Identifying Young Brown Dwarfs Using Gravity-Sensitive Spectral Features07
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Identifying Young Brown Dwarfs Using Gravity-Sensitive Spectral Features07
ep；43；rt；sa；:v；aIDENTIFYINGYOUNGBROWNDW；youngsub-stellarobject,σ；1.INTRODUCTION；Theoreticalmodelspredict；.7others).Forexample,int；352+166?85pc,Hipparcos),；Onewaytodistinguishbetwe；
3 2 epS 32 1v4369 3 /hp-ortsa:viXraIDENTIFYINGYOUNGBROWNDWARFSUSINGGRAVITY-SENSITIVESPECTRALFEATURESMARKR.MCGOVERN1,J.DAVYKIRKPATRICK2,IANS.MCLEAN1,ADAMJ.BURGASSER1,3,L.PRATO1,PATRICKJ.LOWRANCE2ABSTRACTWereporttheinitialresultsoftheBrownDwarfSpectroscopicSurveyGrav-ityProject,tostudygravitysensitivefeaturesasindicatorsofyouthinbrowndwarfs.Low-resolution(R～2000)J-bandandoptical(R～1000)observationsus-ingNIRSPECandLRISattheW.M.KeckObservatoryrevealtransitionsofTiO,VO,KI,NaI,CsI,RbI,CaH,andFeH.Bycomparingthesefeaturesinlate-typegiantsandinold?elddwarfsweshowthattheyaresensitivetothegravity(g=GM/R2)oftheobject.Usinglow-gravityspectralsignaturesasageindicators,weobservedandanalyzedJ-bandandopticalspectraoftwoyoungbrowndwarfs,G196-3B(20-300Myr)andKPNOTau-4(1-2Myr),andtwopossiblelowmassbrowndwarfsintheσOrioniscluster(3-7Myr).Were-porttheidenti?cationoftheφbandsofTiOnear1.24μmandtheA-XbandofVOnear1.18μmtogetherwithextremelyweakJ-bandlinesofKIinKPNO-Tau4.Thisisthe?rstdetectionofTiOandVOintheJ-bandinasub-stellarmassobject.TheopticalspectrumofKPNO-Tau4exhibitsweakKIandNaIlines,weakabsorptionbyCaH,andstrongVObands,alsosignaturesofalowergravityatmosphere.G196-3Bshowsabsorptionfeaturesinbothwavelengthre-gionslikethoseofKPNO-Tau4suggestingthatitsageandmassareatthelowerendofpublishedestimates.WhereasσOri51appearstobeconsistentwithayoungsub-stellarobject,σOri47showssignaturesofhighgravitymostcloselyresemblinganoldL1.5/L0,andcannotbeamemberoftheσOrioniscluster.Subjectheadings:infrared:stars―stars:lowmass,browndwarfs―techniques:spectroscopic―surveys1.INTRODUCTIONTheoreticalmodelspredictthatsub-stellarobjectsaremuchmoreluminousandhotterwhenveryyoung(Burrowsetal.2001).Consequently,deepimagingsurveysofyoungclusterscanyieldverylowmasssub-stellarobjectsthatwillbetoofainttostudyoncetheyhaveaged,unlesstheyarelocatedextremelyclosetotheSun.Hence,theverylowendoftheinitialmassfunctioncanbemoree?ectivelyprobedinyoungclusters.Severalgroupshavereporteddeepinfraredsurveysofyoungstar-formingregionsatdistancesrangingfrom140-450pc(Luhmanetal.ab,2000B′ejaretal.1999;Stau?eretal.1999;Ardilaetal.2000;Lucas&Roche2000;ZapateroOsorioetal.2000;Mainzer&McLean2003,among.7others).Forexample,intheSigmaOrionisopencluster(4.2+2?1.5Myr,Oliveiraetal.2002;352+166?85pc,Hipparcos),ZapateroOsorioetal.