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Location and Origin of Dust in Circumstellar Debris Disks

Permanent Link: http://ufdc.ufl.edu/UFE0022653/00001

Material Information

Title: Location and Origin of Dust in Circumstellar Debris Disks A Mid-Infrared Imaging Study
Physical Description: 1 online resource (161 p.)
Language: english
Creator: Moerchen, Margaret
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: asteroids, astronomy, circumstellar, collisions, debris, disks, dust, dusty, infrared, instrumentation, kuiper, mid, planetary, planets, system, telescopes, thermal
Astronomy -- Dissertations, Academic -- UF
Genre: Astronomy thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Approximately one third of A-type stars host dusty disks beyond the expected timescales for dissipation of the primordial disk material. The primordial dust particles may either be blown out by radiation pressure from the star or they may experience destructive collisions that generate smaller particles that are then blown out of the system. We infer from the sustained presence of the dust that it must be resupplied through collisions of already-formed planets and planetesimals or through the sublimation of cometary bodies, and systems with such dust are called debris disks. Since the 1984 discovery of the debris disk Vega, observations of circumstellar debris disks have revealed the presence planetary systems that would otherwise have remained unknown. In this work, we set out to ?nd asymmetric structures in debris disks that would indicate a physical process sculpting the disk, such as a catastrophic planetesimal collision that generates a bright region of newly-formed dust, or a clumpy pattern comprised of dust that is trapped in an orbital resonance with a giant planet. We obtained high spatial resolution ( < 0.5') images of the thermally emitting dust in 21 debris disk candidates (some of which are now known not to be debris disks), and in most cases we did not detect any brightness asymmetry nor was the source even spatially resolved. However, among the resolved disks, we have discovered several structures that may be analogous to those in our own solar system, such as a potential asteroid belts (in Zeta Lep) and a snow line (in HD 32297). One brightness asymmetry is seen, in the disk of HR 4796A, and we have determined that the bright side of the disk is also hotter than the opposite side. We review the possible origins of such a temperature asymmetry in the dust disk, such as pericenter glow and resonant trapping, and this investigation is ongoing. More generally, two disk archetypes are observed among all of the disks in this sample: Kuiper Belt analogs (four) and asteroid belt analogs (two). The asteroid belt analog is a new archetype among the overall group ( < 20) of spatially resolved debris disks, and its impact on descriptions of planetary system architecture will be better understood as the sample of resolved disks grows.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Margaret Moerchen.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Telesco, Charles M.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022653:00001

Permanent Link: http://ufdc.ufl.edu/UFE0022653/00001

Material Information

Title: Location and Origin of Dust in Circumstellar Debris Disks A Mid-Infrared Imaging Study
Physical Description: 1 online resource (161 p.)
Language: english
Creator: Moerchen, Margaret
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: asteroids, astronomy, circumstellar, collisions, debris, disks, dust, dusty, infrared, instrumentation, kuiper, mid, planetary, planets, system, telescopes, thermal
Astronomy -- Dissertations, Academic -- UF
Genre: Astronomy thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Approximately one third of A-type stars host dusty disks beyond the expected timescales for dissipation of the primordial disk material. The primordial dust particles may either be blown out by radiation pressure from the star or they may experience destructive collisions that generate smaller particles that are then blown out of the system. We infer from the sustained presence of the dust that it must be resupplied through collisions of already-formed planets and planetesimals or through the sublimation of cometary bodies, and systems with such dust are called debris disks. Since the 1984 discovery of the debris disk Vega, observations of circumstellar debris disks have revealed the presence planetary systems that would otherwise have remained unknown. In this work, we set out to ?nd asymmetric structures in debris disks that would indicate a physical process sculpting the disk, such as a catastrophic planetesimal collision that generates a bright region of newly-formed dust, or a clumpy pattern comprised of dust that is trapped in an orbital resonance with a giant planet. We obtained high spatial resolution ( < 0.5') images of the thermally emitting dust in 21 debris disk candidates (some of which are now known not to be debris disks), and in most cases we did not detect any brightness asymmetry nor was the source even spatially resolved. However, among the resolved disks, we have discovered several structures that may be analogous to those in our own solar system, such as a potential asteroid belts (in Zeta Lep) and a snow line (in HD 32297). One brightness asymmetry is seen, in the disk of HR 4796A, and we have determined that the bright side of the disk is also hotter than the opposite side. We review the possible origins of such a temperature asymmetry in the dust disk, such as pericenter glow and resonant trapping, and this investigation is ongoing. More generally, two disk archetypes are observed among all of the disks in this sample: Kuiper Belt analogs (four) and asteroid belt analogs (two). The asteroid belt analog is a new archetype among the overall group ( < 20) of spatially resolved debris disks, and its impact on descriptions of planetary system architecture will be better understood as the sample of resolved disks grows.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Margaret Moerchen.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Telesco, Charles M.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022653:00001


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andtoJohn. 3

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...wearelikedwarfsontheshouldersofgiants,sothatwecanseemorethanthey,andthingsatagreaterdistance,notbyvirtueofanysharpnessofsightonourpart,oranyphysicaldistinction,butbecausewearecarriedhighandraisedupbytheirgiantsize.{JohnofSalisbury,attributingthequotetoBernardofChartres,1159Indeed,thisworkwouldnothavebeenpossiblewithoutthesupportandknowledgebaseofadvisors,colleagues,friends,andfamily.IoerspecialthankstotheMichelsonScienceCenteratCaltechforagraduatefellowshipthatfundedmyresearchforthefullthreeyearsofmydissertationwork.Iamindebtedtomyadvisor,CharlieTelesco,foremostformanyengagingandproductivescienticdiscussions.ThesebeganinearnestinJanuaryof2004,whenIwasthoroughlyspoiledbyspendingmyrstgraduateobservingrunatan8-metertelescope(likewise,forseveralrunssince).Further,withouttheguaranteedtelescopetimeassociatedwiththeinstrumentsOSCIRandT-ReCSattheGeminiObservatory,thedataforthisworkwouldnothavebeenobtainedinsuchanexpedientmanner.Charlie'sadvicewasespeciallynotableinmattersofpushingthelimitsofwhatwassupposedlypossible{thereismorethanonecasewhenIhaddismissedourdataasnear-worthlessbeforediggingfurtherandemergingwithspectacularresults.IwouldalsoberemissnottoacknowledgeCharlie'suniquesenseofhumorandoutgoingpersonality.Hehasnotonlybeenanentertainingpresenceonourvarioustripsaroundtheworld,buthissocialnaturehasledmetonewscienticcollaborationsviaintroductionshehasmadebetweenmeandhiscolleagues.IamalsogratefultoChrisPackhamforhiseternallyopenocedoor.Onsubjectsfrommid-infraredtechniquestoprofessionaletiquette,Chrishasadvisedcopiouslyforthepastveyears.IamfortunatetohavecontributedtoCanariCamwithChrisformanyhoursinthelabwithwhatevertaskswereathand,andIlookforwardtoobservingwithCanariCaminthenearfuturewithhimandtherestoftheCanariCamteam.Most 4

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page ACKNOWLEDGMENTS ................................. 4 LISTOFTABLES ..................................... 10 LISTOFFIGURES .................................... 11 LISTOFSYMBOLS .................................... 15 ABSTRACT ........................................ 16 CHAPTER 1INTRODUCTION .................................. 18 1.1Motivation .................................... 18 1.2SearchingforOtherPlanetarySystems .................... 18 1.3DebrisDisks:IndirectEvidenceofPlanets .................. 21 1.4FormationandEvolutionofDebrisDisks ................... 23 1.5PlanetarySignaturesintheDiskStructure .................. 31 1.5.1Examples ................................. 31 1.5.2ObservingMethods ........................... 35 1.6OpenQuestionsinExoplanetarySystemResearch .............. 36 2SOURCESELECTION ............................... 38 2.1SampleDenition ................................ 38 2.2RecategorizedSources ............................. 40 2.2.1HD21362 ................................ 41 2.2.2HD74956 ................................ 42 3OBSERVATIONS ................................... 43 3.1Mid-infraredObservingTechniques ...................... 43 3.1.1ConstraintsoftheEarth'sAtmosphere ................ 43 3.1.2TheNeedforChoppingandNodding ................. 45 3.1.3Sensitivity&Eciency ......................... 47 3.2Data ....................................... 49 3.2.1FacilitiesEmployed ........................... 49 3.2.2SummaryofImagingObservations ................... 51 3.2.3DataReduction ............................. 53 3.3BasicImageAnalysis .............................. 54 3.3.1Photometry ............................... 54 3.3.2TestingforMarginalSpatialResolution ................ 55 7

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.................. 60 4.1PriorDetectionsofWarmDustinLeporis ................. 60 4.2ObservationsatGemini ............................. 61 4.3ResolvedDiskResults ............................. 61 4.3.1SourceSize&Model .......................... 61 4.3.2ParticleSize,MassandLifetimes ................... 66 4.4DustProductionScenarios ........................... 68 4.5Discussion .................................... 71 5HD32297:SPATIALLYRESOLVEDCENTRALCLEARINGANDSNOWLINE .................................. 73 5.1Mid-IRImagingofDebrisDisks ........................ 73 5.2ObservationsofHD32297 ........................... 74 5.2.1Data ................................... 74 5.2.2DiskGeometryandMorphology .................... 75 5.3Discussion .................................... 77 5.3.1ComparisonwithKeyDebrisDiskArchetypes ............ 77 5.3.2CharacteristicParticleTemperatures ................. 79 6HR4796A:ONTHENATUREOFTHEASYMMETRYINTHEDEBRISRING ............................... 84 6.1LiteratureReview ................................ 84 6.1.1Discovery&Morphology ........................ 84 6.1.2CharacteristicsoftheDustPopulation ................ 87 6.2OurRecentMid-infraredImagesfromGemini ................ 89 6.2.1Data ................................... 89 6.2.2GeneralDataAnalysis ......................... 89 6.2.3CharacteristicDiskTemperatures ................... 90 6.3PotentialOriginsoftheBrightnessAsymmetry ............... 95 7FURTHERCONSIDERATIONOFSOURCESTHATARENOTCLEARLYEXTENDED ..................................... 103 7.1CharacterizingMarginallyandCompletelyUnresolvedSources ....... 103 7.2SourceMeasurements .............................. 104 7.2.1DustColorTemperaturesDerivedfromIRExcessLevels ...... 104 7.2.1.1Sourceswithmarginalornoexcessemissionat12and18microns ............................ 106 7.2.2StatisticalSignicanceofResolutionResults ............. 107 7.3AretheSpatialandPhotometricMeasurementsConsistent? ........ 109 7.3.1UnresolvedSources ........................... 111 7.3.2MarginallyResolvedSources ...................... 114 7.3.3ResolvedSources ............................ 115 7.4OverviewofSampleResults .......................... 118 8

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...................................... 123 8.1Chapter1:Introduction ............................ 123 8.2Chapter2:SourceSelection .......................... 123 8.3Chapter3:Observations ............................ 123 8.4Chapter4:Leporis:AnAsteroidBeltAnalog? ............... 123 8.5Chapter5:HD32297:SpatiallyResolvedCentralClearingandSnowLine ............................... 124 8.6Chapter6:HR4796A:OntheNatureoftheAsymmetryintheDebrisRing ............................ 124 8.7Chapter7:FurtherConsiderationofSourcesThatAreNotClearlyExtended 125 APPENDIX:DETAILEDPROFILEWIDTHMEASUREMENTS ........... 128 REFERENCES ....................................... 152 BIOGRAPHICALSKETCH ................................ 161 9

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Table page 1-1Timescalesrelevanttodustremovalincircumstellardisks ............. 25 1-2Doesthisworkanswerthefollowingopenquestionsintheeld? ......... 37 2-1DebrisDiskCandidateTargetList ......................... 41 3-1ContributionstoUnwantedBackgroundEmission ................. 49 3-2SummaryofImagingObservations ......................... 52 3-3FiltersUsed ...................................... 53 3-4FluxDensities&BackgroundofDebrisDiskCandidates,inmJy ....... 55 3-5FWHMofDebrisDiskCandidates&PSFReferenceStars(Michelle) ...... 58 3-6FWHMofDebrisDiskCandidates&PSFReferenceStars(T-ReCS) ...... 59 5-1FluxDensitiesofHD32297fromGround-BasedObservations .......... 75 6-1PhotometryoftheHR4796ADisk ......................... 89 7-1ExcessEmissionofUnresolvedDebrisDiskCandidates(Michelle) ........ 104 7-2ExcessEmissionofUnresolvedDebrisDiskCandidates(T-ReCS) ........ 105 7-3SummaryofDebrisDiskCandidateIRExcessDetections ............. 107 7-4StatisticalSignicanceofExtendedSourceDetectionsat10{12Microns ..... 109 7-5StatisticalSignicanceofExtendedSourceDetectionsat18Microns ....... 109 7-6DustRadiusLimitEstimates(inAU)forUnresolvedSourceswithIRExcess .. 111 7-7DustRadiusEstimates(inAU)forMarginallyResolvedandResolvedSources 116 10

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Figure page 1-1MassesoftheknownextrasolarplanetsinEarthmassesasafunctionoftheyearofthediscovery. ................................. 20 1-2Schematicdrawingandcorrespondingspectralenergydistributionforasinglestar,astarwithacontinuousdisk,andastar-disksystemwithaninnerclearing. ............................................. 22 1-3Thefourstagesofstarformation. .......................... 24 1-4Dierentdustdestructionand/orremovaltimescalesforatypicaldustgraininthePicdisk,calculatedasafunctionoforbitaldistance. ............ 26 1-5Evolutionofdiskluminositywithtime. ....................... 27 1-6Evolutionofthe24-m(top)and70-m(bottom)fractionalexcessesofAstars. 29 1-7FractionalexcessofAstarsat24masafunctionofage. ............ 32 1-8ResidualopticalimageofFomalhautaftersubtractingamodelofadustbelt. 33 1-9SmoothedimagesofPicrotated58counter-clockwise. ............. 34 2-1Excessratiovs.agefor266A-typemain-sequencestars. ............. 40 3-1ATRANmodelatmospherictransmissionatMaunaKea. ............ 44 3-2Diagramoftypicalchopandnodpositions,regardingwhatportionoftheskyisimagedonthedetector. ............................... 47 3-3Exampleimagesofwhatisseenateachchopposition,thedierencebetweentheseimagesineachnodposition,andthecombinednalimageresultingfromthetwonodpositions. ................................ 48 3-4Schematicrepresentationofareectingtelescopeshowingtherelativelocationoftheprimary,secondary,andtertiarymirror. ................... 50 4-1ProlesofazimuthallyaveragednormalizedintensityforLep(diamonds)andreferencePSFstar(dots). .............................. 62 4-2FWHMvaluesofthesubdividedintegrationsequenceforLep(center,diamonds)andreferencePSFstar(leftandright,circles). .................. 63 4-3Anillustrationoftheradiallysymmetricdouble-annulusmodelusedtoapproximatetheresolved18-mdustemission. ......................... 65 4-4Prolesof3ofthediskmodelsthatweregeneratedtoapproximatethe18.3mimagingobservations. ................................ 66 11

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............. 76 5-2Color-colorplotofmid-IRuxesofHD32297(starsymbol)andthetwoarchetypedisks,Pic(diamond)andHR4796A(triangle). ................. 78 5-3RadialcolortemperatureproleofHD32297,asderivedfrom11.7and18.3muxdensitymeasurementsalongthedisk. ..................... 81 6-118.1-mimage(Michelle)ofHR4796Aand24.5-mimage(T-ReCS)ofHR4796A. 85 6-218.1-mphotosphere-subtractedimage(Michelle)ofHR4796Aand24.5-mphotosphere-subtractedimage(T-ReCS)ofHR4796A. .............. 86 6-318.1-mimageconvolvedwitha2-Dgaussianprolewithawidthcorrespondingtothe24.5-mPSFFWHMand24.5-mimageconvolvedwitha2-Dgaussianfunctionwithawidthcorrespondingtothe18.1-mPSFFWHM. ....... 91 6-4BrightnessproleoftheHR4796Adiskat18.1m(squares)and24.5m(circles). 92 6-5ColorproleoftheHR4796Adisk,basedonthe18.1mand24.5mbrightnessproles. ........................................ 93 6-6ColortemperatureproleoftheHR4796Adiskbasedonphotometricmeasurementsat18.1mand24.5m. .............................. 94 6-7Neworbitsofthefragmentsofacollisioninwhichalargeparentparticle\P"thatwasonacircularorbitaroundastar\S"wasbrokenup. .......... 96 6-8Face-onviewofthesurfacedensitydistributionofdierentsizes(characterizedbytheparameter)createdinthecollisionaldestructionofplanetesimals. ... 98 7-1MIRcolortemperatureofdustagainstsystemage. ................ 120 A-1HD21362at11.2m(night1). ........................... 129 A-2HD21362at10.4m(night2). ........................... 129 A-3HD21362at18.3m. ................................ 130 A-4HD38206at10.4m. ................................ 130 A-5HD38206at18.3m. ................................ 131 A-6HD56537at11.2m. ................................ 131 A-7HD56537at18.1m. ................................ 132 A-8HD71155at10.4m. ................................ 132 A-9HD71155at18.3m. ................................ 133 A-10HD74956at10.4m. ................................ 133 12

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................................ 134 A-12HD75416at11.7m. ................................ 135 A-13HD75416at18.3m. ................................ 135 A-14HD80950at10.4m. ................................ 136 A-15HD80950at18.3m. ................................ 136 A-16HD83808at11.2m. ................................ 137 A-17HD83808at18.1m. ................................ 137 A-18HD95418at11.2m. ................................ 138 A-19HD95418at18.1m. ................................ 138 A-20HD102647at11.2m. ................................ 139 A-21HD102647at18.1m(night1). .......................... 139 A-22HD102647at18.1m(night1). .......................... 140 A-23HD115892at10.4m. ................................ 140 A-24HD115892at11.7m. ................................ 141 A-25HD115892at18.3m(night1). .......................... 141 A-26HD115892at18.3m(night2). .......................... 142 A-27HD139006at11.2m. ................................ 143 A-28HD139006at18.1m(night1). .......................... 143 A-29HD139006at18.1m(night2). .......................... 144 A-30HD141569at11.2m. ................................ 144 A-31HD141569at18.1m(night1). .......................... 145 A-32HD141569at18.1m(night2). .......................... 145 A-33HD161868at11.2m. ................................ 146 A-34HD161868at18.1m(night1). .......................... 147 A-35HD161868at18.1m(night2). .......................... 147 A-36HD172555at11.7and18.3m. .......................... 148 A-37HD178253at11.7m. ................................ 149 13

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................................ 149 A-39HD181296at11.7and18.3m. .......................... 150 A-40HD181869at10.4m. ................................ 151 A-41HD181869at18.3m. ................................ 151 14

