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TheRMSSurveyTestingmodelsofmassivestarformation
James S. Urquhart1, Melvin G. Hoare2, Stuart L. Lumsden2, Rene D. Oudmaijer2, Mark A. Thompson3, Toby J. T. Moore4, Joe C. Mottram5 and Ben Davies2,6
contact: James Urquhart phone: (+61) 9372 4360 email: [email protected] web: http://www.atnf.csiro.au/
Introduction Althoughmassivestars(>8M��and103L�)makeuponlyafewpercentofthestellarpopulation theyplayahugely important role inmanyastrophysicalprocesses fromthe formation of the Dirst solid material in the early Universe (Dunne et al. 2003,Nature 424, 285); to their substantial inDluence upon the evolution of their hostgalaxies and future generations of star formation. However, despite decades ofresearchrelativelylittleisknownabouttheunderlyingphysicalprocessesinvolvedintheformationofmassivestars.Thislackplacesamajorhurdleinthewayofstudiesofgalaxy formation and evolution (e.g. Silk 2003, Where Cosmology & FundamentalPhysicsMeet).Recent theoretical 3D hydrodynamic simulations suggest that massive stars areformed throughdisk accretion (e.g.,Krumholtz et al., 2009, Science, 323, 754), in ascaled‐upversionof low‐massstar formation.Weplantoexploitanewlydevelopedsampleofmassiveyoungstellarobjects(MYSOs)andtwonewcuttingedgefacilities;theAtacamaLargeMillimetreArray(ALMA)andtheHerschelSpaceObservatory.TheRedMSXSource(RMS)surveyprovidesalargeandunbiasedfoundationforthelargescale high resolution investigations that are permitted for the Dirst time by theexquisitesensitivityandangularresolutionofALMA.
Figure1: GalacticdistributionofthecompleteRMSsampleofMYSOsandUCHIIregionswithbolometric luminosites>104L.TheRMSdistribution is superimposedovera sketchofhowtheGalaxyisthoughttoappearifviewedface‐on.ThisimagehasbeenproducedbyRobertHurtoftheSpitzerScienceCenterinconsultationwithRobertBenjaminandattemptstosynthesisemanyofthekeyelementsofGalacticstructureusingthebestdatacurrentlyavailable.
Figure2:Leftpanel:HistogramofthedistributionofRMSsourcesasafunctionofluminosity(bin size used is 0.5 dex). Right panel: Luminosity distribution as a function of heliocentricdistance.ThedarkgreylineandlightgreyshadingindicatesthelimitingsensitivityofRMSanditsassociateduncertainty.
Summary High‐resolutionandsensitiveobservationsofawell‐selectedsampleofyoungmassivestarswill provide crucial information on the frequency and properties of accretiondisks, andmolecular outDlows, as a function of luminosity and provide awealth ofaddition informationwithwhich to investigate the inDluenceof environmenton theformationofmassivestars.ThisprojectwillleaveahighqualitylegacydatasetwithclearlydeDinedsourceparametersagainstwhichtheoreticalmodelscanbetested.
