The Starburst-Interstellar Medium Interaction of NGC 1569 I.(3)

guresare3′′.1by3′′.1,andtheorientationofeachimageisthesameasFigure1.

4.

4.1.DiscussionGlobalPropertiesofSources

WenowcompareourabsolutemagnitudesandcolorsoftheNGC1569WRstars(Ssources;Table2)toaveragedLargeMagellanicCloud(LMC)WRstellarmagnitudesandcolorstakenfromFeitzinger&Isserstedt(1983).ThecomparisonwithLMCstarsisalogicalassumptionsinceNGC1569isclassi edasImandtheirmetallicitiesaresimilar.AverageSabsolutevisualmagnitudes(MV=-7.43)bestagreewithaveragelate-typeWNmagnitudes(MV=-6.26&(B-V)o=-0.07;Feitzinger&Isserstedt1983).ThecolorsoftheSsources((B-V)o=-0.2)arecomparablewithearly-typeWNstars(MV=-4.39&(B-V)o=-0.22;Feitzinger&Isserstedt1983).However,theuncertaintiesduetohighreddeningandtomeasuredmagnitudesallowfortheagreementofcolorsoftheLMCWNLWRstarsandSsources.Therefore,wehypothesizethattheseSsourcesaresimilartoLMCWNLstars.SpectroscopyoftheindividualsourceswouldneedtobeperformedinordertoclassifyeachWRstar.

TheHeiiluminosityofeachSsourcewascheckedtoGalacticandLMCWRstellarluminosities(2.6×1035–2.9×1036ergss 1;Drissenetal.1999andreferencestherein).Forreference,theaverageHeiiluminosityforanearly-typeWN(WNE)WRstaris

5.2±2.7×1035ergss 1and1.6±1.5×1036foralate-typeWNWRstar(Schaerer&Vacca1998andreferencestherein).We ndallSsourcestohavesimilarluminositiestotheGalacticandLMCWRstarswithS3(L(Heii)=4.76×1035ergss 1)atthelowerendoftheluminositiesandS2(L(Heii)=2.04×1036ergss 1)atthetop.S3andS4haveHeiiluminositiessimilartotheaverageWNEHeiiluminosityreportedbySchaerer&Vacca

We present the detection of He II sources in the Im galaxy NGC 1569 from HST WFPC2 imagery. Out of the fifteen detections, seven were Wolf-Rayet stars, five were stellar clusters with associated He II emission, and three were sources of unknown origin. The

(1998),andallotherSsourcesare1σoutsideofthisvalue.Massey&Hunter(1998a) ndthat30DoradushasseveralO3If*starswhichwerethoughttobeWNstarsinprevioussurveys.ThesestarsalsoemitHeiiandhaveabsolutevisualmagnitudessimilartoourSsources.However,theHeiiequivalentwidthofOfstarsistypicallyaround5-10 A(Notaetal.1996),andCrowther&Dessart(1998)stateaHeiiluminosityof1.7×1035ergss 1foroneO3If*/WNstar.Schaerer&Vacca(1998andreferencestherein)alsoreportaHeiiluminosityforOfstarsof2.5×1035ergss 1.Thisamountisafactorof2smallerthanthefaintestSsourceinHeii.Furthermore,thedetectionofO3starswouldsuggestanage～1Myr(Massey&Hunter1998a),whichcontradictsthemostrecentagescalculatedfortheclustersinNGC1569(4-6Myr;Hunteretal.2000).Also,oursensitivitylimitofthreetimesthestandarddeviationofthemeanbackgroundgivesaminimumHeiiluminosityof2.4×1035ergss 1whichplacestheweakestWRstarsandOfstarsatourdetectionthreshold.WemustconcludethattheSsourcesareWolf-Rayetstars(mostlikelyWNstars),butspectroscopywillhavetobeperformedtodeterminetheirspectraltype.

FromFigure2weseethatthestellarsourcesareconcentratedclosetoSSCA.Thissupportstheideathatthemostrecentstarburstingregionisprimarilyconcentratedaroundthesuper-starclustersA&B.ThenumberofWRstarssurroundingSSCAisgreatercomparedtoSSCB(5versus1).Gonz´alez-Delgadoetal.(1997)didnot ndWRsignaturesinthespectraaroundSSCBandKobulnickyandSkillman(1997;cf.theirFigure1)havespectraovertheselocationsandovermostofthestellarsourceslistedhere.They ndnebularHeiiinthosespectraonly.ThisleadsustosuspectthatsomeofthepointsourcesandHeiiemissionarecoincidental,orthatthestarisphotoionizingtheinterstellarmediumtoproduceHeii.However,Conti(1999)bringsupavalidpointinthatsomestarburstgalaxieswiththesameagedonotshowWRstarsandthiscouldbeexplainedbythefactthatcontinuumdilutionofthebackgroundstarlightcouldmaskoverthebroad,faintWRemissionlines.Sincethesespectracamefromground-basedobservatoriesandtheslitused

We present the detection of He II sources in the Im galaxy NGC 1569 from HST WFPC2 imagery. Out of the fifteen detections, seven were Wolf-Rayet stars, five were stellar clusters with associated He II emission, and three were sources of unknown origin. The

inbothpaperswaswide(≥1′′.5),itispossiblethatcontinuumdilutionoverwhelmedtheHeiiemissionfromtheseWRstarsinoneorbothcases.Finally,theWolf-RayetstarspectroscopicallydiscoveredbyDrissen&Roy(1994)isourS7.

Fouroftheclustersarealsofound(seeFigure2)inthelargestregionofstarformation.OneclusterisontheEasternoutskirtsofthelargestarformationregion.ThefourclusterswithinthestarburstshowbothredstarsandWRstars(seeFigure5).ThereasonweknowthatthereareredstarscomesfromDrissenetal.(1999).TheyreportedthatF469N-F555Wwouldproduce“holes”whereredstarswere,andtypicallythereare“holes”atthecenterofeachcluster.Thus,theredstarsareseparatedfromtheHeiiemission.Thiswas rstreportedinDeMarchietal.(1997)asasuperpositionoftwoclusterswhichmakeupSSCA.InFigure1oftheirpaper,theylabeltheWestclusterSSCA1andtheEastSSCA2(theorientationoftheir gureandoursisroughlythesame;seeourC4inFigure5d).SSCA1isslightlyredderthanSSCA2.InourHeiicontours,thissplittingisalsoseen.We ndthatSSCA2iswherethestrongestHeiiemissioniswhileSSCA1iscenteredaroundthe“hole”intheHeiicontours.ThisdetectionofredsupergiantsalongwithWolf-Rayetstarshasbeenwellreportedinthepastandisconsistentwiththeevolutionofmassivestars(e.g.,Massey1999).

Thelocationsofthe“unknownsources”areplottedinFigure2.ThethreeUsourcessurroundSSCA.Thisregionis lledwithHeiiemissionwhichissimilarinsurface

brightnessandmayberelatedtostellarwind-shocks,orevensupernova(e).Therefore,ingtheGalacticandLMCHeiiluminositiesabove,weseethattheHeiiemissionoftheunknownsourcesiscomparable(seeourTable4).

We present the detection of He II sources in the Im galaxy NGC 1569 from HST WFPC2 imagery. Out of the fifteen detections, seven were Wolf-Rayet stars, five were stellar clusters with associated He II emission, and three were sources of unknown origin. The

4.2.NotesonIndividualSources

Below,commentsaregivenoneachofthedetectionsthatwerereportedi …… 此处隐藏：6045字，全部文档内容请下载后查看。喜欢就下载吧 ……