The Near-infrared Spectrum of the Brown Dwarf Gliese 229B

DWARFGLIESE229B

T.R.Geballe

arXiv:astro-ph/9606056v1 10 Jun 1996JointAstronomyCentre,660N.A’ohokuPl.,Hilo,HI96720

S.R.Kulkarni

PalomarObservatory105-24,CaliforniaInstituteofTechnology,Pasadena,CA91125

CharlesE.Woodward1,2

UniversityofWyoming,DepartmentofPhysics&Astronomy,Laramie,WY82071-3905

G.C.Sloan1

NASAAmesResearchCenter,MS245-6,MoffettField,CA94035-1000

1

VisitingAstronomer,UnitedKingdomInfraredTelescope.NSFPresidentialFacultyFellow.

2

Received

–2–ABSTRACT

Amediumresolution1.0−2.5µmspectrumofthebrowndwarf,Gliese229BhasbeenobtainedusingCGS4onUKIRT.Inadditiontothebroadspectralstructureseeninearlierlowresolutionobservations,thenewspectrumrevealsalargenumberofabsorptionlines,manyofwhichcanbeidentifiedwithwatervapor.Waterandmethanearebothshowntobestrongabsorbersinthenear-infraredspectrumoftheobject.SeveralspectralfeaturesinGl229Bthatareattributabletomethanematchonesseeninreflectioninthegiantouterplanetsand,inparticular,Titan.

Subjectheadings:infrared:general—infrared:stars—infrared:linesandbands—stars:individual(Gl229B)—stars:lowmass,browndwarfs

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1.

INTRODUCTION

Recently,Nakajimaetal.(1995)reportedthefirstdetectionofacoolbrowndwarf,Gliese229B(hereafterGl229B),apropermotioncompanionofGliese229A.Oppenheimeretal.(1995)obtainedalowresolution(λ/∆λ≈150)near-infrared(1−2.5µm)spectrumofGl229Bandfoundanumberofstrongabsorptionbands.SimilarabsorptionbandsareseeninthespectrumofJupiterandareattributedtomethane.Methaneisnotseeninstars.Tsuji,Ohnaka&Aoki(1995)concludethatmethanewillbeseenonlyinobjectscoolerthanabout1800K,lowerthantheeffectivetemperaturesofeventheleastmassivestars(Burrows&Liebert1993).Fromthemeasuredbroad-bandspectrumofGl229BandassumingaradiusequaltothatofJupiter,Matthewsetal.(1996)inferTeff=900Kandabolometricluminosityof6.4×10−6L⊙.ThelowluminosityandthepresenceofmethaneinthephotosphererequirethatGl229Bisacoolbrowndwarf.

Herewepresentanew1.0−2.5µmspectrumofGl229Bwithsignificantlyhigherresolutionandsignal-to-noiseratiothantheoriginalspectrumpresentedbyOppenheimeretal.(1995).ThenewdataprovideaconsiderablymoredetailedviewofGl229B.WecompareourspectrumtospectraoftheJovianplanetsandTitanwhichareknowntoshowstrongabsorptionfeaturesduetomethaneinthenear-infrared.

2.OBSERVATIONSANDDATAREDUCTION

WeobtainedspectraofGl229Binthe0.99−2.52µmintervalattheUnitedKingdom3.8mInfraredTelescope(UKIRT)on1995October28-29UTandDecember12-13UT,usingthefacilityspectrometerCGS4(Mountainetal.1990).The75l/mmgratingandthe150mmfocallengthcameraopticsinCGS4imagedaslit90′′longontoa256×256InSbarray.Eachpixelspanned1′′.23,andtheslitwasonepixelwide.Table1providesdetailsof

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theobservations.TheresolutionofCGS4wasapproximately390×λ(µm)infirstorderand780×λ(µm)insecondorder.Tocalibratethespectra,weobservedHR1849(A0V,V=5.55)eachnightjustbeforeobservingGl229B.

