1983ApJ. . .273. .5973 © 1983.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. The AstrophysicalJournal,273:597-623,1983October15 lanic Cloud(LMC).Feast(1953)firstdiscussedthe the giant30DoradusHTiregioninLargeMagel- Agency. Aeronautics andSpaceAdministration, bytheScienceResearch Satellite whichissponsoredand operatedbytheNational Council oftheUnitedKingdom, andbytheEuropeanSpace 1 Radcliffe 136(R136;HD38268)liesatthecenterof GuestObserverswiththeInternational UltravioletExplorer © American Astronomical Society • Provided by the NASA Astrophysics Data System 6 741 _l 3 1 presented. ThedataconsistoflowandhighresolutionspectraobtainedbytheInternational lifetime ofabout2X10yr. vibrational stabilityproblemarediscussed.Thesupermassive starshouldhaveanevolutionary peculiar dustorbythecoalescenceofstarsinaregion highstardensity.Possiblesolutionstothe however, wellknownproblemsassociatedwiththeformationandvibrationalstabilityofsuchstars. properties: L«6X10L,T«75,000K,R*50M*2100MandM«5X10~yr'. properties canbederivedfromtheavailabledata.Itisdeducedtohavefollowingapproximate have adiameteroflessthan1000AU.Suchclusterwouldconsistmore30therarest yielding aterminalspeedof3600kms.ThePCygniSiivXX1394and1403featureisweakor XI640, andNivX1718.TheUnesofv,Civ,Hen,havePCygniprofileswith The inferredstellarstructureisconsistentwithinteriortheoryforsupermassivestars.Thereare, most densestellarregionsknown,andhaveacorecollapselifetimeoflessthan10years.We presented. ThereddeningcorrectionistakenfromanindependentinvestigationbyFitzpatrickand Subject headings:clusters:open—nebulae:individualstars: evolution—stars:interiors It isarguedthattheformationcouldoccurbyordinarystellarcollapseinaregioncontaining consider itmorelikelythattheradiationfromR136aisdominatedbyemissionasingle data span.TheHeilX1640emissionandCivXI550absorptionUnespossiblyexhibitvariabilityat not beenlargechangesintheultravioletcontinuumorlinespectrumofR136aover4years R136a mustbecarefullydesignedtominimizetheeffectsofcontaminationfromneighboringstars. supermassive starorasmallgroupofstars.IfR136aissinglestar,constraintsonits stars foundingalacticsystems,haveastellarmassdensitythreeordersofmagnitudelargerthanthe compact clusterofnormalhotstars,theultravioletdatarequirepresenceabout30veryearlyO of R136aresemblesthatotherhotMagellanicCloudstarssuchasR122(03III).Ifisa following stellarfeaturesaredetected:NvX1240,OX1371,FeXX1420to1490,CivX1550,Hen From anaveragelowresolutionspectrumconsistingofsixindividualsmall(3")aperturespectra,the that fromthebrightcomponentdetectedinvisiblewithspeckletechniques,clusterwould and WNstarswithina3"diameterregion.IftheultravioletemissionfromR136aisdominatedby the ±10%level.TheobservedanddereddenedenergydistributionofR136afrom0.12to20f±mis absent. Theobservedlinespectrumsuggestsaveryhotstarorclusterofstars.Therehave Savage ofdustinthe30Doradusnebula.Theshapedereddenedultravioletenergydistribution Ultraviolet Explorersatelliteovertheperiod1978to1982.Becausefieldiscrowded,studiesof 0 Blair D.Savage,EdwardL.Fitzpatrick,JosephP.Cassinelli,andDennisC.Ebbets Ultraviolet observationsofR136a,thebrightcentralobject30Doradusnebula,are I. INTRODUCTION THE NATUREOFR136a,SUPERLUMINOUSCENTRAL ultraviolet: spectra Washburn Observatory,UniversityofWisconsin-Madison OBJECT OFTHE30DORADUSNEBULA Received 1983February11;acceptedMarch30 ABSTRACT 597 51_l peculiar opticalspectrumofR136,andWalbom(1973&, That suggestionwasinpart basedontheassumption might beasupermassivestarwith250star. Walbom (1981)hasarguedonobservationalgrounds thermore, CMSshowedthatthesepropertiesare pothesis comesfromthespeckleinterferometrymea- compact clusterof15to30veryhotbutnormalstars. form (Kahn1974;Yorke1980)andthattheyarepulsa- are thatstarsmoremassivethanabout100Mcannot star hypothesis.Thetwostandardtheoreticalobjections Although thesespeckleresultsdisagreeonthebrightness conclude thatthedataaremosteasilyunderstoodin carefully examinedtheopticalspectrumofR136aand massive starhypothesis.EbbetsandConti(1982)have that itismorereasonabletoproposeR136aa tionally unstable(SchwarzschildandHärm1959). bright componentisestimatedtobelessthan0702or ing datarelatingtothisunusual sourceandgiveequal ways. Wehaveobtainedmoreinformationaboutthe and positionsoffaintsecondarystars,bothresultsimply surements ofWeigelt(1981)andMeabumetal(1982). strongest newargumentinfavorofthesingle-starhy- terms ofasupermassivehydrogen-burningstar.The region fromFitzpatrickandSavage(1983),wehave line andcontinuumvariability(§V).Usingnewinfor- with muchhighersignal-to-noiseratiosthanthosedis- have producedandanalyzedaverageultravioletspectra presence ofnearbystarsinthecrowdedfield(§III).We ultraviolet dataconcerningR136sofarobtainedbyus. eter. InthecaseofMeabumetal.(1982)data balance totestsofthecluster hypothesisandsupermas- 0 obtained theintrinsicultravioletenergydistributionof mation aboutultravioletextinctioninthe30Doradus cussed earlier(§IV).Wehavesearchedforultraviolet difficulties associatedwithstudiesofR136aduetothe Our analysissupplementsthatinCMSanumberof there isaverybrightcomponentofsmallangulardiam- 0eff R136a (§VI).Finallywehavereevaluatedalltheexist- explanations arealsoconsidered (§VII<7). sive starhypothesis(§VIIZ? and VIIc).Otherpossible 0 1000 AUindiameterforanassumeddistanceof55kpc. R136 wasthefirstextragalacticobjectstudiedinhigh © American Astronomical Society • Provided by the NASA Astrophysics Data System Several objectionscanbedirectedatthesupermassive Several recentobservationshavesupportedthesuper- In thispaperwepresentananalysisofalltheIUE SAVAGE, FITZPATRICK,CASSINELLI,ANDEBBETS lite anditsdetectorsystemarediscussedbyBoggess length coverageandresolutionisprovidedattheendof dispersion spectraofR136listedinTable1.Thesatel- etal. (1978a,b).Summaryinformationaboutwave- necessary toprocesstheminauniformwayorder Table 1.Sincethedataextendover4years,itwas Washburn extractionprogramdescribedbyKoomneef and extractedattheGoddardSpaceFlightCenterusing R136a. Thehigh-dispersionspectrawereallprocessed search forcontinuumandspectmmlinevariabilityof the intensitytransferfile(ITF)ineffectafter1979July olution observationstoabsolutefluxes,thecalibration using theWashburnprogram.Inconvertinglow-res- and deBoer(1979).PixelsaffectedbytheITFproblem length dependentbutislessthanabout8%sincethe curve ofBohlinandHolm(1980)wasused.Thisproce- are automaticallycorrectedpriortoextractionwhen