The Birth of Massive Stars and Star Clusters

In the present-day universe, it appears that most, and perhaps all, massive stars are born in star clusters. It also appears that all star clusters contain stars drawn from an approximately universal initial mass function, so that almost all rich young sta

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aTHEBIRTHOFMASSIVESTARSANDSTARCLUSTERS————–JonathanC.TanInstituteofAstronomy,Dept.ofPhysics,ETHZ-urich,8093Z-urich,Switzerlandjt@phys.ethz.chAbstractInthepresent-dayuniverse,itappearsthatmost,andperhapsall,massivestarsareborninstarclusters.Italsoappearsthatallstarclusterscontainstarsdrawnfromanapproximatelyuniversalinitialmassfunction,sothatalmostallrichyoungstarclusterscontainmassivestars.InthisreviewIdiscussthephysicalprocessesassociatedwithbothmassivestarformationandwithstarclusterfor-mation.FirstIsummarizetheobservedpropertiesofstar-forminggasclumps,thenaddressthefollowingquestions.Howdotheseclumpsemergefromgiantmolecularclouds?Intheseclusteredenvironments,howdoindividualstarsformandgainmass?Canaformingstarclusterbetreatedasanequilibriumsystemoristhisprocesstoorapidforequilibriumtobeestablished?Howdoesfeedbackaffecttheformationprocess?1.IntroductionStarclusters1,arethefundamentalunitsofstarformationingalaxies.MostGalacticstarsareborninclusters(Lada&Lada2003):their gure2implies

thatequalnumbersofstarsareformingineachlogarithmicintervalofclustermass,forclustermassesfrom～50 1000M⊙.Thereisadearthofstarformationinclustersbelow50M⊙.ThesampleofLada&Lada(2003)istoosmalltoconstraintheinitialclustermassfunctionbeyond～1000M⊙.Hub-bleSpaceTelescopeobservationshaveprobedthisrangeinexternalgalaxies, ndingacontinuationofthemassfunctionslope(Larsen2002).ForthedwarfstarburstgalaxyNGC5253,Tremontietal.(2001)haveproposedamodelinwhichallstarformationoccursinclusters,whichthendissolveontimescalesof～10MyrtocreatethesourcesoftheobserveddiffuseUVlight.

Theinitialmassfunctionofstarsinclustersappearslargelyinvariant(Kroupa2002)sothatalmostallrelativelymassiveclusterswillcontainatleastafew1Ide neastarclusterasagroupofstarsthatformstogetherfromagravitationallyboundgasclump.

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