(2000)reportseveralsub-stellarmassobjectswhich,ifmembersoftheassociation,wouldhavemassesintherange8-15Mjupiter.Onewaytodistinguishbetweenyoungandoldbrowndwarfsistolookforgravity-sensitivespectralfeatures.Theradiusof?eldbrowndwarfsvariesonlyslightlywithmassandage,andthereforethesurfacegravity(g=GM/R2)isdeterminedbythemass(logg～5).Atayoungevolutionarystagehowever,lowermassobjectswillhavee?ectivetemperaturescorrespondingtothoseoflateMorearlyLdwarfs.Furthermore,theradiusoftheseyoungobjectscanbeasmuchasthreetimesgreaterthantheireventualequilibriumstate(Burrowsetal.2001).Asaresult,youngobjectscanexhibitsigni?cantlylowersurfacegravities(10-100times)thanthemoremassiveevolveddwarfsofthesamespectraltype.Mart′?netal.(1996)andLuhmanetal.(1997)demonstratedthatCaH,KI,NaI,andVOfeaturescanbeusedasgravity-sensitivediagnosticsintheopticalspectraofbrowndwarfcandidates.Gorlovaetal.(2003)showedthattheneutralpotassium(KI)linesintheJ-bandarealsosensitivetosurfacegravity.Inthiscurrentpaperwepresentthe?rstresultsfromoursurveythatfocusonyoungbrowndwarfsandtheirsurfacegravityspectralindicators.Weshowthatforages??5Myr,notonlyaretheKIlinesandtheFeHfeatureintheJ-bandveryweakforagivenspectraltype,butalsoinfraredTiOandVObands,normallynotpresentattheboundarybetweenMandLdwarfsintheold?eldpopulation,arepresentforthelowestmassobjects.WealsorevealthattheopticalspectraofveryyoungbrowndwarfsexhibitweakerlinesofKI,NaI,RbI,andCsI;weakerbandsofCaH;andstrongerbandsofVOthan?elddwarfsofthesamespectralclass.Theseresults,andtheextensivespectraldatabaseofold?eldM,L(Kirkpatricketal.1999),andT(Burgasseretal.2002)dwarfsintheBrownDwarfSpectroscopicSurvey(BDSS,McLeanetal.2003;hereafterM03)andtheKirkpatrickLDwarfArchive4enableustotestthemembershipofpotentialverylowmassbrowndwarfsinyoungclusters.WehaveappliedthesecriteriatoKPNO-Tau4intheTaurus-Aurigaregion(Kenyonetal.1994a,Brice?noetal.2002),G196-3B(Reboloetal.1998),andσOri47andσOri51intheSigmaOrionisopencluster(B′ejaretal.1999,ZapateroOsorioetal.2000).In§2wedescribetheobservationsandmethods.Resultsarepresentedanddiscussedin§3,andourconclusionsaresummarizedin§4.2.OBSERVATIONSANDRESULTSTheobjectsobservedandtheirrelevantphotometricpropertiesarelistedinTable1.KPNO-Tau4wasdiscoveredbyBrice?noetal.(2002)aspartofanIandz′deepsurveyof8squaredegreesintheTaurus-Aurigadarkcloudcomplex(1-2M140±10pc,Kenyonetal.1994b).AnopticalspectrumobtainedbytheauthorswasusedtoassignaspectraltypeofM9.5±0.5.G196-3BwasdiscoveredbyReboloetal.(1998)fromadirectimagingsearcharoundyoung,nearby,cooldwarfstars.Itsagewasestimatedat20-300MyrbaseduponthechromosphericandcoronalpropertiesofG196-3A.OpticalspectraindicateG196-3BisanL2dwarfwithamassestimatedat25+15?10Jupitermasses(Reboloetal.1998).σOri47was?rstreportedasasub-stellarcandidatememberoftheσOrioniscluster(B′ejaretal.1999)aspartofadeepRIzsurveyinan870squarearcminute?eldaroundtheO9.5VstarσOrionis.FollowupinfraredphotometryandopticalspectroscopyacquiredbyZapateroOsorioetal.