Signatures of planets in the observable structure of circumstellar debris disks PDF
Preview Signatures of planets in the observable structure of circumstellar debris disks
SIGNATURESOFPLANETSINTHEOBSERVABLESTRUCTUREOF CIRCUMSTELLARDEBRISDISKS By MARKC.WYATT ADISSERTATION PRESENTEDTOTHEGRADUATESCHOOL OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT OFTHEREQUIREMENTSFORTHEDEGREEOF DOCTOROFPHILOSOPHY UNIVERSITYOFFLORIDA 1999 ACKNOWLEDGMENTS ThefirstpersonIwouldliketoacknowledgeismysupervisor,StanDermott, notonlyforhisadvice,encouragement,andinfluenceonmywork,butalsoforenticing — meovertoFloridainthefirstplace ithasbeenatremendousexperience. EquallyI wouldliketothankmysupervisoratQueenMaryandWestfieldCollege,CarlMurray, forinspiringmetocontinuewithacareerinAstronomy. Thanksarealsoduetothe FulbrightcommissionforsupportingmeinmyfirstyearinFlorida. ThisdissertationisessentiallyanextensionoftheworkthatStanDermottand hisSolarSystemDynamicsgrouphaveaccomplishedoverthepast15orsoyearsin theireffortstomodelthezodiacalcloud. Itisfairtosaythatthisdissertationcould nothavebeenachievedwithoutthisfoundation. Iwishtothankallmembersofthe group,pastandpresent,especiallyKeithGrogan,forcountlessusefuldiscussions. AnotherpersontowhomIamindebtedisCharlieTelesco,bothforhisdirect inputtothedissertationthroughourworkontheHR4796disk,andforproviding mewiththeopportunitytogoobservingwithhisInfraredAstrophysicsgroup. The perspectiveIgainedfromthisobservingexperiencehasbeenanessentialpartofmy Ph.D. education. IacknowledgeallofthemembersoftheOSCIRobservingteam, especiallyRobertPinaandScottFisher,fortakingthetimetoteachmetheobserving lore. Onapersonallevel,Iwouldliketothankmyparentsfornourishingthegift oflifeinme. Mostofall,Iwanttothankmyfiancee, Maxine, withwhomIhave u sharedthelast8years,andwhoseloveandsupportthroughoutIcouldnothavelived without. Thisdissertationisdedicatedtoher. TABLEOFCONTENTS page ACKNOWLEDGMENTS ii ABSTRACT vii CHAPTERS 1 INTRODUCTION 1 1.1 Motivation 1 1.2 TheSearchforExtrasolarPlanets 2 1.3 ParadigmofStarFormation 4 1.4 ParadigmofPlanetaryFormation 7 1.5 CircumstellarDebrisDisks 10 1.6 TheZodiacalCloud 15 1.7 AimsandOutline 19 2 DISSECTIONOFACIRCUMSTELLARDISKOBSERVATION ... 21 2.1 DiskParticleThermalEmission 21 2.2 DiskStructureDefinition 22 2.3 LineofSightBrightness 23 2.4 ObservingProcedure: RealImages 24 2.5 ModelingDiskImages 26 3 THESTRUCTUREOFADYNAMICDEBRISDISK 28 3.1 Gravity 29 3.2 Collisions 31 3.2.1 CollisionalCascade 31 3.2.2 CollisionalLifetime 32 3.2.3 EvidencefromtheZodiacalCloud 36 3.3 RadiationForces,(3 38 3.3.1 RadiationPressure 38 3.3.2 Poynting-Robertson(P-R)LightDrag 42 3.3.3 EvidencefromtheZodiacalCloud 47 3.4 DiskParticleCategories 49 3.4.1 CategoryDefinitions 49 3.4.2 CategoryCross-SectionalArea 51 3.5 DebrisDiskEvolution 53 3.6 ThePerturbedDynamicDisk 56 IV 4 SIGNATURESOFPLANETARYPERTURBATIONS 57 4.1 SecularPerturbations 58 4.1.1 PerturbationEquations 58 4.1.2 SolutiontoPerturbationEquations 60 4.1.3 OffsetandPlaneofSymmetryofFamilyMaterial .... 