A Systematic Study of X-Ray Flares from Low-Mass Young Stellar Objects in the $ ho$ Ophiuchi Star-Forming Region with Chandra

摘要

WereportontheresultsofasystematicstudyofX-rayflaresfromlow-massyoungstellarobjects,usingtwodeepexposureChandraobservationsofthemainregionofthe$$ ho$$Ophiuchistar-formingcloud.From195X-raysources,includingclassIa€“IIIsourcesandsomeyoungbrowndwarfs,wedetectedatotalof71X-rayflares.Mostoftheflareshavethetypicalprofileofsolarandstellarflares,fastriseandslowdecay,whilesomebrightflaresshowunusuallylongrisetimescales.Wederivedthetime-averagedtemperature($$langlekT angle$$),luminosity($$langleL_{mathrm{X}} angle$$),riseanddecaytimescales($$ au_{mathrm{r}}$$and$$ au_{mathrm{d}}$$)oftheflares,findingthat(1)classIa€“IIsourcestendtohaveahigh$$langlekT angle$$,whichsometimesexceeds5keV,(2)thedistributionof$$langleL_{mathrm{X}} angle$$duringflaresisnearlythesameforallclassesfrom$$sim10^{29.5}$$to$$sim10^{31.5},mathrm{erg},mathrm{s}^{-1}$$,althoughthereisamarginalhintofahigher$$langleL_{mathrm{X}} angle$$distributionforclassIthanclassIIa€“III,and(3)positiveandnegativelog-linearcorrelationsarefoundbetween$$ au_{mathrm{r}}$$and$$ au_{mathrm{d}}$$,and$$langlekT angle$$and$$ au_{mathrm{r}}$$.Inordertoexplaintheserelations,weusedtheframeworkofmagneticreconnectionmodelwithheatconductionandchromosphericevaporationtoformulatetheobservationalparameters($$ au_{mathrm{r}}$$,$$ au_{mathrm{d}}$$,and$$langlekT angle$$)asafunctionofthepre-flare(coronal)electronicdensity($$n_{mathrm{c}}$$),thehalf-lengthofthereconnectedmagneticloop($$L$$),andmagneticfieldstrength($$B$$).Theobservedcorrelationsarewellreproducediflooplengthsarenearlythesameforallclasses,regardlessoftheexistenceofanaccretiondisk.Theestimatedlooplengthisalmostcomparabletothetypicalstellarradiusoftheseobjects($$10^{10}hbox{--}10^{11},mathrm{cm}$$),whichindicatesthattheobservedflaresaretriggeredbysolar-typeloops,ratherthanlargerones($$sim10^{12},mathrm{cm}$$)connectingthestarwithitsinneraccretiondisk.Thehigher$$langlekT angle$$observedforclassIsourcesmaybeexplainedbyaslightlyhighermagneticfieldstrength($$approx500,mathrm{G}$$)thanforclassIIa€“IIIsources(200a€“300G).