01:59:16.7 orib-09 LBS 31,32
10 05:41:43.54 -01:54:16.7 orib-01 NGC 2024
11 05:41:31.54 -02:18:16.7 orib-14 NGC 2023
12 05:41:21.54 -01:44:16.7 orib-04 LBS 37,40
13 05:45:31.54 -01:16:16.7 orib-29 no dense gas
14 05:46:15.54 -01:16:16.7 orib-29 no dense gas
15 05:46:59.54 -01:16:16.7 orib-30 no dense gas
16 05:47:43.54 -01:16:16.7 orib-30 no dense gas


aA designation of LBS indicates the region was identified as a 50 dense
core by Lada et al. (1991a).


Once the fields were chosen, specific sources needed to be targeted for spectroscopic

observation. This was a slightly more complex procedure due to the restrictions imposed

by multi-obj ect observations. MOS mode with FLAMINGOS is provided by a grism

inside the camera dewar and a slit mask (a rectangular stainless steel plate with slits

lasered into it) placed at the telescope focal plane. Each slit in the mask corresponds to

a single position on the detector; unique slit masks must therefore be designed for each

field, with accurate positions of the desired targets known a priori. Sample selection thus

becomes a two step process; the first step is to compile a list of desired targets and the

second step is to design slit masks which maximize the number of these targets on the

mask.

The final consideration in the selection and design process is the choice of telescope.

As part of the NOAO GMC survey, we had the option of carrying out FLAMINGOS

MOS observations on either the KPNO 2.1 m or 4 m telescopes. Due to the intrisic

faintness of the low mass population of Orion B, the maj ority of obj ects targeted in










this study were placed on masks designed for observation on the KPNO 4 m telescope,

making use of the increased sensitivity (1 hour limiting magnitude, K~ 15.0) to maximize

the number of brown dwarf spectra obtained. However, selected bright sources were

targeted for observation on the 2.1 m telescope (1 hour limiting magnitude, K~13.0).

2.4.1.1 4 m Target Selection

Making use of either the images, photometry, and astrometry output from the 2. 1

m imaging survey or a 4 m image with photometry and astrometry also generated by

Pinkpack, target lists for 4 m spectroscopy were constructed according to the following

guidelines: During the 2002-2003 observing season priority was given to brown dwarf

candidates having K < 16.5. These objects were identified by comparing source

positions in H/H K color-magnitude diagrams with a theoretical 1 Myr isochrone.

All obj ects located below the reddening vector extending from the isochrone positioned

at M~= 0.08M.0 (corresponding to the HBL) are candidate brown dwarfs (e.g. Figure

2-13, left). During the 2003-2004 observing season priority was given to infrared excess

(IRX) sources (a common indicator of youth, e.g. Chapter 4, ~4.3.2.3) having magnitudes

brighter than K=16.5. In this case, IRX targets were identified using J H vs H K

color-color diagrams (Figure 2-13, right). In either case, once a target list was compiled

with the available IRX sources or brown dwarf candidates, a secondary list was created

containing all available obj ects having K magnitudes 12.0
IRX or brown dwarf candidate status. Finally, during the 2004-2005 and 2005-2006

observing seasons target lists were compiled using a simple magnitude-limited selection

including as many sources as possible with H magnitudes brighter than 15.5.

2.4.1.2 2.1 m Target Selection

The brighter sources (10.0
telescope as part of a parallel survey to characterize the intermediate mass population of

Orion B (Hernandez, 2006). I include discussion of the bright population in this study












Selec~ion Dagram ior PMS SO Sources


c


d Orp O

b,4 a
~ ~0~9~
F
~~i
..""
~..L r'

~aa~i~~t '.
-'
B~
""
P;=
~

;t~c~
,C
't g
t
n 1 2 3
HK Ca or


Selection Diagram tor R Excess Sources

~I
"~I,I
w8 .
i'A.8' Ir


~;



IV


SCo or


Figure 2-13. Sample selection diagrams for spectroscopic targets in NGC 2071: a) Color-
magnitude diagram (CMD) for all sources down to K <16.5. The leftmost
dotted line is the main sequence from Bessell & Brett (1988). The shorter
dotted line is an abbreviated 1 Myr isochrone of D'Antona & Mazzitelli

(1997). The dot-dashed lines are reddening vectors using the extinction law
of Cohen et al. (1981) placed at 0.08 and 0.02 Me. Selected brown dwarf
candidates are shown as solid red dots. b) Color-color diagram (CC) for all
sources down to K <16.5. The solid lines are the giant colors of Bessell &
Brett (1988) coupled with a combination of Bessell & Brett (1988) dwarf
colors for spectral types down to K7 and Leggett (1992), Leggett et al.

(1996), and Dahn et al. (2002) for spectral types from MO to M6 (Refer to
Ch. 4.4. 1 for an explanation of dwarf color choice). The dot-dashed lines
are the reddening vectors of Cohen et al. (1981) and the dashed line is the
classical T Tauri locus of Meyer et al. (1997). Selected IRX sources are
shown in red.



since a few of the observed obj ects may be extremely young M stars and statistics of the


brighter sources will aid in assessing our overall completeness in the region.

All 2. 1 m targets were selected solely on the basis of a magnitude limit. Sources


selected during the 2003-2004 observing season had magnitudes K <13.0 and sources

selected during the 2004-2005 observing season had H <14.0.


2.4.1.3 Mask Design and Fabrication


Once suitable target lists were assembled, slit masks were designed using M~askde-


sign, a custom C program written by Matthew Horrobin. M~askdesign takes an input









catalog and an associated image as well as a user specified field center (the MOS field

of view on FLAMINGOS is ~ 1/3 of the full imaging field) and places as many slits as

possible in the MOS field down to a specified magnitude limit. It is up to the user to ad-

just the selections to ensure that the desired sources are targeted and also to keep track of

sources which have been placed on previous masks. In addition, the user must also select

at least three bright stars distributed across the field to be used as setup stars. These stars

are required to align the mask at the telescope without the setup stars, it is impossible to

center the science targets in their slits. Finally, it is important to note that for M~askdesign

to run properly, the image used to design the masks must have the plate scale and field of

view corresponding to the FLAMINGOS field at the telescope where the MOS observa-

tions will be taken. The 2. 1 m survey images and catalogs thus required resampling and

trimming prior to their use in mask design this was accomplished using the IRAF task

geotran. In addition, in a few cases (particularly for the 2001-2002 observing season)

select fields were "pre-imaged" with FLAMINGOS at the 4 m. These images were also

run through the pipelines and the output could then be directly used for mask design.

All slitmasks for the Orion B spectroscopic survey were designed as follows: first

a maximum number of the priority obj ects discussed above were placed on the mask.

Once this was completed, any available magnitude-limited "filler" obj ects were added

to maximize the total yield from the multi-obj ect observations. In all cases, the total

number of obj ects targeted on each mask was limited by the spatial distribution of sources

in the regions being targeted. Typically we were able to place 20-30 objects per mask

for fields targeting the densest portions of the clusters and 10-20 obj ects per mask in the

non-clustered regions. Figure 2-14 shows the region file output by M~askdesign indicating

the location of the slits and acquisition stars for a sample mask (n2071a2) targeting the

eastern (low stellar density) region of NGC 2071.










































Figure 2-14. K-band image of NGC 2071 (North is up and East is left) taken with
FLAMINGOS on the KPNO 2.1 m telescope on 2002 Dec 31 and trans-
formed to the 4 m field of view using geotran. The green regions on the
image indicate the slit positions for the mask n2071a2, designed in fall 2004
and targeting 11 obj ects with H< 15.5 and one filler target with H< 16.0.
The bright stars enclosed in boxes are the acquisition stars.


Slit masks designed for the FLAMINTGOS GMC survey were fabricated using an

on-site laser in the University of Florida's Infrared Instrumentation lab and either sent or

taken by the observers to the telescopes.

2.4.2 Spectroscopic Observations

MOS spectra of young sources in Orion B were taken with both the KPNO 2.1Im

and 4m telescopes. For the most part the data acquisition procedures were the same on










both telescopes, however, there were a few differences, thus I have chosen to discuss them

separately. A complete observing log for the masks observed on both telescopes can be

found in Table B-1 in Appendix B.

2.4.2.1 4-m Data Acquisition

Slit masks identified in Table B-1 as 4 m masks were observed on the KPNO 4 m

telescope during four consecutive winter observing seasons from January 2003 through

December 2005 using the following procedure: First, the field in question was imaged

using a short exposure time (typically 10-30s) in the K or K, filter. If the resultant image

was suitably centered with the setup stars visible, a guide star was acquired, and the MOS

mask was put into place. At this point the field was re-imaged (with the slit mask in)

to check mask alignment. A perfectly aligned field will have stars centered in all of the

alignment boxes and some of the slits (Figure 2-15). On the other hand, if the alignment

stars were not centered in their boxes, the IRAF task xbox was used to determine small

correctional offsets to the telescope position. These offsets were applied, the field was

reimaged, and this process was repeated until acceptable mask alignment was achieved.

All spectra were taken using the JH filter (0.9-1.8 pum) coupled with the JH grism,

providing complete spectral coverage in both the J and H bands simultaneously. The

slit width in all cases was 3 pixels or 0"'95 on the 4m, yielding a spectral resolution, R

~ 1300. For each mask, 300s or 600s exposures were taken in sets of four, dithering

between two positions on the chip in a standard ABBA pattern to allow for background

sky subtraction. The separation between positions A and B was 4"'along the long

dimension of the slitlets. Total exposure times for each mask typically ranged from 40-80

minutes and are detailed in Table B-1.

Once the science exposures were completed, internal quartz lamp flat fields and

HeNeAr arc lamp spectra were taken for calibration purposes. Short exposure, long slit

spectra of a nearby G dwarf at similar airmass were also taken immediately following the

science observations to correct for telluric absorption.







































Figure 2-15. Image of a properly aligned MOS plate. The plate shown is n2071a2 (c.f:
Figure 2-14) with North up and East left. Note that all of the setup boxes
contain reasonably centered stars.