64 4.1.4 OffsetandWarpofWholeDisk 70 4.1.5 PhysicalUnderstandingofOffsetandWarp 71 4.1.6 ObservationalEvidenceofOffsetandWarpintheZodiacal Cloud 72 4.2 ResonantPerturbations 75 4.2.1 ResonantTrappingduetoParticleMigration 76 4.2.2 ResonantTrappingduetoPlanetaryMigration 81 4.2.3 ResonanceRemovalMechanisms 84 4.3 Accretion 85 5 DISKPARTICLEOPTICALPROPERTIES 87 5.1 InterstellarDustGrains 88 5.2 PrimordialCometaryGrains 90 5.3 ZodiacalDust 92 5.4 DebrisDiskDust 93 5.5 DustModels 94 6 HR4796LITERATUREREVIEW 99 6.1 ThestarsHR4796AandHR4796B 99 6.2 ThePre-discoveryDisk 104 6.3 DiskDiscoveryImages 106 6.4 ThePost-discoveryDisk 109 7 THEDYNAMICHR4796DISKMODEL 113 7.1 ModelofOffsetDiskStructure,a(r,9,0) 113 7.1.1 DistributionofOrbitalElements,a(a,e,I,Q,0) 114 7.1.2 ConversiontoSpatialDistribution,o(r,6,<j>) 116 7.2 ModelofP(A,r) 118 7.2.1 OpticalPropertiesofDiskParticles 118 7.2.2 Cross-sectionalAreaDistribution 121 7.2.3 PericenterGlow 124 7.3 DiskModelOrientation 124 7.4 ModelingProcessandResults 125 7.4.1 SymmetricalDiskStructure 127 7.4.2 ParticleSize 127 7.4.3 AsymmetricalDiskStructure 129 7.4.4 StatisticalSignificance 131 8 INTERPRETATIONOFTHEDYNAMICHR4796DISKMODEL . 132 8.1 TheDynamicHR4796Disk 132 8.1.1 RadiationForces,/3 132 8.1.2 CollisionalProcesses 134 8.1.3 P-RDrag 135 8.1.4 TheDynamicHR4796Disk 136 v 8.2 InterpretationofLobeAsymmetry: HR4796’sSecularPerturbations137 8.2.1 JustHR4796B 140 8.2.2 HR4796BandaPlanet 140 8.2.3 JustPlanet 141 8.2.4 OtherConsiderations 142 8.3 DiscussionofEmittingParticleCategory 142 8.4 OriginoftheInnerHole 149 8.5 InterpretationoftheResidualStructure 151 9 CONCLUSION 153 9.1 Conclusion 153 9.2 FurtherWork 157 9.2.1 HR4796 157 9.2.2 OtherDisks 159 REFERENCES 161 BIOGRAPHICALSKETCH 169 vi AbstractofDissertation PresentedtotheGraduateSchool oftheUniversityofFloridainPartialFulfillmentofthe RequirementsfortheDegreeofDoctorofPhilosophy SIGNATURESOFPLANETSINTHEOBSERVABLESTRUCTUREOF CIRCUMSTELLARDEBRISDISKS By MarkC.Wyatt December 1999 Chairman: StanleyF.Dermott MajorDepartment: Astronomy Recentadvancesinastronomicalinstrumentationhaveledtoavastincrease inourknowledgeoftheenvironmentsofnearbystars. Inparticular,wearenowable toimagethethermalemissionfromthedisksofdustaround mainsequencestars thatmaybethefossilremnantsofplanetaryformation. Theseobservationsimply thatthedistributionofdustinthedebrisdisksisneithersmoothnorsymmetrical; e.g.,mid-infraredimagesofthediskofdustaroundtheyoungAOVstarHR4796A showtwolobesofemission,oneofwhichmaybe~5%brighterthantheother. The observedstructureofthedebrisdiskinthesolarsystem,i.e.,thezodiacalcloud,also containsasymmetries: ithasanoffsetcenterofsymmetry, itiswarped, andthere isanasymmetricringofdustco-orbitingwiththeEarth. Sincethezodiacalcloud’s asymmetrieshavebeenshowntobesignaturesofthegravitationalperturbationsof thesolarsystem’splanets, itishopedthatitmaybepossibletoindirectlydetect extrasolarplanetarysystemsbytheirsignaturesindebrisdiskobservations. Thisdissertationusesthephysicalprocessesthataffecttheevolutionofde- brismaterialinthesolarsystemtocreateageneralizedmodelfortheevolutionof circumstellardebrismaterial. Itthenshowshowplanetaryperturbationsaffectthat evolution,therebycausingthesignaturesofplanetsseeninthestructureofthezodi- acalcloud. Thismodelcanbeusedtoprovideaquantitativeinterpretationofdebris diskobservations,andthenecessarymodelingtechniquesaredemonstratedbytheir applicationtoobservationsoftheHR4796disk. Aswellasdeterminingthelarge scalestructureoftheHR4796disk,themodelingshowshowasmallbody(>10M®) intheHR4796systemthatisonanorbitwithaneccentricitylargerthan0.02could bethecauseoftheobservedbrightnessasymmetry. Themodelingalsoshowsthat thedisk’smid-IRemittingparticlesarehotterthanblackbody(andthereforesmall), anddiscusseswhethertheyareintheprocessofbeingblownoutofthesystemby