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ApproximatelyonethirdofA-typestarshostdustydisksbeyondtheexpectedtimescalesfordissipationoftheprimordialdiskmaterial.Theprimordialdustparticlesmayeitherbeblownoutbyradiationpressurefromthestarortheymayexperiencedestructivecollisionsthatgeneratesmallerparticlesthatarethenblownoutofthesystem.Weinferfromthesustainedpresenceofthedustthatitmustberesuppliedthroughcollisionsofalready-formedplanetsandplanetesimalsorthroughthesublimationofcometarybodies,andsystemswithsuchdustarecalleddebrisdisks.Sincethe1984discoveryofthedebrisdiskVega,observationsofcircumstellardebrisdiskshaverevealedthepresenceplanetarysystemsthatwouldotherwisehaveremainedunknown. Inthiswork,wesetouttondasymmetricstructuresindebrisdisksthatwouldindicateaphysicalprocesssculptingthedisk,suchasacatastrophicplanetesimalcollisionthatgeneratesabrightregionofnewly-formeddust,oraclumpypatterncomprisedofdustthatistrappedinanorbitalresonancewithagiantplanet.Weobtainedhighspatialresolution(.0.5")imagesofthethermallyemittingdustin21debrisdiskcandidates(someofwhicharenowknownnottobedebrisdisks),andinmostcaseswedidnotdetectanybrightnessasymmetrynorwasthesourceevenspatiallyresolved. However,amongtheresolveddisks,wehavediscoveredseveralstructuresthatmaybeanalogoustothoseinourownsolarsystem,suchasapotentialasteroidbelts(inLep)andasnowline(inHD32297).Onebrightnessasymmetryisseen,inthediskof 16

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Suchknowledgefurtherdrivesustoseekanswerstofundamentalquestions.Ifthesunisnottheonlystartohostplanets,howmanystarsdohavetheirownplanetarysystems?Ifourearthisnottheonlyplanet,arethereotherslikeit?IfthereareotherEarth-likeplanets,cantheysustainlifeaswepresentlyunderstandit?Whilenosinglescienticworkislikelytolayallofthesequestionstorest,ourcollectivestudiesofextrasolarsystemscontinuetoprovidenewinformationabouttheformationandevolutionofthearchitectureofplanetarysystems.Inturn,wemayattempttoplaceourownsolarsystemintocontextandbetterunderstanditsorigins. Radial-velocitymeasurementscontinuetobetheworkhorseapproachforplanetdiscovery(e.g.,Mayor&Queloz1994;Butler&Marcy1996;Marcy&Butler1996),inwhichoneseeksDopplershiftsinphotosphericspectrallinesthatindicatethepresenceof 18

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Photometrictransitobservations(bothprimaryandsecondary)havealsoprovensuccessfulindetectingplanets,with50candidatesatpresent.Inadditiontothedetectionoftheplanetitself,transitlightcurvesallowforinvestigationoftherelationshipoftheplanetaryradiustoitsmass(e.g.,Charbonneauetal.2007).Thenumberofplanetsdetectedbytransitislikelytoexpanddrasticallywiththeadventofnewdedicatedground-andspace-basedfacilities(e.g.,SuperWASP[Streetetal.2003]andCoRoT[Baglinetal.2003]),manyofwhichhavealreadyresultedinseveraldetectionsthathavebeenconrmedbyothermethods.Microlensingevents,whilemorerare,haveyieldedatleastsevenplanetdetectionssofar.Here,astar-planetsystemactsasalensofamoredistantsource,whichcausesadistortedlightcurvethatisdiscrepantfromthelightcurvethatwouldbeobservedifthestaralonehadactedasthelens. Astrometricobservationsmayalsoleadtoplanetdiscoveries,andthisapproachwillseeimprovementinthenearfuturewithhigh-angular-resolutioneortsbothontheground(e.g.PRIMAattheVLTI[Quirrenbachetal.1998,2004])andinspace(e.g.,Gaia[Lindegrenetal.2008],Kepler[Boruckietal.2003],andSpaceInterferometryMission[SIM]). Onlyahandfulofplanetshavebeendetectedthroughdirectimaging,thusfaronlyaroundsubstellarobjects(e.g.,Chauvinetal.2005),butlarge-scaleprojectsutilizingadaptive-opticscorrectionsarecurrentlyunderway(e.g.SPHEREattheVLT[Beuzitetal.2006],GeminiPlanetImager[GPI;Macintoshetal.2006])that,amongtheirgoals,aimtoincreasethenumberofdirectlyimagedplanetarysystems. Pulsartimingisanotherproventechnique,havingenabledtherstdetectionofplanetsoutsideoursolarsystem( Wolszczan&Frail 1992 ).Despitetheinitialsuccessof 19

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MassesoftheknownextrasolarplanetsinEarthmassesasafunctionoftheyearofthediscovery.Yellowandredcirclesdenoteplanetsdiscoveredusingtheradial-velocityandtransittechniques,respectively.Themassesoftheplanetsdiscoveredusingtheradial-velocitytechniqueareminimummasses.Thesizeofthecirclesisproportionaltotheorbitalperiodoftheplanet.Blueandpurplesquaresrepresentthepositionoftheplanetsdiscoveredbymicrolensingandpulsar-timingsurveys.Thepurplearrowindicatesthetimepositionofaverylow-massplanetdiscoveredbythepulsar-timingtechnique.ThemassesofJupiter,Saturn,Neptune,andtheEartharemarkedforcomparison.ThetemporalevolutionofthelightestplanetdetectionpointstowardafuturedetectionofanEarth-massplanet.Credit:Udry&Santos2007. suchobservations,therearealimitednumberofmillisecondpulsarcandidates(130),andthemethodhasonlyyieldedoneadditionalsystemwithaplanet( Thorsettetal. 1993 ).Itmustalsobenotedthattheseplanetsmaybefundamentallydierent(e.g.,notlife-sustaining)thanthoseorbitingamain-sequencestar,duetotheadvanced 20

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Figure 1-1 showsthebiasindetectingrelativelymassiveplanets,butitalsoshowstheprogressofthelowerlimitfordetectablemassestowardthatofanEarth-likeplanet.Somemethods,suchasmicrolensingandastrometry,cansampleabroaderparameterspaceofplanetmassandorbitalradius,andmaybemorelikelytorevealanEarth-massplanetsooner( Udry&Santos 2007 ). Thissecond-generationdustreprocessesstarlightbyeitherscatteringorabsorbingitandre-emittingtheradiationthermally.Circumstellardiskscomprisedofthisdustaretypicallydetectedbyphotometricmeasurementsatmultiplewavelengths.Suchdiskshavetwothermalemissionsources:thestarandthedustydisk.Thespectralenergydistributionforthediskisacompositeofthesetwoemissioncurves(Figure 1-2 ),andthedustemissioncurveisusuallydistinctfromthestellaremissioncurveduetoitsmarkedlycoolertemperature.Thestellarphotosphericuxcanbepredictedbypairingobserveduxesatmultiplewavelengthswithstellaratmosphericmodels,andonecanthencomparetheexpectedphotosphericux(fromthestaralone)withtheactualuxobservedatnear-infraredtomillimeterwavelengths,whereblackbodydustattemperatures10{300Kisexpectedtoemit.Iftheobserveduxisstatisticallyhigher 21

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Schematicdrawingandcorrespondingspectralenergydistributionforasinglestar,astarwithacontinuousdisk,andastar-disksystemwithaninnerclearing.Credit:NASA/JPL-Caltech/T.Pyle(SSC). thantheexpectedphotosphericux,thentheexcessuxislikelyattributabletothepresenceofthermallyemittingdustorbitingthestar.(Backgroundsources,extendednebulosity,andbinarycompanionsmustberuledout.) Ifthestellarageisoldenoughthattheprimordialdiskofdustandgasshouldhavedispersed(duetoprocessesdiscussedinx ),thenthedustsupplymustbecontinuallyresupplied,andthesourceisconsideredtobeadebrisdisk.Wenotethattheboundarybetweenprimordialcircumstellardisksandcompletelyreprocessedlater-generationdebrisdisksisnotwellunderstood,andforyoungstellarages(fewMyr)wemustconsiderthat 22

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Manysourcesappeartohaveevolvedfromtheinsideoutwards(e.g.,Calvetetal.2005),resultingindisksthatmaybecomprisedofreprocessedmaterialintheirinnerregionsandprimordialmaterialintheouterregions.Multipleexplanationshavebeenoeredforthisphenomenon.Intheso-calledUVswitchmodel,theinnerdiskisclearedviaacombinationofphotoevaporationoccurringintheouterdiskandviscousaccretionintheinnerdisk;initiallybothoftheseprocessesarebalanced,butinlaterstagesphotoevaporationhasclearedtheouterdisksuchthatitcannolongerserveasareservoirofmaterialfortheinnerdisk( Alexanderetal. 2006 ; Clarkeetal. 2001 ).Alternatively,theinnerdiskmaybeclearedbyplanet-formingactivity,eitherbycoreaccretionorgravitationalinstability( Quillenetal. 2004 ). 1-3 ).Asthediskcools,particlescondenseoutofthegas,withheavierrockyelementscondensingrstintheinnerhotterregionofthenebulaandmorevolatileicycompoundscondensingfartheroutbeyondtheso-calledsnowlineoriceline.Thesesubmicronparticlesthenbegintoexperiencebenigncollisionswitheachotherandaccretebyelectrostaticforces.Theparticlescontinuetoaccrete,aidedastheygrowbytheirincreasinggravitationalforces,andmaygrowaslargeasakilometerinradiuswithin104years.Theseparticlesthathavegrownbeyondtheiroriginal\primordial"sizesarewhatremainafterthestellaroutowclearsawaythebulkofthecircumstellarmaterial,andtheycomprisetheprotoplanetaryremnantdiskthatmoststarsarethoughttobebornwith( Haischetal. 2001 ). 23

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Thefourstagesofstarformation.(a)Coresformwithinmolecularcloudsasmagneticandturbulentsupportislostthroughambipolardiusion.(b)Aprotostarwithasurroundingnebulardiskformsatthecenterofacouldcorecollapsingfrominside-out.(c)Astellarwindbreaksoutalongtherotationalaxisofthesystem,creatingabipolarow.(d)Theinfallterminates,revealinganewlyformedstarwithacircumstellardisk.Credit: Shuetal. ( 1987 ). Theparticlescontinuetoundergocollisionswitheachotheratlowvelocities,whichcondensethetwoimpactorsintoonelargerbodyuntilradiiofhundredsorthousandsofkilometersarereached.Bodiesofthissizearereferredtoasplanetesimals,andtheycontinuetoincreaseinsizeatthisstagethrougholigarchicgrowthduetogravitationalforces.PlanetesimalsmayfurtheraccretegasesontotherockyoricycorestoformplanetslikeJupiter;thisprocessisthoughttooccurover5x105106years(e.g.,Lissauer1987).Oncethecoresreachsizesof2000{3000kminradius,orbitsofplanetesimals 24

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Timescalesrelevanttodustremovalincircumstellardisks ProcessTimescaleFactors Radiationpressuretper(r)[yr]orbitalradiusr[AU]orbitalperiodtper(r)=p Collisiontcoll=tper(r) 4eff(r)orbitalradiusr[AU](andpotentialbreakup)orbitalperiodtper(r)=p Poynting-RobertsondragtPR(r)[yr]=7x106ar2 resultingfromcollisionalaccretioncrosseachother,andgravitationalinteractionbetweenplanetesimalsisstrong( Chambers&Wetherill 1998 ; Kenyon&Bromley 2006 ).Collisionsnowoccuratmuchhighervelocitiesandeventuallycausefragmentationoftheparentimpactors(e.g.,Davisetal.1985). Thefragmentsresultingfromtheseeventscontinuetocollidewitheachotherandproducesmallerparticlesinacollisionalcascade,afterwhichthesmallestparticlesareremovedfromthesystembyradiationpressurefromthestar,orbyPoynting-Robertson(P-R)dragthatcausesthemtospiralintothestar.Thefrequencyofthesecollisionsdecreasesasplanetesimalsarefragmentedinacollisionalcascadebycollisionsintosmallerbodies,smalldustparticlesareremovedfromthesystem,andthelargestplanetarybodiesachievestableorbits.ThedustremovalprocessesandtheirtimescalesaresummarizedinTable 1-1 .Thedominantprocessdependsonseveralfactors,includingthedensityofdustinthesystemandstellarpropertiessuchasmassandluminosity.Forexample,P-RdragisirrelevantforadiskwhosedustpopulationisdenseenoughthattheexpectedtimescalebetweencollisionsismuchshorterthantheexpectedtimescaleforaparticletomakesignicantprogresstowardthestarunderP-Rdrag.Itisalsoimportanttonotethatthedominantdustremovalprocessmayvaryaccordingtothediskradius,onwhichtheorbital 25

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Dierentdustdestructionand/orremovaltimescalesforatypicaldustgraininthePicdisk,calculatedasafunctionoforbitaldistance. Artymowicz ( 1997 ) periodanddustdensitymaybedependent.ThiscorrelationisillustratedinFigure 1-4 ,whichplotsthetimescalesforatypicaldustgraininPicasafunctionofitsdistancefromthestar( Artymowicz 1997 ).Thetimescalesaccordingtodistancewillvaryamongdisks,accordingtotheirstellarparameters,disksurfacedensity,andparticlepopulationcharacteristics. Thedescriptionofcollisionalevolutionisovergeneralized,however,anditisnotwellunderstoodwhattriggerstheinitialspikeofcollisionalactivity.ThemodelsofKenyon&Bromleysuggestthatthereexistsacriticalminimumsizeofplanetesimals(2000km)thattriggerschaoticinteractionamongthesmallerplanetesimals(<100km),resultingincollisionalcascadesamongthissmaller-sizepopulation.Therefore,the 26

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Evolutionofdiskluminositywithtime,resultingfromcollisionalcascademodelsin Kenyon&Bromley ( 2004a ).Thecurvescorrespondtodierentplanetesimaltensilestrengths(increasingtensilestrengthwithincreasingnumber),andthe\D"indicatesaparameterfromthe Davisetal. ( 1985 )fragmentationalgorithm. 27

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1-5 )predictthatthereshouldbeaninitialplateauorshallowdropinobserveddiskluminosityastheplanetesimalsexperienceoligarchicgrowthbyaccretion,sweepingupwould-beluminousmaterialintheso-calledrunawaygrowthphase.Then,oncethecriticalstirringsizehasbeenreached,thediskluminosityrisessharplyasthecollisionalcascadesbegin.Thisprocessisreferredtoby Wyatt ( 2008 )asself-stirring.Finally,theluminosityexperiencesamoreorlesssteadydecay,possiblypunctuatedbystochasticcollisionalevents(furtherdiscussionbelow),astheparentbodysupplyisdepleted( Kenyon&Bromley 2004a b 2006 ).Inspiteoftherisingnumberofdestructivecollisions,planetformationislikelytobeongoingbeyondthetimewhenthecriticalstirringsizeofplanetesimalisachieved.Forthesolarsystem, Pollacketal. ( 1996 )and Alibertetal. ( 2005 )estimatethatthegasgiantplanetswereformedwithin1{16Myr,dependingontheirlocation,butterrestrialplanetformationmaybesustainedforalongerperiod,upto100Myr( Kenyon&Bromley 2006 ). Thedecayofdiskluminosityhasbeenwelldocumented(e.g.,Hollandetal.1998;Spangleretal.2001;Decinetal.2003),especiallyforAstars( Riekeetal. 2005 ; Suetal. 2006 )thatarecapableofheatingdusttohighertemperaturesandatgreaterdistancesthanforFGKstarsandthereforegenerateIRexcesssignaturesthatareeasiertodetect.Figure 1-6 showsthedecayofIRexcess,indicativeofthermallyemittingdust,associatedwithseveralhundredAstars(blacklledcircles)spanning800Myrinage( Riekeetal. 2005 ; Suetal. 2006 ; Wyatt 2008 ; Wyattetal. 2007b ).ThedecayinIRexcessitselfisnoted,butthedataalsoshowawiderangeofIRexcessesforanygivenage,implyingthatthesystemageisnottheonlyfactordeterminingthediskbrightness. Wyattetal. ( 2007b )demonstratethat,withafewexceptions,thedistributioncanbereplicatedbyasimulatedsampleofdiskswitharangeofinitialmassesthatsubsequentlyundergoasteady-statecollisionalevolution(smallyellowcircles,inFigure 1-6 ).Amoremassivediskwillprocessitsavailableplanetesimalmaterialmorequickly,yieldingthe 28

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Evolutionofthe24-m(top)and70-m(bottom)fractionalexcessesofAstars( Riekeetal. 2005 ; Suetal. 2006 ; Wyattetal. 2007b ).(a)Fractionalexcessofobserveddisks(blackclosedcirclesandredopencircles)andamodelpopulation(yellowdots),inwhichallAstarspossessaprestirredplanetesimalbeltwitharangeofinitialmasses(alog-normaldistributioncenteredon10Mofwidth1.14dex)andradii(3{120AU),andwhichallundergosteady-stateevolution.Credit: Wyatt ( 2008 ). 29

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Suetal. 2005 )andLep( Chen&Jura 2001 ; Moerchenetal. 2007b ; Riekeetal. 2005 )thathaveagesof&200{300Myr,theIRexcessishighenoughtosuggesttransientdust-producingevents,suchasthecataclysmiceventassociatedwiththeformationoftheMooninoursolarsystem(e.g.,Canup&Asphaug2001).Craterrecordsfromourownsolarsystemalsosuggestasharpincreaseincollisionalactivity700Myraftertheplanetswereformed( Hartmannetal. 2000 ),knownastheLateHeavyBombardment.Therefore,thebasictheorydescribedabovecannotfullyaccountfortheformationofthesolarsystem(e.g.,Tsiganisetal.2005).Oneoftheaimsofthisworkistoinvestigatewhethersuchspikesofactivityareananomalyuniquetoourownplanetarysystemor,morelikely,whethersuchtransientandstochasticeventsarecommontotheplanetaryformationprocess.Spatiallyresolvedimagesofdisksmayrevealthesignaturesofcatastrophiccollisions,andarediscussedfurtherinx Recentobservationsby Currieetal. ( 2008 )(centerplotofFigure 1-7 )alsoshowtheexpecteddecayindiskbrightnesswithage.Further,thisworkcorroboratesthepredictedscenarioof Kenyon&Bromley ( 2004b ),inwhichthediskluminosityrisessharplybeforedeclining;inthiscase,thepeakluminosityoccursat10{15Myr.Althoughsuchbroadtrendsindiskbrightnesswithagecanbeinferredthroughsurvey-typephotometricobservations,thelocationofthedustandthephysicalprocessesthatsculptthedisksarestilllargelyunknown.Forexample, Wyatt ( 2008 )notesthatHR4796AandPic,bothAstars,occupysimilarpositionsintheage-vs.-24-mexcessplotfrom Currieetal. ( 2008 ).However,resolvedNIRandMIRimagesofbothsourcesrevealsignicantlydierentdustdistributions:HR4796Ahasawell-deneddustannulusat70AUthatis17AUwide( Schneideretal. 1999 ; Telescoetal. 2000 ; Wyattetal. 1999 )andthedustdiskofPic 30