2.4.2.2 2.1-m Data Acquisition

Data acquisition for the 2. 1 m spectra proceeded in much the same way as for the

4 m spectra, but with slight differences in the guiding and calibration procedures. The

guider at the 2.1 m is not a moveable guide probe (as at the 4 m) but rather a fixed off-

axis mirror which feeds into the guide camera. Consequently, the first step in the 2.1 m

observation procedure was to ensure that a suitable guide star was present in the FOV of

the guide camera and would remain so for the entire ABBA dither pattern. If this was the

case, then the observations could proceed as above.










Spectra of the brighter targets were obtained with FLAMINGOS on the KPNO 2.1Im

telescope during the 2003-2004 and 2004-2005 winter observing seasons. As with the

4 m spectra, all sources were observed using the combination of the JH filter and the

JH grism. Individual 300s exposures were taken in sets of four, yielding typical total

integration times of ~ 1 hour (see Table B-1 for precise exposure times per mask). All

slits were again 3 pixels wide, which on the 2.1 m corresponds to a width of 1"'82 and

results in a spectral resolution ofR ~ 1300. Flat fields were taken using an illuminated

white screen mounted on the inside of the telescope dome. Due to the lack of internal

arc lamps at the 2.1 m, no designated wavelength calibration frames were obtained -

wavelength calibration would be determined using the atmospheric OH emission lines

intrinsic to all NIR spectra. Finally, as with the 4 m data acquisition, a nearby G star was

observed to correct for telluric absorption.

2.4.3 Data Reduction

All FLAMINGOS spectra taken for this study (both 2.1Im and 4m data) were

reduced using a combination of standard IRAF procedures and custom FLAMINGOS

routines. Spectra taken prior to fall 2004 were run through all reduction procedures

by hand. Spectra taken after this point were reduced using the FLAMINGOS spectral

reduction pipeline, dubbed .1/n my wa'rtllr' and authored by Aaron Steinhauer. Either way,

the reduction procedure is outlined below.

We determined that a linearization correction was not necessary as the size of the

correction was < 1% of the raw flux levels for usable data and the high order function

needed would likely introduce larger errors. Consequently, the raw data were first dark

subtracted and subsequently divided by a normalized flat field created by averaging a

number of dark subtracted quartz flats (4 m) or dome flats (2. 1 m). A pairwise subtraction

of adj acent exposures was then employed to remove background sky emission. The sky

subtracted images were aligned and combined using imcombine to create a single image

containing all spectra for each slit mask or standard star (e.g. Figure 2-16).




























Figure 2-16. Reduced (but not extracted) spectra for n2071a2 (left) and the telluric stan-
dard HD 23050 (right) both observed on 2004 Nov 26. The negative regions
are byproducts of the pairwise subtraction


Once basic reductions were complete, the two-dimensional images were converted

to one-dimensional spectra as follows: The long slit (e.g. standard star) spectra were

extracted immediately, using the IRAF task apall to identify and trace the aperture and

extract the flux. The same aperture trace was then employed to extract arc lamp spectra

(4 m data) or background sky spectra (2.1 m data) for wavelength calibration. The

positions of known lines in these spectra were identified and used to derive the dispersion

correction (to convert pixel numbers to wavelengths). The correction was subsequently

applied using the IRAF task dispcor.

Extraction of the multi-slit data was accomplished by first cutting a two-dimensional

image of each slitlet from the final combined image described above (Figure 2-17).





Figure 2-17. Two-dimensional image of a single reduced MOS slitlet prior to extraction
of the one-dimensional spectrum. The image shown is n2071a2_06 the
sixth slitlet on n2071a2.










Each spectrum was then traced and extracted separately using apall. The dispersion

solutions for the multi-slit data were derived using background OH emission lines local

to each slitlet. Once the wavelength calibration was applied, target spectra were divided

by the telluric standard to correct for atmospheric absorption. Features introduced by this

division were removed by multiplying the resultant spectra by the solar spectrum, thereby

yielding the final product ready for spectral classification (Figure 2-18).


: ~' C
LI~:~~~


115~'' -lel..t 1-..1-l.:


Figure 2-18. The final reduced and calibrated spectrum for n2071a2_06.


2.4.4 Final Spectroscopic Sample

Although sixteen fields were originally targeted for spectroscopic observations,

as can be seen from Table B-1, only fields 2 (NGC 2071), 4 (NGC 2068), 6 (dense

core), 8 (dense core), 9 (dense core), 10 (NGC 2024), and 13 (no dense gas) were

observed. In part this was a choice made after the first observing season to increase

the overall number of sources observed (and hence the statistical significance) in the

cluster regions. Unfortunately, in the end, the limited off-cluster data taken proved to be







46

insufficient for analysis, largely due to low signal-to-noise values (SNR<20 as compared

to SNR~-30-100 for the cluster spectra) reducing the overall number of extractable and/or

classifiable sources in these regions. Reasons for these poor SNR values include weather,

large extinction in the cores, and poor plate alignment. Consequently, in the remaining

chapters, I focus solely on the imaging and spectra of the three Orion B clusters: NGC

2024, NGC 2068, and NGC 2071.















CHAPTER 3
SPECTRAL CLASSIFICATION OF YOUNG M STARS

3.1 Classification Strategy

In order to determine the spectral types of the late-type obj ects in our Orion B

sample, it is necessary to develop a classification scheme using standard stars of similar

spectral class. Over the past decade, the astronomical community has assembled a large

library of NIR spectra of field dwarfs (e.g. Cushing et al., 2005; McLean et al., 2003;

Reid et al., 2001; Leggett et al., 2001, 1996; Jones et al., 1994). The most prominent

features in low-moderate resolution NIR spectra of these late-type stars are the narrow

atomic lines of Al I, Na I, K I, and Mg I and broad absorption bands due to steam (H20),

iron hydride (FeH), titanium oxide (TiO), and carbon dioxide (CO). The strength of these

features is strongly dependent on spectral type (e.g. the molecular absorption in Figure

3-1.

The water absorption bands are ideal for classifying M dwarfs (e.g. Wilking et al.,

1999; Reid et al., 2001; Jones et al., 2002; Slesnick et al., 2004) as they produce a very

distinct continumm shape which becomes even more pronounced for later spectral types,

even at very low spectral resolutions (Figures 3-1 and 3-2). However, steam absorption

is significantly stronger in the NIR spectra of young obj ects than in field dwarfs of the

same optical spectral type (e.g. 93.4 and Lucas et al., 2001; McGovern et al., 2004).

Consequently, if we use field dwarf standards to type our young cluster members, the

derived spectral types will be systematically too late. Rather, we must use optically

classified young obj ects to make an accurate comparison.

3.2 FLAMINGOS Late-Type Spectroscopic Standards

Due to the large amount of dust absorption in the region, there is a dearth of obj ects

with published optical spectral types in Orion B. Consequently, to define a set of young











I
~ Fcl~


I


LNS 429 M7

- ---s_
LHS ROG5 Mg


WO HO


#,O
r


1.5
'r~uv~l~riglh y~i


Figure 3-1. A sample of low resolution near-infrared spectra (R ~ 600) for late-type
field dwarfs adapted from Leggett et al. (2001). Note the increase in water
absorption for later spectral types.


spectral standards, we turned to the nearby Perseus molecular cloud and the young

cluster IC 348. IC 348 is a partially embedded young cluster which has been extremely

well studied by multiple authors (e.g. Luhman et al. 2003b; Muench et al. 2003 and

refererences therin). The relatively low extinction in the region has allowed for a number

of known members with established optical classifications. Further, the youth of the

cluster (mean age ~ 2 Myr, Herbig 1998) implies that spectral template stars taken

from IC 348 membership lists will be similar in age and surface gravity to our Orion B

program obj ects (see ~3.4 for further discussion of surface gravity effects). Table 3-1 lists

the identifications, positions, and optical spectral types of the IC 348 standards as taken

from Luhman et al. (2003b).


~PI1~










Table 3-1. Young Spectral Standards
R.A. Dec. Optical Spectral Type

03:44:43.53 +32:07:43.0 Ml
03:44:33.22 +32:15:29.1 M2.25
03:44:30.30 +32:07:42.6 M3.5
03:44:21.91 +32:12:11.6 M4
03:44:18.26 +32:07:32.5 M4.75
03:44:35.52 +32:08:04.5 M5.25
03:44:38.88 +32:06:36.4 M6
03:44:15.58 +32:09:21.9 M7.5
03:44:21.15 +32:06:16.6 M8
03:44:33.42 +32: 10:31.4 M8.5
04:27:28.01 +26:12:05.3 M9.5


ID

1348-052
1348-122
1348-207
1348-095
1348-266
1348-230
1348-298
1348-329
1348-405
1348-603
KPNO-Tau4


Note. Spectral types for the IC 348 objects are the spectral
types adopted by Luhman et al. (2003b) and found in table 2 of that
work. The spectral type for KPNO-Tau4 was determined by Bricedo
et al. (2002).


FLAMINGOS spectra of these standards were obtained on the nights of 2004

October 04 and 05 on the KPNO 4 m telescope and reduced according to the procedures

described in Chapter 2 as part of a parallel survey to classify new members of IC 348

Luhman et al. (2005a, hereafter LOS). In addition, we also obtained a spectrum of the

young Taurus member KPNO-Tau 4 to provide a very late type template (M9.5, Bricefio

et al., 2002). The entire FLAMINGOS M star standard sequence can be seen in Figure

3-2.

3.3 A Reddening-Independent Procedure for Classifying Late-Type Spectra

The deeply embedded nature of our spectroscopic targets guarantees that there

will be significant amounts of extinction towards most sources (The Av in these regions

is typically 5-10 magnitudes and can sometimes be as high as 20-30 magnitudes,

c.f: Table 4-2). Consequently, the resultant spectra will be highly reddened, altering

both the continuum slope and the depth of any absorption features. This is a problem

for traditional M star classification schemes which employ spectral indices based on












co-HO HO HO HO HO HO
Mg I

M16

IM324 0523 8-298






S230n5-tau

1.2 i481.41618 12 .