launch ofIUE(Schiffer1982). dure doesnotallowforinstrumentaldegradationover the 4yearsofobservations.Thedegradationiswave- 11. Alllow-dispersionspectrawereextractedusingthe been producedbyaveragingalllow-resolutionsmall- in individualspectraweredeterminedbyaligninginter- Á, respectively.Priortoaveraging,theindividualspectra were resampledbylinearinterpolationintoauniform aperture spectraandselectedhigh-resolutionlarge-aper- wavelength grid. dispersion observationswerelessthan+4Aand0.06 ture spectra.Smalladjustmentstothewavelengthscales stellar features.Theshiftsappliedtothelowandhigh be inferredfromthelow-dispersion,large(10"X20") it necessarytomodifytheangular scalegiveninFigure Feitzinger etal(1980).Inproducing Figure1wefound aperture IUEobservations.Figure1containsasketchof light distributioninthevicinityofR136.Inthesedata (1980) haveprovideddetailedisophotesofthevisual found inWalbom(1973Z?).Inaddition,Feitzingeretal. field. Asequenceofvisualphotographsthisfieldis ble withtheirFigure2.The adjustmentwemadeis 4 ofFeitzingeretal.inorder for thedatatobecompati- ponents bandccontributetothe“comma”shape b, andc—withcomponentabeingthebrightest.Com- the R136fieldasinferredfromFigures2and4of ponents bandcareapproximately6timesfainterthan R136 appearstobecomposedofthreecomponents—a, component a. associated withtheobject.Atvisualwavelengths,com- The IUEsatellitewasusedtoobtainthelowandhigh To improvethesignal-to-noiseratio,spectrahave Information abouttheultravioletextentofR136can R136 isthebrightestobjectatcenterofacrowded III. ULTRAVIOLETSPATIALEXTENTOFR136 II. OBSERVATIONS Vol. 273 1983ApJ. . .273. .5973 No. 2,1983 plotted withthesameangularscaleasstarfieldand large-entrance aperturefortwosetsofobservations With thisadjustment,theatobandcseparations with theindicatedlarge-apertureorientations.Theyare of theWashburnextractionprogram(seedeBoer, (Bohlin etal.1980).Thescans,whichruninthedirec- R136 fieldfromIUElow-dispersionimagesobtained can befoundinTable1. For allobservationsthepositionangleoflarge-aper- R136 areindicatedbythesolidanddashedovalUnes. approximately perpendiculartothedispersiondirection tion ofthelongaxis10"X20"aperture,are assume anIUEdetectorscaleof17525persquarepixel Feitzinger etal(1980).TheorientationoftheIUE tem inFigure1differsfromthesystemgiven are 271and374,respectively.Thestarnumberingsys- Koomneef, andMeade1981). and weremadeusingthespatialextractioncapabilities ture longaxiswithrespecttothenorth-southdirection tween 2000and2500Àinthe caseoftheLWRimages consistent withimagesofR136obtainedbyChu(1982). and between14101510 Á inthecaseofSWP The spatialscansshowninclude spectraldatabe- Figure 1alsocontainsultravioletspatialscansofthe © American Astronomical Society • Provided by the NASA Astrophysics Data System e e e e e e SWP 1400.. LWR 5584 LWR 3810 LWR 1363 LWR 1362 SWP 1401.. SWP 2766.. SWP 2765.. LWR 11561 LWR 10623 SWP 11174 SWP 8002 SWP 7989.. SWP 6515.. SWP 15069 SWP 15005 SWP 13985 LWR 10624 SWP 13976 SWP 13975 small andlargeaperturedata,thepercentcontaminationofobservedfluxat1520Aisestimatedtobe:SWP8002,38%; SWP 16605 SWP 15006 11174, 38%;SWP1397528%;13985,25%;15005,44%;15069,47%. e d C b a 3 The extractedfluxesfortheselarge-aperturespectracontaincontributionsfromstarsnearR136a.Basedonacomparisonof Positionangleofthelongaxislargeapertures(=73°-spacecraftroll). L =largeaperture(10"X20");Ssmall(3"). L =lowresolution(SWP,6Á;LWR,9Á);Hhigh0.15À;0.2À). SWP =shortwavelengthprimecamera(1170-2000Á);LWRlongredundant(ÀÀ1800-3200À). Image (yr dayhr) 78 78 78 79 79 78 78 78 80 79 81 81 80 82 81 81 81 81 81 81 81 81 Date 269 258 258 269 259 258 264 259 112 112 112 112 136 136 137 136 136 51 27 52 50 82 05 05 20 13 14 15 16 15 14 14 16 18 12 16 14 12 15 4 0 2 4 1 15 Dispersion NATURE OFR136a WE ImagesofR136 L L L L L L L H L H H L H H L H H H H L L L TABLE 1 images. ThesolidUnesincludedwitheachspatialscan profile, andcontributeabout25%ofthelarge-aperture images LWR5584andSWP6515,stars1,2,3,4 produced byR136a.Star2hasbeenclassifiedMelnick radiation detected.Fortheseimagestheenhancedwing contribute tothebroadsecondarybumpinspatial produce thesecondarypeakinspatialscan.The more thanonesourcecontributestothelargeaperture illustrate theIUEspatialresponsefunctionforapoint These threestarscontribute lessthan10%ofthe (1978) asWN+O.Star5isprobablyresponsibleforthe ultraviolet fluxfromthesetwostarsisabout13%that on theapertureorientation.IncaseofimagesLWR flux detectedbytheIUE. the combinedeffectsofstars R136b,R136c,and5. tion collectedfromstarsnearR136adependscritically to thesoutheastofprincipal peakisprobablydueto slightly elevatedwingtothesouthwestofR136a.For as theR136data.Forobservations,clearly stars takenwithapproximatelythesametelescopefocus source. Thesecurveswereobtainedfromobservationsof 1362 andSWP1401,stars23togetherprobably 1410-1510 Áradiationfrom R136a. 0 Aperture In thelarge-apertureIUEdata,amountofradia- L,S L,S L,S L L,S L,S L,S L,S L,S L L L L L S L,S S L,S L,S L,S L S Exposure Time(s) 420.00 420.00 315.00 180.00 180.00 180.00 120.00 180.00 180.00 13.00 4.00, 4.98, 4.00, 9.00, 3.32, 5.00, 5.25 3.33, 3.33, 5.00, 5.00, 1.17 (min) 20.00 10.00 10.50 12.98 10.00 13.00 13.00 13.00 7.00 9.98 7.00 0 Position Angle Large-Aperture -200° -200° -189° -189° -200° -200° -200° -194° -40° -42° -41° -65° -15° 20° 20° 20° 43° 20° 44° 43° 43° 43° 599 1983ApJ. . .273. .5973 =a1/2 2 600 with studyingR136a.Spectraobtainedfromtheground by radiationfromnearbysources.Furthermore,the broadening wouldbeveryevidentinthecoreofour with ^obs(A/£+Ri36a)276.Thisamountof ian function,exp(—6/2of),witho=271(de dominates thecoreofprofiles.TheIUEinstrumen- evident thatanobjectofsmallultravioletextent neighboring objects.Withthe smallIUEaperture(3" extracted signalisproduced bycontaminationfrom or fromspacewillbecontaminatedtovaryingdegrees conclude thattheultravioletextentofR136aislessthan observed profiles.Sincethisbroadeningisnotseen,we tal spatialspreadfunctioniswelldescribedbyaGauss- diameter) centeredonR136a, lightfromtheseneighbor- case oftheIUElarge-aperture spectra,20-50%ofthe spheric seeingwilladdanadditionalcomplexity.Inthe amount ofcontaminationwilldependonaperturesize Gaussian ultravioletbrightnessdistributionwitha Boer, Koomneef,andMeade1981).IfR136ahada the observedspatialspreadfunctionsofFigure1,itis and orientation.Inthecaseofgroundstudies,atmo- that ofaGaussianwith=175. between R136a,b,andchasbeenmodifiedtocorrectforanerrorintheFeitzingeretal.presentation(see§III). spatial spreadfunctionforapointsource.Thescaleandorientationofthescansaresameasstarfield.ForLWR and stars nearR136acanbeevaluatedfromthisfigure.ThestarnumberingsystemdiffersthatfoundinFeitzingeretal.