Gl229Bisabout8′′fromGl229A.IntheJ,H,andKbandsGl229AoutshinesGl229Bbyapproximately10magnitudes.AtUKIRTthehaloofdiffractedandscatteredlightfromGl229AcontributedalargeamountofradiationtothearrayrowscontainingtheGl229Bspectrum.Inordertominimizethedifficultiesofremovingthisbackground,weobservedwiththeslitofCGS4perpendiculartothelinejoiningthetwosourcesontheskyandobtainedalternatespectraofGl229Bbynoddingthetelescope10rowsalongtheslit.Subtractingthesealternatespectralimagesremovedmuchofthehalo,allowingGl229Btobedetectedclearlyatnearlyallwavelengths,typicallyatgreaterstrengththantheresidualhalofromtheprimary.WeextractedthespectrumofGl229Busingstandardinfraredreductionprocedurestocorrectforcurvatureofthespectraandtoremoveresidualbackground.SpectraofHR1849wereutilizedtocorrectforatmosphericandinstrumentalabsorption,aftereditingthemtoremoveatomichydrogenabsorptionlines(withtheexceptionofPaαat1.875µm).

Figure1presentsthecompletespectrumofGl229Bfrom0.99to2.52µm(anumericalfileofthisspectrumisavailablefromtheauthorsuponrequest).AmoreexpandedversionofthespectrumisprovidedinFig.2.Toproducethespectrum,weslightlysmoothedandrebinnedtheobservedspectrainstepsof0.0005µmforthesecondordersegmentsand0.0010µmforthefirstordersegments.Thesmoothinglowersthespectralresolvingpowerby≈10%fromtheactualinstrumentalresolutioncitedabove.Intheregionofoverlapbetweenthetwoorders,wechosetousethefirstorderspectrumbeyond1.585µm.Wescaledtheindividualsegments(byfactorsrangingfrom0.7to1.2)tomatchthespectrainoverlappingsectionsandat1.585µm.Theaveragescalingfactorwasunity,butwe

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scaledthefinalspectrumbyafactorof0.76tomoreaccuratelymatch(towithinabout0.1mag)therecentPalomarphotometryofGl229B(J=14.2,H=14.3,Ks=14.3,K=14.4)asreportedbyMatthewsetal.(1996a).

ThenoiseintheCGS4spectrumvariesconsiderablywithwavelength,asillustratedinFig.2,duetolargevariationsinatmospherictransmissionandinthebackground(causedbyboththermalandOHemissions).Inparticular,inthespectralregionsdominatedbystrongtelluricabsorptionbands(1.12−1.14µm,1.36−1.42µm,1.81−1.93µm,and2.48−2.52µm),apparentspectralfeaturesmaynotbereal.Inthesespectralregionsthedataprobablyareonlyusefulforestimatingtheaveragefluxlevel.

3.DESCRIPTIONOFTHEINFRAREDSPECTRUM

ThenewspectrumofGl229BisconsistentwiththelowerresolutionspectrumofOppenheimeretal.(1995).However,thelatterspectrumwasincorrectlynormalized,asnotedbyMatthewsetal.(1996a).Matthewsetal.(1996b)presentarevisedspectrumwiththepropernormalization,withwhichtheCGS4spectrumagreeswell.

Twonewresultsareimmediatelyapparentfromthenewspectrum.First,emissionisseenacrosstheentireIRbandfrom1.0to2.5µm.Specifically,emissionisdetectedthroughthewavelengthintervalsofstrongtelluricH2Oabsorption(seediscussionabove)andintherange2.2−2.5µmincludinganotableriseinfluxatwavelengthslongwardof2.4µm.ThedataofOppenheimeretal.(1995)wererestrictedtotheusualnear-infraredwindows(Z,J,H,K)andfurthermorewerenoisyintherange2.2−2.5µm.Weemphasizethat,withthepossibleexceptionofthewavelengthinterval1.36−1.40µm,wherethedetectionismarginal,theemissiondetectedisfromGl229BandisnotcontaminationbyGl229A;crosscutsthroughthedifferencedspectralimagesinthesewavelengthintervals

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showlocalmaximaintherowsofthearraycorrespondingtothelocationofGl229B.Second,numerousnarrowlinesareseen(seeFig.2).Theseappearinregionsofhighemissionandonalloftheabsorptionedgeswiththeexceptionoftheverysteepedgeat1.6µm.Broaderfeaturesoccurinthe1.6−1.9µmabsorptiontroughatabout1.63,1.67,1.71,and1.80µm,andat2.17,and2.20µm.MostofthebroadfeatureswereseeninthespectrumofOppenheimeretal.(1995),albeitatlowerresolution.