F igvure3-2. FLAMNGO M sta spcta seuec copie usn yug( -2Mr
memer ofC38adTuu Lh a ta. 03b;Brcfi t l. 00)
Prmien fetue inteelwreouinsetr r niatd oeta
obet withsetrltps alerta 6 r islydatR50 nb
jetslaerthn 6 reshwnwih =20 orth rasnsdicuse blo
(633).Th cetrl rgios f te pecra rebloke ou fo dsply urpse
beas in mos cae h inlt niei hs einsi eylwdet
the ovrhemn telurcbsopton

coninum atis o dterin spctal yp. I thsecasstheextnciontoard a

objet isusualy drive phoometicaly an thespecrumis he "eedend b ta
amut.Te pcta ndxissbsqenl etrindfrmth eedeedsecrm

impling hat he ina lsifcto i eedeto hecluatdetncin(eeFgr


Ths epndnc o secra clsiicto on koing th k-acualecictooa







thgue derived NOSM ta spectral tye o xmlSlqesncket al. e (2004) found tha an uncrrete


spectrumj with A,=mgitudsesta waps enoughr to aus on ae to is-classify =5 and objetb





spctralu rtypes usin aeemn spectral nexbaed on the dept ofe the J-bacind wtewrd fatue











Corrected Spectrum (A =8.6)
1.5 Cont.
Uncorrected Spectrum







0.5-



1.2 1.4 1.6 1.8



Figure 3-3. Reddened and dereddened FLAMINGOS spectra of an M6.25 obj ect in NGC
2068. The J-band H20 absorption feature and nearby continuum peak are
indicated on both spectra. It is clear that the continuum/line ratio for this
absorption feature will depend on the applied reddening correction.


Following the work of LOS, we have elected to use a visual method to classify

our spectra. Generally, this method involves a "pseudo-dereddening" procedure which

dereddens the spectra by arbitrary amounts until all obj ects have uniform continuum

slopes. These "dereddened" spectra are then compared to the spectra of the IC 348/Taurus

standards shown in Figure 3-2. The maj or advantage to this method is that it is largely

independent of the true line-of-sight extinction towards an obj ect and the effect noted

by Slesnick et al. (2004) and discussed in the caption to Figure 3-3 does not apply.

Whether or not the reddening value determined visually is accurate, because all spectra

and standards have the same J-band flux values (see below) we can use the RELATIVE

depths of the water feature to classify obj ects without worrying about the accuracy of

photometrically determined reddening values. Put another way, the visual method of

classification we use here is a robust way to determine spectral types without requiring a

true extinction correction. The details of this visual method are as follows:

1. All reduced spectra (including the standards) are smoothed to a resolution R ~

500 and normalized such that the peak flux in the H-band, located at 1.68 pumhas a










value of 1.0. Note that because we are using a visual classification process based

on broad absorption profiles we are not losing any critical information by lowering

the resolution rather, the increase in S/N makes the continuum shape easier to

discern, facillitating spectral classification.

2. The slopes of the standard spectra are adjusted (if necessary) using the IRAF

deredden task until they have uniform J-band flux values (flux 1.21) at 1.32

pum (cf: Figure 3-2). The points at 1.32 and 1.68 pum were chosen because they

represent the regions least affected by the stellar absorption features and are

therefore closest to the true continuum levels.


Uncorrected Program Spectrum
1.5 =~1.32 p~m --------- Dereddened Program Spectrum
Locus of Standard Spectra






0.5



1.2 1.4 1.6 1.8


Figure 3-4. Example of the visual dereddening process. Objects are dereddened and over-
plotted with the standard spectra until their flux values at 1.32 pummatches the
standard template flux, irrespective of spectral type.


3. The IRAF "deredden" task is then used to visually redden or deredden the program

spectra until the flux value at 1.32 pum matches the flux of the standard templates

(Figure 3-4). Note that although a nominal value of extinction is generated via this

process, it is not necessarily refl ective of the true extinction toward program obj ects

as this "dereddening" is simply an artificial mechanism to ensure that all obj ects

have a relatively uniform appearance.









1.4
----- M5.25 standard (too shallow)
n Dereddened Orion Spectrum
,,.1 -- M7.5 standard (too steep)






I I I I I M




1.2 1.3 1.4 1.5 1.6 1.7 1.8
h(E.rn)

Figure 3-5. Example of the visual classification process. In this case, the J-band fall-off
of the blue spectrum (M5.25) is shallower than the Orion object while the
same water absorption feature in the red spectrum (M7.5) is too deep. The
same effect can be seen in the H-band in the region from 1.5-1.7 pum. Conse-
quently, the true spectral type of this object must be somewhere in between.
(In the end, after plotting this object with the M6 standard and against simi-
larly classified Orion spectra, we estimate its spectral type to be M6.25+0.75
subclasses.)


4. A spectral type for each obj ect is determined by visually comparing the program

spectra to the individual standards until a best fit match is determined. Particular

attention is paid to the slope and depth of the J-band fall-off at 1.3 5 pum, the strong

absorption features in the HI-band on either side of 1.68 pum, and if present, the

strength of the Mg I line (Figure 3-5). If the obj ect appears to be late-type (>M6),

the spectrmm is smoothed to R~-200 to further increase the S/N, making the exact

spectral type more obvious. Finally, once all obj ects have been assigned a first-pass

spectral type, classifications are fine-tuned by placing the spectra in order of their

M subclasses and adjusting the sequence to ensure that the strength of the water

absorption monotonically increases with spectral type.

We estimate this method of spectral classification to be quite robust, with typical errors in

spectral type of +0.5-1 subclasses.










Table 3-2. Surface Gravity Standards
ID R.A. Dec. SpT' Gravity Obs.Date Telescope Obs.TypeZ zex Nex
HD 39045 05:51:25.0 +32:11:09 113 III very low 2004 Dec 18 2.1m slit 3s 7
1348-207 03:44:30.3 +32:07:43 113.5 IV low 2003 Jan 16 4m mos 600s 12
GL 388 22:28:00.3 +57:40:06 113 V high 2004 Oct 08 4m slit 10s 7
HD 196610 20:37:55.6 +18:20:24 116 III very low 2004 Dec 31 2.1m slit 2s 9
1348-298 03:44:38.9 +32:06:36 116 IV low 2003 Jan 16 4m mos 600s 12
GJ 1111 08:29:48.9 +26:48:32 116 V high 2004 Dec 12 2.1m slit 20s 10
KPNO-Tau4 04:27:28.0 +26:12:05 119.5 IV low 2004 Oct 07 2.1m slit 300s 12
LHS 2065 08:53:36.4 -03:27:32 119.5 V high 2004 Dec 13 2.1m slit 60s 15

'Spectral Type for each object, including luminosity class.
2Indicates whether data were taken at the KPNO 2.1m or 4m telescope and whether the data were taken in mulit-object (mos)
or single slit (slit) mode.


3.4 Surface Gravity

In addition to their obvious usefulness in general spectral classification, the low

resolution NIR spectra of M stars can also be employed to yield rough estimates of an

object's surface gravity (e.g. Gorlova et al., 2003; McGovern et al., 2004). In particular,

certain narrow absorption features as well as the broad water absorption bands discussed

above are especially sensitive to surface gravity effects. This sensitivity provides a natural

method for distinguishing young sources (which have intermediate surface gravities) from

relatively high surface gravity field dwarfs or low gravity background giants. In order to

evaluate the surface gravity of the Orion sources and ultimately assist with membership

assessment (see Chapters 4 and 5), I have assembled a small selection of surface gravity

standards observed with FLAMINGOS. The sources and observing log are listed in Table

3-2 and the reduced spectra are shown in Figure 3-6.

Figure 3-6 illustrates the progression of the strongest gravity sensitive features

visible in our data-the broad water absorption features in both the J and H bands and

a narrow potassium doublet at 1.243/1.252 pum-as a function of both surface gravity

and spectral type. For the M3 obj ects both the J and HI-band H20 induced fall-offs

are steepest for the young star. This effect becomes more dramatic for the M6 and M9

obj ects where the field dwarf continuum profiles have broad HI-band plateaus versus

a distinct triangular shape for the young obj ects. (The same effect was also noted by

Lucas et al. (2001) and LOS). Looking at the M6 and M9.5 obj ects, the field stars have












M3III -._Iv M6III
1.5 --


M3.5 ~2Myr 1~' M6 ~2Myr wl' i M9.5,~1Myr





0.5 H03 HM6.5VH. I H20M9.5V
KI KI
1.2 1.4 1.6 1.2 1.4 1.6 1.2 1.4 1.6


Figure 3-6. FLAMINGOS spectra of the young obj ects 1348-207 (M3.5), 1348-298
(M6), and KPNO-Tau4 (M9.5), shown with spectra of the field dwarfs GL
388 (M3V), GJ 1111 (M6.5V), LHS 2065 (M9.5V) and the M Giants HD
39045 (M3III) and HD 196610 (M6III). The most prominent gravity sensitive
features at R~500 are labeled. The spectra of both giants appear to have a
much higher H-band line frequency than the young obj ects or field dwarfs. In
addition, water absorption causes the younger obj ects to have a much more
triangular H-band shape which can be used to distinguish field stars from
young cluster members.


a strong potassium doublet which is weak or absent in the lower gravity atmospheres

of the young stars and giant. Finally, it is also apparent that the two giants have a flatter

continuum profile and a much higher frequency of H-band absorption lines. Consultation

of the literature (e.g. the low resolution infrared spectral libraries of Lancon & Rocca-

Volmerange (1992)) confirms that this is a hallmark of giant stars and is likely caused

by overtones of CO and OH as well as blended molecular lines only visible at very low

surface gravities.

These trends in spectral shape and line frequency will be used in upcoming chapters

to assess the surface gravity of each Orion spectrum. As in Table 3-2, sources will be

assigned a designation of low (indicating a low surface gravity young obj ect), high

(indicating a field dwarf), and very low or giant (indicating a very low gravity giant).

