A theoretical look at the direct detection of giant planets(2)

Sincetheindirectradial-velocitytechniqueforEGPdiscoveryselectsforthecloservari-ety,itislikelythatalargereservoirofgiantsexistsatdistancesandorbitalperiodsbeyondthereachofDopplerspectroscopy.Furthermore,thebesttheoryfortheorbitsoftheclosestEGPsisthattheymigratedinfromfurtheroutduringtheearlyphaseofstarandplanetfor-mation[26].ThistoowouldimplythatalargepoolofEGPsresidesatlargerseparations.Indeed,itmaybethatthemajorityofstarsinthesolarneighborhoodharborplanetary

Astronomy is at times a science of unexpected discovery. When it is, and if we are lucky, new intellectual territories emerge to challenge our views of the cosmos. The recent indirect detections using high-precision Doppler spectroscopy of now more than on

systems,thatonlynewtechniquescanreveal.Thisiswherethedirectplanetdetectionmethods,moste ectiveatlargeangulardistancesfromtheparentstar,willcomeintotheirown.

III.THEORETICALATMOSPHERESANDCHEMISTRYOFEGPS

Afterformation,withoutanysigni cantinternalsourcesofenergy,anEGPgraduallycoolsandshrinks.Itsrateofcoolingcanbemoderatedbystellarirradiation,orbyhydro-gen/heliumphaseseparationwhenoldandlight[27],butisinexorable.JupiteritselfisstillcoolinganditstotalinfraredplusopticalluminosityisabouttwicethepowerinterceptedfromtheSun.Therateofcoolingisafunctionofmassandcomposition,withmoremassiveEGPscoolingmoreslowly.Hence,theinstantaneousstateofanEGPisafunctionofmass,age,composition,orbitaldistance,andstellartype,notjustmassandcomposition.

Unlikeastar,EGPatmospherictemperaturesaresu cientlylowthatchemistryisdes-tiny.Thisisadistinguishingcharacteristicofsubstellar-massobjects(SMOs).Theat-mosphereofagaseousgiantplanetisthethinouterskinofmoleculesthatregulatesitsemissionspectrumandcoolingrate.Molecularhydrogen(H2)istheoverwhelmingcon-stituent,followedbyatomichelium.AnEGP’se ectivetemperature(Te ,thetemperatureofits“photosphere”)canvaryfrom～1500KforthemoremassiveEGPsatbirthto～50KfortheleastmassiveEGPsafteraHubbletime.Thiswiderangetranslatesintoarichvarietyofatmosphericconstituentsthatforagivenmassandelementalcompositionevolvessigni cantly.Atbirth,JupiterhadaTe near600-1000KandtheappearanceofaTdwarf

[28]browndwarf.Ithadnoammoniaorwatercloudsand,duetothepresenceofatomicsodiuminitshotatmosphere,hadamagentacolorintheoptical[29].Itsatmospherewasdepletedofaluminum,silicon,iron,calcium,andmagnesiumduetotheformationandsettlingtodepthoftherefractorysilicates(“dirt”)thatcondenseinthetemperaurerange～1700-2500K[30–32].Watervapor(steam)wasthemajorreservoirofoxygen,gaseousmethanewasthemajorreservoirofcarbon,gaseousammoniaandmolecularnitrogenwere

Astronomy is at times a science of unexpected discovery. When it is, and if we are lucky, new intellectual territories emerge to challenge our views of the cosmos. The recent indirect detections using high-precision Doppler spectroscopy of now more than on

thereservoirsofnitrogen,andH2Swasthereservoirofsulfur.Asitcooled,thelayerofalkalimetalswasburiedbelowthephotospheretohigherpressures,butgaseousH2,H2O,NH3,andCH4persistedtodominatetheatmosphericcomposition.AtaTe of～400K,watercondensedintheupperatmosphereandwatercloudsappeared.Thisoccurredwithinits rst100millionyears.Withinlessthanagigayear,whenTe reached～160K,ammoniacloudsappearedontopofthewaterclouds,andthislayeringpersiststothisday.Stellarirradiationretardscloudformation,asdoesalargeEGPmass,whichkeepstheEGPhotlonger.AroundaG2VstarliketheSun,at5GyrandforanEGPmassof1.0MJ,watercloudsformat1.5AU,whereasammoniacloudsformbeyond4.5AU[33].Jupiter’sandSaturn’scurrente ectivetemperaturesare124.4Kand95K,respectively.Jupiter’sorbitaldistanceandageare5.2AUand4.6Gyr.Theorbitaldistance,mass,andradiusofacoevalSaturnare9.5AU,0.3MJ,and0.85RJ.However,asanEGPofwhatevermasscools,itsatmosphericcompositionevolvesthroughasimilarchemicalandcondensationsequence.Figure1depictstheatmospherictemperature/pressure(T/P)pro leforasequenceof1-MJ,5-GyrmodelsasafunctionoforbitaldistancefromaG2Vstar.Astheplanet“moves”outward,itsatmospherictemperatureatagivenpressuredecreases.SuperposedontheplotaretheH2OandNH3condensationlines.Inanapproximatesense,agivenatmosphericcompositionandtemperaturecanresultfrommanycombinationsoforbitaldistance,planetmass,stellartype,andage.ThislendsanaddeddegreeofcomplexitytothestudyofEGPswithwhichthestudyofstarsdoesnotneedtowrestle.

Theatmospheresofclose-inEGPs(“roasters”)atorbitaldistancesof～0.02-0.07AUfromaG,F,orKstarareheatedandmaintainedattemperaturesof1000-2000K,roughlyindependentofplanetmassorcomposition.Anedgestarofthesolar-composition,hydrogen-burningmainsequence(M ～75MJ)hasaTe of～1700K.Therefore,anirradiatedEGP,witharadiuscomparabletothatofsuchastar,canbeasluminous.ItsatmosphericcompositionispredominantlyH2,He,H2O,Na,K,andCO.Athightemperatures,carbonisgenerallyincarbonmonoxide.ThisisthedominantmoleculeofcarbonforMdwarfswithTe sof2200-3500K.AtthehighestTe s,cloudsofironparticulatescanformand

Astronomy is at times a science of unexpected discovery. When it is, and if we are lucky, new intellectual territories emerge to challenge our views of the cosmos. The recent indirect detections using high-precision Doppler spectroscopy of now more than on

persistintheupperatmosphere,asmaybethecaseinHD209458b.Thereare,however,signi cantday/nightdi erencesanduniquere ectivepropertiesthatdistinguisharoasterfromaloneandisolatededgestar.Exoticgeneralcirculationmodels(GCMs)[34–36]maysoonbenecessarytounderstandtheequatorialcurrents,jetstreams,day/nightdi erences,terminatorchemistry,andglobalwinddynamicsofseverelyirradiatedroasters,inparticular,andoforbiting,rotatingEGPs,ingeneral.

ItisusefultonotethatayoungEGPinawideorbitwithamassof1.0to5.0MJhasanatmosphereandspectrumthataresimilartothoseofanoldbrowndwarfwithamassof30-60MJ.Asitevolves,thespectroscopicclassofagiantplanetcantransitionfromthatofahotMdwarf,intoanLdwarf(wherethesilicatecloudsareintheatmosphere),thenintoaTdwarf,endingupintheterritory,asyetunexplored,betw …… 此处隐藏：6496字，全部文档内容请下载后查看。喜欢就下载吧 ……