4.

4.1.

DISCUSSION

Methane:comparisonwithSolarSystemobjects

WehavecomparedthenewspectrumofGl229Btothesolarreflectancespectraofthreeofthegiantgaseousplanets(Jupiter,Saturn,andUranus)andTitan,usingthedataofFink&Larson(1979),andrecentCGS4spectraofTitan(Owen,unpublished)andSaturn(Geballe,unpublished).Methanedominatesthe1.0−2.5µmspectraofalloftheaboveouterplanetsandTitan,butthedegreeofdominationvaries,increasinginthesequenceJupiter,Saturn,Titan,andUranus.

ThespectrumofTitanprovidesthebestoverallspectralmatchtothatofGl229Bandthespectraofthetwoexhibitseveralsimilardetails.Mostremarkably,theirverysharpcut-offsat1.61µmareessentiallyidentical(seeFig.3).ThematcheswiththespectraoftheotherSolarSystemobjectsdonotappearasprecise;forexample,thecut-offinSaturnisshiftedby0.01µmtolongerwavelength.Between1.62and1.72µmthewavelengthsofsomeoftheabsorptionmaximainTitanandGl229Bagree.BothTitanandGl229Bshowabsorptionbandsat∼2.17µmand2.20µm(asdoJupiterandSaturn).InTitaneachofthesefeaturesarisesfromCH4(Fink&Larson1979;C.Griffith,pers.comm.).Thus,despitetheconsiderablydifferentphysicalconditionsoftheCH4inGl229Bcompared

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tothoseintheatmospheresofthegiantgaseousplanetsandTitan,detailedcomparisonprovidesclearconfirmationofthestronginfluenceofmethaneonthespectrumofGl229B.

4.2.H2OinGl229B

TheoreticalmodelspectraofbrowndwarfswitheffectivetemperaturessimilartothatofGl229Bshowconsiderableabsorptionduetosteam(Marleyetal.1996;Allardetal.1996).Wenote,however,thatthesemodelspectraonlyincludelowresolutionmethaneopacitiesshortwardof1.6µm(R.S.Freedman,personalcommunication).Jonesetal.(1994)havecompiledlowresolutionspectraofthecooleststarsandattributeabroadabsorptioncenteredatabout1.4µmanditssharpabsorptionedge,whosewavelengththeyreportas1.34µm,toH2O.The1.4µmwaterbandalsoappearsstronglyintheabovemodelbrowndwarfspectra.FollowingJonesetal.,Oppenheimeretal.(1995)identifiedtheabsorptionedgeseeninGl229BataboutthiswavelengthasduetoH2O.

Inthepresentspectrum,theabsorptionedgeinGl229Bisseentobecenteredatabout1.31µm.TheH2OabsorptionedgeinlateMstarsactuallyoccursat1.33µm(Walkeretal.1996)andismuchsteeperthantheabsorptionedgeinGl229B;indeeditisexpectedtostrengthenandsteepenwithdecreasingtemperature.ThemethaneabsorptionedgesofTitanandSaturnarecenteredat1.30µmand1.31µm,respectively.TheworkofJonesetal.isrestrictedtostarswhicharecool,butcertainlynotcoolenoughtohavestrongphotosphericmethanebands.Thus,confusionbetweenCH4andH2Odoesnotariseintheinterpretationofstellarspectra.Weconcludeonthebasisofthelowresolutionspectraalonethattheabsorptionedgeandtroughlongwardof1.3µminGl229Bcannotbeattributedunambiguouslytowateraloneandthatmethanecouldcontributesignificantly.However,whencomparedtothespectraofallofthegiantouterplanetsandTitan,theemissionbumpat1.5−1.6µmisbadlyeatenawayonitsshortwavelengthside(Fig.3).