(1980).spacing 5584 andSWP6515{dashedoval).Large-aperturespatialscansfromimagesLWR13621401areshownontheright-handside of are plotted.TheovalsrepresenttheWElarge-apertureorientationforimagesLWR1362andSWP\AQ\{solidoval) SWP spectra,thespatialscansrefertowavelengths2000-2500Aand1410-1510A,respectively.Large-aperturefluxcontributionsfrom the figurewhilespatialscansfromimagesLWR5584andSWP6515areshowntoleft.Dottedcurves,actualscans;solid the UEIUE = 175,wewouldexpecttoobserveaGaussianprofile R136a While variousstarsotherthanR136acontributeto © American Astronomical Society • Provided by the NASA Astrophysics Data System Figure 1illustratesquitewelltheproblemsassociated Fig. 1.—ThecentralpartofthisfigureshowstheR136fieldasinferredfromdataFeitzingeretal.(1980).Onlybrighteststars SAVAGE, FITZPATRICK,CASSINELLI,ANDEBBETS Figure 2.Lineidentifications areprovidedontheaver- contamination fromneighboringstarsandhaving Koomneef, andSavage(1980).Inthediscussionto persion spectra.Inthecaseofhigh-dispersiondata, ture short-wavelengthspectra ofR136aareshownin average spectrawithhighsignal-to-noiseratios. follow, webenefitfromhavingmoreinformationabout spectrum ofR136aarefoundinCMS.Adiscussion mizes thecontaminationproblem. persion spectrawithanapertureorientationthatmini- emphasize inourdiscussionsthesmall-aperturelow-dis- ing objectsincludingR136bandR136cismostlyex- the ultravioletinterstellarlinesisfoundindeBoer, our emphasiswillbeonthoselarge-aperturehigh-dis- cluded. the small-aperturespectraareoflowquality.Therefore, The individualandaveragelow-resolution small-aper- Previous discussionsofthestellarultravioletline Because ofthesecontaminationproblems,wewill IV. THEULTRAVIOLETLINESPECTRUMOFR136a a) Low-ResolutionSmall-Aperture Spectra Vol. 273 1983ApJ. . .273. .5973 No. 2,1983 on The Ridentifiesadetectorreseau.uppermostspectrumisanaverage ofthehigh-dispersionlarge-aperturespectraSWP13975andbWF individual spectra.Linesofmostlystellaroriginaremarkedabovethe averagespectrum.Linesofmostlyinterstellar^naremarkedbekm are individuallow-resolutionsmall-aperturespectra.Thesespana 4yeartimeinterval.Theaveragespectrumisanofthesix marked belowthespectrumwhilestellarlinesare echellè ripplecorrectionalgorithmofAke(1982)wasusedinprocessing thehighresolutionspectrum. age spectrum.Linesofmostlyinterstellaroriginare above thespectrum.Thefluxes plottedforthesmall- measured toallowforlightloss atthesmallaperture.In aperture spectrawereincreased afactorof2overthose this sectionwewillconcentrate onthespectralline 13985, smoothedbya0.1ÁFWHMGaussian.Thislarge-aperturespectrum iscontaminatedataboutthe25%levelbystarsnearR136a.The © American Astronomical Society • Provided by the NASA Astrophysics Data System 2 Fig. 2.—VariousIUEspectraofR136aareplottedasabsoluteflux (ergscm's'A')versuswavelength(A).Thelowersixspectra NATURE OFR136a information intheaveragespectrum.Searchesforline variability inindividualspectrawillbediscussed§V. An expandedplotofthe longwavelengthregion be seenintheenergydistribution shownanddiscussed (\X 1900-3200)isnotshown, althoughvariouslinescan in §VI.IntheIUElong-wavelength regiontheonly 601 1983ApJ. . .273. .5973 -1 _1 lines seenatlowresolutionarethoseduetointerstellar Very stronginterstellarHiLyaabsorptionblendswith N vXI240,CivXI550,HenX1640,andX1718. produced bystarsotherthanR136a.Thespatialextrac- km s),butthelineisnotparticularlydeep.TheUne olution averagespectrumincludethePCygniUnesof Mg I,U,Feii,andMnn. high-dispersion spectrum.Theseestimateswereob- resolution hasacentraldepthoflessthan0.20. depth, F(line)/Fcontinuum),foundinthesmall-aper- unusual inthatitindicatesahigh-speedflow(~3600 or collectionofsuchstars.IntheIUEstandardstar estimates ofthiscontaminationforeachlargeaperture neighboring starsofthoselarge-aperturehigh-dispersion stars exhibitingPCygniCiv,theUneobservedinlow stars 04andearlier.TheR136aCivPCygnilineis the absorptionpartofNvPCygnifeature.O 602 make thevariousstellarspectra directlycomparable. in thelarge-apertureIUEspectracomesfromHD Walbom andassumingthatbothstarsexperiencethe tion fromthelow-dispersionlarge-apertureimage,SWP about 25%ofthedetectedfar-ultravioletradiationis flux levels,correctedforsmallapertureUghtloss,with ture dataatlowresolutionis0.38.Inmosthotluminous spectra ofWuetal.(1982),OvX1371isstrongonlyin mostly interstellarinorigin. absorption isfoundat1371A.Stellarfrom many linesofFevismainlyresponsibleforthefeature of -250kmswasapplied totheR136aspectrum reflect thoseofthe03If*star,andweshallreferto Way starsHD303308(03V((f)))atthetopand the large-aperturefluxes.ForSWP13975and13985 tained bycomparingthesmall-aperturelow-dispersion spectra sofarobtained.CommentefromTable1gives of Figure2.TheIUElargeapertureorientationforthis ever, thehigh-dispersiondatarevealthattheseUnesare For allthreestarstheeffects ofHiLyaX1216absorp- 93129B. Thespectralcharacteristicsofthedatamostly (Walbom 1973û).FromtheVmagnitudesgivenby HD 93129B(03V((f)))whichislocated3"away HD 93129AinFigure3containsacontributionfrom spectrum providestheminimumcontaminationfrom from imagesSWP13975and13985isshownatthetop extending fromabout1420to1490À(see§TVb).The spectrum asthatofHD93129A. Avelocitycorrection same reddening,weestimatethatabout20%ofthelight Figure 3alongwithcomparisonspectraoftheMilky Si ivdoubletnear1394and1403Âisdetected.How- 93129A (03If*)atthebottom.Thespectrumof 13976, confirmsthisestimate. x The stellarUnesthatcanberecognizedinthelow-res- The spectrashowninFigure2implyaveryhotstar An expandedplotoftheR136aspectrumisshownin © American Astronomical Society • Provided by the NASA Astrophysics Data System The averageIUEhigh-dispersionspectrumofR136a b) High-ResolutionSpectra SAVAGE, FITZPATRICK,CASSINELLI,ANDEBBETS 21 1 -2 1 -1 by Bruhweiler,Kondo,andMcCluskey(1981) in OandWolf-Rayetstarspectrahasbeendiscussed high-dispersion spectrumofR136aillustratedinFigures have beenobtainedforsections ofthespectracontain- rection. WeakCmabsorptionisfoundcenterednear in Bohlin,Savage,andDrake(1978)theHicolumn McCluskey. ManyoftheseUnesappearintheR136a Fe vUnesandblendsUstedbyBruhweiler,Kondo, above