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Telescoetal. 2005 ).Thus,thefractionalIRluminosityalonecannottellthewholestory. Resolvedimagesofdiskscanprovidecrucialadditionalinformationthatnarrowstherangeofscenariosthatcouldgiverisetoadiskofaparticularbrightness.Likewise,suchinformationaboutthephysicalprocessesatworkinadiskmayservetosupportonediskevolutiontheoryoveranother.Forexample,ifoneassumesasteady-stateevolutioninwhichacertainsizeofplanetesimalmustbegrowninordertostirthediskcollisionally,adiskofagivenbrightnessandagewould,inthemostgeneralcase,implytheinitialmassthatthepresentdiskhadevolvedfrom.Incontrast,onemightthenobtainresolvedimagesofthesamediskthatshowedanobviouscentralclearing.Thismorphologicalevidencecouldimplythepresenceofaplanetthat,throughresonantperturbationsormigrationandsecularperturbations,hadinducedaspikeofcollisionalactivitysubstantiallylater(duetoplanetstirring)thanattheexpectedpeakofcollisionalactivityintheself-stirringmodel.Theeectsoftheunseenplanetsandplanetesimalsaremorepreciselyprobedwithhigh-spatial-resolutionimagingobservations(whenpossible)thanwithunresolvedphotometricmeasurements.Somecaseswhereresolvedimagesofthediskshaverevealedthephysicalprocessesthereinarereviewedinthefollowingsection,x 1.5.1Examples Forexample,diskdistortionortruncationbyastellarorplanetarycompanionshouldaectallsizesofdustparticlesequally,withlittleifanyobservablewavelengthdependence(e.g.,Artymowicz&Lubow1994).ThisscenariohasbeensuggestedfortheFomalhaut 31

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FractionalexcessofAstarsat24masafunctionofage( Currieetal. 2008 ).Thefractionalexcessisshownasthedierence,in24mmagnitude,betweentheobservedbrightnessofthesystemandthebrightnessexpectedfromthephotospherealone,wheretheexcessisattributabletodust.Thelinesshowtheself-stirredmodelsof( Kenyon&Bromley 2005 )foraplanetesimalbeltextending30{150AUthathasamassdistribution3xand1/3xMMSN(minimummasssolarnebula).MIRimagesofspecicdisksareoverplottedtoillustratewherethisemissioncomesfromatdierentages:HD141569( Fisheretal. 2000 );HR4796( Telescoetal. 2000 );Pic( Telescoetal. 2005 );49Ceti( Wahhajetal. 2007 );HD32297( Moerchenetal. 2007a );Fomalhaut( Stapelfeldtetal. 2004 );Lep( Moerchenetal. 2007b );Vega( Suetal. 2005 );Vel( Gasparetal. 2008 ),althoughnotethatthisexcessemissionisthoughttoarisefrominteractionwiththeinterstellarmedium).Thelightbluelineonallimagesis100AUunlessotherwiseindicated.Credit: Wyatt ( 2008 ). 32

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ResidualopticalimageofFomalhautaftersubtractingamodelofadustbelt.Greenlinestracetheprojectedboundariesofthedetectedbelt(133{158AU).Thewhitediamondandasteriskmarkthecentresofthebeltandthestar,respectively.Thehorizontalgreenlinetracesthebeltsemimajoraxis,whereastheredlinetracesthevectorbetweenthebeltandstarcentres.Whiteboxesandcirclesmarkextendedobjectsandbackgroundstars,respectively.( Kalasetal. 2005 ). disk( Quillen 2006 ),basedonitseccentricityandsharpinneredgeseeninopticalscatteredlight(Figure 1-8 ;Kalasetal.2005). Incontrast,ithasbeenshownthatanincreaseinthenumberofverysmallparticles(whichshouldbewarmerthantheequilibriumpopulation)thatresultsfromaplanetesimalcollisionandbreakupappearsasa\hot"asymmetry,i.e.,morepronouncedatshorterwavelengths.ThiscasehasbeenobservedinMIRimagesofthePicdisk(Figure 1-9 ),and Telescoetal. ( 2005 )suggestthatacatastrophiccollisionmayberesponsibleforthepresentdustdistribution,whereaclumpofdustontheSWsideappearshotter,andthuslikelyiscomprisedofsmallerparticles,thantherelatively 33

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SmoothedimagesofPicrotated58counter-clockwise;NEtoleft,SWtoright,smoothedpoint-sourceFWHMcontoursatright.Theverticalsolidlineisatthestar(center)andtheverticaldottedlinesareatNEandSW52AU.Thecentralwavelengthofthelterusedforeachimageisgivenatright.( Telescoetal. 2005 ). quiescentpopulationontheNEside.Wenotethattheprobabilityofobservingarecentcataclysmiccollisionassociatedwithadebrisdiskchosenatrandomislow(e.g.,Wyatt&Dent2002;Dominik&Decin2003;Kenyon&Bromley2005). Kenyon&Bromley ( 2005 )estimatethatthecloudofdebrisresultingfromsuchacollisionshouldonlybesustainedfor102orbitalperiods;thus,thetimeforwhichthecloudpersistsdependsonitsdistancefromthestarandthestellarmass.Nonetheless,thedebrisdiskcandidateschosenforhigh-spatial-resolutionimagingfollow-upobservationsarenotselectedatrandom.Thediskschosenforthissample(andlikewise,thedisksthathavebeenresolvedbypriorobservations)werechosenonthebasisoftheirhighIRluminosity,andsothereisagreaterprobabilitythatthesediskshaveexperiencedrecentcollisionalactivity. 34

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Wyatt 2005 ),buttheP-Rdragprocessmaybeimportantforolderdisks. Inoursolarsystem,thedebrisdiskiscomprisedofsublimatedcomets,theso-calledzodiacaldustresultingfromasteroidalcollisions,andalessmassivepopulationofdustfromKuiperBeltObjectcollisionsandinterstellardust.Thezodiacaldustservesasanotherexamplewheretheduststructurebearsthesignatureofplanetsinthesystem.TheforemosteectisthatthelocationoftheasteroidsisdictatedbyorbitalresonanceswithJupiter,resultingintheKirkwoodgaps( Dermott&Murray 1981 ; Kirkwood 1867 ).ThezodiacaldustalsoexhibitsaresonantclumpthattrailsbehindEarthinitsorbitduetotheinwarddriftofparticlesexperiencingP-Rdrag( Dermottetal. 1994 ).However,mostoftheplanetsthatarelikelypresentandinuencingthemorphologyinexosolardebrisdisksarestillundetected.Therefore,studyingthestructureofobservabledustdisksinthesesystemsisavaluabletechniqueforinferringthepresenceofplanetsandphysicalprocesseswithinadisk,andultimatelyfordevelopingplanetaryformationtheory. ,thedetectionofaninfraredexcessistherstindicationthatadebrisdiskmaybepresent.Indeed,thiswasthebasisoftherstdebrisdiskdiscovery,withIRAS:Vega( Aumannetal. 1984 ).Initally,thedustwasassumedtoresideinasphericalshellaroundthestar.SoonaftertheIRASdiscovery, Smith&Terrile ( 1984 )performedcoronagraphicimagingobservationsofPic,asourcewithasimilarIRexcess,andrevealedthatthedustassociatedwiththesourcewasactuallycoplanarandlikelytohavebeengeneratedbycollisionsofplanetesimals.Thepossibilitiesforanon-disk 35

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Backman&Paresce ( 1993 ). Thewarmdustybyproductofplanetesimalcollisionsandcometsublimationiseasiertoobservethantherelativelycolderandlower-surface-areaplanetsthemselves,andcanrevealsubstantialinformationabouttheevolutionofplanetarysystems.ThroughrelativelylargesurveysofIRphotometry,asdescribedinx ,wecanassesstrendsinexcessIRluminosityfordebrisdisksspanningalargerangeofages.Bygoingastepfurthertoimagesinglesourceswithhighspatialresolution(withcurrentlyavailablefacilities,<0.5"isconsidered\high"),wemaydiscoverstructuresinthedustdiskthatareindicativeofthephysicalprocessesoccurringinthem. Therearepresentlyseveralhundredphotometricdetectionsofdebrisdisks(e.g.,Oudmaijeretal.1992;Mannings&Barlow1998),butonlyasmallfraction(20)havebeenspatiallyresolved(e.g.,Figure 1-7 ).DebrisdiskscanbeimagedinscatteredlightatopticalorNIRwavelengths,butsuchobservationssuerfromstrongphotosphericcontaminationfromthecentralstar,andacoronagraphtypicallymustbeemployed(e.g.,thegrayscaleimageofHD141569inFigure 1-7 ;Weinbergeretal.1999).ThisproblemislargelyavoidedbyobservingatMIRwavelengthswherethereislessemissionfromthephotosphere,andthermaldustemissiondominates(Figure 1-2 ). TheMIRregimeoerstheadditionalbenetofdiraction-limitedobserving,andbyimagingfromtheground,asinthiswork,wecanexploitlargetelescopestoachievehighspatialresolution.Forexample,the=Ddiractionlimitat11.7mand18.3matthe7.9-meterGeminiObservatorytelescopesis0.24"and0.39",respectively. 36

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Doesthisworkanswerthefollowingopenquestionsintheeld? QuestionYesNoPartially

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Thisstudy'ssampleof21debrisdisksisdrawnfromasampleof266A-typestarsdescribedby Riekeetal. ( 2005 )(withthreeexceptions:HD32297,HD172555,andHD181296.)Thisbroadersampleconsistsofstarswithmassesof1.2{3.8Manduxdensitymeasurementsat24or25m.In Riekeetal. ( 2005 ),the24or25muxdensityforeachofthesestarswasmeasuredwiththeSpitzerMIPSinstrument( Riekeetal. 2004 ),IRAS,orISO(onlyonesource). Thestellaragesspan5{560Myr.Theagesforstarsinthissamplewereestimatedinthreewaysby Riekeetal. ( 2005 ):(1)byassociationwithaclusterormovinggroupofawell-knownage,(2)matchingthepositionofthestarontheH-Rdiagramtoevolutionarytracks,or(3)fromtheliterature,primarilyviaStromgrenphotometry(e.g.,Songetal.2001).Therstmethodtypicallyhasthesmallestuncertainty,becausesuchestimatesarebasedonstellarparametersfromarangeofstarsandnotsolelyonparametersfromthestarforwhichanageissought. Riekeetal. ( 2005 )notethattheredoesnotappeartobe 38

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Theexcessratio,theobserved24(or25)muxdividedbytheexpectedphotosphericux,isplottedagainsttheestimatedageforeachstarinFigure 2-1 Riekeetal. ( 2005 )drawtwomainconclusionsfromthesedata:(1)thereisanupperenvelopetotheobservedexcesses,approximatedbyaninversepower-lawdecaywithstellarage,and(2)thereisawidedispersionofexcessesforanygivenagebin.(Thersttrend,inparticular,hasbeennotedpreviously,e.g.,byDecinetal.[2003].)Theauthorspresenttwopotentialexplanationstoaddressthelatterresult:eitheralldisksevolvesimilarlyinasteadystatebuthavedieringinitialconditions(e.g.,Wyattetal.2007),oralldisksareinitiallysimilar,andtheapparentdispersioninexcessresultsfromstochasticcollisionalevents,whichcausetransientincreasesintheemittingdustmass(e.g.,Dermottetal.2002).SuchaneventhadalreadybeenproposedforthedebrisdiskofPictoris( Telescoetal. 2005 ). Manyofthesourcesinthe Riekeetal. ( 2005 )samplearetoofaintforground-basedobservations,andwehavechosenthesourceswiththehighestpredicteduxfromemittingdustasthedisksthatwearemostlikelytobeabletoresolvestructurewithin.Thereareafewsources,suchasVegaandFomalhaut,whichbyouruxcutopointwouldhavebeenincludedinthesample,butareknowntohavesurfacebrightnessestoolowtodetectwithground-basedobservations.(Thesesourceshavebeenspatiallyresolvedwithspace-basedobservations,bySuetal.[2005]andStapelfeldtetal.[2004],respectively.)Sincethissampleisasubsetofsourceswhichwereclustermemberswithparticular 39

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Excessratiovs.agefor266A-typemain-sequencestars.Credit: Riekeetal. ( 2005 ). characteristicsknown,itisnotstatisticallycomplete.However,wedoconsiderthesampletobeindicativeoftheAstarpopulationthathostsIRexcesses. 40

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DebrisDiskCandidateTargetList NameTypeAgeAged24or25umFFluxExcessRatio[Myr]Ref.[pc][Jy]Ref.[Ftotal=F?] HD21362B6Vn8011700.32448.37HD32297A03021120.2111139.05aHD38206A0V93690.11543.34HD38678A2Vann231,3301,4221.160112.43HD56537A3V5604290.586111.32HD71155A0V169,2401,4380.32141.54HD74956A1V390,3301,4241.990121.1HD75416B8V55970.12843.51HD80950A0V803810.12143.79HD83808A5V+4004411.140111.16HD95418A1V300,358,3806,1,4241.400111.21HD102647A3V50,5201,4112.320111.42HD115892A2V3504180.70541.2HD139006A0V314,3501,4231.686111.29HD141569B9.5e5(PMS)7,8991.81911162.4aHD161868A0V184,3051,4290.525111.47HD172555A5IV-V129291.092119.66aHD178253A2V254,3201,4400.348111.45aHD181296A0Vn129480.491118.32HD181869B8V1104520.280111.46HR4796AbA0V8,2010,1673.3801197.2 .AllotherexcessratiovaluesaretakenfromRiekeetal.2005.bMIRimageshadalreadybeenob-tainedforHR4796AwiththeGeminiObservatoryinstrumentspriortostartingthiswork;hencethistargetdoesnotappearinthesourcetableofdatatakenexpresslyforthisproject. References{(1)Songetal.2001;(2)Kalas2005;(3)Gerbaldietal.1999;(4)Riekeetal.2005;(5)deZeeuwetal.1999;(6)Kingetal.2003;(7)Weinbergeretal.2000;(8)Mernetal.2004;(9)Zuckermanetal.2001;(10)Staueretal.1995;(11)Moshiretal.1989;(12)IPAC1986 Suetal. ( 2006 )concludedthattheinfraredexcesspreviouslythoughttobeduetoadebrisdiskpresencewasactuallyduetoafast-rotatingB-typestarwithastrongstellarwindcreatingacircumstellargasdisk(alsoknownastheBephenomenon.) 41

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Gasparetal. 2008 ).Thedustinthisoverdenseregion(15xLocalBubbledensity)iscompressedattheshockfrontgeneratedbyphotonpressureandisheatedbythestar,whichgivesrisetoanarc-shapedmorphologythatisresponsiblefortheinfraredexcess. 42

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3-1 illustratesthesewindows( Glass 1999 ). Thesensitivityofground-basedMIRinstrumentsislargelydependentontheatmosphericconditionsatthesiteselectedfortheobservatory.Thecolumndensityofatmosphericgasesanductuationsthereinmustbemonitoredatanysite,butthese 43

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ATRANmodelatmospherictransmissionatMaunaKea.Notethestrongabsorptionfeaturesat7m(H2O),9.6m(O3),14{16m(CO2),andseverallongwardof18m(H2O).Credit: Glass ( 1999 ). negativeeectscanbeminimizedbychoosingacold,drysiteathighaltitude.Mostworld-classobservatorysitesarechosenatlatitudesneartheedgeofaHadleycell,wherethereisadownowofcold,dryair.IntheHadleycells,convectiontransportswarm,moistairupwardandpolewardfromtheequator,andmostofthewatervaporcontentislosttocondensationandprecipitationastheairmassrisesinaltitude.Theairthencoolsasitmovespoleward,creatingahigh-pressurezonewherethecooler,drierairsinksat30

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Despitethetypicallocationoftelescopesabovetheinversionlayer,someatmosphericturbulencemaystillaectimagequality.Changesinthedensityoftheatmosphere(i.e.,turbulence)alongthepathoftheincomingradiationyieldchangesintheindicesofrefractionandthereforechangetheapparentlocation(resultinginsmearingduringlongintegrations)andintensity(resultinginscintillation)ofthesourceonthesky.Thiseectisknownasseeing,andisinverselydependentonwavelength(/0:2)andthereforelessimportantatlongerwavelengthsintheinfrared.Apointsourceontheskyshould,intheory,appearasanAirypattern;however,withseeingeects,apointsourceimageatopticalwavelengthscantypicallybeapproximatedasatwo-dimensionalGaussianfunctionwithafull-widthathalf-maximum(FWHM)ofseveraltimesthetheoreticalvalue.Thelevelofimpairmentduetoseeingishighlyvariableaccordingtoweatherconditions.SeeingisquantiedbythevalueoftheFWHM,inarcseconds,ofapointsource;thissmearedimageisalsoknownastheseeingdisk. 45

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Glass 1999 ). Inordertodistinguishtheastronomicalsourcephotonsfromthesignalproducedbytheskyandthedetectoritself,MIRobservationsrequirethetechniqueofchopping.Thistechniqueconsistsoftiltingthesecondarymirroralternatelyon-andslightlyo-axis,typicallyafewtimesasecond,toproduceano-sourceimageofthenearbyskythatcanbesubtractedfromtheon-sourceimage.TheGeminitelescopeshaveamaximumchopangleof15arcsecondsbetweenthesetwopositions. Oncetheo-sourceimagehasbeensubtractedfromtheon-sourceimage,thereisstillstrongresidualemissionintheimagefromthetelescopeitself.Thetelescope,itssupportstructures,andtheinstrumententrancewindowallsitattheambientairtemperature(300K)andthereforeradiatesubstantiallyatMIRwavelengths.Bychangingtheopticalpathintheprocessofchopping,theamountandpatternofemissionfromthesesourceschangesaswell.Theresultingdierencebetweenthetwochoppositionimagesisknownastheradiativeoset,anditismuchsmallerthantheskyemissionbutnotnegligible.Toremovemostoftheradiativeoset,thetelescopeismovedevery30stothesamepositionasthepreviouso-sourcepositionreachedbychopping. Thisprocessisreferredtoasnodding,andfromthisalternatenodposition,thesecondarymirrorchopsbetweenthesourcebeamandano-sourcebeamonoppositesideoftheo-sourcebeamintheinitialnodposition(\nodA").Thechop-and-nodpatternshouldbesymmetricabouttheopticalaxis.Thus,theresultingdierencebetweenthetwochoppositions'imagesatthenewnodposition(\nodB")willshow(nearly)exactlythe 46

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Diagramoftypicalchopandnodpositions,regardingwhatportionoftheskyisimagedonthedetector.EachnodpositionhasanAandBchopposition,andtheon-sourcechoppositionforeachnodisindicatedbyabox. oppositeradiativeosetpattern.Thedierencebetweenthetwosetsofchoppedimagesateachnodpositioncorrespondstothesameamplitudeoftelescopepositionshift,butintheoppositedirectionawayfromthesource.Bycombiningthedierencesinthechoppedimagesfromeachnodposition,theradiativeosetisnearlycompletelyremoved,andonlythesourcesignalremains.Anexampleoftheappearanceofthese4componentimagesandhowtheyarecombinedisshowninFigures 3-2 and 3-3 47

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Exampleimagesofwhatisseenateachchopposition(rstrow),thedierencebetweentheseimagesineachnodposition(middlerow),andthecombinednalimageresultingfromthetwonodpositions(bottom). thedetectorsystem,suchthatwearelimitedonlybyphoton(or\shot")noisefromtheskyandastronomicalsource.Innearlyallcases,theskyemissionexceedsthesourceemissionbyseveralordersofmagnitude,soinpractice,thelimitisthephotonnoiseofthebackground.Thisregimeisreferredtoasbackground-limitedperformance,orBLIP.Thesignal-to-noiseintheBLIPregimecanbeexpressedas N=Sr nB=fp whereNsisthenumberofobjectimages,eachwithintegrationtimet,nisthenumberofpixels,andBthebackgroundperpixel( McLean 1997 ).ThesecondexpressioninEq. 3{1 alternatelyexpressesthesignal-to-noisewiththeratio,f,ofsourcecountstobackgroundcountsperpixel,andthetotalintegrationtime,T. 48