The results of this gravity assessment will then be used to assist in establishing the

membership status for each obj ect.















CHAPTER 4
M STARS AND BROWN DWARFS IN NGC 2024

4.1 Introduction

In this Chapter I present results from FLAMINGOS photometry and spectroscopy

of the young cluster NGC 2024. Specifically, I characterize the individual spectra,

subsequently combining them with the photometry to construct Hertzprung-Russell

diagrams for the region. With the aid of PMS evolutionary models, I derive a new age

and IMF for the low mass cluster population and examine the properties of the substellar

population. In forthcoming chapters these results will be compared with those in other

regions of low mass star formation both within and outside the boundaries of Orion B.

NGC 2024 is a young (<1 Myr) HII region that is deeply embedded in the cloud

material of Orion B. The distance to the region is estimated to be 415pc (Anthony-

Twarog, 1982). Optical images show a spectacular, flame-shaped nebula with a dark dust

lane at the center obscuring the heart of the cluster and most of the stars in the region

(Figure 4-1).

The center of the optical nebula is a bright radio source which exhibits both radio

continuum emission and recombination lines, indicating the presence of a massive star

(spectral type ~O09) responsible for the ionizing radiation ( 1.3, Kruegel et al., 1982;

Barnes et al., 1989). Due to the large amount of dust in the region, this ionizing source

was only recently identified to be the late O to early B star IRS2b (Bik et al., 2003).

The full extent of the cluster associated with the HII region was revealed by the K-band

imaging survey of Lada et al. (1991Ib) who detected ~300 sources down to K< 14.0.

Multi-wavelength infrared photometric studies of the cluster show that maj ority of

detected objects exhibit near-infrared excess emission indicative of hot circumstellar

material (Comeron et al., 1996; Haisch et al., 2000). The proximity, extreme youth, and





































Figure 4-1. Optical image of NGC 2024 (The Flame Nebula) taken from the digital sky
survey. North is up, East is left, and the field of view is 20' x 20'.


indicators of active star formation in NGC 2024 combine to make this an ideal region to

study the young, low mass population.

4.2 New Photometry for NGC 2024

In addition to the survey photometry described in Chapter 2, J, H, and K-band

images of NGC 2024 were obtained on 2001 November 19 using FLAMINTGOS on the

KPNO 4m telescope. The data were taken using a 16-point dither pattern with individual

exposure times of 60s for J and H and 30s for K, yielding total exposure times of 16

minutes in J and H and 8 minutes in K. Typical seeing at all wavelengths was 1 171-1('2

FWHM and the 100- detection limits are J=19.4, H=18.8, and K=17.8.

These images were reduced and assessed using the pipelines and routines described

in ~2.3.2 and 2.3.3.1. The JHK photometry for ~400 sources having color errors <0.1













000

m co
ao oo Bo o`

?s o 00 Pe


0 .o acoo o o 0 o


:000 0 ol


Figure 4-2. Color-magnitude (left) and color-color (right) diagrams for all classified
obj ects in NGC 2024. In the CMD, the leftmost dotted line is the main se-
quence from Bessell & Brett (1988) and to the right is the 1 Myr isochrone
of D'Antona & Mazzitelli (1997). The dot-dashed lines are reddening vec-
tors using the extinction law of Cohen et al. (1981) placed at 3, 0.08, and
0.02 Me0. In the color-color diagram, the dotted lines are the giant colors of
Bessell & Brett (1988) combined with Bessell & Brett (1988) dwarf colors
for spectral types down to K7 and Leggett (1992), Leggett et al. (1996), and
Dahn et al. (2002) dwarf colors for spectral types from MO to M6. The dot-
dashed lines are the reddening vectors of Cohen et al. (1981) and the dashed
line is the classical T Tauri locus of Meyer et al. (1997). Open circles are
general photometric catalog sources, filled circles are spectroscopic targets,
and the red triangles represent the final classified sample.


magnitudes is shown in Figure 4-2. In regions with little to no nebular emission, the

photometric accuracy (as indicated by a comparison to 2MASS) is estimated to be 0.03

magnitudes. For regions with large amounts of nebula, as in the center of NGC 2024, the

scatter with respect to 2MASS was much larger (~0. 15 magnitudes) than that expected

from purely photometric noise. Reasons for this difference are discussed in ~2.3.5. The

scatter with respect to 2MASS was also larger for objects on the edge of detector where

the data rapidly degrade due to a delamination of the engineering array. For obj ects in this

region (generally non-nebular), the 2. 1 m photometry from the imaging survey was used.

The survey field corresponding to NGC 2024 is orib-01. Given that the sources requiring










Table 4-1. Slit Masks Observed in NGC 2024
Mask ID Telescope Target Selection Method Nsi~tt Nartracted a Nclassified b Nduplicates c

n2024bd1 4m Brown Dwarfs 23 22 (+1) 8 0
oc24mf11 4m IRX Sources 27 22 16 0
oc24mf21 4m IRX Sources 22 19 (+3) 17 0
n2024f31 2.1m K magnitude 23 11 (+1)d 5 1
n2024b2 4m H magnitude 25 19 9 0
n2024b3 4m H magnitude 28 22 (+2) 19 4

Totals: 148 115 (+7) 74 5

aln cases where multiple sources fell on a single slit it was possible to extract stars in
addition to those originally targeted. These objects are indicated in the Nartractio column
by (+n).
bNclassified refers to the number of M stars classified in each mask in the 4 m field of
view. In other words, stars with spectral types earlier than M are not counted.
CThis column indicates the number of independently classified duplicate sources on the
given mask
dOf the 23 slits on the 2.1 m plate, only 11 fell within the 4 m field of view.


this photometry are in predominantly non-nebular regions, the mean survey scatter value

ofo-=0.05 magnitudes applies. The field-specific statistics (which include the extremely

nebular portions of the cluster) can be found in Appendix A.

4.3 Spectroscopy of NGC 2024

4.3.1 Sample and Observations

The spectroscopic sample for NGC 2024 was selected according to the guidelines

described in Chapter 2. In the end, 148 sources were targeted for observation on 6 differ-

ent slit masks. The targeting breakdown can be found in Table 4-1 and the photometry is

shown by filled circles in Figure 4-2. The spatial distribution of these targets can be seen

in Figure 4-3.

FLAMINGOS observations of the 4m slit masks were taken on the nights of 2003

January 19, 2003 December 06, 2003 December 10 and 2004 December 01. Spectra from

the 2. 1 m slit mask were obtained with FLAMINGOS on the night of 2003 November

29. The full details of the observing procedures at both telescopes can be found in ~2.4.2.

The specific integration times by mask can be found in Appendix B. All spectroscopic












































Figure 4-3. Three-color image of NGC 2024 taken with FLAMINTGOS on the KPNO
4m telescope. North is up, East is to the left, and the field is approximately
10' on a side. Circled objects are all 4m spectroscopic targets and rectangles
enclose the 2.1m targets.


data were reduced using the procedures detailed in ~2.4.3 and classified according to

the methods developed in Chapter 3. Totals by mask of the number of sources targeted,

extracted, and classified are listed in Table 4-1.

4.3.2 Results

4.3.2.1 Spectral Classification

The final classifications yielded 65 unique obj ects from the 4m sample with

identifiable M type spectra (ranging from M1 to >M8) and 2 sources with spectral










types earlier than MO. In addition, 4 duplicate sources were also extracted which when

independently classified yielded spectral types in agreement with the original source to

within 0.25 subclasses. Of the ~40 4m extracted objects which were not classified, 14

were filler targets with K magnitudes > 15.0 which are typically too faint to classify with

our current exposure times. The remaining unclassified sources while bright at K, were

typically highly reddened obj ects with poor signal to noise in the J and H bands after

dereddening. The 2.1Im sample yielded 4 new M type obj ects, 6 sources with spectral

types earlier than MO, and one duplicate classification (which agreed with the 4m source

to within 0.5 subclasses). All classified spectra are shown in Figure 4-4 along with

selected obj ects from the FLAMINGOS M star standard sequence (@3.2). Obj ects with

spectral types M6 have

been further smoothed to R~200 to aid in the classification process.

4.3.2.2 Surface Gravity Assessment

Applying the surface gravity diagnostics discussed in Chapter 3, 53.4 to the NGC

2024 spectra, I find the majority of sources display the distinct triangular continuum

profiles indicative of youth and are thus assigned low gravity designations. I also find

that strong J-band potassium lines are absent from all spectra. Two sources (60 and 64)

display enhanced absorption in the H-band and may be background M giants.

Results of the assessment for each individual obj ect are noted in Table 4-2. An

ellipsis in the gravity column indicates that the gravity assessment was not completed,

typically because the spectral type was too early to exhibit a distinct triangular profile.

However, for all uncertain cases the HI-band was sufficient to rule out background giants

and the relatively large values of Av imply that these sources are not foreground obj ects

(refer to 54.5.1).

