Correlation between the spatial distribution of th

arXiv:astro-ph/0610401v323Oct2006Astronomy&Astrophysicsmanuscriptno.examplecESO2008February5,2008CorrelationbetweenthespatialdistributionofcircumstellardisksandmassivestarsintheopenclusterNGC6611⋆.Compiledcatalogandclusterparameters.M.G.Guarcello1,2,L.Prisinzano2,G.Micela2,F.Damiani2,G.Peres1,andS.Sciortino21DipartimentodiScienzeFisicheedAstronomiche,Universit´adiPalermo,PiazzadelParlamento1,I-90134PalermoItaly2INAF-OsservatorioAstronomicodiPalermo,PiazzadelParlamento1,90134PalermoItalyABSTRACTContext.Theobservationofyoungstarswithcircumstellardiskssuggeststhatthedisksaredissipated,startingfromtheinnerregion,bytheradiationofthecentralstarandeventuallybytheformationofrockyplanetesimals,overatimescaleofseveralmillionyears.ItwasalsoshownthatstrongUVradiationemittedbynearbymassivestarscanheatacircumstellardiskuptosomethousanddegrees,inducingthephotoevaporationofthegas.Thisprocessstronglyreducesthedissipationtimescale.Aims.WestudywhetherthereexistsacorrelationbetweenthespatialdistributionofstarswithcircumstellardisksandthepositionofmassivestarswithspectralclassearlierthanB5,intheopenclusterNGC6611.Methods.Wecreatedamultibandcatalogofthecluster,downtoV∼23m,usingopticaldatafromaWFIobservationat2.2mofESOintheBVIbands,the2MASSpublicpointsourcecatalogandanarchivalX-rayobservationmadewithCHANDRA/ACIS.Weselectedthestarswithinfraredexcess(duetotheemissionofacircumstellardisk)usingsuitablecolorindicesindependentofextinction,andstudiedtheirspatialdistribution.Results.ThespatialdistributionofthestarswithKbandexcess(duetothepresenceofacircumstellardisk)isanticorrelatedwiththatofthemassivestars:thedisksaremorefrequentatlargedistancesfromthesestars.WearguethatthisisinagreementwiththehypothesisthatthecircumstellardisksareheatedbytheUVradiationfromthemassivestarsandphotoevaporated.Keywords.stars:formation-planetarysystems:protoplanetarydisks-stars:pre-mainsequence-openclusterandassociations:individual(NGC6611)-infrared:stars-X-ray:stars1.IntroductionCircumstellardisksareimportantfeaturesofstarformationprocessformanyreasons.Forexample,theyregulatethean-gularmomentumevolutionofthestarsandcanevolveinplan-etarysystems.Disksformduringthegravitationalcontractionoftheprotostellarnebulaandaredissipatedovertimescaleofseveralmillionsyears(see,forexample,Haischetal.2001).Whenitisnotpossibletodirectlyobservethecircum-stellardisks,asintheneareststar-formingregions,thediskdustemissionintheinfraredbandsisusedtodetectandstudythem.However,observationsshowadeclineintheemis-sionoftheinnerregionofthedisks(mostlyNIRandmid-infraredbands)between∼1and∼10Myr(see,forexample,Sicilia-Aguilaretal.2005;Calvetetal.2005).Thisdeclinecanbeexplainedasthenormalevolutionofthedisks,disruptedfirstSendoffprintrequeststo:[e-mail]⋆BasedonobservationsmadewiththeEuropeanObservatorytele-scopesobtainedfromtheESO/ST-ECFScienceArchiveFacility.intheinnerregionbytheradiationofthecentralstarandbytheformationofrockyplanetesimals,thatarethoughttooccurfastestintheinnerdisks(Podolaketal.,1993).Starswithtran-sitiondisksthatarenotcharacterizedbyNIRemissionbutthathavestrongmid-andfar-infraredemissionhavebeenobserved.Theevolutionofcircumstellardisks,however,dependsstronglyontheenvironmentwheretheyformed.Itwasob-servedthatstrongUVradiation(mostlythefarUV,withphotonswithenergybetween6.0eVand13.6eV)fromthemassivestarscanheatthetemperatureofcircumstellardisksuptothousandsofdegrees,inducingthephotoevapo-rationofthegas(Hollenbach,1994;Johnstoneetal.,1998;St¨orzer&Hollenbach,1999;Adamsetal.,2004).Thediskmasslossrateisindependentofthedistancefromthecen-tralstars(Hollenbach,1994)andsincethesurfacedensityde-creaseswithincreasingdistancefromthecentralstar(Pringle,1981),thedisksaredisruptedstartingfromtheouterregions.Sotheevolutionandthethermalstructureofdisksnearmassivestarsisverydifferentfromthoseofisolateddisks.Forexam-2M.G.Guarcelloetal.:SpatialdistributionofthecircumstellardisksinNGC6611ple,Johnstoneetal.(1998)shownthattheouterregionofacircumstellardiskwithamassequalto0.2solarmassesandadistancelessthan1017cmfromthestarΘ1Ori(spectralclassO6)isdisruptedin106yearsuptoaradiusofabout1AUfromthecentralstar.Toinvestigatesucheffects,westudythestarswithcircum-stellardisksintheyoungopenclusterNGC6611.Thisclus-terischaracterizedbyapopulationofpre-mainsequencestarsyoungerthan3millionofyears(seeSect.4.2)andbyalargenumberofmassivestars(56starswithspectralclassearlierthanB5,Hillenbrandetal.1993).Themassivestarsaredis-tributedirregularlyintheregion,thereforetheUVfluxisnon-uniforminthecluster.NGC6611isanidealtargettostudytheeffectsofUVradiationontheevolutionofthecircumstel-lardisksoflowmassstars.TheclusterisalsowithintheEagleNebulathatstronglyabsorbstheradiationofbackgroundstars.Thiswillbeveryhelpfultoestimatethedistanceoftheclusterandtoavoidalltheproblemsrelatedtocontaminatingback-groundstars.Hillenbrandetal.(1993)(thereafterH93)estimatedtheageofthepre-mainsequencestarsofNGC6611asbe-tween0.25and1millionofyears,theclusterdistanceas2.0±0.1kpcandderivedananomalousextinctionlaw,withR=3.75.Belikovetal.(1