The onset of X-ray emission in young stellar objec

arXiv:astro-ph/0611520v116Nov2006Astronomy&Astrophysicsmanuscriptno.6424cESO2008February5,2008TheonsetofX-rayemissioninyoungstellarobjectsAChandraobservationoftheSerpensstar-formingregionG.Giardino1,F.Favata1,G.Micela2,S.Sciortino2,andE.Winston31AstrophysicsDivision–ResearchandScienceSupportDepartmentofESA,ESTEC,Postbus299,NL-2200AGNoordwijk,TheNetherlands2INAF–OsservatorioAstronomicodiPalermo,PiazzadelParlamento1,I-90134Palermo,Italy3Harvard-SmithsonianCenterforAstrophysics–60GardenStreetCambridge,MA02138,USAReceiveddate/AccepteddateABSTRACTAims.TostudythepropertiesofX-rayemissionfromyoungstellarobjects(YSOs)throughtheirevolutionfromClassItoClassIIIanddeterminewhetherClass0protostarsemitX-rays.Methods.AdeepChandraX-rayobservationoftheSerpensstar-formingregionwasobtained.TheSerpensCloudCoreisideallysuitedforthistypeofinvestigation,beingpopulatedbyadenseandextremelyyoungclusterwhosemembersarefoundinallevolu-tionarystages,includingsixwell-studiedClass0sources.Results.NoneofthesixClass0protostarsisdetectedinourobservations,excludingthepresenceofsourceswithtypicalX-raylumi-nosities∼0.4×1030ergs−1(forcolumndensitiesoftheorderof4×1023cm−2,orAV∼200).Atotalof85X-raysourcesaredetectedandthelightcurvesandspectraof35YSOsarederived.ThereisacleartrendofdecreasingabsorbingcolumndensitiesasonemovesfromClassItoClassIIIsources,andsomeevidenceofdecreasingplasmatemperatures,too.Weobserveastrong,long-duration,flarefromaClassIIlow-massstar,forwhichwederiveaflaringlooplengthoftheorderof20stellarradii.Weinterprettheflaringeventasoriginatingfromamagneticfluxtubeconnectingthestartoitscircumstellardisk.Thepresenceofsuchadiskissupportedbythedetection,inthespectrumofthisstar,of6.4keVFefluorescentemission.Keywords.ISM:clouds–ISM:individualobjects:Serpenscloud–Stars:pre-mainsequence–X-rays:stars1.IntroductionYoungstellarobjects(YSOs)arewell-knownX-raysources,andX-rayobservationshavebecomearoutinetoolforthestudyofstar-formingregions.Besideitsintrinsicastrophysicsinterest,X-rayemissionintheearlystagesofstellarevolutionmayhaveasignificantinfluenceonthecircumstellarenvironment.Asaion-izingsourceforthecircumstellaraccretiondisk,X-rayemissionmayplayanimportantroleinregulatingthecouplingbetweenthediskandthemagneticfieldandaffectthechemistryofthediskitself.X-raysareanimportantcomponentinthecomplexfeedbackprocessesregulatingstar-formation:Nattaetal.(2006)speculateonthepossibilitythattheobservedspreadinaccretionratesobservedinclassicalTTauristars(CCTS)ofanygivenage(whichistoolargetobeexplainedby“classical”accretiondisktheory)mightbeexplainedbythelargespreadinX-rayluminos-ityamongotherwisesimilarpre-mainsequencestars,ifthemainsourceofdiskionizationisindeedprovidedbythestellarX-rayemission.Additionally,asthehotX-rayemittingplasmaneedstobeconfinedbystrongmagneticfields(B≥100G),tracingtheX-rayemissionanditsspatialextentprovidesameanoflocatingstrongmagneticfields,which(inthemagnetosphericaccretionmodeloflow-massYSOs)haveakeyroleinchannelingtheac-cretingplasmaandthusinregulatingtheaccretion.OneopenquestionregardstheonsetofX-rayemission:atwhichevolutionaryphasedoYSOsstarttoemitX-rays?CurrentevidenceshowsthatX-rayemissioniscommon(althoughper-hapsnotuniversal)inClassIsources(e.gOzawaetal.,2005)–Sendoffprintrequeststo:G.GiardinoClassIsourcesareprotostarswhichalreadyhaveanaccretiondiskbutarestillsuppliedwitharelativelymassivecircumstellarenvelope.InClassIIsources(orCTTS,i.e.youngstarsactivelyaccretingfromacircumstellardisk,butwithoutacircumstel-larenvelope)X-rayemissionisanuniversalfeature,asshown,forinstance,bythedeepChandraobservationsoftheOrionNebulaCluster(Flaccomioetal.,2003b;Preibischetal.,2005).Inmoreevolvedsources,thatarenolongeraccreting(ClassIII,orweak-linedTTauristars–WTTS),X-rayemissionisalsoanuniversaloccurrence,withcharacteristicsandluminositysimi-lartoactivemain-sequencestars(e.g.Flaccomioetal.,2003b;Preibischetal.,2005).Class0sourcesareyoungprotostarsatthebeginningofthemainaccretionphase.Observationally,theyarecharacterizedbystrong,centrally-condenseddustcontinuumemissionatsubmil-limeterwavelengths,powerfuljet-likeoutflows,andverylittleemissionshortwardof10μm.AccordingtothedefintiongiveninAndreetal.(2000),theirsubmillimeterluminosity(measuredlongwardof350μm),shouldbegreaterthan0.5%oftheirbolo-metricluminosity–therebysuggestingthattheenvelopemassexceedsthecentralstellarmass.TheevidenceforX-rayemis-sionfromClass0protostarsisstillfragmentary.Tsuboietal.(2001)claimedthediscoveryofX-raysfromtwohighlyembed-dedsourcesintheOMC-2/3clouds,withinafewarcsecsfromtheClass0candidatesMMS2andMMS3.ThetwodetectedsourcesshowsomeClass0characteristics:averylargeabsorp-tion(N(H)=1−3×1023cm−2),nonear-IRcounterparts,andas-sociationswithmillimeterradioclumps.However,follow-upra-dioandnear-IRobservationsbyTsujimotoetal.(2004)didnot2G.Giardinoetal.:TheonsetofX-rayemissioninyoungstellarobjectsunambiguouslyclassifyeithersourceasClass0,andassociatedoneofthemwithemissionfromaproto-stellarjetoriginatinginaClassIsource.AnotherclaimforX-rayemissionfromClass0sourceswasmadebyHamaguchietal.(2