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ContributionstoUnwantedBackgroundEmission Contributions atmosphericemissivityemissionoftelescopemirrorsemissionoftelescopestructureinbeamemissivityofwarmwindowsemissivityofsurfaceswithinthecryogenicvesselscatteredlightwithininstrument Glass ( 1999 ) 3-1 .SomestepshavebeentakentominimizetheemissionassociatedwiththeobservatoryitselfattheGeminitelescopes.Forinstance,thekeymirrors{primary,secondary,andtertiaryfold{atGeminiSoutharecoatedwithsilvertoyieldathermalemissivityoflowerthan2%.Incontrast,thepreviouscoatingmaterialofaluminum(whichcontinuestobethestandardcoatingatother8-meter-classfacilities)hasatypicalemissivityatMIRwavelengthsof10%.Theinstrumentsthemselves(T-ReCSandMichelle)arecooledtoliquidheliumtemperaturesandthereforedonotcontributetothebackgroundexceptfortheirentrancewindows,whichsitatambientairtemperature.Thematerialsusedfortheentrancewindowshaveemissivitiesof10%.Straylightenteringthecamerasystemthatarisesfromtherelativelywarmtelescopestructurecanalsobeavoidedbytheuseofapupilstop.Thepupilstop(orcoldstop,socalledbecauseitsitsintheinstrumentatcryogenictemperatures)isanellipticalaperturesituatedatthepupilimageofthesecondarymirror,eectivelypreventinganyradiationfromoutsidethebeamfromcontaminatingthesignal.Scatteredlightwithoriginsinsidethedewarispreventedfromfurtherpassagebyaseriesofbaeswithin. 3.2.1FacilitiesEmployed 49

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Figure3-4. Schematicrepresentationofareectingtelescopeshowingtherelativelocationoftheprimary,secondary,andtertiarymirror.TheinstrumentsusedinthisworkaremountedattheCassegrainfocus.( McLean 1997 ) 50

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3-4 )arehyperbolicsurfaces.Thisallowsalargereldofviewwithgoodimagequality,incontrasttotheclassicalCassegrainlayoutwithaparabolicprimary,whichsuersfromcoma. TheinstrumentMichellewasemployedatGeminiNorth,whichsitsatopMaunaKea(Hawaii,US)at13,800feet.TheinstrumentT-ReCS(ThermalRegionCamera&Spectrograph)wasemployedatGeminiSouth,whichsitsatopCerroPachon(Chile)ataslightlylowerelevationof8,900feet.MichellewasdesignedandbuiltattheUKAstronomyTechnologyCentreinEdinburgh,Scotland.ItwasoriginallyintendedforuseattheUKInfraredTelescope(UKIRT),butafterayearofdutytherewastransferredtoGeminiNorthin2004forlong-termloan.T-ReCSwasdesignedandbuiltbytheUniversityofFlorida,anditwasdeliveredtoandcommissionedatGeminiSouthin2003. 51

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SummaryofImagingObservations ObjectGeminiProgramFilter DatesObserved HD21362NGN-2006A-Q-10N0 13Oct2006HD32297SGS-2005B-DD-8Si-5 3Feb2006,4Mar2006,9Mar2006,10Mar2006Qa 24Jan2006,3Feb2006,4Mar2006,9Mar2006,10Mar2006HD38206SGS-2005A-Q-2N 19Sep2005Qa 5Feb2006HD38678SGS-2005A-Q-2N 3Feb2005Qa 3Feb2005HD56537NGN-2006A-Q-10N0 4Apr2006,7Apr2006HD71155SGS-2005A-Q-2N 4Mar2006Qa 5Feb2006HD74956SGS-2005A-Q-2N 6Feb2006Qa 5Feb2006HD75416SGS-2005A-Q-2Si-5 22May2005Qa 22May2005HD80950SGS-2005A-Q-2N 8Mar2006Qa 5Feb2006HD83808NGN-2006A-Q-10N0 6Apr2006HD95418NGN-2006A-Q-10N0 7Apr2006HD102647NGN-2006A-Q-10N0 10May2006,15May2006HD115892SGS-2005A-Q-2N 21May2005Si-5 22May2005Qa 21May2005,22May2005HD139006NGN-2006A-Q-10N0 30Apr2006,12May2006HD141569NGN-2006A-Q-10N0 10May2006,14May2006HD161868NGN-2006A-Q-10N0 30Apr2006,20May2006HD172555SGS-2007A-Q-23Si-5 1Jul2007Qa 27Jun2007,1Jul2007HD178253SGS-2005A-Q-2Si-5 22May2005Qa 22May2005HD181296SGS-2007A-Q-23Si-5 28Apr2007Qa 28Apr2007HD181869SGS-2005A-Q-2N 20Aug2005Qa 20Aug2005,19Mar2006 52

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Table3-3. FiltersUsed InstrumentFilterNamec[m][m]50%cutobounds[m] MichelleN011.22.410.1{12.5Qa18.11.917.13{19.06 T-ReCSN10.365.277.70{12.97Si-511.661.1311.09{12.22Qa18.31.5117.57{19.08 AsummaryofthetargetsandtheirrespectiveprogramIDsandobservationdatesisgiveninTable 3-2 .AdescriptionofthebandpassesoftheltersusedineachinstrumentisgiveninTable 3-3 .EachdebrisdisktargetobservationwassandwichedbyobservationsofaPSF(point-spreadfunction)referencestarinordertocomparetheextentofthedebrisdisktargetwithaknownpointsource.Inafewcases,thePSFreferencecouldonlybeobservedonce,eitherbeforeorafterthetargetobservation.Foreachnight,atleastonestarofknownuxdensitywasalsoobservedtocalibratetheuxdensityofthesciencetargets.Initially,asintheGS-2005A-2program,separatestarswereusedforthePSFreferenceandtheuxstandardreference.AfterdiscoveringthatmanyoftheMIRCohenstandardsweresuitableforuseasPSFreferencestars(J.DeBuizer,personalcommunication),theobservingsequencewasmademoreecientbysandwichingthedisktargetwithareferencestarthatservedbothasauxandaPSFstandard. 53

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3.3.1Photometry Oncecalibrated,theuxdensityforeachdebrisdisksourcewasmeasuredwiththesamecircularapertureandskysubtractionmethodaswasusedfortheuxstandards.Thebackgroundvariationwasassessedbymeasuringthestandarddeviationofpixelvaluesinanapertureofthesameradius(asthatofthesourcemeasurement),nowplacedonaregionintheimagecontainingonlyblanksky.Theuncertaintyintheuxdensitymeasurementattributabletobackgroundvariation,,isthestandarddeviationofthebackgroundmeasuredinano-sourceaperturemultipliedbythesquarerootofthenumberofpixels(p 54

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MeasureduxdensitiesanduncertaintiesduetothebackgroundaregiveninTable 3-4 Table3-4. FluxDensities&BackgroundofDebrisDiskCandidates,inmJy Source Instrument HD# (N) (N0) (Si-5) (Qa) (Qa) 21362 Michelle 6832 4566 32297 T-ReCS 531 901 38206 T-ReCS 2021 1167 38678 T-ReCS 21472 93710 56537 Michelle 14321 59712 71155 T-ReCS 10832 37510 74956 T-ReCS 86104 238024 75416 T-ReCS 2293 927 80950 T-ReCS 19113 10910 83808 Michelle 36143 156713 95418 Michelle 35882 174310 102647 Michelle 58223 231717 115892 T-ReCS 25403 25212 102617 139006 Michelle 40772 179515 141569 Michelle 3381 88313 161868 Michelle 11051 44311 172555 T-ReCS 11552 109411 178253 T-ReCS 7702 36012 181296 T-ReCS 3952 34316 181869 T-ReCS 6952 20214 55

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Asdiscussedinx ,thebrightnessproleofanastronomicalobjectcanvarywithchangesintheatmosphericconditions.ItisforthisreasonthataPSFreferencestarisobservedinclosetemporalproximitytotheactualsource.Thus,ifthetargetobjectappearsbroaderthanexpectedforagivenwavelengthandprimarymirrorsize,onecanconrmbycomparisontothePSFwhetherthisbroadeningisinherenttothesourceorifitissimplyanatmosphericeect.Ideally,thePSFisobservedbothbeforeandafterthetargetobservations;inthisway,deterioratingskyconditionsdonotleadtoafalse-positiveresolvedsourceifthePSFisobservedbeforethetargetobject. Arst-ordercomparisoncanbeperformedbymeasuringtheFWHMofthenalstackedsourceimageandthenalstackedPSFimages.Ifthemeasurementsareequal(orwithinuncertainties,asdiscussedinthenextparagraph),thenthescienceobjectisunresolved.IftheFWHMofthesciencetargetisgreaterthantheFWHMofthePSF,furthercheckscanbemadetotestwhethertheextensionisduetothesourceitselforanothereectthathasdegradedtheimage.Thedebrisdiskssourcesstudiedinthisworkaretypically>10xfainterthanthePSFreferencestars.Therefore,theimagesforthetargetobservationsmusthavesignicantlylongerintegrationtimestoachievesimilarS/Nlevels.Pupilrotation,incorrectguidingcorrection,andchangesinthequalityofseeingthroughoutlongerobservationscanresultinanalimagebroadenedbyeectsthatthePSFdoesnotsuerfromwithshorterintegrationtimes. AfairassessmentofthedierenceinprolewidthbetweenthedisktargetandthePSFreferencecanonlybemadewithimagesofequalintegrationtime.Fortunately,thisissimpletoachieve,becausethemulti-extensionFITSleforeachrawimageiscomposedofsub-images,knownassavesets,whicharestackedframescomposedofseveralchopcycles,correspondingtointegrationtimesof10s.Eachsavesethasthesameintegrationtime,regardlessoftheoverallintegrationtimeoftheimage.(Theintervaltimebetweennodsis 56

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AsummaryoftheFWHMmeasurementsforthedebrisdisktargets(thatarenotclearlyresolved)andtheirrespectivePSFreferencestarsisgiveninTables 3-5 and 3-6 .ThesevalueswereobtainedbymeasuringtheFWHMofMoattstotheazimuthallyaveragedproleofeachsource,usingtheimexamroutineinIRAF.FurtherdiscussionofthesemeasurementsfollowsinChapter7(FurtherConsiderationofSourcesThatAreNotClearlyExtended). 57

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FWHMofDebrisDiskCandidates&PSFReferenceStars(Michelle) N0FWHM[arcsec]QaFWHM[arcsec] NameSourcePSFSourcePSF HD213620.4250.0460.3860.0040.5830.0870.5520.0870.3640.0090.3860.041HD565370.3660.0020.3980.0130.5450.0190.5390.004HD838080.3470.0010.3800.0090.5370.0040.5290.005HD954180.3400.0010.3280.0020.5400.0030.5440.003HD1026470.3610.0020.3530.0080.5340.0050.5340.0020.5310.0030.5350.003HD1390060.4190.0030.3640.0060.5740.0070.5560.0040.5170.0040.5440.003HD1415690.4350.0040.3740.0160.8070.0290.5400.0050.8180.0660.5230.003HD1618680.3860.0030.3660.0060.5550.0310.5280.0030.5370.0170.5250.006 58

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FWHMofDebrisDiskCandidates&PSFReferenceStars(T-ReCS) NFWHM[arcsec]Si-5FWHM[arcsec]QaFWHM[arcsec] NameSourcePSFSourcePSFSourcePSF HD382060.4420.0080.4270.0140.5930.0420.5330.006HD711550.3480.0030.3310.0030.6460.0400.5830.025HD754160.4950.0140.4720.0080.8760.0560.6480.024HD809500.3720.0080.4140.0070.4870.1250.6180.015HD1158920.4140.0100.4610.0060.5800.0160.5970.0120.6000.0110.6160.022HD1725550.3690.0020.3780.0060.5910.0140.5590.013HD1782530.4390.0030.4450.0080.5300.0530.5840.019HD1812960.3840.0020.3800.0060.5840.1920.5190.012HD1818690.3780.0030.3530.0110.5060.0240.6120.037

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Backmanetal. 1992 ; Wyattetal. 1999 ).Suchdisksarereferredtoasdebrisdisks.TheA-typemain-sequencestarLep(HD38678)wasidentiedasadebrisdiskcandidatebytheIRASdiscoveryofitsinfraredexcess( Aumann&Probst 1991 ).Itisoneofseveralstarswherethediskparticlesaresignicantlywarmerthanistypicalfordebrisdisks( Aumann&Probst 1991 ; Cote&Waters 1987 ),implyingthattheparticlesarerelativelyclose(<10AU)tothestar.Forexample,the320Kmid-infraredcolortemperatureofLep( Chen&Jura 2001 ; Moerchenetal. 2007b )contrastswitharchetypessuchasthediskofPic,whichhasamid-infraredcolortemperatureof160Kfordust80AUfromthestar( Telescoetal. 1988 ).Furthermore,Lepisoneofthefewsuchstarswithwarmdisksthatarecloseenoughtous(21.5pc)( Perrymanetal. 1997 )thatground-basedobserverscanreasonablyexpecttospatiallyresolvethedisk.TworecentobservationsofthediskdrewourattentiontotheLepdiskandmotivatedourprogramofmid-infraredimagingatGemini:astrongconstraintonthesizeofthediskat18matKeck( Chen&Jura 2001 )andaSpitzerMIPSphotometricsurveyofA-typestarsshowingthattheinfraredexcessofLepishighamongstarsofcomparableage( Riekeetal. 2005 ). Chen&Jura ( 2001 )havenotedthatthediskradiusmustbesmallerthan9AU,andthereforeprobablycomparableinsizetoourownasteroidbelt,whichmakesituniqueamongdebrisdisksdiscoveredsofar.Aswereporthere,wehaveimagedLepwithGeminitoconstrainmoretightlythedisksizeandfurtherexaminedustproductionmechanisms. 60

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Chen&Jura 2001 ; Fajardo-Acostaetal. 1998 ).Weobservedapoint-spread-function(PSF)comparisonstar,HD42042,bothbeforeandafterthetargetobservationsineachlter.HD42042,anMstarlocated6.6awayfromLep,is10timesbrighterthanLepat10.4mand6timesbrighterat18.3m.WedividedthePSFandtargetintegrationsintopairsofnodsets(imagesofequalintegrationtime)toexaminethepotentialeectofairmassonthefull-widthathalfmaximum(FWHM)value,andthereisnocorrelationwithtime. 4.3.1SourceSize&Model 4-1 ).Wehavepartlycharacterizedthisextensionbycomparingthefullwidthhalf-maximum(FWHM)oftheprolesdeterminedfromaMoattgeneratedinIRAF.At10.4m,thetstothePSFstarandLepprolesmeasure0.31"0.02".At18.3mtheproletsgive0.54"0.02"and0.60"0.02",respectively,asshowninFigure 4-1 .EventhoughweconsidertheFWHMradiusoftheproles,wehavedeterminedthosevaluesfromanalytictstothefullproles.ForLep,theindicateduncertaintyisthestandarddeviationofthemeanoftheseriesofFWHMmeasurementsmadethroughouttheintegrationset,asdescribedingreaterdetailbelow.ForthePSF 61

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ProlesofazimuthallyaveragednormalizedintensityforLep(diamonds)andreferencePSFstar(dots);verticallinesindicatetheFWHMvaluesofproletstothePSFstarandLep. 62

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FWHMvaluesofthesubdividedintegrationsequenceforLep(center,diamonds)andreferencePSFstar(leftandright,circles).Eachpointrepresentsanon-sourceintegrationtimeof120s(6minelapsedtime,or4\nodsets").ThehorizontallinesindicatethemeanFWHMvaluefortheseries,withverticalbarsindicatingthestandarddeviationofthemeanFWHMvalueforthecorrespondingPSForsourceintegrationsequence. star,wehaveonlysixFWHMmeasurements,andtheuncertaintygivenishalfoftheexcursionbetweenthemaximumandminimumvalues.Like Chen&Jura ( 2001 ),wedonotresolveLepat10.4m,butwedoresolvethesourceat18.3m(Figure 4-1 ). Totesttherobustnessofthelatterresult,weexaminedtheFWHMofthePSFstarandLepthroughouttheintegrationsequence.Pupilrotation,incorrectguidingcorrection,andchangesinthequalityofseeingduringlongobservationsonaprogramobjectcanresultinanalimagedegradedbylower-frequencycomponentsthatarenotaccuratelyrepresentedinthePSFdeterminedfromgenerallyshorterintegrationtimes.Thus,wehavedividedthetotal18.3mintegrationsequenceintobinsof120sofon-sourceintegrationtimeandhavemeasuredthewidthoftheMoatprolet 63

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4-2 ).ThedisplacementbetweenthemeanprolewidthofthePSFandthemeanprolewidthofthesourceimpliesthatthesourceisresolved.QuadraticsubtractionofthederivedFWHMvalueofthePSFstarfromthatofLep,asdeterminedfromthe18.3mimages,indicatesthattheaveragediskradiusis0.14"0.01",or3.00.3AU. Chen&Jura ( 2001 )estimateabest-tvalueof2.15AU,buttheuncertaintiesintheirobservationsallowforvaluesaslargeas9AU(C.Chen,privatecommunication).Ourobservationsmadeundernearlydiraction-limitedconditionsimplythatthesizeoftheemittingregionisindeedcomparabletothatoftheasteroidbeltinoursolarsystem. ToconstrainthegeometryoftheLepdisk,wegenerated2-Dmodelsoftheexpected18.3mbrightnessdistributionwitharangeofdiskparameters.SubtractionofthePSFimagenormalizedtothepeakbrightnessfromtheimageofLepshowsaresidualringofemissionthat,giventheasymmetriesevidentinthePSFitself,isconsistentwiththediskbeingapproximatelyazimuthallysymmetric.Wethereforeassumeaface-ondiskgeometryforourmodelsandcalculations,butthisassumptionhasnoeectonourconclusions.(Ourobservationsprovidenoconstraintonthediskorientation.) Ourmodelsconsistofacentralsource(thestar)witheitheroneortwoannuliofdustemissioneachwithuniformsurfacebrightness,wheretheconstanttotaluxdeterminedbyobservationsat18.3mwasequallydividedbetweenthecentralsourceanddustemissionaccordingtotheestimatedexcessabovethephotosphere( Chen&Jura 2001 ).Forthesingle-annulusmodels,thefreeparametersofannuluswidth(2,4,or6AU)andannulusinneredgedistancefromthestar(2,3,4,6,or8AU)arecombinedtoyieldfteenmodels.Wealsogeneratedmodelswithtwoconcentricandcontiguousannuliwithdierentbutuniformbrightnesses.Inallofthesemodels,theinnerannulusextendsfrom2to4AU,whereastheouterannulusextendsfrom4AUtoeither6AUor8AU.Theratiosofthetotaluxesfromtheinnerandouterannuliareeither1:1or3:1,respectively,resultinginfourdouble-annulimodels.Thedouble-annulusisasimplegeometricconstruct 64