(1999)suggestedasubstellarnatureofσOri47.ZapateroOsorioetal.presentedaKeckLRISspectrumofthisobjectandconcludedthatthestrengthsofthehydrideandalkalilinesarelowerthanina?eldLdwarfofthesametype.ThiswouldindicatelowergravityandhenceayoungageforσOri47.Theyalsoclaimedalithiumdetectionof4.3±0.5?AEW,anotherindicationofyouth.AspectraltypeofL1.5±0.5wasassignedbasedupontheopticalspectrum.σOri51(ZapateroOsorioetal.2000)wasdiscoveredduringadeeperfollowupsurveyoftheinitialB′ejaretal.(1999)sample.Sub-stellarstatuswasdeterminedfrom?tsofthecolordatatomodelisochronesforyoungbrowndwarfs.SubsequentlowresolutionopticalspectroscopywasconductedbyBarradoyNavascu′esetal.(2001)whodetermineditsspectraltypetobeM9.0±0.5.Finally,andmostimportantly,weobservedtheMiravariableIOVirginis(M9.5III+)asatemplateforverylowgravities(logg～0).Near-infrared(near-IR)observationsweremadewithNIRSPEContheKeckIItele-scope,usinga0.43′′slittoyieldaresolutionofR=λ/?λ～2000atJ-band(1.14-1.36μm).TheJ-bandwaschosenbecauseitcontainsfourstronglinesofneutralpotassium,FeHbands,andastrongH2Obandusefulforspectralclassi?cation(M03).AdetaileddescriptionofNIRSPECisgivenelsewhere(McLeanetal.).Observationsweremadeasasequenceofnoddedpairsbypositioningtheobjectattwolocationsalongtheslitseparatedby～20′′.Eachexposurewas300sandatleasttwonoddedpairswereobtainedforeachsource,givingtotalintegrationtimesof20-60minutes.Thetypicalsignal-to-noiseratioperresolutionelementfortheseobservationsis20-30,withtheexceptionofσOri51whichhasasignal-to-noiseof～10.DataextractionandreductiontechniqueswerecarriedoutusingREDSPEC,theNIRSPECreductionpackage(seeM03fordetails).OpticalspectrawereobtainedattheKeckItelescopeusingtheLowResolutionImagingSpectrograph(LRIS;Okeetal.1995).FormostoftheobservationslistedinTable1,thesetupandreductionprocedureswereidenticaltothosediscussedinKirkpatricketal.(1999).TheonlyexceptionisthespectrumofG196-3B,whichispiecedtogetherfromtwodi?erentLRISsetups.The?rstsetupwasusedon1999Mar04/05,resultinginan9?A-resolutionA.On2001Feb19weusedthe300lines/mmspectrumofG196-3Bfrom?gratingblazedat5000?Atocovertheintervalfrom?A.Reductionswereidentical,andtheresultingspectrumhasaslightlylowerresolutionof11?A.Becausethebluespectrumwastakeninbetterskyconditions,wehaveconstructeda?AspectrumforG196-3Bbycombiningbothdatasets:1999Mardataforλ&8500?Aand2001Febdataforλ&8500?A.Spectraobtainedon2003Jan02/03and2001Feb19havebeencorrectedfortelluricabsorption.ThiswasaccomplishedbyobservingaG0dwarfatsimilarairmassshortlybeforeorafterthetargetobservationandinterpolatingthecontinuumacrossthetelluricbandstodeterminethecorrection.3.DISCUSSIONFigure1showstheNIRSPECJ-bandspectrumofIOVircomparedwiththatofKPNO-Tau4andG196-3B.AremarkablefeatureintheJ-bandspectraistheappearanceofabroadabsorptionbandnear1.24μminboththeMiravariableIOVirandtheyoungM9.5dwarfKPNO-Tau4.ThesebandsareprobablynottheresultofFeH,asrelatedfeaturesat1.20μmareveryweakorabsent.Weinsteadidentifythesefeaturesastheφ(?ν=?1)bandheadsofTiO(Galehouseetal.1980).Asecondfeaturenear1.18μmisattributedtotheA-X(?