A theoretical look at the direct detection of giant planets(3)

Nevertheless,themolecularmixdescribedin§IIIdeterminestheemergentandre ectedspectrum.ThoughH2isabundant,ithasnopermanentelectricdipolemoment,and,hence,averylowphotonabsorptioncrosssectionintheopticalandinfrared.Similarly,heliumisallbuttransparent.Theresultisthatgaseouswatervapor,withitsstrongabsorptionfeaturesfrom0.94µmto～7µm,cande nemuchofanEGP’sspectrum.Becausewater

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

residesinboththeEarth’sandanEGP’satmosphere,thewaterbandsthatbracketanddeterminetheEarth’sphotometricwindowsat～1.0µm(Z),～1.25µm(J),～1.65µm(H),～2.2µm(K),～3.45µm(L′),and～4-5µm(M),throughwhichground-basedinfraredas-tronomyispossible,areexactlythesamewindowsinanEGPorbrowndwarfatmospherethroughwhichemergent uxcanpour.Thus,andfortuitouslyforbrowndwarfobservations,theemissionpeaksforSMOscoincidewiththeclassicTerrestrialatmosphericbandpasses.Inlieuofmeasurements,theory llsthevacuum.Figure2,takenfromBurrows,Sudarsky,&Hubeny[33],depicts“phase-averaged”[39]planet/star uxratios(f)from0.5µmto30µmfora1-MJ/5-GyrEGPinacircularorbitatvariousdistancesfromaG2VstarliketheSun.ThesemodelsarethesameasthosedepictedinFig.1.Similarplotsfordi erentassumedparameterscanbegenerated.Thewaterabsorptiontroughsaremanifestthroughout.ForthecloserEGPsathigheratmospherictemperatures,carbonresidesinCOandmethanefeaturesareweak.Fortheseclose-inEGPs,theNa-Dlineat0.589µmandthecorrespondingresonancelineofKIat0.77µmareimportantabsorbers,suppressing uxinthevisiblebands.Otherwise,theoptical uxisbuoyedbyRayleighscatteringofstellarlight.Asaincreases,methaneformsandthemethaneabsorptionfeaturesintheoptical(mostoftheundulationsseeninFig.2fora>～0.5AUshortwardof1µm),at～3.3µm,andat～7.8µmappear.Concomitantly,NaandKdisappearfromtheatmosphereandthe uxesfrom～1.5µmto～4µmdrop.Forallmodels,themid-infrared uxeslongwardof～4µmareduetoself-emission,notre ection.AsFig.2makesclear,forlargerorbitaldistancesabifurcationbetweenare ectioncomponentintheopticalandanemissioncomponentinthemid-infraredappears.Thisseparationintocomponentsisnotsostraightforwardforthecloser,moremassive,oryoungerfamilymembers.FortheseEGPs,eitherthelargeresidualheatcomingfromthecoreorthesevereinsolationpropupthe uxesfrom1to4µm.ThemoremassiveEGPs,or,foragivenmass,theyoungerEGPs,havelargerJ,H,andKband uxes.Asaresult,thesebandsarediagnosticofmassandage.ForEGPswithlargeorbitaldistances,thewavelengthrangefrom1.5µmto4µmbetweenthere ectionandemissioncomponentsmaybetheleastfavorablesearchspace,unlesstheSMOismassiveoryoung.

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

Whenwaterorammoniacloudsform,scatteringo themenhancestheoptical uxes,whileabsorptionbythemsuppresses uxesatlongerwavelengthsin,forexample,the4–5µmwindow.Becausewaterandammoniacloudsforminthemiddleofthisdistancesequence,there ectione ciency(or“albedo”;§V)isnotamonotonicfunctionofa.Thesee ectsareincorporatedintoFig.2,buttheirprecisemagnitudedependsuponunknowncloudparticlesize,composition,andpatchiness.Asaconsequence,directspectralmeasurementsmightconstraincloudproperties.

Importantly,tracenon-equilibriummolecularspecies,di culttomodel,canbepresentinquantitiessu cienttoaltercolors.Sucha“chromophore,”whosemolecularnatureisnotyetknown,absorbsintheblueandcreatesthereddishcastofbothJupiterandSaturn,loweringtheiralbedosshortwardof0.55µmbyafactorof～1.5–2.(ChromophoreswerenotmodelledtoproduceFig.2.)

AsFig.2suggests,theplanet/starcontrastratioisbetterinthemid-tofar-infrared,particularlyatwideseparations.Forsuchseparations,thecontrastratiointheopticalcansinkto10 10.Fortheclosest-inEGPs,suchasHD209458b,OGLE-TR56b,51Pegb,andτBoob,thecontrastratiointheopticalisbetween10 5and10 6andismorefavorable.SuchEGPsarenotshowninFig.2;thereareinfactabout20knownEGPswithorbitaldistanceslessthan0.08AU.Duetothepossibleformationofironcloudsintheiratmospheres,HD209458bandOGLE-TR56bmaybebrighterintheopticalthan51Pegbandmayhavehigherre ectionalbedos.Figure3portraysagenericabsolute uxspectrumat10parsecsofaclose-inEGP(“ClassV”inthenomenclatureofSudarskyetal.[39]),notunlikeHD209458b.Highlightedarethepositionsofsomeoftheimportantspectralfeatures.Sincemoderntelescopescaneasilydetect uxesatthemilliJanskylevel,Fig.3demonstratesthatthe uxesthemselvesarenotsmall.Theproblemisseeingtheplanetfromundertheglareofthestar(§VI).At10parsecsandanorbitalseparationof0.05AU,themaximumangularseparationisachallenging～5milliarcsecs.

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

V.PHASEFUNCTIONSFOREGPSANDORBITALORIENTATION

TheplanetaryalbedoandthephasesexecutedasplanetstraversetheirorbitsarecentralquantitiesinthetheoryofEGPlightcurves.Inaddition,aswediscussinthissection,thewavelength-dependentalbedosarestrongfunctionsoforbitaldistanceaswell.Furthermore,thechangingorientationoftheilluminatedfaceofaplanetfromtheEarth’sperspectivetranslatesintoalightcurvethatcanshowsigni cant uxandcolorvariations.InFig.2,thesevariationswereaveragedoutovertheorbit,assumedcircular.Thelongitudeindepen-denceofJupiter’sT/Ppro leresultsinlittleday/nightvariationinthemid-infraredand,hence,littlephasevariation,butsuchaplanetistoocoldtobeself-luminousenoughintheopticalforitsre ectedcomponentnottodominateattheseshorterwavelengths.Hen …… 此处隐藏：6310字，全部文档内容请下载后查看。喜欢就下载吧 ……