On the origin of brown dwarfs and free-floating planetary ma

(abridged) Sub-stellar mass objects have fundamentally different binary properties than stars implying that they cannot have the same formation history. They are rather unfinished stars having either been ejected by their siblings from compact groups of ac

Mon.Not.R.Astron.Soc.,accepted19thSept.2003,000,000–000(0000)Printed2February2008A(MNLTEXstyle lev2.2)Ontheoriginofbrowndwarfsandfree- oatingplanetarymassobjects

1,2,31PavelKroupa,JeromeBouvier1

2Institut

arXiv:astro-ph/0309645v1 23 Sep 2003Laboratoired’Astrophysiquedel’ObservatoiredeGrenoble,BP53,F-38041GrenobleCedex9,Francef¨urTheoretischePhysikundAstrophysikderUniversit¨atKiel,D-24098Kiel,Germany3HeisenbergFellow2February2008ABSTRACTBricenoetal.reportasigni cantlysmallernumberofbrowndwarfs(BDs)perstarintheTaurus-Auriga(TA)pre-mainsequencestellargroupsthaninthecentralregionoftheOrionNebulacluster(ONC).Also,BDshavebinarypropertiesthatarenotcompatiblewithastar-likeformationhistory.ItisshownherethattheseresultscanbeunderstoodifBDsareproducedasejectedembryoswithadispersionofejectionvelocitiesofabout2km/sandifthenumberofejectedembryosisaboutoneperfourstarsborninTAandONC.TheBricenoetal.observationisthuscompatiblewithauniversalBDproductionmechanismandauniversalIMF,buttherequirednumberofBDsperstarismuchtoosmalltoaccountfortheoneBDperstardeducedtobepresentintheGalactic eld.TherearetwoothermechanismsforproducingBDsandfree- oatingplanetary-massobjects(FFLOPs),namelytheremovalofaccretionenvelopesfromlow-massproto-starsthroughphoto-evaporationthroughnearbymas-sivestars,andhyperboliccollisionsbetweenproto-starsindenseclusters.ThethirdBD avour,thecollisionalBDs,canbeneglectedintheONC.ItisshownthattheobservedIMFwitha atteningnear0.5M⊙canbere-producedviaphoto-evaporationofproto-starsiftheyaredistributedaccordingtoafeaturelessSalpeterMFabovethesub-stellarmasslimit,andthatthephoto-evaporatedBDsshouldhaveasmallerve-locitydispersionthanthestars.Thenumberofphoto-evaporatedBDsperstarshouldincreasewithclustermass,peakinginglobularclustersthatwouldhavecontained

manystarsasmassiveas150M⊙.Therequirednumberofembryo-ejectedBDsinTA

andtheONCcanbeaslowas6ejectedBDsper100starsifthecentralONCcontains

0.23photo-evaporatedBDsperstar.Alternatively,iftheassumptionisdiscardedthat

embryoejectionmustoperateequallyinallenvironments,thenitcanbearguedthat

TAproducedaboutoneejectedBDperstarleadingtoconsistencywiththeGalactic-

eldobservations.Thedispersionofejectionvelocitieswouldbeabout3km/s.Inthe

centralONCthenumberofejectedBDsperstarwouldthenbeatmost0.37,orlessif

photo-evaporatedBDscontribute.Thisnon-universalscenariowouldthusimplythat

theGalactic- eldBDpopulationmaymostlystemfromTA-likestarformationor

modestclusters,theONCnotbeingabletocontributemorethanabout0.25±0.04

BDsperstar.

Keywords:stars:formation–stars:low-mass,browndwarfs–binaries:general–

openclustersandassociations:general–Galaxy:stellarcontent

1INTRODUCTION

Thefailureuntilafewyearsagoto ndsub-stellar-massobjects(m<0.08M⊙)posedalong-standingunsolvedproblembe-causetheoreticalconsiderations(e.g.Boss1986,2001;Kumar2001)suggestedthatopacity-limitedfragmentationmayproceeddownto0.001 0.01M⊙.Accordingtothisargument,adensemolecularcloudregioncontractswithconstanttemperatureandrisingdensitysothattheJeansmassbecomessmallerallowingsmallerfragmentstoformastimeprogresses.Thecollapse

(abridged) Sub-stellar mass objects have fundamentally different binary properties than stars implying that they cannot have the same formation history. They are rather unfinished stars having either been ejected by their siblings from compact groups of ac

2Kroupa,Bouvier

leadstoanincreasingopacitywhichultimatelyincreasessu cientlytoprohibitradiativecoolingofthecore.ThecoreheatsupandtheJeansmassincreases,thusleadingtotheaboveminimumfragmentationmass.Ontheotherhand,argumentshadbeenputforward(e.g.Adams&Fatuzzo1996)thatthehydrostaticcore,whichformswithinafragmentonceopacitystopssigni cantradiativecooling,wouldcontinuetoaccretefromanenvelope,whichalwaysexceedsthehydrostaticcoremassbylargefactors,untilfeedbackreversestheinfall.Feedbackenergycomesfromcollimatedout owsandtheluminosityoftheprotostarduetoaccretionluminosity,deuteriumburningand nallyhydrogenburning.Thetypicalconditionsindensemolecularcloudregionsaresuchthattheformationofsub-stellarmassobjects,thatrequireveryfeeblemass-accretionrates,isunlikely.This ttedratherwellwiththenon-detectionoftheseobjects,andonecouldbeginfeelingsomewhatcomfortablewiththisnullresult.

However,sinceafewyearsagosub-stellar-massobjectsarebeingdiscoveredinincreasingnumbers(e.g.Basri2000).<Sub-stellar-massobjectscanbesplitintotwobroadcategories,browndwarfs(BDs,0.01<～m/M⊙～0.08)andlow-massBDsorbitingastaroraBD,orfree- oatingplanetary-massobjects(FFLOPs,m<～0.01M⊙≈10MJ,MJ=Jupitermass).Di erentiatingbetweenmassiveplanetsandlow-massBDsorbitingastarisdi cultobservationally.Theoretically,theycanbedistinguishedaccordingtowhethertheirformationinvolvedtheinitialcondensationofsolidsinacircum-stellardisk(yieldingplanets)orthecollapseofacloudordiskfragment(e.g.Kumar2002).TheinternalstructureandconstitutionofBDsandFFLOPsisover-viewedbyChabrier&Bara e(2000).

BDsandFFLOPshavebeenfoundinanumberofveryyoungclusterswheretheyaremosteasilydetectedgiventhattheyfadewithtime(Lucas&Roche2000;ZapateroOsorioetal.2000,Muench,Lada&Lada2000;Bouvieretal.2002;Morauxetal.2003;Muenchetal.2002,2003).Measurementsofthemassfunctions(MFs)in veclustersindicatethattheirdistributioncanbedescribedwithapower-lawfunction,ξ(m)∝m α,whereξ(m)dmisthenumberofstars(orBDsandFFLOPs)inthemassintervalm,m+dm,andα=+0.5±0.1betweenabout0.03and0.3M⊙(Bouvieretal.2002).TheinitialMF(IMF)ofstarscanberepresentedbythestandardformα=+1.3±0.5,0.08≤m/M⊙≤0.5andα=+2.3±0.3,0.5<m/M⊙(Kroupa2002).SuchstructureintheIMFcontainsinformationontheprocess …… 此处隐藏：54465字，全部文档内容请下载后查看。喜欢就下载吧 ……