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Anillustrationoftheradiallysymmetricdouble-annulusmodel(priortoconvolutionwiththePSF)usedtoapproximatetheresolved18-mdustemission.Thepoint-sourcephotosphericcontributionatthecenteriscutoinordertobettershowtheannuluslevels. thatlikelyapproximatesamorecomplexradially-dependentbrightness.(AnillustrationofthismodelisshowninFigure 4-3 .)Themodelswerethenconvolvedwitha2-DrepresentationofthePSFthatwasgeneratedfromtheazimuthallyaveragedPSFstarobservationat18.3m.A-squaredminimizationcomparisonoftheresultingmodelprolestothesourcedatashowsthatthesourceisbestapproximatedbyamodeloftwoannuli.Acomparisonof3models,includingthebesttoftheset,isshowninFigure 4-4 .Inthismodel,a2-4AUannulusemits75%ofthethermaldustux,anda4-8AUannulusemitstheremaining25%.Ourmodelsarenotexhaustivebutdoillustratethenatureofsatisfactorytstotheobservedproles.Wealsonotethatthedustemittingtheunresolved10.4mexcessmustlieinteriortotheresolved18.3muxandmaybe 65

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Figure4-4. Prolesof3ofthediskmodelsthatweregeneratedtoapproximatethe18.3mimagingobservations.TheprolesinthisgurerepresentthePSF-convolveddiskmodelswiththefollowinggeometries:asingleannulus8-10AU(teal,top),twoannuli2-4AUand4-8AU(uxratio3:1;pink,center),andasingleannulus2-4AU(magenta,bottom).ThesolidlineistheazimuthallyaveragedproleofLepat18.3m. Telescoetal. 2005 ; Weinbergeretal. 2003 ).Blackbodybehaviorrequiresparticleradiiofafewmicronsorlarger.Additionalevidencethatthe 66

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Chenetal. 2006 ).Silicatesarecommonlyfoundincircumstellardisks,butthespectrumofLepshowsnoevidenceofadistinctsilicateemissionfeature.Silicateparticlesaslargeas1-2mcanresultinabroadsilicatefeature,whichisexpectedtodisappearasparticleradiiincreaseaboveafewmicronsandtheparticlesbecomeopticallythickinthemid-infrared( Przygoddaetal. 2003 ). Chenetal. ( 2006 )concludethat,ifsilicatesarepresent,themajorityofparticleradiiarelargerthan10m. Weestimatethesizeofthesmallestparticlesexpectedtoremainonboundorbits(=Fr=Fg<0.5)inresponsetotheforcesofgravityandradiationpressure( Burnsetal. 1979 ),basedonthestellarluminosity,14L,thestellarmass,1.9M,andradiationpressureeciency,1.Adoptingthestandardsilicatedensityvalueof2.5gcm3,wendthattheminimumradiusis3.4m.Particlessmallerthanthisareblownoutofthesystembyradiationpressurewithinafeworbitaltimescales.TheKeplerianorbitalperiodat3AUfromLepis3years,sograinssmallerthantheblowoutradiusof3.4mshouldberemovedonadecadetimescale.Withthisminimumparticlesizethreshold,weestimatethenumber-density-weightedaveragedustparticleradiusof4.7mbyassumingthattheincrementalnumberofgrainsperradiusintervaldaisproportionaltoa3:5,whichdescribestheequilibriumsizedistributionofthedustparticlepopulationinacollisionalcascade( Dohnanyi 1969 ). Assumingthatallparticleshavetheaverageradius4.7mandareinathinshell3.0AUfromthestar,weestimatethedustmassfromtheamountofultravioletandopticaluxreprocessedasthermalemission( Juraetal. 1995 ).WithvaluesforthefractionalinfraredluminosityLIR/L=1.7104( Fajardo-Acostaetal. 1998 )andparticledensity2.5gcm3,wendatotaldustmassof6.71021g,or0.37%ofthetotalmassoftheasteroidsinoursolarsystem.Thisvalueis40%ofthemassestimatein( Chen&Jura 2001 )duetothesmallerdisksize(andthereforehotterdust)determined 67

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Largedustparticles(a&3:4m)aresubjecttolossbyspiralingintothestarunderP-Rdrag( Burnsetal. 1979 ).However,thisprocessisonlyrelevantifthetimeintervalbetweendestructiveparticlecollisionsismuchlongerthantheP-Rdragtimescale( Wyatt 2005 ).Thecharacteristiccollisionallifetimeoftheaverageradiusparticleatadistancerfromthestaristcoll(r)=tper(r)=(4e(r)),wheretperistheKeplerianorbitalperiodandeistheeectiveopticaldepth.Weestimatetheeectiveopticaldepth,denedastheratioofthetotalcross-sectionalareaofdusttothetotalareaofthedisk( Wyattetal. 1999 ),fromthetotalcross-sectionalareaofaverage-sized(4.7m)particlesnecessarytoproducetheinfraredluminosityandtheareawithinthe2{8AUannulus;forLep,thisvalueis2104,approximatelyequaltotheobservedvalueforLIR/L. Assumingaconstantparticlenumberdensity,theratiooftheP-Rdragtimescaletothecollisionaltimescaleisaweakfunctionofdistancefromthestar,soweexpecttherelativesignicanceofthedominantprocesstobesimilarthroughoutthedisk.Forexample,theexpectedtimeforaparticletomigratebyP-Rdragfrom6AUto3AU(thecenterofeachannulus)is16000y,severaltimeslongerthanthecollisionaltimescaleat4AU(thenominaldivisionbetweentheannuli)ofonly2000years.Therefore,weconsideritunlikelythattheparticlesorbitingLepmigrateasubstantialdistancetowardthestarbeforebeingbrokenupinacollisionalcascade.Weconcludethat,followingtheinitialproductionofdust,radiationpressureandsubsequentinterparticlecollisionsarethetwodominantprocessesdeningthepopulationanddistributionofparticlesinLep'sdisk. Riekeetal. 2005 ).Inthispaperwehaveadoptedthevalueof231Myr,estimatedwithphotometry-determinedstellarparameterstottostellarevolutionarytracks( Song 68

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2001 );thisestimatealsoincorporatestheeectsofrapidrotation,andasurveyofA-typestarsshowsthatLep'spositionintheH-Rdiagramisclosetothatofotherstarswithcomparableageestimates( Juraetal. 1998 ).However,iftheageis100Myrorless( BarradoyNavascues 1998 ; Songetal. 2001 ),theexcesslevelforthediskiswithintherangeofvaluestypicalforthoseagebins.Therefore,thedustproductionscenariothatweconsidermostprobabledepends,tosomeextent,ontheageweassumeforLep:ifthediskisolderthan100Myr( Lachaumeetal. 1999 ; Songetal. 2001 ),theexcessisindeedanomalouslyhighandmayindicatearecentcataclysmiccollision,but,ifthediskismuchyounger,wemaybeobservingthedustproductionanddecayassociatedwitharelativelysteadycollisionrateofsmallerbodies.Weexaminebothofthesescenariosbelow. RecentcataclysmiccollisionsmayaccountforsomeoftheexcessemittingdustaroundPic( Telescoetal. 2005 )andVega( Suetal. 2005 ),andmayalsocontributetotherangeofexcessesobservedforstarsofsimilarages( Riekeetal. 2005 ).Thedestructionofoneplanetesimalofradius85kmcouldaccountfortheentireobservedmid-infrared-emittingdustmassinLep.Toassesstheplausibilityofasinglecataclysmiccollisionofsuchabodyresultinginadiskresemblingourbest-tmodel,weconsiderthedynamicsofthedustparticlesimmediatelyfollowingtheirproductioninsuchanevent.Theinitialorbitalevolutionofthenewlyformeddustparticlesisdominatedbyradiationpressure,whichactstoincreasethesemi-majoraxesoftheirorbits.Ifweassumethattheseparticlesareproducedbythedestructionofabodyonacircularorbitat2AU(theinneredgeoftheinnerannulusinthebest-tmodeltoourobservations),wecanestimatetheradiusforparticlesexpectedtomigratetoagivendistance,usingstandardrelations( Kortenkamp&Dermott 1998 ; Wyattetal. 1999 ).Sincesmallerparticleswillhavelargerapastrondistancesundertheinuenceofradiationpressure,thesizeofaparticlethatcanmigratetoagivenapastrondistancecorrespondstoalowerlimitonthesizeofparticlesinteriortothatlocation.Theradiusofparticlesexpectedtoreach4AU(5.1m)willbeboththeupperlimitforparticlesinthe4{8AUannulusandthelowerlimitforthe2{4AU 69

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Tocharacterizetheparticledistributionthatwouldyieldtheuxratio(3:1)betweentheannuliofthemodelttoourobservations(x ),wesumovertheareasofalltheindividualemittingparticles,asdescribedbyapower-lawparticledistribution,assumingtheradiiare>5.1mintheinnerannulusand3.9{5.1mintheouterannulus.(Sincetheradiioftheparticlesthatareexpectedtoremainonboundorbitswithin8AUarelargerthanafewmicrons,weassumethattheparticlesareblackbodyemitters.)Toestimatetheirtemperatureandux,wethenassumethatalloftheparticlesintheinnerandouterannuliareatacharacteristicdistanceof3and6AUfromthestar,respectively.Wendthat,tomatchthemodelofourobservations,thesizedistributionofthedustpopulationresultingfromacollisionat2AUmustbedescribedbyapowerlawwithanexponentof6.4,muchsteeperthanthe Dohnanyi ( 1969 )valueof3.5.Indeed,thereisnoreasontoexpecttheimmediatepost-collisionpopulationtoresembletheDohnanyidistributionthatisthoughttoapplytoapopulationthathasevolvedtocollisionalequilibrium( diMartinoetal. 1990 ; Durda&Flynn 1999 ).Thepost-collisiondistributionpresumablydependsheavilyonthedetailsofthecollisionandthecompositionoftheparentbody.Forexample,withregardtothelatter,\rubblepiles"anddierentiatedasteroidsmayproducedierentrelativeamountsofsmallandlargeparticles( Groganetal. 2001 ).Aparticlesizedistributionconsistentwithourdouble-annulusmodelthereforesuggeststhattheproportionofsmallparticlesproducedinthehypotheticalcollisionislargerthanthatimpliedbytheDohnanyidistribution. Incontrasttothecatastrophiccollisionscenario,dustproductionbyrelatively\steady-state"grindingcollisionsmaybemorelikelyiftheageofLepislessthan100Myrandtheexcesslevelisnothighcomparedtootherstarsofcomparableage.Indeed,( Chen&Jura 2001 )proposethatdustisreplenishedinLepinanasteroidbelt 70

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.Aplotofsimulateddustproductioninoursolarsystemagainsttimeshowsaslowdecreaseofdustareaovertime,punctuatedbyspikesfromdust-producingcollisions( Dermottetal. 2002 ).Thisillustratesthattheareaofdustalonecannotindicatehowlongithasbeensincethemostrecentcataclysmiccollision.MuchgreaterspatialresolutionthatpermitstheparticleorbitstobeconstrainedisneededforustobetterunderstandthecollisionalhistoryoftheLepdebrisdisk.Theapparentlyrestrictedspatialextentofthisdiskmaybeduetoplanetaryperturbationssuchasresonanttrapping,ascenariothathasalsobeenproposedforthedebrisdiskaroundtheKstarHD69830.Thisdisk'slimitedextentwasinferredbySpitzerphotometry( Beichmanetal. 2005 )andmaybeduetotheinuenceofitstripleplanetsystem( Lovisetal. 2006 ).Thepresenceofplanetsmayalsoassistinredistributionofthedustazimuthallybysecularperturbations,resonantinteractions,gravitationalscattering,oraccretion. OurkeyconclusionisthatwehaveresolvedthediskofLepat18.3m,whichhasacharacteristicsizecomparabletothatofourownasteroidbelt.Thisistheonlyspatially 71

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Herewereporttherstmulti-wavelengthmid-IRimagesthatresolvethedebrisdiskofHD32297,andwenotetheindependentdiscoveryofresolvedthermalemissionby Fitzgeraldetal. ( 2007 ).Theseobservationsweremotivatedbyhigh-contrastcoronagraphicimagingofscatteredlightintheextendeddustdiskaroundHD32297innear-IRandopticalbandpasses( Kalas 2005b ; Schneideretal. 2005 ).Thehighfractionalinfraredluminosity(0.0027;Schneideretal.2005)ofthisA0starindicatesaconsiderablecontributionfromthermalemissionthat,itseemedtous,wouldlikelyberesolvableatadistanceof112pc(Perrymanetal.1997,hereafterP97)atmid-IRwavelengths.Overahundreddebrisdiskcandidateshavebeenidentiedphotometrically(e.g.,Riekeetal.2005),butonlyaboutadozenhavebeenspatiallyresolvedatanywavelength.Ofthe 73

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Backmanetal. 1992 ; Fisheretal. 2000 ; Jayawardhanaetal. 1998 ; Lagage&Pantin 1994 ; Marshetal. 2002 ; Telescoetal. 1988 2000 2005 ; Wahhajetal. 2007 );Lepremainstheexceptionamongresolveddisks,beingcomparableinsizetoourasteroidbelt( Chen&Jura 2001 ; Moerchenetal. 2007b ).HD32297isthereforeanimportantadditiontothesmallbutgrowingsampleofdebrisdisksresolvedatmid-IRwavelengths. 5.2.1Data 74

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Table5-1. FluxDensitiesofHD32297fromGround-BasedObservations FluxDensity[mJy]11.7m18.3m Total 5-1 )withatotalextent(atthe3-level,whereistheuncertaintyassociatedwiththebackgroundvariation)of291AU(2.58")and341AU(3.02")at11.7and18.3m,respectively.Withellipsettingofisophotesinthe11.7mimage,wendapositionangle(PA)forthemajoraxisof51.1o2.4o(eastofnorth),consistenttowithin2owiththevaluemeasuredfortheentirediskinnear-IRimages( Schneideretal. 2005 )andforthesouthwestsideofthediskinopticalimages( Kalas 2005b ).Themid-IRspatialextentisonlyhalfthatmeasuredatnear-IRwavelengths( Schneideretal. 2005 ). At11.7m,theintegrateduxoftheNEsideofthediskis1.340.09timesasbrightastheSWside;thestatisticalsignicanceofthedierenceinthebrightness 75

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11.7m(left)and18.3m(right)imagesofHD32297.Contoursarelinearlyspacedat3,6,9,12,and15timesthe1-noiselevel(mJypixel1)inthe11.7mimage,andat3,6,and9timesthe1-noiselevel(mJypixel1)inthe18.3mimage.The11.7mand18.3mimagesaregaussian-smoothedby2and3pixels,respectively.ThecircleattheupperrightineachimagecorrespondstothesizeoftheFWHMofthePSFateachwavelength. betweenthetwosidesis6.1.Theuxesusedinthiscalculationmeasurethetotaluxoneachsideexteriortothe0.75"regioncorrespondingtotherstAiryring,inordertoexcludemostofthephotosphericux.Thisasymmetryisoppositetothatdeterminedby Schneideretal. ( 2005 ),wherethescatteredlightinthecoronagraphicimageisbrighterintheSWbyadierenceof2.6.At18.3m,theSWsideis1.280.29timesasbrightastheNEside(oppositetothatat11.7m),butthisdierenceisnotsignicant(1.5).Theuxesforeachsideat18.3maremeasuredfromthecentralstarpositionoutward,sincethephotosphericcontributionatthiswavelengthissmall(seeTable 5-1 ).Otherthanthemodestdierenceinbrightnessbetweensides,therearenoobviousmorphologicalfeaturesinthediskateitherwavelength.Asymmetriesareseeninmanydisks,andtheymayhavediverseoriginssuchasresonanttrappingorcataclysmiccollisions( Kalasetal. 76

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; Telescoetal. 2005 ).However,untildeepermulti-wavelengthimagesareavailable,detailedconsiderationsofanyasymmetriesinHD32297seemtoustobeunwarranted. 5.3.1ComparisonwithKeyDebrisDiskArchetypes Kalas ( 2005b )notesthattheopticallyobservedmorphologycouldbeinterpretedastheresultofoutowssuggestiveofamuchyoungerage.ThefractionalIRluminosityofHD32297(0.0027;Schneideretal.2005)issimilartothoseofPicandHR4796A:0.0015( Decinetal. 2003 ),and0.005( Schneideretal. 1999 ),respectively.HD32297ismuchfartheraway(112pc[P97])thanPic(19pc[P97])andHR4796A(67pc[P97]),whichresultsinalowertotalsignal-to-noiseratioandpoorerresolutioninitsimages.Therefore,itisnotpossiblewithourcurrentimagestostudytheinnerregionofthediskclosesttoHD32297(.30AU)inasgreatdetailastheseothertwosources.Wecan,however,usefullycompareforthersttimeitsmid-IRcolorstothoseofthetwodisks. WeestimatethephotosphericuxdensitiesforHD32297at11.7mand18.3mbyextrapolatingtheK-bandmagnitudeof7.59( Cutrietal. 2003 )to10m.Weassumethattheuxdensityvariesas1:88overthiswavelengthrange,asisestimatedby Kurucz ( 1979 )tobeappropriateforanA0star(e.g.,Juraetal.1998).Beyond10m,weadoptedthestandardRayleigh-Jeans(2)relationtoestimatephotosphericuxdensitiesandresultingexcessuxdensitiesasgiveninTable 5-1 .The24.5muxdensityof19832mJywasinterpolatedfromour11.7mmeasurementandtheIRASdetectionat25m. 77

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Color-colorplotofmid-IRuxesofHD32297(starsymbol)andthetwoarchetypedisks,Pic(diamond)andHR4796A(triangle).FluxesanduncertaintiesforHD32297areinTable 5-1 Thecolor-colorplot(Figure 5-2 )ofHD32297,PicandHR4796AshowsthatallthreesourcespossesssimilarF(24.5m)/F(18.3m)colors,buttheF(18.3m)/F(11.7m)colorofHR4796AistwicethatofbothPicandHD32297( Telescoetal. 2000 2005 ; Wahhajetal. 2005 ).ThehighvalueforthislattercolorforHR4796Aislikelyduetoadeciencyofhot11.7m-emittingdustveryclosetothestar,asisindeedseen 78

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,weprovideadditionalevidenceanddiscussionregardingthedetailedgeometryofthecentralclearing. Przygoddaetal. 2003 ).Forcomparison,wealsocomputethetemperatureofthedustparticlesassuminganemissioneciencyproportionaltofrequency(Q/),whichyields15610K. Wemeasuredtheradialbrightnessprolesofthephotosphere-subtracteddiskbysummingovera0.8"swathalongthediskaxisandthensummingthetwosidesofthedisk.Theradialbinwidthsare20AU,withtheexceptionoftheoutermostdatapointthatreectsthemeanvaluefora40AUbintoincreaseS/N.Foreachlter,thePSFwasscaledtothelevelofthephotosphere(seeTable 5-1 )andthensubtractedfromthediskimage.Toachievethesameresolutioninthetwoimages,eachimagewasthenconvolvedwithagaussianprolehavingawidthcorrespondingtothePSFFWHMintheotherlter.Weusedthesebrightnessprolestogenerateacolortemperatureproleofthedisk,showninFigure 5-3 .Weseethatthecolortemperatureremainsconstanttowithinafewkelvinsfromthecenterofthediskoutto80AU.Particletemperatureshoulddecreaseasdistancefromthestarincreases.Therefore,thiszoneofconstanttemperature 79