theHD303308spectrumdenotelocationsof Fitzpatrick (1982).Thenumerousunlabeledtickmarks feature isstronglyinfluencedbytheechelleripplecor- 2 and3.AsymmetricUnesofOivarefoundat1338.6 spectrum. TheFevUnesarenarrow.Cross-correlations and 1343.5À.Unfortunately,theprofileofX1343 XI640 emission. larger forthelow-dispersionsma/Z-aperturespectrathan XI240 Unecannotbeseen.TheedgevelocityoftheCiv lengths ofvariouspotentialstellarabsorbersareindi- respectively. InFigure3,stronginterstellarfeaturesare densities V(Hi)=2.3xl0,8xl0and3X10 resonance lineprobablyrepresentsthewindterminal with another.Partofthisproblemmayrelatetothe rection schemeofAke(1982),thecorrectionfor marked belowthespectrumofHD93129A.ForR136a tion hasbeenremovedbyusingthetechniquesdiscussed Figure 2.Thisdifferenceiscausedbycontaminationof for thehigh-dispersion/arge-aperturespectrashownin km s“.TheHenX1640andNivX1718Uneshave extended natureoflarge-apertureR136adataandthe R136a dataoccasionallyintroducespronouncedstruc- cated abovethespectrumofHD303308. atoms cmforHD303308,R136a,and93129A, speed forR136a. Lya absorptiontheshort-wavelengthedgeofNv respectively. Unfortunately,becauseofthestrongHi velocities. TheCivabsorptionedgeisat—3600 transition wavelengthsfromoneechelleordertoanother. effect thishasonthestandardtechniquesforestimating ture atthewavelengthswhereoneechelleorderisjoined intensity ofthecross-correlation functionforR136ais ing thestrongestFevfeatures. Thefullwidthathalf- the large-aperturedatabysourceswithweakHen emission equivalentwidthsareUstedalongwithHmiting Figure 3.ForUneswithPCygniprofiles,absorptionand stellar UnesfoundintheR136aspectrumillustrated the bottomofFigure3,extratickmarksdenote the backgroundbetweencrowdedechelleorders.At tions fromtheMilkyWayandLMC.Restwave- the interstellarfeaturesaredoublebecauseofcontribu- absorption extendingto—2000and—2700kms“, 1175.6 A(seeFig.2).ThepresenceofFevabsorption 180 kms.Similarcorrelations werecomputedfor A numberofweakstellarfeaturesareapparentinthe The HeilXI640emissionequivalentwidthis2A Table 2givesmeasuredparametersforthestrongest Even thoughweusedtheupdatedechelleripplecor- 1983ApJ. . .273. .5973 © American Astronomical Society • Provided bythe NASA Astrophysics Data System 1983ApJ. . .273. .5973 1 _ 1 604 normal Ostarswithvsiniestimatesavailablefrom km s“. v sini.Iftheobservedcorrelationwidthisattributedto rotation, theempiricalrelationimpliest;sin/~110+40 Conti andEbbets(1977).Thesedatawereusedtoderive the empiricalrelationbetweencorrelationwidthand photospheric Siivabsorption.NobroadPCygni undoubtedly producedintheHhregionsurrounding R136a. However,whatappeartobedampingwingson feature isapparent. these sharpUnesmaybeproducedbyrelativelyweak filled inCivabsorption.AtthebottomofFigure4we HD 303308and93129A.Exceptionstothisinclude similar ifnotintermediateinappearancetothosefor show thesmall(SWP2766)andlarge13975 comparison betweenthelargeandsmallaperturepro- contains lesscontaminationfromobjectsnearR136a.A the verybroadHeilXI640.4PCygniprofileand files suggeststhatsomeofthefilling-inlarge- tation ofthetrueR136aCivprofilesincespectrum the small-apertureprofileisprobablyabetterrepresen- hour beforeSWP2766. However, spectrumlinevariabilitymightalsoinfluence aperture profilemaybetheresultofcontamination. larger thanthatofR136a; the remainingstarshave all low-dispersionspectra.SWP2765wasobtained1 that theCivabsorptioninSWP2765isdeepestof this comparison.Wenotefromthelow-dispersiondata 93129 AandR122haveCiv edgevelocities300kms 04 starsobservedinhighdispersion withtheIUE.HD Cygni profilesforotherLMC andMilkyWay03 13985) apertureprofilesofR136a.Althoughverynoisy, © American Astronomical Society • Provided by the NASA Astrophysics Data System The strongnarrowSiivlinesat1394and1403Àare Many ofthelinesinspectrumR136aarequite For comparisonpurposes,Figure4alsoshowsCivP d Nv ....1238.82,1242.80 Niv ...1718.551.9 Civ.... 1548.20,1550.777.6 He il...1640.401.4 Ov ....1371.292.1 influenced bycontaminationfromstarsnearR136a.Thisisestimatedtobe 26% ofthemeasuredfluxat1520À. ments istheresultofcontaminationlarge-aperturedatabysourcesotherthanR136a. average spectrumis6.0À.Thedifferencebetweenthelargeandsmallaperturemeasure- 13975 and13985(seeFig.3).Theemissionabsorptionequivalentwidthswillbe -1 c b a e d Ion (À)(kms) StellarOvemissionisnotapparent. Interstellar HiLyaabsorptionblendswithNvstellarabsorption. Measurementsarefromthehigh-dispersionco-addedlarge-aperturespectra,SWP The Henemissionequivalentwidthmeasuredfromthelow-dispersionsma//-aperture Assumes absorptionbetween1624and1629AisnotproducedbyHen. SAVAGE, FITZPATRICK,CASSINELLI,ANDEBBETS absem X f*x(abs)^(em)udge()^edge( 0e RI36a StellarLineParameters“ TABLE 2 in thesmall-apertureR136adatabothitsterminal nent ofthePCygniprofile. HD 93250(03V((f)))isquitesimilartotheonefound similar orlowerterminalvelocities.TheCivprofilefor speed andintheresidualfluxabsorptioncompo- but theresultsarenotconvincing.Avariabilitysearch provided byfindingconvincingevidenceforcontinuum variability arefoundinEbbets,Savage,andMeade must carefullyaddresstheproblemsassociatedwith reported (FeitzingerandSchlosser1977;Moffat1982), or spectrallinevariability.Visualvariabilityhasbeen observation todependingonsuchthingsas contamination fromobjectsnearR136a.Thiscon- (1982). etc. Thefirstresultsrelatingtooursearchforultraviolet telescope focus,atmosphericseeing,aperturealignment, tamination isaseriousproblemsinceitcanvaryfrom have beenadjustedtoyieldthesamecontinuumflux between 1750and1900À asfoundintheaverage average lowdispersionsmallaperturedataforthe spectra (see§III).Figure2showstheindividualand tamination problemsassociatedwiththelarge-aperture ture datainourvariabilitysearchbecauseofthecon- were lessthan±10%,which issimilartothevariable short-wavelength IUEdetector.Themeasurementsspan approximately 4years(seeTable1).Thespectrashown spectrum. Thiscorrectsforvarying amountsoflightloss at theIUEsmallentrance aperture. Theadjustments ed b 4.0 —2000+2800 Clues aboutthephysicalnatureofR136amightbe 2.3 +1600 5.1 -3600+2400 We haveemphasizedthelow-dispersionsmall-aper- c 1.1 -2700+2200 -800 a) Low-DispersionSmall-ApertureSpectra V. SEARCHFORULTRAVIOLETVARIABILITY Vol. 273 1983ApJ. . .273. .5973 data topermitadirectcomparisonwiththeMilkyWaystardata. are compared.VelocityshiftshavebeenapphedtotheLMCstar Spectral typesindicatedarefromWalbom(1972,1977,1982).The No. 2,1983 spectra illustratedfrombottomtotopinclude:R136a(small-aper- images SWP13975+13985);HD269810 (alsoR122,SWP10755); HD 269676(SWP5086+13908+ 14002); HD269698(SWP6967 ture datafromimageSWP2766);R136a(large-aperture (SWP 14747);HD93250 14746); andHD46223(SWP 10757). + 8011);HD93129A(SWP13992+14007+15026); HD93205 Fig. 4.