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AbsorptionbyH2OinGl229BcanexplainthissystematicdifferencebetweenGl229BandthegiantplanetsandTitan.Thesharpestportionofthe1.3µmabsorptionedgeofGl229B,at1.33µm,wouldthenbeduetoH2O.

Althoughthelowresolutionspectraareambiguousastothepresenceofwatervapor,comparisonofthenewhigherresolutionspectrumofGl229BwithplotsofH2Oopacityatsimilarresolution(kindlysuppliedbyD.Saumon,R.S.Freedman,andD.Schwenke)demonstrateitspresenceclearly.AnillustrationofthisisprovidedinFig.4.AlmosteveryabsorptionlineinthespectrumofGl229Bintheintervals1.30−1.34µm,1.53−1.58µm,and1.95−2.07µmhasacounterpartinthewateropacityspectrum.Moreover,thereisgoodoverallcorrelationbetweenobservedandmodelledlinestrengths.Theopacitiesofthelinesintheabovewavelengthintervalsarerelativelylow,implyingthatthedepressionsinGl229Batabout1.4µmand1.9µmareindeedduetoabsorptionbyH2O,whichhasmuchhigheropacityatthosewavelengths.

Thus,thenewspectrumallowsafirmidentificationofwatervaporinthephotosphereofGl229B.Itisclearthat,aspredictedinthemodels,bothH2OandCH4producestrongabsorptionsinthenear-infraredspectrumofGl229B.However,untilthelineparametersofmethanearebetterknown,itwillbedifficultbothtodisentangletherelativecontributionsofmethaneandwaterandtosearchforotherabsorbersatnear-infraredwavelengths.

5.Acknowledgements

Wethankthemanyindividualsresponsiblefortheexcellentconstruction,maintenance,andoperationalsupportofCGS4andthestaffofUKIRTforitssupportoftheseobservations.WearegratefultoR.S.Freedman,D.Schwenke,andD.SaumonforprovidinguswithdetailedopacityplotsofH2OandCH4andtoT.Owenforpermittingustopublish

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aportionofthespectrumofTitan.Wethankthereferee,F.Allard,forseveralhelpfulcomments.WealsothankD.Cruikshank,M.Marley,K.Matthews,T.Nakajima,andB.Oppenheimerfortheirassistance.TheUnitedKingdomInfraredTelescopeisoperatedbytheJointAstronomyCentreonbehalfoftheU.K.ParticlePhysicsandAstronomyResearchCouncil.SRK’sresearchissupportedbyCaltech,theUSNSF,NASA,andthePackardFoundation.CEWacknowledgessupportfromtheUSNationalScienceFoundation.

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6.

REFERENCES

Allard,F.,Hauschildt,P.H,Baraffe,I.&Chabrier,G.1996,ApJ(Letters),inpress.Burrows,A.&LiebertJ.1993,Rev.Mod.Phys.,65,301Fink,U.,&Larson,H.P.1979,ApJ233,1021

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7.

FIGURECAPTIONS

Fig.1.—Combined1.0–2.5µmspectrumofGl229B.Theintensityscaleislogarithmic;negativevaluesnear1.4µmwereremovedpriortoplotting

Fig.2.—SpectralsegmentsforGl229B,covering0.99−2.52µm.Representativenoiselevelsareshownasverticallinesalongthebottomofeachpanel.a.0.99−1.41µm.b.1.39−1.91µm.c.1.−2.52µm.

Fig.3.—SpectraofGl229B(darkerline)andTitan(scaledbyafactorof∼0.003)nearthe1.6µmmethaneabsorptionedge.

Fig.4.—SpectrumofGl229Bandtheopacityofwatervaporinthe1.95−2.10µmregion.TheopacityisforP=1barandT=700K,convolvedtoaresolvingpowerof1000,andiscourtesyofR.S.Freedman,D.Schwenke,andD.Saumon.

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Table1.ObservationLog

Date(UT)Gratingorderλ(µm)Int.TimeConditions