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5-2 .However,inapparentcontradictiontoFigure 5-2 ,inwhichthecolorsofHD32297arenearlythesameasthoseofPic,thisinferredcentralclearingsizeisalmostidenticaltothatdeterminedforHR4796A(e.g.,Jayawardhanaetal.1998;Koerneretal.1998;Telescoetal.2000).WesuggestthatbothndingscanbereconciledinapicturewherethezonemarkedlydecientindustisapproximatelythesamesizeastheoneinHR4796A,butthatthiszonestillcontainssignicantlymoredustthantheclearedregionofHR4796A,whichisalmostcompletelyfreeofdebris.ThuswemaybeobservingadebrisdiskwithamorphologyintermediatebetweenPicandHR4796A.Wedonotmeantosuggestthatthesestarscorrespondtoatemporalsequence,sincethatisinconsistentwiththeknownagesofPicandHR4796A.However,thediskinHD32297appearstobroadenourappreciationoftherangeofmorphologiesthatcanoccurduringdiskevolution. WeplottemperaturecurvesinFigure 5-3 forablackbodyandforparticlesofthreecharacteristicradii(0.05m,0.075m,and0.2m).Theselattercurveswere 80

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RadialcolortemperatureproleofHD32297,asderivedfrom11.7and18.3muxdensitymeasurementsalongthedisk.Largeerrorbarsreectthecombinedphotometricandmeasurementuncertaintiesandsmallerrorbarsreectonlythemeasurementuncertainties.Thedottedlineindicatestheexpectedblackbodytemperaturechangewithdistancefromthestarandthethreesolidlinesindicatepredictedtemperatures(asdiscussedinBackman&Paresce1993andx )forcharacteristicgrainsizes:0.25m(left),0.075m(center),and0.05m(right). calculatedusingequationspresentedin Backman&Paresce ( 1993 ),whichenableaparticletemperatureestimateatagivendistancefromthestarforparticleswitharange 81

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Backman&Paresce 1993 ).WendthatHD32297'scolortemperatureproledoesnotcoincidewiththatexpectedforparticlesofonlyonecharacteristicsize.Theproleiswellapproximatedbythecurvefor0.075-mparticlesbetween80{120AU,butatlargerdistancesthecolortemperaturefallsbelowthiscurve.Sinceoreectsbothparticlesizeandcomposition,achangeineither(orboth)ofthesecharacteristicsofthedominantpopulationat120AUcouldproducethedeviationweobservefromthetemperaturecurveforasingleovalue. Sucharadialtransitionmightoccuratthesnowline,beyondwhichtemperaturesarecoolenoughthatvolatilegasesmaycondenseoutintosolidparticles.Thesnowlineinprotoplanetarydisksisexpectedtoliewheretemperaturesfallto145{170K(e.g.,Hayashi1981;Sasselov&Lecar2000).Weestimatecolortemperaturesbelowthislevelforthetwooutermostpointsinthedisk,beyond140AU.Anincreaseinparticlesurfacedensitybeyondtheputativesnowlinewouldlikelyresultinthegrowthoflargericyparticlesthanwouldbefoundintheinnerregionofthedisk.Suchparticleswouldshowdierentabsorptionandemissionbehaviorcomparedtonon-ice-enhancedparticlesclosertothestar,notonlybecauseoftheirlargersize,butalsobecauseofthesignicantchangeincomposition,whichdictatestheparticleheatingandcoolingprocesses(e.g.,Podolak&Zucker2004).Thisideaispurelyspeculativeandmustbeinvestigatedfurtherwithspatiallyresolvedspectroscopicobservationsandsubsequentmodeling. WhilethediskofHD32297mustbeexploredwithdeeperdirectimagingandspectroscopytoclarifythedistributionandcompositionofdust,wehaveestablishedthekeyconclusionthatHD32297isyetanotherdebrisdiskwithacentralclearing.The 82

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6.1.1Discovery&Morphology Jura 1991 ).Itsageof82Myr( Staueretal. 1995 )(basedonthelithiumabundanceofHR4796B)isintheso-calledtransitionalrange,closerinagetopre-main-sequencestarswithages1Myrthantomoreevolvedandlessmassivedebrisdiskswithagesupto100sofMyr.HR4796Aisat67pc,andhasanM-starcompanionwithaseparationof7.7"( Juraetal. 1993 ). Jura ( 1991 )rstsuggestedthemainlocationofdustintheHR4796Adiskat77AU,basedontheSEDconstructedfromNIRandMIRphotometry.Indeed,MIRdiscoveryimagesofthesourceatKeckandCTIOrevealedahighlyinclinedringlikediskwhosedustdistributionpeaksnear70AU( Jayawardhanaetal. 1998 ; Koerneretal. 1998 ).Follow-upcoronagraphicNIRimageswithHST/NICMOStightlyconstrainedthewidthoftheringto17AU,andthedustpopulationisseverelydepletedbothinteriorandexteriortothisregion( Schneideretal. 1999 ). Furtherimagingobservationsofthethermalemissionconrmastrongpeakinthedustdensityat70AU( Telescoetal. 2000 ; Wahhajetal. 2005 ). Telescoetal. ( 2000 )alsonotea1.8-brightnessasymmetryinwhichthenortheastsideofthediskisbrighterthanthesouthwestside.Inacompanionpaper, Wyattetal. ( 1999 )demonstratethatsuchanasymmetrycouldarisefromthephenomenonofpericenterglow.Asecondbodyinthesystemonaneccentricorbitaboutthestarmaycausepericenterglowwhen,throughsecularperturbations,iteectivelyshiftsthecenterofthedustringawayfromthestarandclosertotheapastronoftheperturbingcompanion'sorbit.Thusthesideoftheringclosertothestarbecomesrelativelyoverheatedandoverluminous( Dermottetal. 84

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Top:18.1-mimage(Michelle)ofHR4796A.Contoursaredrawnat3,6,9,12,15,and18bkd,wherebkdisthemeasurementuncertaintyassociatedwiththebackgroundvariation.Bottom:24.5-mimage(T-ReCS)ofHR4796A.Contoursaredrawnat3and6bkd,wherebkdisthemeasurementuncertaintyassociatedwiththebackgroundvariation.Notethatthe18.1-mimagewastakenwithMichelle(platescale:0.1005"/pixel)andthe24.5-mimagewastakenwithT-ReCS(platescale:0.089"/pixel). 1998 ).Atpresent,whileseveralworkshaveacknowledgedtheexistenceofabrightnessasymmetry(e.g.,Wahhajetal.2005,Debesetal.2008),noalternativeexplanationsfor 85

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Top:18.1-mphotosphere-subtractedimage(Michelle)ofHR4796A.Contoursaredrawnat3,6,9,12,15,and18bkd,wherebkdisthemeasurementuncertaintyassociatedwiththebackgroundvariation.Bottom:24.5-mphotosphere-subtractedimage(T-ReCS)ofHR4796A.Contoursaredrawnat3and6bkd,wherebkdisthemeasurementuncertaintyassociatedwiththebackgroundvariation.Notethatthe18.1-mimagewastakenwithMichelle(platescale:0.1005"/pixel)andthe24.5-mimagewastakenwithT-ReCS(platescale:0.089"/pixel). 86

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)( Debesetal. 2008 ; Schneideretal. 1999 ; Telescoetal. 2000 ; Wahhajetal. 2005 ). Thereissomedebateonthedustdistributioninthediskinteriortothedominantannulusat70AU. Koerneretal. ( 1998 )ndthattheexcessemissionat=12.5misrelativelyconcentratedcomparedtothatat=20.8m,andndthatthecolortemperatureimpliedfordustatornearthestellarpositionis200{300K.TheysuggestthatthisresultinindicativeofatenuousdustcomponentwithinafewAUofthestar,whichcouldrepresentananalogtothezodiacalcomponentofoursolarsystem.However, Telescoetal. ( 2000 )investigatedtheinnerregionintheir10.8-and18.2-mimagesbysubtractingthescaledPSFtoremovethephotosphericcontribution,andtheyconcludedthattheaccuracyoftheprocedurewasnothighenoughtoconrmanyinnerstructure.In2005,Wahhajetal.modelednewthermalemissionimagesandtheMIR-to-submillimeterSEDinconjunction,andagaininvokedtherequirementforahotzodiacaldustcomponent.Furthercommentaryonwhetherdustpopulationmodelsimplythepresenceofsuchadiskcomponentisgiveninx Augereauetal. 1999 ; Jayawardhanaetal. 1998 ; Juraetal. 1998 1993 ; Koerneretal. 1998 ; Li&Lunine 2003 ; Telescoetal. 2000 ; Wahhajetal. 2005 ).Blackbody-likeparticlesatthisdistanceonlyreachatemperatureof60Kinradiativeequilibrium,andthustheycannotserveastheprimaryconstituentoftheoverallpopulation( Juraetal. 1998 ). Thereisgeneralagreementthattheeectiveradiusofthedominantparticlepopulationmustexceedafewmicrons.Avarietyofargumentssupportthisresult, 87

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Juraetal. 1998 1993 )andcollisionaltimescales( Telescoetal. 2000 ),expecteduxesfromvaryingsizesofMiesilicatespheres( Wyattetal. 1999 ),SED-basedmodeling( Augereauetal. 1999 ; Debesetal. 2008 ; Juraetal. 1998 ; Li&Lunine 2003 ; Wahhajetal. 2005 ),andmodelingofthemorphologyseeninimagesoftheresolvedthermalemission( Koerneretal. 1998 ; Wahhajetal. 2005 ).AtleasttwoworksthatincorporateSEDmodelingcitetheneedfortheparticlestobesignicantlyporous(P&0.6,vacuumfraction)toreplicatetheobservedemission,buttheparticlecomposition(e.g.,icyvs.refractory)inthemodelsvaries( Augereauetal. 1999 ; Li&Lunine 2003 ). Modelsofthedustparticlepopulation,likethoseoftheresolveddiskmorphology,oerconictingconclusionsregardingthepresenceofa\hot"zodiacalcomponentintheinnerregionoftheHR4796Adisk.DetailedmodelsoftheSEDandNIRandMIRimagesby Augereauetal. ( 1999 )requireapopulationofhighlyporouscomet-likegrainsat9AU,whichsupporttheinitialsuggestionofsuchacomponentbytheMIRimageanalysisof Koerneretal. ( 1998 ).Incontrast, Li&Lunine ( 2003 )modeltheMIR{submillimeterSEDexclusivelyandndnoevidencefordustwithinafewAUofthestar.Theauthorsclaimthatpreviousmodelscouldhavereproducedtheexcessemissionat12mwithoutthezodiacalcomponentbyconsideringabroaderdistributionofparticlesizes(inthecaseofKoerneretal.1998)orsizedistributionsotherthanthestandardDohnanyipowerlaw(inthecaseofAugereauetal.1999). Wahhajetal. ( 2005 )presentthemostrecentworkfocusingonthermalemissionandagainassertthepresenceofasignicantpopulationofdustwithin10AUofthestarbasedonmodelsofboththeSEDandimagingdata.Theauthorssuggestthattheresultsof Li&Lunine ( 2003 )arerelativelyinsensitivetothedisputedparticlepopulation,becausethelattergroup'smodelsdonotincorporatetheresolvedimages. InrecentstudiesofHR4796AinscatteredNIRlight, Debesetal. ( 2008 )presentestimatesofthescatteringeciencyasafunctionofwavelength,asdeterminedfromthe 88

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6.2.1Data 6-1 .Theuncertaintiesreectonlymeasurementerror.Photometricuncertaintiesaredrivenprimarilybyvariableskytransmissionthroughoutthenight,andlackingmultiplestandardstarmeasurements,weadopttypicalphotometricuncertaintiesof15%forbothlters. Table6-1. PhotometryoftheHR4796ADisk 18.111067481058724.533074733327447

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Juraetal. ( 1998 )byextrapolatingtheK-band(2.2m)uxdensityusinga1:88powerlawasgivenbythe Kurucz ( 1979 )modelatmospherewithT=9500Kandlogg=4.0.PhotosphericvaluesatlongerwavelengthswereestimatedwiththeassumptionthatHR4796AandVegahavethesameslopetotheirphotosphericSEDsatMIRwavelengths,becausetheyarebothA0-typemain-sequencestars( Koerneretal. 1998 ; Telescoetal. 2000 ). ThePSFwasscaledtothephotospherelevelanditscenterwasshiftedtothepositionassumedforthestarinthetargetsourceimages.Forthe18.3-mimage,theadoptedstellarpositionwasthecentroidpeakofthe2-pixel-smoothedcentrallobeofemission(whichweassumearisesfromthepresenceofthestar)betweentheNEandSWansae,whosepeakswerealsodeterminedfromacentroidmeasurementofthe2-pixel-smoothedimage.Thestellarcentroidpeakpositioninthe18.1-mimageisdisplacedfromtheactualmidpointbetweentheNEandSWpeaksofemissionby1pixel.Onepixelinthe18.1-mimagecorrespondsto0.1005",or7AU.Forthe24.5-mimage,thecentralstarpositionisnotapparent,andsothemidpointbetweenthetwocentroidpeaksoftheNEandSWansaewasadoptedasthestellarposition.ThedistancefromtheNEpeaktotheSWpeakis17.3pixels(1.74")at18.1mand18.2pixels(1.62")at24.5m. ThescaledPSFimagesfromeachbandpasswerethensubtractedfromtheimagesofHR4796A.Thetotalphotosphere-subtracteduxdensityofthesourcewasmeasuredateachwavelength,andtheresultsaregiveninTable 6-1 Juraetal. 1993 ).Thecolortemperature 90

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Top:18.1-mimageconvolvedwitha2-Dgaussianprolewithawidthcorrespondingtothe24.5-mPSFFWHM.Bottom:24.5-mimageconvolvedwitha2-Dgaussianfunctionwithawidthcorrespondingtothe18.1-mPSFFWHM.Notethatthe18.1-mimagewastakenwithMichelle(platescale:0.1005"/pixel)andthe24.5-mimagewastakenwithT-ReCS(platescale:0.089"/pixel).Theimageshavebeenscaledtoshowthesameareainsquarearcseconds,butthenumberofpixelsineachimageisnotthesame. ofthedustasafunctionofdiskradiuswasalsoestimatedtoexamineanypotentialtrends.Thetechniquesusedforthisestimationaredescribedbelow. Foreasyinspection,theimagesofHR4796Awererotated.Thepositionangle(PA),eastofnorth,wasestimatedastheanglebetweentheverticalaxisandthelineconnectingthetwocentralpeaksofemissionofthetwoansae:28.1inthe18.1-mimageand29.9

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BrightnessproleoftheHR4796Adiskat18.1m(squares)and24.5m(circles).Measurementuncertaintiesarewithinthesizeofthedatapoints.Photometricuncertaintiesareassumedtobe15%ofthemeasureduxdensity. inthe24.5-mimage.WeadoptaPAvalueof29,theaverageofthetwovalues.Thephotosphere-subtractedimageswererotatedcounter-clockwisebyanangleof61(90minusthePAofthediskEofN),toorientthediskplaneparalleltothex-axis. Aftertheremovalofthephotosphericcontribution,theangularresolutionofeachimagewasdegradedtoachievethesameresolutioninbothimagesforaccuratespatialcomparison.UsingthegaussroutineinIRAF,eachofthetargetimageswasconvolvedwithagaussianprolehavingthesameFWHMasthePSFfromtheotherbandpass.ThePSFFWHMwasestimatedbyagaussianttotheazimuthallyaveragedprole.TheFWHMvalueswere5.19pixels(0.52")inthe18.1-mimageand8.08pixels(0.72")inthe24.5-mimage.ThePSFimagesthemselveswereconvolvedinthesamemanner,whichconrmedthattheresultingresolution(estimatedbyFWHMmeasurements)was 92

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ColorproleoftheHR4796Adisk,basedonthe18.1mand24.5mbrightnessproles.Combinedmeasurementuncertaintiesfromthetwobrightnessprolesarewithinthesizeofthedatapoints. identicalforbothimages.Thenalresolutionofthecross-convolvedimagesis0.9",whichcorrespondsto60AUforthesourcedistanceof67pc. Theimageswerecroppedtoaswathofy3"alongthex-axis(thex-axisisparalleltothediskelongation).Thepixelvaluesintheseimagesweresummedalongthey-axis,resultinginaone-dimensionalbrightnessproleforthediskateachwavelength(Figure 6-4 ).Fromthesebrightnessproles,acolorprole(Figure 6-5 )andacolortemperatureprole(Figure 6-6 )wereconstructedtoillustratevariationsintemperaturealongtheextentofthedisk.Asatest,the24.5-mbrightnessprolewasshiftedby1pixel(6AU)ineitherdirectionalongthex-axisrelativetothe18.1-mbrightnessprolebeforethetwobrightnessproleswerecombinedtodeterminethecolorprole.Thetworesultingcolorprolesthatincorporatedthisosetyieldedcolortemperatureprolesthat 93

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ColortemperatureproleoftheHR4796Adiskbasedonphotometricmeasurementsat18.1mand24.5m.TheNEproleisrepresentedbyredcircles,andtheSWproleisrepresentedbybluesquares.Combinedmeasurementuncertaintiesfrombothbandpassesaresmallerthanthesizeofthedatapoints. didnotdierfromtheresultspresentedinFigure 6-6 within60AUofthestarbymorethan2K. TheNEandSWansaeproleareoverplottedonthesameradiusscale(x-axis)tobettershowdierencesintemperatureatlikedistancesfromthecentralstar.Weseenosignicantvariationincolortemperaturewithin20AUofthestar.Thisisasmallerspatialregionofconstanttemperaturethanmightbeexpectedwiththepriorknowledgethatthedustdensitypeaksinanannulusat70AU.However,weexpectthatthisresultisattributabletothedegradationofresolutionthatwasnecessarilyperformedtocarryoutthesecalculations.Thiseectisalsoseenintheseparationofthetwoansaepeaks,whichdecreasedforbothimagesfollowingthecross-convolution,from1.74"to1.1"at18.1m, 94

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Thereisacleartemperatureasymmetry,inwhichthedustintheNEsideofthediskhassignicantlyhighercolortemperaturesatgreaterthan30AUfromthestar.Thetemperaturedierencepeaksat9Knear55AU.Theuncertaintiesduetomeasurementerrorinthetemperatureestimatesarelessthan2K.Thecolortemperatureuncertaintiesthatincorporatephotometricuncertaintiesare8K,butinthisanalysiswecomparetherelativetemperaturesofthetwoansaeandarethereforemostconcernedwithuncertaintieswithintheimagesandnotabsolutephotometricuncertainties. Wehaveexaminedtheeectofover-andunder-subtractionofthephotosphericuxdensityfromthediskimagebyperformingthefollowingtest.ThepeakuxdensityvalueofthePSFscaledtothephotospherewasmeasured,1.4mJyat18.1mand0.34mJyat24.5m,andwecalculatedthephotosphericuxdensitythatwouldbeyieldedifthisvalue(thepeak)weresustainedacrossthespanoftheAirydiskofthePSF(10pixels).Usingthebrightnessprolesofthephotosphere-subtractedimages,thecoloratthecenterofthediskproleis0.33.IfthetwoMIRbrightnesslevelsareassumedtohavetheoverestimateduncertaintiesdescribedabove(i.e.,suchthatthepeakbrightnessvalueoccursthroughouttheAirydisk),thenthecolorwouldhaveanassociateduncertaintyof0.05.However,evenifthephotosphere-scaledPSFwereover-orunder-subtracted,theeectontheestimatedcolorwouldbenegligiblebeyondtheboundsoftheAirydisk,whichcorrespondsto35AUindiskradius.Indeed,asignicantcolorasymmetryisstillapparentatradiigreaterthan35AU,andsoourconclusionofatemperatureasymmetryremainsunaected. Telescoetal. 2005 ).ThePictoris\clump"hasaclearwavelengthdependence,andthetemperatureestimatesfortheparticlepopulationinthatregion 95