—CivPCygniprofilesforR136aandvariousOstars © American Astronomical Society • Provided by the NASA Astrophysics Data System NATURE OFR136a _1 1 (Holm l982).Thus,thereisnoevidenceinthesedatafor plots are,ofcourse,verynoisy.Theratioshow bottom ofthestronginterstellarHILyalineratio variation exceedingapproximately±10%.Asimilar The ratiospectrashownoevidenceforcontinuumshape level. PossiblyHeilÀ1640emissionisweakerthan about ±15%. transmission ofthesmallaperturefoundforsteadystars line appearsdeeperthannormalinSWP1401and2765. normal inSWP1401andstrongerthan line strengthinemissionorabsorptioncansimply À1640 emissionandCivXI550absorption.Amore conclusion wasreachedonexaminingratiospectrafrom from Figure5whichshowstheaveragespectrumand changes inthestellarabsorptionUnesorPCygnipro- Figure 2showsthattherehavenotbeensignificant of varyingamountscontaminationfromneighboring year timespan.Someoftheindividualspectrasuggest 2765. TheabsorptionpartoftheCiv\1550PCygni aperture observations.Atthosewavelengthsnearthe the ratio,F/(average),forallindividualsmall- assessment ofpossiblelinevariabilitycanbedrawn files overthe4yeartimeframe.Amorequantitative ever, thecontaminationshouldnotappreciablymodify sured fluxintheworstcase,differencesmeasured stars inthedifferentlarge-aperturespectra.Becauseof extensive variabilitysearchshouldbecarriedoutwith that, ifvariabilityispresent,itoccurringatalow C ivPCygniprofile.Inthesixlarge-aperturespectrawe the detectedshort-wavelengthedgevelocityofstrong arise fromthevariableamountsofcontamination.How- this contamination,whichapproaches50%ofthemea- ture IUEdataisseverelycompromisedbythepresence the IUEtoseeiftheseindicatedchangesarereal. dispersion small-aperturedataforstrongvariationin the LWRdatawhichspanwavelengthrangefrom that thewindspeedofR136aisnotchangingwithtime. find u(Civ)=3600±100kms.Thus,itappears the continuumorlineradiationfromR136aovera4 1800 ÁcontinuumvariabilityofR136atoalevel 1800 to3200À. heliocentric velocitiesbyvelocity shiftingeachspectrum variations ofR136a.Asafirststepinthissearch,the x which isthevelocityof Hi21cmMilkyWay in §III6canbeusedtosearchforradialvelocity ±10% to±15%variationinthestrengthofHeII until thevelocityofMilky Wayinterstellarabsorp- edge tion bytheSntripletnear 1255 Áwas±13kms“, five individuallarge-aperturespectrawereadjustedto A searchforvariabilityofR136abyusinglarge-aper- A casualinspectionoftheprominentstellarlinesin In conclusion,thereisnoevidenceintheIUElow- The relativelynarrowFevabsorptionlinesdiscussed b) High-DispersionLarge-ApertureSpectra 605 1983ApJ. . .273. .5973 -1 _1 in widthcontainingtheFevUneswerechosencentered near 1376,1409,1419,1436,and1469Á.Usingthe emission observedbyMcGee(1979)inthedirectionof 606 The cross-correlationsimpliednostellarradialvelocity R136. Selectedwavelengthintervalsapproximately5A computed byintercomparingthedatafromimageSWP less thanabout20years. contamination isnotaffectingthisresult,thesedata selected intervals,cross-correlationswithvelocitywere spectra toaprecisionof±15kms.Assumingthat shifts oftheFevabsorptionlinesbetweenindividual 8002 withthatfortheotherfourlarge-apertureimages. comment onlyappliestobinarymotionswithperiodsof trum. Sincethedataspanapproximately2years,this the objectproducingR136aabsorptionlinespec- appear toruleoutlarge-amplitudebinarymotionsfor similar dataforHD93129A implies£>3—t>93^9 = 250+10kms.Conti,Niemela, andWalbom(1979) R16HD At thetopoffigureaveragespectrumisalsoshown. A cross-correlationbetween the R136Fevdataand © American Astronomical Society • Provided by the NASA Astrophysics Data System Fig. 5.—TheratioF/(average)isplottedversuswavelengthforalltheindividualsmallaperturelowdispersionspectrafrom2. x 1200 1300140015001600170018001900 SAVAGE, FITZPATRICK,CASSINELLI,ANDEBBETS WAVELENGTH -1 -1 1 1 lines. ThereforefromtheFevlines,u(helio)«240 Wesselink (1960). estimate 251:kmsfromFeast(1961)butdiffers find u93J29(helio)=-12kmsfromvisualNv from thevalue274kms“Feast,Thackeray,and were obtainedwiththeIUEusingsmall(3"diame- Á to2.2/xmisshowninFigure6a.Theultravioletdata by thestandardfactorof2 to accountforlightlossin ments plottedinFigure6a wereobtainedfromthe R136 sources UstedinTable3. The outstandingfeatures the smallaperture.Thevisual andinfraredmeasure- ter) aperture.Theresultantfluxeshavebeenscaledup HD ± 15kms“.Thisvelocityiscompatiblewiththe The observedenergydistributionofR136from1200 VI. CONTINUUMENERGYDISTRIBUTION a) ObservedEnergyDistribution Vol. 273 1983ApJ. . .273. .5973 No. 2,1983 b blackbody curveshavebeendrawnthrougheachultravioletenergydistribution. continuum levelofR136ais16XbrighterthanthatR122.R122hasnearlythesameluminosityasHD93129A,mostluminousknown data. ForcomparisonpurposestheenergydistributionforHD269810(R122),mostluminousLMC03starisshowninFig.6b. The data obtainedwith15"and20"aperturesarecontaminatedbyradiationfromsourcesnearR136a.Thesameproblemmayaffecttheinfrared low-dispersion small-apertureWEspectraobtainedafter1981.0(seeTable1).ThestandardGoddardextractionwasusedalongwith the Milky Waystar.ThedatashowninFig.6bhavebeencorrectedfortheeffectsofinterstellarHiLy«absorption.Inaddition,40,000 K correction appliedtoproduceFig.6bisdiscussedin§VIZ?.TheR136opticalandinfrareddataplottedarereferencedTable3.visual Bohlin andHolm(1980)photometriccalibration.Afactorof2correctionwasappliedtoallowforsmall-aperturelightloss.Theextinction J . J . B . B . I . R . H. K. K. H. U. Tarenghi 1981.(5)McGregorand Hyland 1981. Bergh andHagen1968.(3)Mendoza 1970.(4)Panagia,Tanzi,and isophotes ofthecoreregion30 Doradus. V. F V. a b Filter © American Astronomical Society • Provided by the NASA Astrophysics Data System Fig. 6.—Observed(Fig.6a)anddereddened(Fig.-6b)energydistributionsofR136a.Theultravioletdataareanaveragethe Source.—(1)Schmidt-Kalerand Feitzinger1981.