(1)AustraliaTelescopeNationalFacility,CSIROAstronomyandSpaceScience,POBox76,EppingNSW1710,Australia,(2)SchoolofPhysicsandAstrophysics,UniversityofLeeds,Leeds,LS2,9JT,UK,(3)CentreforAstrophysicsResearch,ScienceandTechnologyResearchInstitute,UniversityofHertfordshire,CollegeLane,Hatfield,AL109AB,UK,(4)AstrophysicsResearchInstitute,LiverpoolJohnMooresUniversity,TwelveQuaysHouse,EgertonWharf,Birkenhead,CH41,1LD,UK,(5)SchoolofPhysics,UniversityofExeter,Exeter,EX74QL,UK,(6)RochesterInstituteofTechnology,54LombMemorialDrive,Rochester,NY14623,USA
Research outline and goals Akeypredictionof themonolithic collapseandcompetitiveaccretionmodels is thepresenceofacircumstellardisk,a common featureof low‐massstar formation (e.g.Lada 1999, OSPS, 143). While the detection of disks around massive young starswouldconDirmtheaccretionscenario,relatively fewhighmassdiskcandidateshavebeenfoundtodate(Cesaronietal.2008,ApSS,313,23),andnonehavebeenfoundtowardsO‐typestars.TotestthesemodelsforthemostmassiveO‐typestars,whereradiationpressureandphotoevaporation of the accretion disk and protostellar envelope may play asigniDicant role in the formation process, requires an increase in sensitivity andangularresolutionofanorderofmagnitudethaniscurrentlyavailable—ALMAwillprovide the sensitivity and angular resolution necessary. We plan to use ALMA toconductasystematicstudyofcircumstellardiskstowardsalargeluminosityselectedsampleofthemostmassiveO‐typestars.Thesedatawillnotonlybeusedtomeasurethe frequency of MYSO circumstellar accretion disks, but will also correlate diskpropertieswiththoseoftheirMYSOhosts,providingstringentobservationaltestsofmassivestarformationtheories.Inparticular,wewilltestthefollowingpredictionsofthemassivestarformationmodels:1) Diskmasstostellarmassratio,diskradiusandspiralstrengthareallproportional
tostellarmass,andhenceluminosity(Krumholzetal.2007,ApJ665,478).2) Disk frequency shouldbe a strong function ofwhether competitive accretion or
monolithic collapse is thedominantprocess (Bonnell&Bate2007,MNRAS370,488;Krumholzetal.2007,ApJ665,478).
3) Monolithic collapse models predict a high degree of collimation in molecularoutDlows(Arceetal.2007,Protostars&PlanetsV,245).
TheRMSsamplehasanumberofadvantagesovercurrentsamples,mostofwhicharebasedonlowerresolutiondataobtainedbytheIRASsatellite(~25timeslowerthanMSXat25µm)andarethereforebiasedtowardsbright,isolatedsourcesandtendtoavoiddenseclusteredenvironmentsandtheGalacticmid‐planewherethemajorityofMYSOs are expected to be found. The RMS survey provides awell‐selected samplewithsufDicientnumbersofsourcesineachluminositybintoallowstatisticalstudiesasafunctionofluminosity(seeFig.2forluminositydistribution).WewillalsoextendtheRMStolongerwavelengths,inparticulartoencompassthe70‐500µmHerschelHi‐Galsurvey(Molinarietal,2010,PASP122,314)anddatafromtheSCUBA2legacysurveys.WhencombinedtheHiGal,SCUBA2andarchivalSpitzer
The RMS survey TheRMSsurvey(Urquhartetal.2008,ASPCS,387,381)has identiDieda largewell‐selected and well‐characterised sample of young massive stars (e.g., distance andluminosity; www.ast.leeds.ac.uk/cgi‐bin/RMS/RMS_DATABASE.cgi). By combininginfrared,millimetreandradiowavelengthobservationswithcomplementaryarchivaldata we have identiDied ~1300 MYSOs and ultra‐compact HII regions locatedthroughouttheGalacticplane(b<5°;seeFig.1)—anorderofmagnitudelargerthanpreviouslyavailable.
data provide complete coverage from the mid‐infrared to the submillimetrewavelengths with comparable angular resolution at the longer wavelengths. ThesedatasetswillcomplementthedataalreadycompliedfortheRMSsampleandwillbeused to improvemodel Dits to source spectral energydistributions (SEDs), allowingthedust temperatures,massesand luminosities tobebetter constrained (seeFig.3forschematicofwavelengthcoverage).
Figure 3: Spectral energy distribution forthe RMS source G011.9454‐00.0373. TheSED has been Dit using the 2MASS, MSX,MIPS 70 µm, SCUBA 850 µm andMAMBO1.1 mm which are shown as Dilled circles.The coloured bands show the wavelengthcoverageofanumberof theGalacticPlanesurveysthathaverecentlybeencompleted,areunderwayorareduetobegin.