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Neworbitsofthefragmentsofacollisioninwhichalargeparentparticle\P"thatwasonacircularorbitaroundastar\S"wasbrokenup.Fragmentsofdierentsizeshavedierentradiationpressureforces,characterizedbyaparticle's,actingonthem,andsohavedierentorbits:thosewith<0.1,the\large"particles,haveorbitsthatareclosetothatoftheparent;thosewith0.1<<0.5,the\critical"particles,haveorbitsthathavethesamepericenterdistanceastheparentbutlargerapocenterdistances;andthosewith>0.5,the\meteoroids,"havehyperbolicorbits.Credit: Wyattetal. ( 1999 ) aresignicantlyhigher(by50K)thantheassumedducialpopulationelsewhereinthedisk.Theimplicationforthedustpopulationinthisregionisthatithasfundamentallydierentproperties,eitherinmorphologicaltraitssuchassizeorporosity,orinchemicalcomposition. Telescoetal. ( 2005 )proposethatsuchabrightnessasymmetry,inwhichthedustcharacteristicsvarysubstantiallyfromtherestofthedisk,maybeindicativeofarecentcatastrophiccollision. TherelativebrightnessasymmetryinHR4796Aisalsowavelengthdependent,whichisobservedinboththebrightnessandtemperatureprolesofthetwoansae. 96

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6-7 .Instead,theentiredustannulusofHR4796A(especiallyasseeninNIRimages)appearstoberelativelynarrowandwell-constrained,apparently17AUinannuluswidth( Schneideretal. 1999 ).Additionally,theparticlesresultingfromacataclysmiccollisionwill\see"dierentmassesforthecentralstaraccordingtotheirvalue,whichwillresultinarangeofsemi-majoraxesandorbitalperiods. Krivovetal. ( 2007 )and Kenyon&Bromley ( 2005 )thereforesuggestthatanasymmetryresultingfromacataclysmiccollisionshouldbesmoothedoutduetoKeplerianshearwithin102orbitalperiods,or104{105years,andsotheprobabilityofobservingthediskwithinthistimeframeislow. Anestimateofthecollisionrateforagivensizeofplanetesimalcanalsobefoldedintotheconsiderationofthelikelihoodofthedustpopulationbeingattributedtoasinglecatastrophiccollision.Ifthecollisionistheresultofthecollisionoftwocomparableparentbodies,thenweestimatethattheyare2000kminradius(usingthemassofemittingdustestimatedtobe0.01Mlaterinthissection).Equationsfrom Kessler ( 1981 )wereadaptedby Wyattetal. ( 1999 )fortheHR4796Adisk,andthecollisionallifetimeforagivenplanetesimalsizeisestimatedas whereDismeasuredinkilometers.A2000-kmbodyhasacollisionallifetimeofapproximately180Myr,whichismuchlongerthanthelifetimeofthesystemitself. 97

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Face-onviewofthesurfacedensitydistributionofdierentsizes(characterizedbytheparameter)createdinthecollisionaldestructionofplanetesimalswitheccentricities0.28trappedina3:2resonancewitha30Mplanetthatmigrated45{60AUfroma2.5-Mstar.Credit: Wyatt ( 2003 ) Therefore,itisimprobablethatbodiesofthesizerequiredtoproducetheMIR-emittingdustinasinglecollisionhaveyettoexperiencesuchacollision. Pericenterglowisbeingreviewedasaviabletheoryforexplainingthebrightnessandtemperatureasymmetryseeninthissetofimages(asmentionedpreviouslyinx ).Ifthecenterofthediskwereindeedosetfromthepositionofthestar,thentheportionofthediskclosertothestarwouldbepreferentiallyheatedandcouldproduceawarmerregionofdust,whichwouldproduceawavelength-dependentasymmetry.However,astrometricmeasurementsbasedonhigh-resolutionNIRimages(fromHSTNICMOS)maybeabletorevealthedegreetowhichthediskcenteris(orisnot)osetfromthestar.Throughdynamicalmodels(afterthisproject),wewillattempttosetlimitsontheminimumosetrequiredtogeneratetheobservedasymmetryandcomparethislimittotheobservedastrometricresults.Theosetlimitdeterminedbythemodelswillbealowerboundduetothesimplest-caseassumptionthatweareviewingthediskperpendiculartothedirectionoftheforcedpericenter.Iftheviewingangleislessthan90totheaxisofoset,thenthetrueosetwouldneedtobegreaterbyafactorof1/cosioftheangletoproducetheosetseeninprojection. Diskasymmetriesmayalsoresultfromdustproducedbycollisionsofplanetesimalsthataretrappedinresonancebyalargerplanet(e.g.,Holmesetal.2003;Wyatt2003).Thediskstructuresthatresultfromthesecollisionsdieraccordingtothevalueofthedustproduced.Forexample,thelargestparticles(lowvalues)mayremaintrappedin 98

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6-8 ). DusttrappedinorbitalmeanmotionresonancesmayalsohavedriftedinwardtoitscurrentlocationbyP-Rdrag.However,asymmetriesarisingfromdustpopulationsthathavemigratedintoaresonancemaybeerasediftheparticledensityishighenoughfordestructivecollisionstooccurfrequently.Inahighdensityenvironment,collisionswillconverttherelativelylargeresonantlytrappedparticlesintosmallerparticlesthataresubjecttoblowoutbyradiationpressure. Krivovetal. ( 2007 )estimatethatasymmetriesduetodustthathasdriftedinwardandissubsequentlytrappedshouldnotbeseeninsystemswithopticaldepthsgreaterthan104{105.Therefore,iftheasymmetryinHR4796A(LIR/L?=5x103)isattributabletoresonanttrapping,thenitismorelikelytoresultfromtrappingofplanetesimalsexperiencingongoingcollisionsthanfromdustthathasmigratedintotheresonance. Weconsidersomeoftheplanetesimalpopulationsinourownsolarsystemthatarethoughttoberesonantlytrappedasaresultofplanetarymigration,suchastheTrojanasteroidsina1:1resonancewithJupiter( Morbidellietal. 2005 ),andthePlutinos,whichareKuiperBeltObjects(includingPluto)ina3:2resonancewithNeptune( Malhotra 1995 ).TheKuiperBeltispossiblyamoreaptanalogythantheasteroidbeltformostextrasolarsystems,becausedustassociatedwithitisatleastanorderofmagnitudemoreluminousthanthatassociatedwiththeasteroidbelt,anditisthereforemorelikelytobeobservedrstinsystemsotherthanourown( Moro-Martn&Malhotra 2003 ; Stern 1996 ). WealsoexpectthatthecompositionofdustinHR4796AshouldbemorecomparabletodustfoundintheKuiperBeltthantoasteroidaldust,becauseboththeKuiperBeltandthe70-AUannulusofdustinHR4796Afalloutsidetheirrespectivesnowlines.TheannulusofdustinHR4796Aisslightlylargerinradius(70AU)thantheKuiperBelt(40AU),andthesnowlinefallsfartherfromanA-typestar,at20AU,thanfrom 99

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Hayashi 1981 ; Sasselov&Lecar 2000 ). ToexaminethequestionofwhetheraKuiperBeltanalogandanypotentialresonancesthereinmightberesponsiblefortheobserveddustbrightnessandmorphologyinHR4796A,wehavemadesomesimpleestimatesforthemassoftheparentbodiesinthesystem.AminimumemittingdustmasswasestimatedwiththeassumptionthatalloftheparticlesareofradiushaiandtheylieatadistanceD(70AU)fromthestar, 3LIR wheretheparticleshaveadensitys,andthefractionalIRluminosityofthesystemLIR=L?isknown.Weadoptatypicalparticlesizeof2.5m,basedonthemodelsof Wyattetal. ( 1999 )and Telescoetal. ( 2000 ),andaparticledensityof2.5g/cm3.ThefractionalIRluminosityof5x103isknownfromIRASobservations(12{100-mcoverage)( Jura 1991 ).Thesevaluesyieldamassestimateof5.8x1025g,or0.01M. Thisestimateisforthethermallyemittingdustpopulation,however,andthedominantsourceofthethermalemissionisparticlessmallerthanafewmicrons.Forapopulationincollisionalequilibriumwitha Dohnanyi ( 1969 )distribution(n(a)da/a3:5),thebulkofthetotalmassinthesystemiscontainedinbodiesthatarelargerandcoolerthanthosethatproducethermalemission.Thus,weadoptanestimateforthetotalcurrentmassbasedonsubmillimeteruxdensityobservations,whichislikelytobeamoreaccurateassessment.Using800-mphotometricdata, Juraetal. ( 1995 )concludesthatthetotaldustmassis0.1{1M. Ifsteady-statecollisionalgrindingisresponsibleforproducingtheobservedemittingdustmass,wecanextrapolateaparentbodymassestimatebyassumingthatthedustseencurrentlycouldbeobservedatanypointintimeinthesystem'spast.Theestimatedtypicalparticleradiusof2.5mhasa>0:5,whichmeansthattheparticleswillbe 100

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Thesubmillimeter-observation-basedestimateforthecurrenttotalmassmaybemoreaccuratethantheIRAS-observation-basedestimate,butthisisnotasstraightforwardtoextrapolateforthelifetimeofthesystemsinceitisunlikelythatmuchofthesubmillimeter-emittingpopulationhas>0:5andisblownoutbyradiationforces.Itwouldthereforebeinappropriatetoapplythepreviouslydescribedcorrectionfactor(10{100),whichwouldyieldparentbodymassestimatesofover2000M.Nonetheless,arealisticparentbodymassestimateprobablyliesbetweenthetwoestimatesdescribedabove,between200Mand2,000{20,000M,or4{2,000timestheamountofmasspredictedfortheprimordialKuiperBeltpopulation(10{50M)( Stern 1996 ). Wyattetal. ( 1999 )estimateahighervaluefortheparentbodypopulation(7x104M)byinvokingamass-lossratethatisproportionaltothemassofthediskthroughoutitshistory. Thesemassestimatescoveralargerange,andsoasamoregeneralcomparison,wealsocomparethemtotherangeofmassestimatesfortheamountofmaterialrequiredtoproducethetotalplanetarymassinourownsolarsystem.Thisisreferredtoastheminimummasssolarnebula(MMSN),whichis0.01{0.1M,or3,300{33,000M( Weidenschilling 1977 ).OurparentbodymassestimatesforHR4796AdofallwithintheboundsoftheMMSNestimates,andtheywouldcompriseasignicantfractionoftheMMSNanalog.ItisatleastreasonablethatsuchaplanetesimalbeltcouldbepresentaroundHR4796A,butattheupperboundsofthemassestimate,itmaybeunlikely 101

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102

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),andthesourcemeasurementsandtheirsignicancesaregivenanddiscussedinx Thephotometricmeasurements,whencombinedwithestimatesofthephotosphericcontribution,allowconrmationthatthereisexcessemissionlikelyattributabletodust.Indeed,forsomeofthesourcesthatareknowntohave24-mexcessemission,wedonotobserveexcessemissioninthe10-and/or20-mwindows.Thesecasesarediscussedspecicallyinx .Wecanfurtherconrmthattheexcessemission(whenpresent)isspatiallycoincidentwiththestaranddoesnotoriginatefromabackgroundobject.Thesesourceswerechosenbasedonspace-basedobservationsoftheirinfraredexcess,andthelowerresolutionofthoseimages(duetothe10xsmallerprimarymirror)ismorepronetoconfusionwithinthebeam. Inx ,weconsiderwhethertheimplicationsforthelocationofthedustfromphotometricmeasurementsandspatialextentmeasurementspresentaconsistentpictureforeachofthesources. 103

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ExcessEmissionofUnresolvedDebrisDiskCandidates(Michelle) HDTotalStarExcessTotalStarExcess 565371432143115427814359791440157918380836143612985628361156723511384292359541835883593650-623591743262139235126210264758225825297525582231734820202973481390064077408392415340817952621496299262141569338345628234883147228611471618681105111106441111443724063772 3-4 7.2.1DustColorTemperaturesDerivedfromIRExcessLevels Cutrietal. 2003 )to10m.Theuxdensitywasassumedtovaryas1:88overthiswavelengthrange,asisestimatedby Kurucz ( 1979 )tobeappropriateforanA0star(e.g.,Juraetal.1998).Beyond10m,thestandardrelation(2)fortheRayleigh-Jeanstailwasused.ThephotosphericuxdensityestimateandthecorrespondingexcessuxdensityestimatearegivenforeachsourceinTables 7-1 & 7-2 .Asdiscussedinx ,photometricuncertaintiesof10%and15%areassumedforthe10-and18-mwindows,respectively. Thefollowingtermsaredened,foruseincharacterizationofthemeasuredIRexcesses: 3-4 )andthephotometricuncertaintyforacalibratedimage.Thisvalueisreferredtoasphotinthefollowingdiscussionofphotometricmeasurements. 104

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ExcessEmissionofUnresolvedDebrisDiskCandidates(T-ReCS) HDTotalStarExcessTotalStarExcessTotalStarExcess 38206202201693320.........11619546219711551083108815268108.........375572611145775416.........229231418823921558341580950191231622923.........1091952571911589225402542748-208254252125221553662521026155881145155172555.........11551165206351161094164213881164178253.........7707765511577360552689255181296.........3954027112440343541112325418186969570731-3670.........20234234-3334 3-4

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7-3 .AsdiscussedinChapter2(SourceSelection),thedebrisdiskcandidateswerechosenbasedonexcessemissionobservedat24or25m.Weestimatedtheexcessemissionat12and18mwithonlythe25-muxdensitymeasurementandthepriorknowledgeoftheMIRcolorsofawellstudieddebrisdisk,Pictoris.WeassumedthatthesourcesinthissamplehadthesamecolorsfortheirIRexcess(F[24m]/F[18m]andF[24m]/F[12m])asthisarchetypaldisk. However,itisnotsurprisingthatallofthedebrisdiskcandidatesinthesampledonotsharethesameexcesscolorsasPictoris.Pictorishasawiderangeofdusttemperaturesduetoitsrelativelybroaddisk(30{100AU;e.g.Telescoetal.2005)andyieldsdierentcolorsthan,forexample,amorewell-constrainedringofdustsuchasthatfoundinHR4796A( Moerchenetal. 2007a ; Telescoetal. 2000 ; Wahhajetal. 2005 ).AnIRexcessat24mmaybeassociatedwithwarmerdustemittingatshorterwavelengths(e.g.,10{20m),butinsomecasesthepredominantdusttemperatureiscool,andmostofthethedustemissionoccursatwavelengthslongerthan24m.OneexampleofadiskinthissamplethathostsprimarilycooldustisHD181869,whichwasrecentlyspatiallyresolvedat24and70mwithSpitzerMIPSimagesperformedby Suetal. ( 2008 ).Photometryandexcessemissionestimatesatthesewavelengthsdemonstrate 106

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SummaryofDebrisDiskCandidateIRExcessDetections HDN(10.4m)N0(11.2m)Si-5(11.7m)Qaa(18.1m)Qab(18.3m) 38206none.........signicant56537...none...none...71155marginal.........marginal75416......signicant...marginal80950none.........signicant83808...none...none...95418...none...none...102647...none...none...115892none...none...none139006...none...none...141569...signicant...signicant...161868...none...none...172555......signicant...signicant178253......none...none181296......signicant...signicant181869none.........none Thisvalueisreferredtoasextinthefollowingdiscussionofextensionmeasurements. 107

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2ext
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StatisticalSignicanceofExtendedSourceDetectionsat10{12Microns 10.4m11.2mFWHM[arcsec]FWHM[arcsec]HDSourcePSFN(ext)aSourcePSFN(ext) 711550.3480.3314.4.........95418.........0.3400.3286.8139006.........0.4190.3648.6141569.........0.4350.3743.7161868.........0.3860.3662.81818690.3780.3532.2......... StatisticalSignicanceofExtendedSourceDetectionsat18Microns 18.1m18.3mFWHM[arcsec]FWHM[arcsec]HDSourcePSFN(ext)SourcePSFN(ext) 565370.5450.5392.1.........75416.........0.8760.6483.71390060.5740.5562.2.........1415690.8070.5409.1.........0.8180.5234.5......... 7-4 & 7-5 TheplotsofFWHMpersub-imageforeachsourceandbandpassareshownintheappendix.UponinspectionoftheFWHMvaluesofsubsetimagesthroughoutthetotalintegrationtime(asdemonstratedinx ),manysourcesshowdegradingimagequalityoverthetimespanoftheobservation. 109

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whereB1(T)=B2(T)istheratiooftwopointsonthePlanckfunctionforatemperatureT,andQ1=Q2isunity.Thesearerelatedtothetwomeasureduxdensitiesfromthetwoobservedbandpasses(F1=F2)bytheemissionecienciesatthetwowavelengths,Q1andQ2.Iftheparticlesbehaveasblackbodies,thentheratioQ1=Q2isunity.Iftheparticlesaresmallenoughtobehaveasinecientemitters,thentheiremissioneciencyisfrequentlydescribedbytherelationQem/n,withn=1{2,andif1>2,thenQ1=Q2willbegreaterthanunity.Inthisnon-blackbodycase,theratioofthetwopointsonthePlanckfunctionB1(T)=B2(T)wouldhavetobelower,andthusoriginatefromalower-temperaturesource,inordertoproducethesameobserveduxdensityratios.Realdustparticlesaregenerallynotblackbodies,andthecolortemperatureisthereforeanoverestimate. Diskextent(rAU)isestimatedbyquadraticsubtractionofthePSFFWHMfromthesourceFWHM.TheresultingestimateforthedusttemperatureTd,forparticlesatthatdistancefromthestar,isgivenby 4?r1 2AU;(7{6) wherethestellarluminosityL?isinunitsofL,solarluminosity,rAUistheradiusofthedustannulusinAU,andtheequilibriumtemperatureat1AU(Earth)is278K.Uncertaintiesforthistemperatureestimatearecalculatedbypropagatingtheuncertaintyinthediskextent(rAU)throughEquation 7{6 .Thistemperatureestimateisalowerlimit,becausewehaveassumedthatthedustiswellapproximatedbyablackbodywithperfectabsorptionandemissioneciencies.Inreality,manysmallsubmicronparticlesareecientabsorbersbutinecientemittersandarethusheatedtohighertemperaturesthan 110

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DustRadiusLimitEstimates(inAU)forUnresolvedSourceswithIRExcess HDN(10.4m)N0(11.2m)Si-5(11.7m)Qaa(18.1m)Qab(18.3m) 38206............10.856537...2.2.........71155............6.275416......8.6......80950............8.1161868.........3.3...172555......1.4...3.1178253......2.4...7.5181296......2.3...17.6181869............8.9 7{7 Incaseswherewedetectexcessemissionbutitisnotspatiallyextended,weestimateatemperaturewithEquation 7{6 andthe2-extlimitfortheobservation.Thatis,weassumethatanyextensionofthesourceFWHMminusthePSFFWHMthatislessthan2extcouldescapedetection.Thus,weestimatetheupperlimitfordiskextentas 2q Thisestimatealsoyieldsalowerlimittothetruedusttemperature,becausethetemperaturewillincreaseif(1)thedustisanyclosertothestar,or(2)theparticlesaresmallenoughthattheydonotbehavelikeblackbodyemitters. HD38206{Thesourceisunresolvedatboth10.4mand18.3m.Thereisnoexcessemissiondetectedat10.4m.Weestimateatemperaturefortheunresolved18.3-m-emittingdustbycalculatingtheblackbodytemperatureattheorbitaldistance 111

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7{7 .Thistemperaturelimitis201K. 7-1 ).IfwebelievethattheSpitzerdetectionofexcessemissionisrealandthereisdustpresentassociatedwiththesource,thentherearetwopossibilitiesforwhywedonotdetectanyexcessat12or18m:(1)thedustisdiuseenoughthatwedonothavesucientsensitivitytodetectitabovethebackgroundnoise,or(2)thedustistoocooltoemitsignicantlyat12and18m.