(2)vanden This estimateoftheVmagnitude isbasedonastudyof (/xm) 0.44 0.36 0.44 0.55 0.55 0.55 0.90 0.70 2.2 2.2 ^eff 1.65 1.25 1.65 1.25 R136 Photometry Magnitude 10.77 TABLE 3 9.56 9.42 8.81 9.15 9.32 9.18 8.62 8.83 8.78 8.98 8.65 8.92 8.45 Diameter Aperture 20 20 20 20 15 15 15 10 10 10 10 10 10 (") 2.2 NATURE OFR136a 3 Source bump intheultravioletandlackofcontinuity ing subsections. Table 3).TheinterstellarextinctionofR136andthe between andwithinthevariouswavelengthregimes, neighboring starsinthevariouslysizedapertures(see evidently causedbydifferentialcontamination of Figure6aarethestrength2200Áextinction contamination problemwillbediscussedinthefollow- within about5'(80pc)ofR136andcomparisonstars employed, usinganumberofreddenedstarslocated of the30Doradusnebulahavebeeninvestigatedby region. from avarietyoflocationsoutsidethe30Doradus Fitzpatrick andSavage(1983).The“pairmethod”was reddening law,attributedto“LMC foregrounddust”by etal (1981),withanaverage £(#-K)«0.12.This those derivedbyKoomneefand Code(1981)andNandy the regionarereddenedbyan extinctionlawsimilarto The ultravioletpropertiesofextinctioninthecore The resultsofthisinvestigationshowthatallstarsin b) DereddenedEnergyDistribution 607 1983ApJ. . .273. .5973 which accountsfortheaverageGalacticextinctionlaw. is oftenthoughttoreflectanincreaseinthenumberof values inthefar-ultraviolet.Thistypeofextinctionlaw crease inextinctionshortwardof2000Átoverylarge 608 small particlesinthegrainsizedistributionoverthat Fitzpatrick andSavage,ischaracterizedbyasteepin- ponent reddeningasR145andR147,i.e.,aforeground identical (seeFig.2inFitzpatrickandSavage1983). The commonfeatureofthecurvesfromtheseregionsis nebula (BlessandSavage1972;Bohlin behind, ratherthanembeddedin,anextendednebular is encouragingforourextrapolationofthoseproperties which liethemoreheavilyreddenedstarsR136,R145, LMC componentwith£(i?-K)«0.12plusanebular This suggeststhatR136experiencesthesametwo-com- have adoptedtheformervalue because,incaseofre- indicates R[=A/E(B-V)] «3.7forthenebulardust. gas anddustsheetisconsistentwiththelackofa ultraviolet extinctionpropertiestowardR145andR147 reaching outtoR145,whichisataprojecteddistanceof nebular gasanddustabovewhichliethelightlyred- emerges fromtheextinctionstudyisofasheet component with£(i?—F)«0.18.Thepicturewhich fact, acrossthe2200Aregion,threearenearly modification, havingalargerfractionofsmallgrains might beduetotheinitialsizedistribution,before from the30Doradus“nebulardust”isnotasweakin size distributiontolargersizes(MathisandWallenhorst Galactic curve,due,perhaps,toaskewingofthegrain Savage 1972),andtheM8complex(Hechtetal1982). those seenindustyGalacticregions,suchastheOrion the LMCforegroundextinction,equivalent\oE(B-V) and R147,experienceadditionalreddening,apartfrom reddening R136moreheavilythanothernearbyregions. (1981) thatthereisunlikelytobealocaldustcloud R136. Ifthenebulargasanddustarepatchy,rather 23 pceastofR136.Itseemsunlikely,however,thata dened foregroundstarswith£(Æ—K)=0.12andbelow For R147theresultisR«5.0. IndereddeningR136we far-infrared sourceatthepositionofR136(Werneretal single sheetextendsasfarR147,80pcnortheastof and R147.Thesheetmaybecontinuousacrossthecore, similar tothoseofR145(WN6)andR147(WN6).In the far-ultravioletascurvesdescribedabove.This smaller grainsorthegrowthoflargerones.Thecurve their weaknessintheultravioletrelativetoaverage than intheGalacticcases. to theR136lineofsight.Theplacement than asingleuniformsheet,thenthesimilarityof 1981; Panek1983),thepOphdarkcloud(Blessand 1978) andtheconclusionofKoomneefMathis 1981). Thismayarisefromeitherthedestructionof V «0.18. Theextinctionlawresponsibleisreminiscentof It wasalsoshownthattwoofthestarsstudied,R145 The observedultravioletcontinuumofR136isvery © American Astronomical Society • Provided by the NASA Astrophysics Data System Extrapolation ofnear-infrared photometryofR145 SAVAGE, FITZPATRICK,CASSINELLI,ANDEBBETS -1 in theLMC(Walbom1982)andisoneofbrightest comparison oftheslopesR136andR122spectra required. The40,000Kblackbodycurveisalsoshown with theIUEdata,hasbeencorrectedforGalactic distribution ofHD269810,03III(f*)(Walbom1982). more “normal”object,weshowinFigure6btheenergy with theresultsofMorganandNandy(1982) below, wouldiiicreaseinbrightnessby0.23mag.For by emissionUnesorfree-freefromthe gional variations,R145islocatedmuchnearertoR136 obscuring regionsandthevalue ofE(B-V)thatap- distributions. (open circles)isfromRousseauetal(1978)and,along HD 269810,orR122,isthebrighteroftwo03stars differential contaminationoftheinfraredmeasurements R144, thecomparisonstarusedinderivingextinc- than isR147.AlsoR145acloserspectralmatchto dening ofR136includeboth theuncertaintyin fitted totheR122ultravioletdataenableadirect flux levelofR136,16starssuchasR122wouldbe foreground reddeningof0.04.Tomatchtheultraviolet UV objectsinthatgalaxy.TheU,B,Vphotometry Notice thattheright-handverticalaxisofFigure6Z?isin The wavelengthdependenceofthisforegroundextinc- 0.03 mag(seeTable2inMcNamaraandFeltz1980). Koomneef (1982). shapes oftheextinctioncurves derivedforthevarious and tostressthesimilarityoftwocontinuousenergy well. Itisintendedmerelyasaguidetotheeyein units of\L(ergss),assuminganLMCdistance and infraredphotometryarethesameasinFigure6a. extinction lawsdescribedabove.Symbolsforthevisual dereddened by^(R-F)=0.34andthethree which wasdeterminedfromuvbyßphotometryofMilky have usedtheaverageGalacticlaw(SavageandMathis E(B —V)forthisextinctionrangefromabout0.1to Milky Wayforegrounddust.Thesuggestedvaluesof the LMCforegrounddustwetakeR=3.1,consistent stellar winds.IfthelargervalueofRhadbeenadopted, tion curve,thusminimizinganyuncertaintydueto K blackbodywhichfitstheultravioletdatareasonably Way AandFstarsinthedirectionofLMC. consistent withtheMcNamaraandFeltzvalueof0.034 the resultantdereddenedenergydistribution,discussed very neartotheSchmidt-KalerandFeitzinger(1981) 55 kpc.ThedashedlineinFigure6brepresentsa40,000 tion. OuradoptedvalueofE(B—K)=0.04is tion isnotknown.Forlackofabetteralternative,we estimate ofF. etry ofR136,butitisinterestingthatthecurvepasses assessing contaminationproblemswithexistingphotom- 1979; Seaton1979)tocorrectfortheGalacticcontribu- X TOTAL foreground The potentialsourcesoferror involvedinthedered- As withallotherLMCstars,R136isreddenedby In ordertocomparethebrightnessofR136witha Figure 6bshowstheenergydistributionofR136 Vol. 273 1983ApJ. . .273. .5973 plies toeachregion.Consideringfirstthe30Doradus lines ofsightintheMilkyWayplaneshowultraviolet blackbody energydistribution,thiserrorencompassesa À theuncertaintyincoloris0.25mag.Intermsofa uncertainty inthe(m—V)coloris0.35mag.Byfar