radiationpressure. viii CHAPTER 1 INTRODUCTION 1.1 Motivation Oneofthemostconsistenthumanendeavorssincethedawnoftimehasbeento understandthenatureoftheuniverseinwhichwelive.Weareconstantlyquestioning howtheuniverse, andespeciallylifeonEarth,cametobeasitis. Essentiallythis isasearchforanunderstandingofourplaceintheuniverse,whichinevitablyleads ustoponderwhetherlifecouldexistelsewhereinthecosmos. Eversincethemiddle ofthe sixteenth century when the Copernican heliocentric model ofthe universe superseded thegeocentricview, thissearchhasbeenahumblingexperience aswe begintograspthegargantuanscaleoftheuniverse. Theobservationaldataobtained overtheprevious450yearspaintsapictureoftheuniverseasweseeittodaythat — placestheEarthinafarmoreinsignificantrolethanwaspreviouslyimagined we nowknowthatnotonlyistheEarthnotthecenteroftheuniverse, butneitheris theSun,sinceitisjustoneofanestimated400billionstarsthatmakeuptheMilky Way,whichisitselfjustonegalaxyoutofatleast~80billionothersintheuniverse. Howevermuchwehavelearnedaboutthenatureoftheuniverse,thesearch forlifeoutsidetheconfines ofthe Earth has so farproved fruitless. Even in the corneroftheuniverseaboutwhichwehavethemostinformation,thesolarsystem, thedebatecontinuesastowhetherthereis (orindeedhasbeen) lifeinsomeform oranotheronMars. TheoceanunderthefrozenicesofJupiter’smoonEuropaand thesurfaceofSaturn’smoonTitanalsoprovidepotentialhabitatsforlifeinthesolar system, possibilitiesthatremaintobetested. However, thesolarsystemisbut a smallportionoftheuniverse. Sincestarsaresupposedtohaveformedinthesame 1 2 mannerastheSun,andthesolarsystem’splanetsarethoughttobeabyproductof thatformationprocess,thishasledustospeculatethatsomeoftheO(1022)starsin theuniversecouldalsohaveplanetsorbitingthem,someofwhichmay(past,present, orfuture)harborlife. Thus,ithasbeenoneoftheholygrailsofrecentastronomy tofindhabitableplanets(i.e.,onesthatcouldpotentiallysupportlife)orbitingstars otherthantheSun. 1.2 TheSearchforExtrasolarPlanets Fromanobservationalpointofview,thedirectdetectionofhabitableplanets aroundeventhecloseststarsisbeyondourcurrenttechnologicalcapabilities(Back- man1998;Woolf&Angel1998). Therearetworeasonsforthis. First,technological constraintsmeanthatuntilrecentlytelescopesdidnothavetheresolvingpowertosee theregionsaroundthestarswhereitwasexpectedthatplanetswouldhaveformed. Evennow,theregionsclosetothestar(~1AU)whereweanticipatethatterrestrial planetswouldformareobscuredfromviewbythediffractionhaloofthestellarimage. Second,planetsbydefinitiondonothaveaninternalsourceofenergy,whichmeans — thattheyarenotverybright anylightobservedfromthemiseitherreflectedor reprocessedstarlight. Thus,planetsarefardimmerthanthephotosphericemission fromstaraboutwhichtheyareorbitingandaretoodimtobedetecteddirectlyat present(Woolf&Angel1998). Thisledtothedevelopmentofsomeingenioustechniquestoindirectlydetect thepresenceofplanetsaroundstars. Themostproductiveofthesetechniquesuses the gravitationaleffect ofaplanetarysystem onthepositionofastar relativeto thebarycenterofthestar/planetsystem. Thiscausesawobbleinthemotionofthe starrelativetothe Sun. Thiswobblecanbedetected inthreeways: astrometric detection,bymeasuringthechangeinpositionofastaronthesky(e.g.,Gatewood 1987); dopplershiftdetection, bymeasuringthevariationintheradialvelocityof star(e.g.,Mayor&Queloz1995;Marcy&Butler1998);bymeasuringvariationsin
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