ν=?1)bandofVO(Cheungetal.1982).TheseoxidebandsarecommoninthelatetypeJ-bandspectraofMiravariables(Hinkleetal.1989;Joyceetal.1998)andapparentlyalsopresentinthelowgravityatmospheresofyoungbrowndwarfs.TheKIdoublets(1.168,1.177μmand1.243,1.254μm)areabsentinthecool,low-gravityatmosphereofIOVirandonlyweaklypresentinthespectrumofKPNO-Tau4,consistentwiththebehaviornotedbyGorlovaetal.(2003).TheH2Obandat1.34μminKPNO-Tau4isdeeperthanexpectedforanM9.5object,beingmoreconsistentwithaL2dwarf(M03).Thise?ectcouldlikelybetheresultoftheH2Obandshowingsomeslightsensitivitytogravity,asisthecaseingiantstaratmospheres,butthee?ectismuchlessnoticablethanfortheKIlines.ThespectrumoftheL2dwarfG196-3Bexhibitssimilarfeatures.Thedeepabsorptionintherange1.14-1.20μmcanbeattributedtoamixtureofVO,TiOandpossiblyH2O.Wehavecon?rmedtheidenti?cationoftheTiOandVObandsbyobservingtheseobjectsatshorterwavelengths(z-band)wherethefundamentaltransitions(?ν=0)areeasilydetected.WeconcludethatthepresenceofTiO&VO,weakKIlinesandweakFeHabsorptioninthenear-IRspectraofKPNO-Tau4andG196-3Bareindicativeoflowsurfacegravityandhencesupporttheiridenti?cationasyoung,low-massbrowndwarfs.Figure2showsLRISspectraofIOVir,KPNO-Tau4,2MASSWJ613(hereafter,2M0149+29;fromLiebertetal.1999),G196-3B,andKelu-1(fromKirkpatricketal.1999).Thetopthreespectrashowthee?ectsofdi?eringgravityinlateMstars.TheMgiantIOVirexhibitsthehallmarksoflow-gravity,withveryweakornon-existentabsorptionsbyKIandNaI,weakabsorptionbyCaH,andstrong,bowl-shapeddepressionsbyVO.Thepresumablysolar-age?elddwarf2M.5V)showsthehallmarksofhighergravityCstrongabsorptionsbyKIandNaI,strongabsorptionbyCaH,andlesspronouncedfeaturesattributabletoVO.ThespectrumofKPNO-Tau4showsspectroscopiccharacteristicsintermediatebetweenthe?elddwarfandthegiant(cf.Brice?noetal.2002).Suchagravitysignatureisexpectedforadwarfthatismuchlessmassiveand/ormoreex-tendedthanahighermass?eldcounterpartofsimilartemperature.ThisresultisconsistentwithKPNO-Tau4beingyoung.ThebottomtwospectraofFigure2showtheyoungL2dwarfG196-3Bcomparedtothepresumablyolder?eldL2dwarfKelu-1.Againthespectrumoftheyounger(lower-mass)objectshowsthesignaturesoflowergravity,mostnotablytheweakerlinesofthealkalielementsKI,RbI,NaI,andCsI.TheLiIlineshowstheoppositee?ect,aslithiumhasbeenlargelydepletedinthemoremassiveKelu-1,butisretainedinthelowermassbrowndwarfG196-3B(Reboloetal.1992).WenowconsidertheσOrionisobjects.Basedonthenear-IRandopticalsignaturesoflowgravityestablishedinFigures1and2,thecomparisonoftheJ-bandandopticalspectraofσOri47and51inFigures3and4,respectively,isrevealing.TheJ-bandspectrumofσOri47containsstrongKIlines,moderateFeHabsorptionat1.20μmandrelativelydeepH2Oabsorptionat1.34μm.Ontheotherhand,theJ-bandspectrumofσOri51hasveryweakKIlinesandveryweakFeHabsorption.AlthoughtheTiOandVObandsarenotdetected,theweaknessoftheKIlinesandFeHbandintheJ-bandspectrumofσOri51is包含各类专业文献、文学作品欣赏、外语学习资料、生活休闲娱乐、幼儿教育、小学教育、应用写作文书、Identifying 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