Advances in the Theory of Extrasolar Giant Planets

arXiv:astro-ph/9706080v19Jun1997AdvancesintheTheoryofBrownDwarfsandExtrasolarGiantPlanetsA.Burrows,D.Sudarsky,andC.SharpDepartmentofAstronomyandStewardObservatory,UniversityofArizona,Tucson,AZ85721M.MarleyDepartmentofAstronomy,NewMexicoStateUniversity,Box30001/Dept.4500,LasCrucesNM88003W.B.HubbardandJ.I.LunineLunarandPlanetaryLaboratory,UniversityofArizona,Tucson,AZ85721T.GuillotDepartmentofMeteorology,UniversityofReading,P.O.Box239,Whiteknights,ReadingRG66AU,UnitedKingdomD.SaumonDepartmentofPhysicsandAstronomy,VanderbiltUniversity,Nashville,TN37235R.FreedmanSpacePhysicsResearchInstitute,NASAAmesResearchCenter,MoffettFieldCA94035Abstract.Wehavedevelopedanewnon–graytheoryoftheevolution,spectra,andcolorsofextrasolargiantplanets(EGPs)andbrowndwarfsthatrevealstheirexoticnatureanduniqueness.WehavediscoveredthatthefluxesofsuchobjectsforTeffsfrom1300Kto100Kcanbespectacularlyhigherinthenearinfraredbandsthanblackbodyvaluesandthattheirinfraredcolorsarequiteblue.Asaconsequence,EGPsandbrowndwarfsresideinhithertounoccupiedrealmsoftheH–Rdiagramandmaybemoreeasilyfoundwithcurrentandplannedtelescopesthanpreviouslyimagined.1.IntroductionDopplerspectroscopyhasnowrevealedabout20objectsinthegiantplanet/browndwarfregime,includingcompanionstoτBoo,51Peg,υAnd,55Cnc,ρCrB,70Vir,16Cyg,and47UMa(Butleretal.1997;Cochranetal.1997;Marcy1&Butler1996;Butler&Marcy1996;Mayor&Queloz1995;Lathametal.1989).Furthermore,thebrowndwarf,Gl229B,hasbeendiscovered(Oppen-heimeretal.1995;Nakajimaetal.1995;Matthewsetal.1996;Geballeetal.1996).Gl229Bisamilestonebecauseitdisplaysmethanespectralfeaturesandlowsurfacefluxesthatareuniquetoobjectswitheffectivetemperatures(inthiscase,Teff∼950K)belowthesolar–metallicitymainsequenceedge.In1995and1996,wepublishedagraytheoryoftheevolutionofextrasolargiantplanets(EGPs)withmassesfrom0.3MJto15MJ,whereMJdenotesaJupitermass(∼0.001M⊙)(Burrowsetal.1995;Saumonetal.1996;Guillotetal.1996).We(Burrowsetal.1997)havenowdevelopedanon–graytheorythaten-compassestheEGP/browndwarfdomainsfrom0.3MJto70MJ,inaidofthedirectsearchesforsubstellarobjects,bethey“planets”orbrowndwarfs,beingplanned(TOPSandExNPSreports;Legeretal.1993).Wehavelimitedthem-selvestosolar–metallicityobjectsinisolationandignoredtheeffectsofstellarinsolation(Guillotetal.1996).Inthiscommunication,wesummarizesomeoftheresultsofthisextensivenewstudy,towhichwereferthereaderfordetails.2.ModelingTechniqueTheopacitiesweemployedarefromextensionsoftheHITRANdatabase(Roth-manetal.1992,1997),theGEISAdatabase(Hussonetal.1997),andtheoreticalcalculations(Tyuterevetal.1994;Goorvitch1994;Tipping1990;Wattson&Rothman1992;L.R.Brown,privatecommunication).Forwater,weusedthenewPartridge&SchwenkeH2Odatabase.Ourlinelistincludes1.9×106linesforCH4andCH3D,99,000linesforCO,11,400linesforNH3,11,240linesforPH3,and179,000linesforH2S.ModeledcontinuumopacitysourcesincludeH−andH−2opacityandcollision–inducedabsorption(CIA)ofH2andhelium(Borysow&Frommhold1990;Zheng&Borysow1995).ThelatterisadirectfunctionofpressureandamajorprocessinEGP/browndwarfatmospheres.Tocalculateatmosphereprofilesandspectra,weusedthek–coefficientmethod(Goodyetal.1989;Lacis&Oinas1991),widelyusedinplanetaryat-mospheremodeling(seeM.Marley,thisvolume).ThisisnottheODFtechnique(Saxner&Gustafsson1984)andgivesexcellentagreementwithfullline–by–linecomputationsofatmospherictransmission(Grossman&Grant1992,1994a,1994b).ChemicalequilibriumcalculationswereperformedwiththeATLAScodeanddatafromKurucz(1970).TheKuruczreactionconstantsareinaccurateatlowtemperatures,buttheNH3→N2andCH4→COconversionsthatoccurinEGPsandbrowndwarfsdosoinregionsofT−PspaceforwhichtheKuruczreactionconstantsareaccurate.CondensationofNH3,H2O,Fe,andMgSiO3wasincludedusingdatafromvarioussources,includingEisenberg&Kauzmann(1969),theHandbookofChemistryandPhysics(1993),andLange’sHandbookofChemistry(1979).FollowingFegley&Lodders(1994,1996),weassumedthatAl,Ca,TiandVwereremovedeitherbycondensationorweredissolvedinsilicategrainsatabouttheMgSiO3condensationtemperature.2Figure1.Evolutionoftheluminosity(inL⊙)ofsolar–metallicityMdwarfsandsubstellarobjectsversustime(inyears)afterformation.Thestars,“browndwarfs”and“planets”areshownassolid,dashed,anddot–dashedcurves,respectively.Inthisfigure,wearbitrarilydes-ignateas“browndwarfs”thoseobjectsthatburndeuterium,whilewedesignatethosethatdonotas“planets.”ThemassesinM⊙labelmostofthecurves,withthelowestthreecorrespondingtothemassofSaturn,halfthemassofJupiter,andthemassofJupiter.3.EvolutionaryModelsInBurrowsetal.(1995)andSaumonetal.(1996),wepublishedcoolingcurvesforEGPsandsmallbrowndwarfsthatwerebaseduponourthen–currentatmo-spheremodels.InBurrowsetal.(1997),wehaveupdatedtheH2CIA,H2O,andCH4opacitiesandtheradiativetransferalgorithm,aswellasthechemicalequilibriumcalculations.Consequently,theevolutionarytrackshavechanged,butgenerallybynomorethan10%inluminosityatanygiventime,foranygivenmass.Figure1depictstheupdatedluminosityversustimeplotsforobjectsfromSaturn’smass(0.3MJ)to0.2M⊙.Itcoversthreeordersofmagnitudeinmassandencapsulatesthecharacteristicsoftheentirefamilyofsubstellarobje