4.3.2.3 Infrared Excess and Spectroscopic Veiling

The presence of excess flux in the near infrared is commonly taken to be an indicator

of thermal emission by warm dust in a circumstellar disk. This type of emission can result















HO H,0 H,O


1aL


M2.5-M4
f21-16

M3.5


Pa Mg

M1


icMO-M2
f21-17a



M1-M225

; ~bd1--13 V -

S ILGV M1.25-M2.25
bd1-12 V

M1.25--M2.25
bdl-16a

M2.25


SM3-M5
b2-20

M3.5-M4.5
fi 21-06

SM3.5-M4.5
f21--15a

M3.5-M4.5
b3-14

Y M3.5-M4.5
I.b3-12
.M3.5-M4.5
f11-29


b3-11

, M3.75-M4.b-5
"~ f21-01

SM4-M5


. A


: i



v- -r.iYyj~r


.r


5~~


~W*T`UrCI-72 M2-MZ
bd1-18

,M2-M3
n fZ1-13

: j~M2-MZ.5
b2-06

M2-M3.5
Sb2-11

M2-M3.5
~f21-14


.L M2-MZ.5


1.8 1.2 1.4 1.6


Figure 4l-4-l. NIR spectra of all classified M stars in NGC 4'-i (labeled with both spectral
type and ID)) shown with the IC 348 optically classified young standards
(labeled with spectral type only). Prominent spectral features are identified at
the top. (:i .] : having spectral types and objects '>M6 have been smoothed to R~2--' to aid in the ii:Ti i.:
..... T~ 7he central regions of the spectra are blocked out for 1.1 i o pur-

poses because in most cases the :-::.:1 to noise in il:. regions is very low
due to the overwhelming telluric absorption. In some cases, the J-band has
also been blocked these i.. i .: were i: r: i: ::l.:: Ty noisy and classified using
their H-band continuum shape only.


d ~:M2.25-M4
fil-16

M2.25--M4
b3-04





















H,O


co :