roughly afactorof3brighterthantheestimate respectively, differinbrightnessbyabout30%,andare van denBerghandHagen(1968)Mendoza(1970), level typically1.7timesthatofthecorrectedsmall-aper- is obtainedusingasingle,small,aperturesize.The (1983) andMcGregor Hyland (1981)wereboth while orientationdependent(seeFig.1),haveaflux from objectsotherthanR136a. existing datawererecordedthroughavarietyofaper- reddening lawswhichdeviatesystematicallyfromthe ground extinction.Intheintervalfrom3000Ato1500 and LMCforegroundextinction,weestimatethatthe recorded through10"apertures yetdifferbyabout0.3 other nearbystars.Thelow-dispersiondataobtained mostly acceptsradiationfromR136aitself.Largeraper- by thegenerallylargeextinctiontowardMilkyWayO likely errorinourMilkyWayreddeningcorrectionis Way canbeseeninMassa,Savage,andFitzpatrick The widerangeofreddeninglawsfoundintheMilky the valueofE(B-V)thatapphestoLMCfore- the largestcontributorto.thiserrorisuncertaintyin Schmidt-Kaler andFeitzinger fortheinner272. ture data.The15"and20"aperturemeasurementsof ture sizes,andallcontainsomedegreeofcontamination between a50,000Kand35,000ratherthan40,000 wavelength rangethiserrorcouldmeanthedifference (1983). Fromtheselawsweestimatethatthemaximum average (MeyerandSavage1982),itwouldnotbe applicable totheLMCUneofsight.However,numerous sumes thattheaverageMilkyWayextinctionlawis temperature rangeofabout32,000-60,000K. through theIUElarge(IO"X20")aperture,forexample, tures includethecomponentsR136bandR136c termined untilphotometryoveralongwavelengthrange to-infrared energydistributionofR136acannotbede- early-type MilkyWaystarsisadditionallycomphcated correction aslongtheforegroundreddeningisuni- continuua isindependentoftheMilkyWayforeground surprising ifthelow-densityMilkyWaygasanddustin stars. form. ComparisonofeitherR136orR122with K blackbody.ThecomparisonoftheR122andR136a the LMClineofsightalsodeviatesfromaverage. No. 2,1983 1500 ±0.15 maginthe(m-F)color.WithinIUE 1500 The MilkyWayforegroundreddeningcorrectionas- The infrareddataofPanagia, Tanzi,andTarengi The IUEsmallaperture(diameter=3")fortunately It isclearfromFigures6aandbthattheultraviolet- © American Astronomical Society • Provided by the NASA Astrophysics Data System c) TheContaminationProblem NATURE OFR136a -1 51 52-1 These differencesmaybecausedbycalibrationdif- in pointing,resultingtheexclusionofoneormore in Figure2ofFeitzingeretal.(1980).Slightdifferences mag. Thisismuchlargerthanthequoteduncertainties. ionizations s(McGee,Brooks,andBatchelor1972)to information derivedfromtheIUEsmall-aperturespec- nitudes willcertainlydependontheamountofcon- ment betweenthePanagiaetal.andMcGregor contamination. A10"aperturewillmarginallyinclude ferences butmightalsobeaneffectofdifferential nebula, themostluminousHnregioninLocal ing starcontamination. Hyland data. ionization ratesarenotlikelytobesignificantlyin- of thenebularionizationraterangefrom5x10 contamination. Whenconsideringthephysicalnatureof stars 4and5inFigure1,otherfaintershown Group ofgalaxies.Radiocontinuummeasurements R136a below,wewillmainlyuseenergydistribution types anddegreesofreddeningthestarscausing tamination intheapertureusedandalsoonspectral these stars,mayberesponsibleforsomeofthedisagree- produced byabout10005starsor6004(Panagia fluenced bynonthermalradioemission.Mills,Turtle, (Peimbert andTorres-Peimbert1974)stronglysupports fact thatthistemperatureagreeswiththetemperature, nebular electrontemperatureof11,900±1800K.The Huchtmeier andChurchwell(1974)haveestimateda and Watkinson(1978)reachedthesameconclusionsby tra inordertoavoidtheproblemscausedbyneighbor- brighter thany=15maginthe central5'Xregionbut tinuum radiationisthermal.Therefore,theestimated the ideathatalargeproportionofnebularcon- high-luminosity star(§Vile).Otherpossibleexplana- obtained Strömgrenfour-color photometryforallstars obtained forthe30Doradus nebula.Westerlund(1964) competing ideasarethatR136aisaverycompact examining thespectrumofradioradiation. cluster ofnormalhotstars(§VII6)orisasingle object requiredtoexplainthesedata.Thetwomain existing observationaldatarelatingtoR136a(§Vila) tions areconsideredin§VIId. 1978). Fromtheratiorecombinationlinedata, 1 X10ionizationss(Mills,Turtle,andWatkinson and thendiscussthephysicalnatureofobjectsor 10,500 K,derivedfromnebular[Om]measurements 1973). Acompletehotstar inventoryhasnotbeen 51_ The colorsderivedfromthevisualandinfraredmag- R136 liesatthecenterof30DoradusorTarantula An ionizationrateof5x10ionizationssisthat In thefollowingsubsectionswebrieflysummarize VII. THEPHYSICALNATUREOFR136a a) TheObservationalData 609 1983ApJ. . .273. .5973 50-1 35 -1 21 brightest stars(excludingR136)collectivelyprovide ever, withoutknowingthespectralclassificationsor gram shows76starswithb-y^0and<15.0.How- only publishedacolor-magnitudediagram.Thisdia- This estimatewouldincreasesignificantlyifWolf-Rayet calculate theoutputofionizingradiationfromthese dereddened colorsandmagnitudesitisimpossibleto large numberofthemmustbeveryhotstars.AnLMC erally accepted.Possiblythefainterstarsinregion stars. Feitzingeretal.(1980)haveestimatedthatthe16 610 Therefore, ifthefainterstarsprovideionization,a from ananalysisofthenebularemissionUnes30 are responsibleformostoftheionization.However, stars havehighereffectivetemperaturesthanthosegen- about 10ionizationssor1-2%ofthatrequired. vicinity ofR136arediscussedbyMeabum(1981).He ionization intheinnerpartof30Doradusnebula. bers ofthe30Doraduscluster.Sinceaccessible like R136awithA=1.2wouldhavem«13.2.pro- Doradus, Mathis(1982)hasconcludedthattheionizing literature appeartoincludesignificantamountsofcon- concludes thatR136amayberesponsibleformuchof 03 starwithT«50,000K,M=-6.5,andreddened sources haveanunusuallyhightemperaturecompared lished specklemeasurementsofWeigelt(1981)suggesta ponent aexceedsthatofbandcbyatleastfactor ultraviolet radiationfromR136aisequivalenttoabout gram shouldbeundertakentoclassifythefaintermem- to theotherHnregionsinLMCandMilkyWay. § IIIdemonstratethattheultravioletbrightnessofcom- R136 intocomponentsa,b,andc.Thespatialscansof the organizeddynamicalactivityinnebularcore. are largeandcomplex.Thosemotionsintheimmediate that thisuniqueobjecthasthemajoreffecton with L(observed)«2X10 ergs s(Long1982).This measurements ofR136aaredesirable. our considerations.However,wenotethattheunpub- detection ofasecondobject078fromR136abutabout et al.