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Wyattetal. ( 2007b )notesthatthefractionalIRluminosityofHD172555isanomalouslyhigh,or86xtheexpectedmaximumfractionalIRluminositybasedonsteady-stateevolutionmodels.Anestimateof6AUisgivenforthediskradius,basedon24mand70mSpitzerphotometry;thisradiuscorrespondstoablackbodyequilibriumtemperatureof200K.Thistemperatureis75KlowerthanthecolortemperatureestimatebasedonMIRcolor,butitispossiblethatthedustpresentinthediskspansarangeofradii,andthereforetemperatures,and(atleast)twodierentpopulationsofdustarebeingsampledbythe12m/18mcolorandthe24m/70mcolor. 113

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Sinceourinitialobservations,HD181296hasbeenresolvedinimagestakeninJuly2007atMIRwavelengthswithlongerintegrationtimes(Smith,Wyatt,Churcher,Moerchen&Telesco,submittedtoMNRAS)Itislikelythattheinitialobservationsdidnotresolvethesourcebecausetheintegrationtimewasnotsucienttodetectthesurfacebrightnessofthedisk. Smithetal. ( 2008 )discussesingreaterdetailtherelationshipbetweendiskmorphologyandthepredictedlengthofintegrationrequiredtoresolvedagivensource. Modelsofthe18.3-mimagesofthesourceindicatethat50%oftheexcessemissionoriginatesintheresolvedcomponentinanapparentlyedge-ondiskwitha24-AUradius,whichcanbetbyamodiedblackbodyoftemperature100K.Therestoftheexcessemissionresidesinanunresolvedcomponentoftemperature310K,consistentwithdustat3.9AU.Discrepanciesintemperatureestimatesbasedontheinitialandfollow-updatasetsexistprimarilybecauseofa13%loweruxdensitymeasurementat11.7m(fromthemorerecentimages);however,thesetwomeasurementsarestillwithin1ofeachother. HD56537{HD56537appearsmarginallyresolvedat18.1m,atasignicancelevelof2.1ext.ThediskextentisestimatedfromthequadraticsubtractionofthePSFFWHMfromthesourceFWHM.Thecorrespondingblackbodytemperatureatthediskradiusimpliedbythisextensionis4108K.Thistemperatureestimatebasedonextensionmeasurementsisconsistentwiththecolortemperatureforthissourceof787457K. HD56537isnotresolvedat11.2m.However,ifthe11.2-m-emittingdustisassumedtoliewithinaregioncorrespondingtothe2-extlimitofthesourceFWHM,thenthetemperatureestimatefordustatthatlocation(433K,alowerlimit)isalsoconsistentwiththeupperlimit(tothetruedusttemperature)ofthecolortemperature. 114

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Suetal. ( 2008 )recentlyresolvedthisdebrisdiskwithSpitzerMIPSimagesandestimatedadiskradiusof520AUfromthe24mimagesandadiskradiusof&260AUfromthe70mimages.Thecolortemperaturebasedonthe24mand70mphotometryis81K,whichoverestimatestheuxdensityat28{35mand55{65m,andtheauthorsnotethatthissuggestsarangeofdusttemperatures.Nonetheless,the81KblackbodytstheavailableSEDpointsreasonablywell.TheIRSspectrumisalsoshown,whichindicatesuxdensitiesof<100mJyatwavelengths<20m;thesespectralmeasurementsconrmthatourestimatedMIRexcesslevelsarewithinexpectations.GiventhatthecolortemperatureandtheIRSspectrum(andMIPSimages)indicatethepresenceofpredominantlycolddust,itisnotsurprisingthatwedonotndstrongevidenceofexcessemissionorresolvedspatialstructureinMIRimages. HD141569{WereviewHD141569rst,becauseitisknowntoshowspatialextentinMIRimages( Fisheretal. 2000 ; Marshetal. 2002 )andshouldthereforeprovideabenchmarkasaconsistentbodyofevidencethatdescribesthedebrisdisk. 115

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DustRadiusEstimates(inAU)forMarginallyResolvedandResolvedSources HDN(10.4m)N0(11.2m)Si-5(11.7m)Qaa(18.1m)Qab(18.3m) 56537.........2.4...711552.1............75416............28.595418...1.1.........139006...2.4...1.6...141569...11.0...30.4...161868...1.8.........1818693.5............ Marshetal. ( 2002 )forthisdisksuggestthatthedominantparticlepopulationiscomposedofinecientemittersthatwouldbeheatedtotemperaturesabovethoseexpectedforperfectblackbodyemitters,whichimpliesthatourextent-impliedtemperaturesshouldalsobehigher. ThespatiallyresolvednatureoftheHD141569diskisknownapriori.Nonetheless,thedusttemperaturesthatwereestimatedusingtwodierentmeasurementspresentaconsistentpicture,andthiscanbeconsideredatestcase. 116

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Backman&Paresce 1993 ).Intheliterature,anemissioneciencyoftheformQem/n,withn=1{2,isoftenassumed.Thecolortemperatureforsuchinecientemitterswithn=1is487129K,whichisconsistentwiththetemperatureimpliedbythe10.4-mextension. Theunresolved18.3-mresultisconsistentifthisemissionoriginatesfromthesamelocationasthe10.4-memission;thequadraticsumofthe10.4-m-basedsourcesizeandthe18.3-mPSFFWHMsizeislessthanthenominaldetectionlimitforextension,FHWMPSF+2ext.Iftheunresolved18.3-m-emittingdustisassumedtoliewithinaregioncorrespondingtothis2-extlimitofthesourceFWHM,thenitsequilibriumtemperatureisatleast281K,whichisalsoconsistentwiththeupperlimitsetbythecolortemperature. Toestimatethecolortemperatureof653272K,weassumedthattheparticlesbehaveasecientabsorbersandinecientemitters,becausethecolorcouldnotbettedbyasingle-temperatureperfectblackbody.Sincethecolortemperatureisanupperlimittothetruedusttemperature,thetemperatureestimatesbasedonspatialextensionmeasurementsareconsistentwithit. 117

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ThesameconsiderationappliesforHD139006aswasdiscussedaboveforHD75416andHD95418.Whilethephotometricuncertaintyextrenderstheobservedexcessemissionstatisticallyinsignicant,themeasurementsarewellwithin2photofaformalexcessdetection,sotheresultsareconsistent. 7-1 ,theMIRcolortemperatureisplottedagainsttheageforeachdebrisdiskcandidate.Theresolvedsourcesarerepresentedbystars,andtheunresolved(orpossiblymarginallyresolved)sourcesarerepresentedbylledcirclesandsquares(seegurecaptionfordetailedexplanationofsymbols). WiththeexceptionofLep,thesourcesthathavebeenresolvedwithground-basedMIRimagingobservationsappeartohavethecoolestcolortemperaturesforthesample.Weexpectcoolerdusttobemoredistantfromthestarandthereforebepartofamoreextendeddisk.Thus,thefactthatmostoftheresolvedsourcesexhibitrelativelycooldustisnotsurprising.InthecaseofLep,thestariscloseenoughatonly21pcaway(onlytwoothersourcesinthesamplearecloser)tospatiallyresolvethedisk. 118

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7-1 ,oneshouldbecarefulinmakinganyinferencesabouttrendswithage.Oursampleislimitedinnumber(8,afternon-disksourcesandnullexcesseshavebeenculled)andthesampleisbiased.Thesourceswerechosenonthebasisoftheir24-mexcess,soitisalreadyknownthattheyharboratleastsomewarmdust,althoughpossiblynothotenoughtoemitat12or18m,asthenulldetectionssuggest.Further,thereareagreaternumberofsourcesingeneralwithhighfractionalluminositiesatyoungerages,especiallylessthan20Myr,andoursample,whichwaschosenwithabrightnesscriterion,reectsthattrend.Therearethereforeagreaternumberofsourcesofyoungageswithuxdensitiesthatweselectedaslikelytobedetectablewithourground-basedobservations.Theclusterofsourcestowardtheleftoftheplot,atageslessthan100Myr,mustbeconsideredwiththesebiasesinmind. Althoughthesampleobservedforthisworkisnotstatisticallysignicant,wedoseeanapparentbimodalityintheplotteddistributionofagesagainstcolortemperatures.Intheclusterofsourcesatyoungages,wehaveresolvedthreesourcesthatarecomparableinextenttotheKuiperBelt,andallofthesesourceshaveMIRcolortemperaturesoflessthan300K.WiththeexceptionofHD75416(5Myr),thesourcesinoursamplewithsignicantlyhotterdustpopulationsarealsosignicantlyolder:HD38678(Lep)at230MyrandHD71155at200Myr.Lephasaninferreddustdiskradiusof3AU,andithasbeencomparedby Chen&Jura ( 2001 )and Moerchenetal. ( 2007b )totheasteroidbeltinourownsystem(seeChapter4forfurtherdiscussion).HD71155isanewspatiallyresolvedsourcewhosedustdiskradiusimpliedbyits10.4-mextentis2AU,andthissizeisalsocomparabletothesizeofthesolarsystem'sasteroidbelt. Whilefar-IRexcessemission(suchasthatoriginatingfromaKuiperBelt-typestructure)appearstobesustainableatgreaterdiskagesthanMIRexcess( Riekeetal. 2005 ; Suetal. 2006 ),theobservationofapparentasteroid-beltanalogsinoursamplesuggeststhattheolder(>few100Myr)sourcesthatdohostMIR-emittingdustmaybeproducingthisdustinanasteroid-belt-typecollectionofplanetesimalsrelativelyclose 119

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MIRcolortemperatureofdustagainstsystemage.Agevaluesaretheaverageofallestimatesquotedby Riekeetal. ( 2005 ),withtheexceptionsofHR4796A(8Myr)andHD38678(230Myr).Sourcesrepresentedbylledcirclesarestandardcolortemperatureestimatesforunresolvedsources.Sourcesrepresentedbystar-shapedpoints(blue)havebeenspatiallyresolvedbyMIRimagesfromthisstudy(andinthecaseofHD141569andHR4796A,alsopriorworks).Forreference,Pichasanageof12Myrandacolortemperatureof180K. tothestar.Whetherthecollisionshavebeenoccurringinasteadystateisnotobvious,butthelevelofIRexcessrelativetothephotospheremayhelptoanswerthatquestion.Forexample,the24-mexcesslevelofLepfallsabovethemaximumexcessexpectedifcollisionshadbeenoccurringinasteadystate,buttheexcesslevelforHD71155fallswithintheenvelopeofexpectedvaluesforsteady-statecollisions(Figure 1-6 )( Wyattetal. 2007b ).Therefore,itmaybemoreplausibletoinvokeaprocesssuchasdelayed 120

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Withtheadditionofmoreresolvedsourcestothealready-knownsample,wemaybeabletodiscernwhetherthereabimodaldistributionofasteroidbeltanalogsandKuiperBeltanalogscontinuestoexist,butweremaincautiousregardingthisresultduetothesmallnumberofsourcesinthesample.ItisworthnotingthatthefractionalIRluminositiesofthesourcesthatweconsidertobeKuiperBeltanalogs(HD32297,HR4796A;LIR=L?103)are100timeshigherthanthatofthesourcesthatweconsidertobeasteroidbeltanalogs(HD38678[Lep],HD71155;LIR=L?105).Thus,inasystemthathostsbothasteroid-belt-likeandKuiper-Belt-likestructures,thepresenceofaKuiperBeltislikelytoimpairinitialdetectionofanasteroidbelt(seealsoLiou&Zook1999). FortheremainingunresolvedsourcesthatsustainstatisticallysignicantIRexcesses,whatcanthepresenceofthewarmdusttellus?Ultimately,wewouldliketoknowthedistributionofthedustbothradiallyandazimuthallyinordertoinvestigatetheplanetarysystem'sarchitecture:theproductionofthedustandwhatmaintainsitinitscurrentlocation,suchasthepresenceofshepherdingplanets.Theadditionofotherknowncharacteristicsofthediskcancontributetoamoremeaningfuloverallpicture.Forexample,thereare(asofthetimeofwriting)approximately10disksthatareknowntoharborplanets(e.g.,Wyatt2008).Itiscurrentlyeasiertomakeradialvelocityplanetdetections(currentlytheprimarydetectiontechnique)aroundFGK-typestars,whileit ( 2007a )setthecutofordeterminingthetransienceofdust-producingeventsatdisksthathavegreaterthan1000timesthemaximumfractionalIRluminositypredictedfortheirageiftheyhaveevolvedwithsteady-statecollisions. 121

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Ifinformationabouttheplanetaryorbitsisknown,however,dynamicalsimulationscanbeperformedthatmayindicatewherethedustislikelytobestable,whichfurthersthestudyofhowandwherethedustwasinitiallyproduced.SuchsimulationshavebeenmadefortheK-stardebrisdiskHD69830,whichsustainsasurprisinglyhighamountofdustforits4{10GyrageinadditiontothreeNeptune-massplanets( Beichmanetal. 2005 ; Lisseetal. 2007 ; Lovisetal. 2006 ). Lovisetal. ( 2006 )showedthat,giventhelocationsoftheplanetsasdeterminedbyradialvelocitymeasurements,therearetwostableradiifordustannuli. Ifsimilarstudiesareperformedformanymoresystemslikethisasthenumberofknowndisk+planetsystemsincreases,thenourinterpretationsoftheapparentpresenceofhotdustinourunresolvedsystemsarelikelytobecomemorerened.Intheabsenceofadditionalinformation,though,wemaydrawthesimpleanalogytotherelativelywarmdustinourownsystem,generatedbycollisionsintheasteroidbelt.Futurespectroscopicobservationsofthesesourcesmayalsoassistindescriptionsofthesedebrisdisksasextrasolarasteroidbelts,bycomparingtheirparticlepopulationstothoseofmorewell-knownorigins(e.g.,dierenttypesofasteroids)inourownsolarsystem. 122

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andSnowLine intheDebrisRing 124

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FurtherdebrisdiskimagingintheIRat8-meter-classtelescopeswillcontinuetoyieldadditionalresolvedsources,especiallyaswereneourobservingtechniques.Extensivephotometricgroundworkhasbeenachievedbythespace-basedobservationsofSpitzer,whosecryogensupplyisexpectedtorunoutin2009.Overitslifetime,theobservatoryhasculledmanycandidatesourcesthatcanbefruitfullypursuedfromtheground.Wewillalsogainanewground-basedfacilityfordiskobservinginthespringof2009,theGranTelescopioCanarias,a10.4-metertelescopewhoserstmountedscienceinstrumentwilllikelybeCanariCam,themulti-modeMIRcamerabuiltattheUniversityofFlorida.CanariCamperformsimaging,spectrometry,polarimetry,andcoronagraphicobservations,andweespeciallylookforwardtoexploringtheuniquepolarimetryandcoronagraphicmodesinourcontinuedinvestigationsofdebrisdisks.Observationstakeninthespectroscopicmodewillbetterinformourknowledgeofthedustcompositionandmayconrmthenatureofdiskfeaturessuchassnowlines.Wehavesofarnotemployedspectrometryinourdiskstudies,butsuchobservationsareplannedforthenearfuture,andindeedoneprogramisalreadyinthequeueatGemini. Inthemoredistantfuture,weeagerlyanticipatethefactorof3{5improvementinspatialresolutionthatwillbeaordedbythenextgenerationofground-basedtelescopes,theThirtyMeterTelescope(30m)andtheEuropeanExtremelyLargeTelescope(42m).Suchresolution(andsensitivity)capabilitieswillallowustoprobealready-resolveddiskstructuresingreaterdetailandwillpresumablyunlockalargesampleofdisksthatareonthevergeofbeingspatiallyresolvedwithcurrentfacilities.Withanincreasein 126

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HereweprovidethedetailsoftheFWHMmeasurementsofthedebrisdiskcandidatesandtheircorrespondingPSFreferencestars.Asdiscussedinx ,thetotalintegrationtimeforanimagewasbrokenupintosub-imageseachcorrespondingtoafractionofthetotaltime,suchthattheFWHMofthesourcecouldbesampledasfrequentlyaspossible. WhenS/Nlevelsallowed,thesmallestunitoftimeforasub-imagewasthatcorrespondingtoasaveset,10s.Asavesetisastackofchoppedimages(on-ando-source),andtherearetypicallythreesavesetspernodposition.Forformalandnalimagestacking,imagesfrombothnodpositionsmustbecombinedtoremovetheradiativeoset.However,theS/Nofthesourceswashighenoughthattheradiativeosetdidnotaecttheproletstothesources.MeasuringtheFWHMinsinglesavesetshadtheadditionalbenetofnotincorporatingpositionalerrorsarisingfromtelescopemotion.Whenimagesaretakenattwonodpositions,weexpectthatthesourcelocationisthesameinbothimages.However,theremaybeaslightpositionalinaccuracyoccurringbetweeneachnodswitch,andthisisavoidedbynotcombiningimagesfromtwonodpositions. WhentheS/Nlevelswerenothighenoughtoperformareasonableprolettothesourceinasinglesaveset,theseframeswerebinnedupuntilasucientS/Nlevelwasreached.Inthefollowingplots,thetotalnumberofsavesetsisshownasatemporalseriesalongthex-axis.IftheFWHMwasmeasuredineachsavesetimage,thenthenumberofdatapointsequalsthenumberofsavesets.If,forexample,sixsavesetshadtobebinnedforaFWHMmeasurement,thentherewillonlybeonedatapointforeverysixsavesets,andthedatapointwillbeshownatthecenterofthebinnedsavesetgroup,e.g.,savesets1{6arebinned,sotheFWHMvalueisplottedabovethe\saveset#3"tickmark. 128

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MargaretMoerchenwasbornonLaborDayin1979inSanAntonio,Texas,andlivedtherefor18years.In1997,shemoved70milesnorthtoAustin,toattendtheUniversityofTexasonaNationalMeritScholarship,andgraduatedmagnacumlaudewithaBachelorofArtsdegreeinAstronomyinMay2001.SheworkedasaserverandpastrychefatanitaliantrattoriainAustinforsixmonthsbeforereturningtotheastronomydepartmentattheUniversityofTexas,wheresheworkedwithDr.JohnLacyonamid-IRinstrumentdesignforamairborneobservatory.Afteranotheryearandahalfinbothofthesepursuits,shebegangraduateschoolattheUniversityofFloridainGainesvilleinAugust2003.ShereceivedaMaster'sdegreeinAstronomyinMay2005.HerdissertationresearchwassupportedbyaMichelsonFellowship,andshecompletedthePh.D.inAugust2008,withDr.CharlesTelescoasheradvisor.SheismovingtoSantiagodeChileinSeptember2008,whereshewillcontinueherresearchasapostdoctoralfellowattheEuropeanSouthernObservatory,spendingthreeyearsinChileandoneinEurope.Shewillbeaccompaniedbyherpartner,JohnRobertson,whomshemetatatoystoreinAustinin2002. 161