M4-M5


?:M4-M5
:..: bd1-D9


M4.25-M5.2

Sb3-17

i--~j M4.75


'~M4.5-M5.5
f31-10

" iM5-M5.5
b2-03

1 M5-M5.5
b3--02

j~M4.75-M5.;
bZ-10

M;~ 4.5-M6E
b3-25

M5.i~. y25


r






~~~^I"^"?,

~Gj~ i
,


SM4.75-M5.75
bd1-21


b3-01
SM5-M6
b3-24

M4.75-M5.75



f21-20

SM5-M6
i11-10~L

1 M5.2-M6.25


C
"21~
i,
:': :
''' :' :
I

;t*nihL~




lin
i


r
ii: : ::






r


II



I;


75


I ~ M4-M .5

~ ~'M24-M b5 j

M4-M5.5
f11-23

.M4--M5.5


M4.25-M5.25
f11-02

Md415-M5.5

N M4-M6
fli-08

,- M4.5-M5.5
"bd1-11


'LJ""YX/


~i~[


:


i


. *.


i:


1.2 1.4


1.8 1.2


1.4 1.6


Figure 41-4i. continued
































































1.2 1.4 1.6 1.8 1.2 1.4 1.6


Figure 4l-4. continued









in the weakening or veiling of both narrow and broad-band spectral lines. If the amount of

veiling is significant, it can affect spectral classification causing an object to appear earlier

than its true spectral type. In this section I attempt to quantify the effect of veiling on our

classification process by visually inspecting our sample and examining its infrared excess

(IRX) properties.

Visual inspection of the spectra yielded one obj ect (source 5, M1.75) with obviously

weak Mg I absorption likely caused by veiling. In addition, excluding the possible giants

(see above), 27 out of 67 obj ects or 40%+9% of our classified M star sample exhibit

an IRX as determined via a comparison of each obj ect's expected intrinsic H K color

(inferred from spectral type) with its dereddened observed H K color. (The reader is

referred to 64.4. 1 for an explanation of intrinsic color choice and dereddening methods.)

What fraction of IRX sources can be expected to have significant veiling? To answer this

question I have calculated rk, the K-band veiling index for each source.l The typical

errors on rk are +0.08 magnitudes, implying that we should not place too much weight

on individual values of rk which are close to zero. However, since the primary purpose of

this rk analysis is to identify the sources with veiling strong enough to bias our spectral

classification, this error is acceptable. As can be seen from Table 4-2, only source 38

(rk=0.62) exhibits an amount of veiling near rk=0.6, the median value for Classical T

Tauri stars (Meyer et al., 1997). Consequently, I note the potential bias towards an earlier

spectral type for this obj ect.

4.4 The Hertzsprung-Russell Diagram

In this section I combine spectral types and infrared photometry to derive visual

extinctions, effective temperatures and bolometric luminosities for all classified objects. I




SThe veiling index rh is defined as Fh,/Fh,. Converting to photometric K-band excess
yields rk = [( t H) 10((H-K)-(H-K)-0.065.,)/2.5 1]. Note that because I am deredden-
ing to intrinsic dwarf colors our values for rk are lower limits since rh is assumed to be
zero (Meyer et al., 1997).










then place these data on the Hertzprung-Russell (H-R) diagram and use theoretical PMS

evolutionary models to infer masses and ages for the sample.

4.4.1 Extinction

Because NGC 2024 is so deeply embedded in its natal cloud, there is a large and

variable amount of extinction in the region which acts to differentially redden source

magnitudes. In order to properly estimate physical parameters and infer masses and

ages, this reddening must be accounted for. The amount of extinction towards a given

source is typically derived by dereddening its broadband colors, however, care must be

taken when choosing passbands. The redder infrared bands are less sensitive to variations

in extinction but may also be contaminated by infrared excess emission arising from a

circumstellar disk. Optical bands suffer from contamination due to UV excess emission

from the stellar photosphere. It is generally agreed upon that bands between R and J are

most sensitive to extinction while minimizing the effects of excess emission (cf: Meyer

et al. 1997; Luhman et al. 2003b). As I do not have reliable optical photometry for NGC

2024, I elected to use the bluest infrared bands to derive extinction estimates for each

source.

Extinction estimates were determined by comparing our observed J- H colors with

the empirically determined intrinsic M dwarf colors of Leggett (1992); Leggett et al.

(1996) and Dahn et al. (2002) and then converting the color excess to an Av measurement

using the reddening law of Cohen et al. (1981). The choice of both the intrinsic colors

and the reddening law was based primarily on photometric system. FLAMINGOS

filters closely approximate the CIT system (Elston et al. (2003) and the FLAMINGOS

web pages) thus I opted for a reddening law and intrinsic color set derived in the same

system. I opted against using theoretical PMS colors since at young ages these are

highly dependent on model input physics. For the one source lacking J-band photometry,

an extinction estimate was derived using H K colors. It should be noted that for

comparison I also estimated visual extinctions for all sources using H K intrinsic colors










and by dereddening obj ects to a model isochrone in both J/J H and H/H K color

magnitude diagrams. These methods yielded Av values which deviated from the J- H

intrinsic color estimates by as much as 1-2 magnitudes for J/J -H and 3-4 magnitudes

for H/H K. Effects of this deviation will be discussed in ~4.4.2 and 4.5.2.

12111 1. 11. 1.1 11.1.1 11 ,,,|,,,,


5 10 15 20 25


Figure 4-5. Distribution of Av for the NGC 2024 spectroscopic sample. Av values were
derived by comparing observed J H colors with the intrinsic J H colors
of Leggett (1992); Leggett et al. (1996); Dahn et al. (2002).


Figure 4-5 shows the distribution of visual extinctions derived from J -H intrinsic

colors. Values range from ~1-30 visual magnitudes with a mean Av of 10.7 magnitudes.










This is in good agreement with the survey of Haisch et al. (2000), who find a range of A,

from roughly 0-30 visual magnitudes and a mean A,- of 10.4.

4.4.2 Effective Temperatures and Bolometric Luminosities

Spectral types were converted to effective temperatures using a linear fit to the

adopted temperature scale of Luhman et al. 2003. This temperature scale, derived from

the young quadrouple system GG Tau, falls between dwarf and giant temperature scales

and is thus appropriate for the intermediate surface gravity objects studied here. Absolute

magnitudes were calculated by dereddening K magnitudes (see below) using the A,-

derived in ~4.4.1 and applying a distance modulus of 8.09 (Anthony-Twarog, 1982).

Bolometric magnitudes and luminosities were derived using the bolometric corrections of

Leggett (1992); Leggett et al. (1996) and Dahn et al. (2002) as they were observationally

determined using CIT photometry.

While J-band is typically the preferred wavelength for deriving bolometric lumi-

nosities as contaminating excess effects are minimized (Luhman, 1999, see also ~4.4.1), I

have elected to use the K-band since luminosities derived from K magnitudes are far less

sensitive to errors in dereddening. As discussed above, photometrically derived extinction

values can have errors as large as 3-4 magnitudes. A change in A,- of 3 magnitudes

corresponds to nearly a magnitude of uncertainty in dereddened J-band magnitudes but

yields a much smaller AK (<0.3 mag). Although K magnitudes are more sensitive to

excess emission from a warm circumstellar disk, this effect is small in log-Luminosity

space (average AlogL ~0.06 dex). Even when combined with the A,- uncertainty, the net

uncertainty in K-derived bolometric luminosities (+ 0.17 dex) remains smaller than the

corresponding uncertainty using J-band to derive bolometric luminosities (+ 0.32 dex).

4.4.3 H-R Diagram

Figure 4-6 shows H-R diagrams for the classified sources in NGC 2024 along with

the PMS evolutionary models of D'Antona & Mazzitelli (1997) and Baraffe et al. (1998).

The triangular points represent 2.1m classifications and sources with diamonds were





















~02M 8M.0.8
.OM 2Me .02M


0.1Me OOM

3.65 3.60 3.55 3.50 3.45 3.40 3.65 3.60 3.55 3.50 3.45 3.40
Log Te Log Tef f

Figure 4-6. H-R Diagrams for NGC 2024 shown with the pre-main sequence models of
D'Antona & Mazzitelli (1997) (left) and Baraffe et al. (1998) (right). The
diamonds represent points with 4m spectra and the triangles are sources
classified with 2.1Im spectra. Asterixes are potential background giants. Rep-
resentative error bars for an MS obj ect are shown. The solid line accounts
for errors in derived spectral type, distance modulus, and photometry and
the dashed line incorporates an additional error of +3 magnitudes of visual
extinction (see ~4.4.1).


classified using 4m spectra. The two asterixes represent the possible background giants

(see ~4.3.2.2). Individual object data are tabulated in Table 4-2. Two typical error bars for

an M5 dwarf are shown in the lower left corner. The solid line was derived by classically

propagating the measured errors in the photometry, spectral type (0.75 subclasses),

and distance modulus. In this case, the error in luminosity is dominated by error in

the distance to NGC 2024. The dashed line incorporates an additional error of + 3

magnitudes (corresponding to +0.2 dex) in the reddening estimate (refer to 64.4.1) which

dominates the error bar and leads to a larger uncertainty in the calculated luminosity.











Table 4-2. Data for Classified Sources in NGC -= '4
Source J H- K M : i Gravity- Av rK i 'T Ff lo0g i i MaSS -

01 12.06 1 1.07 10.30 02 12.25 11.07 10.37 1.75 ... 4.73 0.19) 3 :.0.125 0.72
03 13.71 11.9)1 11.07 4.00 low~ 10.82 -0.11 3 1 -0.012 0.29)
04 14r.11 12.20 11.29) 1.75 ... 11.36 -0.03 3.5583 -0.004r 0.72
05 15.54 12.831 11.29 1.75 ... 18.82 0.09 3.558 0.2641 0.72
06 13.64 12.0)0 11.37 1.75 ... 8.91 -0.13 3.558 -0. 124 0.70
07 13.22 11.96 11.43 2.50 low 5.59) -l 3.544 -0.29)2 0.J5
08 14.64 12.69 11.615 3.50 low : -0.01 3.524 -0. 195 (
09 15.30 12.80 11.66 4.00 lowc 17.18 -0.20 3.514 -0.019 0.29
10 15.10 12.87 11.69C 4.75 low 14.70 -0.06 3.. '--= -0.146 0.17
11 15.29 13.01 11.69) 2.75 lowh 14.93 0.12 3 -0.067 0.54
12 14.06 12.49 1 1.75 4.50 lowv 8.64 -0.09 3.504 -( 0.20
13 14.77 12.83 11.79 2.75 lowc 11.84 0.04 3.539 -0.218 0.54
14 13.57 12.241 11.81 1.50 ... 6.09) -0.14 3.562 -0.393 0.68
15 14.18 12.55 11.84 2.75 lowr 9.32 -0.12 3.539) -0.350) 0.44
16 13.58 12.38 11.9)2 5.25 low~ 5.48 -0.17 3.488 -t 0.13
17 14.36b 12.94 12.20 5.25 lowc 7.48 -0.05 3. -: -0.627 0.13
18 14.131 12.78 12.23 2.75 low 6.73 -0.13 3.539 -0.6231 0.28
19 14.42 13.08 12.4;1 4.75 lowh 6.61 -0.04; 3.1 i -0.725 0.17
20 14t.412 13.13 12.49) 5.50 low 6.36 -0.08 3.483 -0. 1 0.12
21 17.57 14.30 12.52 4.50 low 241.09) -0.06 3.5041 -0.131 0.20
22 16.19) 13.832 12.65 23 16.81 14.37 12.67 5.50) lowr 16.82 0.30 3.483 -0.487 0.12
24 18.04t 14.65 12.73 4.7.5 low 25.25 -0.01 3.499 -0.182 0.17
25 14r.33 13.36 12.77 7.75 low 3.49 -0.05 3.431 -1.084 0.03
26 16.18 14.20 12.94 4.75 lowc 12.43 0.16 3. -: *** -0.727 0.17
27 14.91I 13.64 12.98 5.00 low 6.05 -0.03 3 -0.9)82 0.17
28 15.60 13.93 12.98 7.25 lowh 10.25 -0.10 3.443 -0.9)07 0.04
29 14.25 13.45 : 8.00 lowv 1.15 -0.05 3.425 -1.264 0.03
30 16.15 14.24 13.00 3.25 lowc 11.86 0.24 3.529 -0.723 0.35
31 15.55 13.84 13.01 4.25 low 9.95 -0.08 3.509) -0.8283 0.25
32 15.59) 13.96 13.03 5.75 lowT 9.59 -0.02 3.478 -0.900) 0.10
33 15.12 13.84 13.10 5.25 low~ 6.20 0.03 3.488 -1.033 0.15
34 16.33 14.28 13.12 5.00 low 13.141 0.01 3.494 -0.783 0.15
35 15.10 13.835 13.19 5.25 low 5.93 -0.03 3.488 -1.0783 0.15
316 15.57 14.07 13.22 7.25 low 8.70 -0.10 3.443 -1.059 0.04
37 1( **: 14.22 13.25 5.25 lowh I1.02 -(* 3. i :. -0.9)19 0.13
38 16.83 14.84 : 3.00 lowv 12.36 0.612 3.534 -0.810 0.37
39 18.86 15.30 13.37 7.00 lowc 27.05 -0.18 3.449 -0.450 0.04
40 15.55 14.20 13.37 8.00 low 6.16 -0.01 3.425 -1.236 0.03
41 16.13 14.i 13.4;2 5.50 lowh 9.91 0.06 3. i -1.036 0.12
42 17.66 14.98 13.43 6.00 low~ 19).27 -0.04 3.-0. 0.09)
43 15.71 14.33 13.50 7.50 low 8.00 -0.09) 3.437 -1.205 0.03
44 16.11 141.541 13.56 6.75 low 8.98 0.02 3.455 -1.1683 0.05
45 15.77 14.38 13.57 5.50) lowr 7.27 0.02 3.483 -1.19)1 0.13
46 15.75 14.35 13.57 5.75 low~ 7.. -0.03 3 .: -1.19)1 0.11
47 15. -- 14.47 13.62 4.75 lowc 7.98 0.04 3. -: *** -1.160 0.19
483 16.99) 14.93 13.65 7.75 low 13.40 -0.01 3.431 -1.079) 0.03
49) 17.02 14.97 13.67 6.50) lowr 13.36 0.07 3.461 -1.045 0.06
50 17.28 14.96 13.69) 4.7.5 low~ 15.52 -0.03 3.499 -0.916 0.20
51 15.60 14.35 13.70 4.00 low 5.82 0.01 3.5141 -1.244r 0.23











Table 4-2--Continued


Source J H K M Subclass"


Gravity

low
low
low
low
low
low
low
low
giant
low
low
low
giant
low
low
low
low
low
low
low


9.00
9.12
18.86
29.11
9.52
5.36
12.91
7.00
14.64
6.66
7.98
11.98
11.23
7.27
8.85
8.70
8.05
6.98
4.68
14.25


logTeff loI LL I Mass (L)


rK

0.08
0.10
0.11
-0.14
0.02
0.01
0.07
-0.02
0.24
0.16
-0.12
0.19
0.15
-0.01
-0.06
0.05
-0.14
-0.09
0.16
0.03


16.30 14.73
16.54 14.86
18.05 15.36
15.88
16.40 14.77
15.91 14.64
17.24 15.32
16.13 14.86
17.86 15.64
16.40 15.03
16.44 15.02
17.56 15.63
17.44 15.60
16.68 15.31
17.10 15.54
17.74 16.11
13.74b 12.25
13.83 12.48
13.88 12.72
15.54 13.43


13.70
13.71
13.75
13.76
13.76
13.82
14.00
14.02
14.19
14.21
14.24
14.37
14.42
14.50
14.61
15.05
11.59
11.75
12.04
12.07


6.50
8.25
4.25
7.00
6.75
8.50
7.50
7.50
4.00
2.75
7.25
4.75
5.00
6.25
7.75
8.00
5.00c
6.75c
2.50c
7.25c


3.461
3.419
3.509
3.449
3.455
3.412
3.437
3.437
3.514
3.539
3.443
3.499
3.494
3.466
3.431
3.425
3.494
3.455
3.544
3.443


-1.215
-1.274
-0.803
-0.531
-1.228
-1.461
-1.228
-1.449
-1.123
-1.373
-1.493
-1.316
-1.371
-1.588
-1.626
-1.816
-0.399
-0.516
-0.568
-0.399


"Spectral types are listed as M subclasses, thus a table entry of 0.0=MO.0, 7.50=M7.50, etc.

bSources 17, 47, and 68 have J magnitudes derived from FLAMINGOS imaging on the 2.1m telescope.

CSources 68-71 have spectral types derived from 2.1m spectroscopy.


4.4.4 Masses and Ages

In order to derive mass and age estimates for young obj ects, sources must be placed

on an H-R diagram and their positions compared with pre-main sequence evolutionary

models. The most frequently used models for low mass stars and high mass brown dwarfs

(rather than planetary mass obj ects) are those of D'Antona & Mazzitelli (1997, hereafter

DM97) and Baraffe et al. (1998, hereafter BCAH98). The primary differences between

these two models are their treatment of convection (mixing-length theory for BCAH98

and full spectrum turbulence for DM97) and the assumption of grey atmospheres in

DM97 vs. non-grey in BCAH98. BCAH98 and references therein argue that the grey

atmosphere approximation is inappropriate for stars whose effective temperatures fall










below ~4500-5000 K as molecules present in the atmospheres will introduce strong non-

grey effects. There is some evidence supporting this claim as both White et al. (1999) and

Luhman et al. (2003b) used empirical isochrones definined with low mass members of

IC 348 and Taurus and the young quadrouple system GG TAU to show that the BCAH98

models agree better with observational constraints. Consequently, while I present H-R

diagrams using both sets of tracks, for the remaining discussion we will focus primarily

on results derived from the BCAH98 models.

Mass and age estimates were derived from the BCAH98 models by interpolating

between the isochrones and mass tracks shown in Figure 4-6. Sources falling above the

youngest isochrone (1 Myr) were assumed to have an age <1 Myr and were dropped

down to the 1 Myr isochrone along a line of constant effective temperature to derive a

mass estimate. In this manner, I derived masses spanning a range from 0.02 to 0.72 M.,

(with 23 obj ects falling below 0.08 M..) and ages ranging from < 1 to ~-30 Myr.

4.5 Properties of the Low Mass Cluster Population

4.5.1 Cluster Membership

Prior to drawing any conclusions regarding the age of NGC 2024 or its substellar

population, it is necessary to evaluate the membership status of sources in our sample.

In the absence of proper motion data, we must rely on other diagnostics to determine

whether obj ects are bona fide cluster members or foreground or background sources

proj ected on the cluster area. The discussion of surface gravity effects in ~4.3.2.2 rules

out foreground or background dwarf contamination in our spectroscopic sample as there

are no potassium lines present in our spectra. In addition, NGC 2024 is deeply embedded

in a core of dense gas (Lada et al., 1991a, 1997) which will obscure background field

stars, limiting the number of field contaminants in the photometric sample. The average

column density of hydrogen in a 0.6 pc clump centered on NGC 2024 has been estimated

from C180 emission to be N(H2)=4.6 x 102 cm-" (Aoyama et al., 2001). Given that

a molecular hydrogen column density of 102 cm2 corresponds to 1 magnitude of










visual extinction (Bohlin et al., 1978), background sources in this region will be viewed

through 46 magnitudes of visual extinction, or ~4.1 magnitudes of K-band extinction.

The spectroscopic sample includes sources down to K ~ 15. Background obj ects

contaminating this sample are seen through the cloud and thus will have unreddened

magnitudes K
control field as well as a similar area from the 2MASS database, I estimate that there

are no more than 5 background sources with K< 11. As this is a relatively insignificant

contribution to the total photometric luminosity function, I conclude that a background

correction is unnecessary. I do note the possibility of giant contamination for two

spectroscopic sources (60 and 64) which display enhanced absorption in the H-band, thus

these obj ects are excluded from further analysis.

4.5.2 Cluster Age

Sources in the H-R diagrams in Figure 4-6 do not fall along a single isochrone

but rather show a scatter in age ranging from <(1 Myr to ~-30 Myr, irrespective of the

PMS models used. This type of width in the evolutionary sequence of young clusters is

common and is usually attributed to a variety of effects including: real age differences

between sources, errors in luminosity derived from uncertainties in the derived reddening,

photometric uncertainty (these effects are represented by the error bars in the figure), as

well as distance variations between sources, variability due to accretion and rotation of

young obj ects, and unresolved binaries. The median age of the entire sample however

should be representative of the median age of the cluster population in the mass range

detected here.

Using the models of BCAH98 the maj ority of sources fall above the 1 Myr

isochrone. Consequently, the median age of the cluster can only be constrained to

< 1 Myr. However, the DM97 models extend to younger ages than those of BCAH98.

Even though I have elected to place more weight on results derived with the BCAH98

models (see ~4.4.4), the DM97 models provide us with additional information on the age










of the cluster population as well as a means to compare our results with previous surveys

of NGC 2024 and other regions where authors have used the DM97 models to derive an

age. Using the DM97 models we derive a median age of 0.5 Myr. If we factor in errors

in the distance modulus (8.09+0.17, Anthony-Twarog, 1982), this leads to a possible age

range of 0.4-0.6 Myr. Including a 3 magnitude shift in reddening (refer to 64.4. 1) yields

a larger range of 0.2-0.9 Myr. All of these results remain consistent with the age derived

from the BCAH98 models, placing NGC 2024 at <1 Myr.

Our derived age of 0.5 Myr for NGC 2024 is in good agreement with ages found by

previous surveys of NGC 2024. Both Meyer (1996, hereafter M96) and Ali et al. (1998)

used infrared photometry and spectroscopy with the models of D'Antona & Mazzitelli

(1997) to derive mean ages of 0.3 and 0.5 Myr respectively. Further, M96 used a distance

modulus of 8.36 magnitudes. Increasing the distance to the cluster acts to increase the

derived bolometric luminosity of sources, making obj ects appear younger. Indeed, using

the larger distance modulus with the models of DM97, I derive a median age of 0.3 Myr

which is in excellent agreement with the results of M96.

A few sources in our H-R diagrams appear to have ages which deviate significantly

from the median. Using either set of PMS models there is a small, lower luminosity

population with inferred ages >3 Myr. In order to attribute these low luminosities to

general scatter caused by photometric errors and uncertainties introduced by variability

(generally no more than +0.2 mag at K), derived reddening (+0.3 mag, see above), and

distance modulus (+0.2 mag), these effects would have to combine to produce at least a

1-2 magnitude shift at K. On the other hand, it has been noted by multiple authors (e.g.

Luhman et al., 2003b; Slesnick et al., 2004; Wilking et al., 2004) that a circumstellar

disk can act to occult the central source, resulting in an underestimate of the obj ect's

luminosity and thus an overestimate of the obj ect's age. I have examined the infrared

excess properties of the subsample in question and, irrespective of the model isochrones










used, all but one of the obj ects with an inferred age >3 Myr have excess flux, indicating

the presence of circumstellar material.

Looking at Figure 4-6, it would appear that object age may be slightly mass

dependent with the less massive sources appearing younger. To quantify this trend, I have

divided our sample into two populations: objects with masses lower than the median mass

and objects with masses higher than the median mass (Mnzedian -0.15 Mo.). Using the

BCAH98 models there does not seem to be an age difference between the low and high

mass samples as both have median ages <1 Myr. However, the median ages indicated by

the DM97 models are somewhat different from one population to the other. The low mass

sample has an age of 0.3 Myr and the higher mass sample has an age of 0.9 Myr. There

are a number of possible explanations for this effect. First, the trend may be an artifact

arising from uncertainties in the evolutionary models at very young ages and low masses.

No two sets of PMS tracks look alike in the brown dwarf regime thus it is a distinct

possibility that the apparent age segregation is caused by a problem with the tracks. On

the other hand, the observed mass dependence could be a selection effect caused by the

intrinsic faintness of the older sub stellar population according to the BCAH98 models

even the highest mass brown dwarfs will be undetectable by our survey by the time they

reach ages of 2-3 Myr and this limit becomes younger for lower mass obj ects. Finally, it

is also possible that this effect is real. If so, this may be evidence for sequential formation

as a function of mass where lower mass obj ects form later in the evolutionary sequence of

a young cluster. Unfortunately, our data are not sensitive enough to distinguish between

these possibilities deeper spectroscopic observations are needed.

4.5.3 Spatial Distribution of Sources

Figure 4-7 presents the spatial distribution of all sources classified using FLAMIN-

GOS spectra. Open circles are obj ects with M > 0.08 M0,, filled triangles have masses

M < 0.08 M0,, and asterixes are the possible giants. The star at the center of the cluster

represents IRS2b, the likely ionizing source for the region (see below). It can be seen











-1.8









1.85


00O


O





OO


O


O O


o X-

O OO


Oo








o


G- 6,OO


O ~-


O


-1.95









--2




Figure 41-7


~i o M > 0.08 Me

s~ ~3M < 0.08 Me

-j B3G Giants?

g IRS2b


5.7 5 I 5.696 5. i 5.692
Right Ascension (J2000)


Spatial i: i::i::i:1: of both i Ii in and substellar objects in NGC: 20241. The
open circles i... obj ects with masses MV > 0.08 M~.,, stars are 1.1 i
with M < 0.:^ 1:. I asterixes are I 111: background giants, and the large
dot :i i.. IRS2.


from this figure that the substellar obj ects are not localized to one region but rather appear

to be distributed similarly to the stellar mass obj ects classified here. It should be noted

that the dearth of classified obj ects (either stellar or substellar) in the center of the cluster

is a selection effect caused by the high extinction in this region blocking much of the J

and H-band flux.










4.5.4 Substellar Disk Frequency

As discussed in ~4.3.2.3, the presence of an infrared excess is commonly taken

to be an indicator of thermal emission from a circumstellar disk. Disks around brown

dwarfs are of particular interest because their presence or absence has implications for

the likelihood of planet formation (planets form within circumstellar dust disks) and the

formation mechanism of brown dwarfs (accretion disks play an important role in the star

formation process). Combining our spectral classifications with H K intrinsic colors we

find 40%+9% of sources in our total sample have an H K color excess. This method

of selecting excess sources has been shown by Liu et al. (2003) to be more sensitive to

small IR excesses (as opposed to the traditional JHK color-color diagrams) and is thus

well suited for investigating the disk properties of brown dwarfs, which are expected

to have smaller excesses than their stellar counterparts. Approximately one third of the

excess sources detected using the color-spectral type analysis have masses which place

them below the hydrogen-burning limit (spectral types >M6). This yields a substellar

HK excess fraction for NGC 2024 of 9/23 or 39%+15%, where quoted errors are derived

from Poisson statistics.

Substellar excess fractions have been compiled for a number of other regions.

For example, Muench et al. (2001) used JHK color-color diagrams to examine a set of

photometrically selected brown dwarfs in the Trapezium cluster and found a substellar

excess fraction of ~65%+15%. In a follow-up L' study of the same region, Lada et al.

(2004) find a K L' excess fraction of 52+20% for their spectroscopically selected brown

dwarf sample and 67% using a JHKL color-color analysis for the larger photometric

sample. More recently, Luhman et al. (2005b) used the Spitzer Space Telescope to obtain

mid-infrared photometry for low mass members of the IC 348 and Chamaeleon I clusters,

finding that 42%+13% of brown dwarfs in IC 348 and 50%+ 17% of brown dwarfs in

Chamaeleon exhibit excess emission, consistent with our result for NGC 2024.










Based on the above results (placing more emphasis on studies with spectroscopic

information), we can conclude that 40-50% of brown dwarfs are surrounded by cir-

cum(sub)stellar disks. Note though in many cases the quoted substellar excess fractions

are deemed lower limits to the true substellar disk fraction (e.g. Lada et al., 2004; Luh-

man et al., 2005b). This is also true for NGC 2024. Disk modeling by Liu et al. (2003)

shows that the maximum expected K-band excess for a disk with no inner hole is 0.42

magnitudes for an M6 dwarf and 0.3 1 magnitudes for an obj ect classified as M9. How-

ever, the L' observations of Liu et al. (2003) are more consistent with disks having an

inner hole Riz m (2 3)R,. The K-band excess for these obj ects would be very small or

undetectable using the H K analysis I present here.

The choice of intrinsic colors may also lead to an underestimate of the substellar disk

fraction in NGC 2024. I have used an intrisic color set derived from observations of field

dwarfs. Our targets are pre-main sequence obj ects which have lower surface gravities

than field dwarfs (e.g. 93.4) and thus bluer H K intrinsic colors for a given spectral

type (refer to the low surface gravity giant sequence plotted in Figure 4-2 as compared

to the dwarf sequence plotted in the same figure). The assumption of the redder dwarf

colors will preclude obj ects with a K-band excess similar to or smaller than the difference

between PMS and dwarf colors from being counted as excess sources. Combining this

effect with the fact that H K excess is a poor indicator of disk emission for sub stellar

obj ects (see above), I conclude the true sub stellar disk fraction for NGC 2024 may be

significantly higher than 39%. This yields further weight to the idea that the maj ority of

brown dwarfs form through a disk accretion process similar to their stellar counterparts.

4.5.5 Low Mass IMF

Prior to constructing a mass function for NGC 2024, it is crucial to ensure that

the subsample under consideration is representative of the overall cluster population.

The left-hand panel of Figure 4-8 shows the uncorrected K-band luminosity function

(KLF) for the photometric sample with the KLF of the final classified spectroscopic











sample. Without placing any limits on the data, it can be seen that our spectroscopic


-- Photometric Catalog KLF - Photometric Catalog KLF, A,$15
60 Classified Sample KLF Classified Sample KIF, A,515



I III


II
20 --






10 121 6 k 1 12 14 16
K magnitude K magmtude

Figure 4-8. Left: K-band luminosity function for all classified objects in the spectro-
scopic sample (solid line) shown with the KLF for the photometric catalog
(dashed line). Right: KLFs for both samples with an imposed extinction limit
of Av < 15. It can be seen from the figure that in the magnitude range 1 1.25 <
K < 14.75 the extinction-limited spectroscopic sample is representative of the
extinction-limited main cluster population.


survey is typically only 10-20% complete in the magnitude range from K=10.5-15.0. As

discussed in ~4.5.1, correcting for background field stars will have little effect. Rather,

much of our incompleteness is caused by high reddening within the molecular cloud

itself ( 4.4. 1). Imposing an extinction limit on the data yields a higher completeness

fraction and gives a more controlled sample from which an IlVF can be constructed. The

right-hand panel of Figure 4-8 shows the K-band luminosity functions for all sources

having Avl<15 in both the photometric catalog and the final sample of classified obj ects.

Disregarding the bins on either end (as they contain only one obj ect each), it now appears

that the spectroscopic KLF is a good representation of the total photometric KLF in

the magnitude range 1 1.25
on K=12.5, the completeness fraction in the same magnitude range now extends from

~25-60% with a median value of 35%. Following the work of Slesnick et al. (2004),










I corrected for this incompleteness by adding sources to each deficient magnitude bin

according to the object mass distribution in that bin.

Figure 4-9 shows the spectroscopically derived mass function for NGC 2024. The

solid line is the mass function for all obj ects with spectral types >MO, excluding the two

possible giants (@4.5.1). Error bars are derived from Poisson statistics. The dashed line

shows the IlVF for the same sample corrected for the incomplete magnitude bins down to

K=14.75. We estimate that for our extinction-limited sample, this corresponds to a mass

completeness limit of 0.04 M~.. The mass function rises to a peak at ~-0.2 M., before

declining across the stellar/substellar boundary. There is an apparent secondary peak

around ~0.03 M., although the error bars are also consistent with a relatively flat INIF in

this regime. The implications of this mass function will be discussed in Chapter 6.

It should be noted that the exact shape of the substellar IlVF is somewhat dependent

on the choice of bin centers and sizes. For a bin width of 0.3 dex, shifting the bin

centers in increments of 0.05 dex shifts the location of both the primary and secondary

peaks through a range of masses from ~0.25-0. 1 M., and ~0.03-0.04 M., respectively.

Additionally, in some cases the secondary peak disappears and the substellar IlVF

becomes flat. Decreasing the bin width by 30% emphasizes the secondary peak, however,

the errors remain consistent with a flat INIF. Increasing the bin widths by 30% either

preserves the secondary peak, flattens the substellar mass function, or causes it to decline

throughout the brown dwarf regime depending on the choice of bin centers.

4.5.6 The Ratio of Brown Dwarfs to Stars

A more robust tool for quantifying the IlVF is the ratio of brown dwarfs to stars

as this quantity is independent of the detailed structure and exact shape of cluster mass

functions. Bricefio et al. (2002) define the ratio of the numbers of stellar and substellar

objects as
N(0.02 < M/M., < 0.08)
R = (4.1)
ssN(0.08 < M/M., < 10)










































0).5









0


0 -0.5 -1 -1.5 -2
Log (M/'

Mass f:::- 11 :: for all i.: :r-: i objects in NGC 20241 whose .i : i:.: indicate
that they are cluster members rbl: p : ?1ili '>MO. The solid line is
the uncorrected rawv mass function shownn wi~ith Pfoisson error bars and the
dashed line has been corrected for magnitude r:.. ..:i7? ... in the range
from 11.25< K <14.75.


Figure 4l-9).










In our completeness-corrected, extinction-limited mass function for NGC 2024

there are 45 objects with masses 0.02M.,
M~ > 0.08M~.. In addition, there are 36 sources in our photometric catalog with K

magnitudes brighter than the bright limit of our mass function (K=11.25, ~4.5.5). Since

the youngest (and thus brightest) object classified as sub stellar has a K magnitude of

1 1.75 with the maj ority of brown dwarfs falling below K=12.75 it is reasonable to

infer that all of these bright photometric sources have masses greater than 0.08 M~..

Finally, I include 9 sources from the 2MASS catalog with magnitudes brighter than the

FLAMINGOS saturation limit which are also expected to be far more massive than the

substellar limit. This yields a value of Rss=45/148 or 0.30+0.05 assuming Poisson errors.

A comparison of the Rss for NGC 2024 with that of other low mass star forming regions

can be found in Chapter 6.

4.6 Summary

This Chapter presents results from FLAMINGOS photometry and spectroscopy for

71 obj ects in NGC 2024. Derived spectral types yield 67 young M stars ranging from

~MI to >M8, excluding two sources with spectral types
Spectral types for these 67 obj ects were then converted to effective temperatures and

photometry was used to calculate extinctions and bolometric luminosities. Sources were

then placed on H-R diagrams and masses and ages were inferred with the assistance of

pre-main sequence evolutionary models.

The median age of M stars in NGC 2024 is 0.5 Myr using the evolutionary models

of D'Antona & Mazzitelli (1997). This value is consistent with a median age < 1 Myr as

derived from the models of Baraffe et al. (1998). Estimated masses range from 0.02 M.,'

to 0.72 M., using the Baraffe et al. (1998) models, with 23 of the 67 obj ects falling below

the stellar/substellar boundary. The spatial distribution of sources indicates that the brown

dwarfs appear to be evenly distributed relative to their stellar counterparts and thirty










nine percent of the classified brown dwarfs appear to have an infrared excess, possibly

indicative of thermal emission from a warm disk.

Using an extinction limited subsample of the spectroscopic sources (Av< 15.0),

I constructed the low mass IMF for the region. The IMF for NGC 2024 peaks at ~-0.2

Me and then declines into the brown dwarf regime. There is a possible secondary peak

around 0.035 Me0. Finally, the ratio of stellar to substellar objects in NGC 2024 is

Rss=0.30+0.05.















CHAPTER 5
M STARS AND BROWN DWARFS IN NGC 2068 AND NGC 2071

5.1 Introduction

In this Chapter I present results from FLAMINGOS photometry and spectroscopy of

the young clusters NGC 2068 and NGC 2071. Both of these clusters are associated with

prominent refl section nebulae located in the northern part of the Orion B cloud (Figure

5-1) and have long been known to be areas of active star formation. Early spectroscopic

and photometric studies detected a number of Hot emission line stars and confirmed the


Figure 5-1. Optical Image of NGC 2068 and NGC 2071 from the Digital Sky Survey.
North is up and east is left.










presence of a population of very young (z~0. 1 Myr) stars (e.g. Herbig & Kuhi, 1963;

Strom et al., 1975). More recent work in the region includes the molecular line maps of

Maddalena et al. (1986) in CO and Lada et al. (1991a) and Lada et al. (1997) in CS, the

2.2 pum survey of Lada et al. (1991Ib), and dust continuum mapping at 850 pum by Mitchell

et al. (2001) and Johnstone et al. (2001). However, to date, there have been no detailed

studies of the low mass cluster populations. This chapter presents an in-depth study of the

M star population of NGC 2068 and NGC 2071, including new H-R diagrams, ages, and

mass functions for both clusters.

5.2 Photometry of NGC 2068 and NGC 2071

J, H, and K-band images of NGC 2068 and NGC 2071 were obtained with

FLAMINGOS on the KPNO 2. 1 m telescope as part of the Orion B imaging survey

(Chapter 2, ~2.3) on the nights of 2002 January 13 and 2002 December 31i. All data were

taken using a single pass of the standard 9-point dither pattern described in ~2.3.1, with

the exception of the H-band for NGC 2068 which was observed twice to account for

bad reads. The exposure time at each point was 35 seconds, yielding a total exposure of

~8 minutes for the N2068 H-band dither set; the other image sets all had totals of ~5

minutes on source. The seeing for NGC 2068 was 1('7-11'9 FWHM and the seeing for

NGC 2071 was 1('7-1('8 FWHM.

All imaging data were reduced and photometered using the FLAMINGOS data

reduction and analysis pipelines described in Chapter 2. The resultant zero point-

corrected catalogs for the entire 2. 1 m field of view (20'x 20') yielded ~800 sources in

N2068 and ~700 sources in N2071 with PSF-fitting color errors less than 0. 1 magnitudes.

This photometry is shown in Figures 5-2 and 5-3.

Photometric quality was assessed in the standard manner for the survey ( 2.3.5). The

mean photometric scatter with respect to 2MASS in all bands is 0.06-0.07 magnitudes

(e.g. Table A-2). The 100- detection limits for the cluster data are J=19.3, H=18.8, and

K=17.8 and the photometric catalog luminosity functions for both clusters turn over at