(1982)demonstratethatanobjecthavingadiame- measurement isinfluenced by interstellarabsorption. ments indicatethatR136is an extendedX-raysource secondary component6timesfainterthantheprimary from R136a.Meabumetal.alsocommentedaboutthe For V(HI)=8X10cm~ (see §TVb)andT«5 to belocatedinacomplexbackground.Furtherspeckle and withaseparationof0746.Boththeseobjectsseem ter <0702or1000AUdominatesthevisualoutput tamination fromsourcesnearR136a. 80 timesfainter.Thisfaintobjectshouldnotinfluence 15 ofthemostluminous03stars(see§VI),itislikely 10. Schmidt-KalerandFeitzinger(1981)haveestimated v effv V forcomponentatobe10.77.Othervaluesofinthe x source The visualimageryofFeitzingeretal.(1980)resolves The motionsofionizedandneutralgasin30Doradus The speckleinterferometrymeasurementsofMeabum © American Astronomical Society • Provided by the NASA Astrophysics Data System Einstein satellitehigh-resolutionimagingmeasure- SAVAGE, FITZPATRICK,CASSINELLI,ANDEBBETS -1 -1 l l 63 likely forthesemeasurements. km sand4.4days,respectively. However,thehydro- velocity. Theinferredamplitude andperiodare±43 he interpretsasperiodicshifts intheemission-lineradial were obtainedwitha10"aperture.Contaminationseems blackbody curve.TheinfrareddatashowninFigure6b bution ofR136aisshowninFigure6b.Itresembles weaker inR136andHD93129AthantypicalWN N iv7103-7123emission.TheemissionUnesaremuch by Vreux,Dennefeld,andAndrillat(1982).Thespec- contaminated bylightfromneighboringstars.However, ability ofR136.ThemeasurementsFeitzingerand If*/WN6-A). Paschen absorptionUnes,HenX10124emission,and lines ofHeIandnaredetected.Thevelocity there arechangesintheR136 Hen\4686profilewhich Schlosser (1977)arenotconvincing.Moffat(1982)claims stars. differences amongthevariouslinesreportedbyEbbets in absorptiontoHllwithdepthsofabout20%.Weak WN stars.Hen\4686andHaexhibitvelocityextents Une spectrumofR136adiscussedin§IVarethestrong ergs s.Theimagingproportionalcounter(IPC)spec- the Schmidt-KalerandFeitzinger(1982)estimateof stars suchasR122(03III(f*))andSk-2267° shape thedistributionfoundforotherveryhotLMC trum resemblesthatofHD93129A(03If*)byshowing and Conti(1982)arenotconfirmedbyChu(1982). of about±1500kms~.TheupperBalmerUnesappear ions whicharefrequentlyseeninthespectraofearly interpreted asrotation,theFevlinewidthsimplya cause ofcontaminationfromothersourceswithinthe1' (1953) andWalbom(19736,1977a).Quantitativemea- v siniofabout110kms~. km s“Inaddition,weakerfeaturesofOiv,v,Cin, may dominatetheR136measurement. nebula (SewardandChlebowski1982).ForCarina, of R136isabout3timeslargerthanthattheCarina IPC angularresolution.TheintrinsicX-rayluminosity surements arefoundinEbbetsandConti(1982).The P CygnilinesofNiv,v,CandHen,withthe Diffuse X-rayemissionfromgasheatedbystellarwinds diffuse emissionwhiletherestisfromearly-typestars. tral informationforthissourcewillbeambiguousbe- spectrum showsbroadbutratherweakemissionlineof C ivlineimplyingaterminalvelocity«3600 about 80%oftheX-rayluminosityisproducedby and FevaredetectedPCygniSiivisabsent.If V =10.77seemstobecompatiblewiththe40,000K X10 K,theintrinsicX-rayluminosityis2X10 The visualdatashowninFigure6bappeartobe The dereddenedultravioletcontinuumenergydistri- The infraredspectrumofR136to1.1jumisdiscussed Several authorshavereportedcontinuumorUnevari- The visualspectrumofR136isdescribedbyFeast The mostnoteworthyaspectsoftheultravioletstellar Vol. 273 1983ApJ. . .273. .5973 X1550 andHenX1640maybeoccurring,butmore velocity variability.Thevisualspectrumasdescribedby Moffat arealsolikelytobeconsistentwithnoradial In ouropinion,theHenmeasurementspresentedby a resultcompatiblewiththeFevmeasurementsof§V. gen absorptionlinespectrumshowsnovelocityshifts, XX1548.2, 1550.8,HeilXI640.4,NivX1718.6,and XXI 175.6,1247.4,NvXX1238.8,1242.8,OivXX1338.6, case. WemustconcludethatvariabilityofR136ahas measurements ruleoutlargechangesincontinuumshape has notchangedbetween1978and1982.Theultraviolet results from§VimplythattheterminalspeedofR136a dence forlargevariationovera30yeartimeframe.The (1982) arequahtativelysimilar.Thus,thereisnoevi- Feast (1953),Walbom(19736),andEbbetsConti various OstarshasbeenanticipatedinFigure3,namely, IUE databankviatheNationalSpaceScienceData numerous UnesofFev. variety ofionizationstagespresentintheultraviolet cluster ofnormalstars,wefirstexaminethetypes not yetbeenconvincinglydemonstrated. or linestrengthsoverthissametimeinterval.However, with MilkyWaystars. limited. Therefore,manyofourcomparisonswillbe very earlyLMCOstarsobservedbyIUEisquite IUE SpectralAtlasofWuetal(1982)andfromthe IUE datatodetermineconstraintsonpossiblecontribu- of variousobservationalconstraints.Becausethewide stars requiredtoreproducetheobservedspectrumand accurate datawillberequiredtomakeaconvincing the linestrengthvariabilityat10-15%levelinCiv subclasses. Dataforthisprojectwereobtainedfromthe spectra ofearly-typestars,emphasiswillbeplacedon between theLMCobjectand theMilkyWaystars,but HD 93129A,whiletheOivandvlinesresemble 93129A (03If*)andthe03V04stars.The LMC stars.Hutchings(1982)hasnotedthatultraviolet Center. ThedataincludespectraforMilkyWayand spectra ofOstarsalltemperatureandluminosity tral featuresthathavebeenexaminedincludeGm tors totheemergentfluxofR136a.Theprincipalspec- then discusstheplausibilityofsuchaclusterinlight No. 2,1983 Fe vspectrumofR136aissimilarinstrengthtothat the spectrumofR136acloselyresemblesthatHD than forMilkyWaystars.Unfortunately,thenumberof stellar windlinesforLMCstarsaregenerallyweaker R136a mightbeduetoachemical abundancedifference those ofHD303308.Theweakness oftheFevUnesin other effectsmaybeimportant becausetheLMCstar 1343.5, OvX1371.3,SiivXX1393.8,1402.8,C In consideringthepossibilitythatR136aisacompact We haveexaminedbothhighandlowdispersionIUE Our conclusionfromacomparisonofR136awith © American Astronomical Society • Provided by the NASA Astrophysics Data System b) TheClusterInterpretation NATURE OFR136a _1 large entranceaperture.Whiletheaveragespectrum profiles, likeR136aandHD93129A,buttheyalsoshow positive velocitysideofUnecenter,whileR136ahas XI 175.6(visibleinFig.2)issimilartothosethe03V illustrated containstheleastamountofcontamination emission onbothpositiveandnegativesides.Theevolved like thatofHD93129A,superposedontopthebroad velocity, 3600kms,isintherangeofearlyO profiles. TheemissionhalfoftheR136aCivprofileis mately asstronginHD303308.Stellarwindoptical from sourcesoutsidethe3"areameasuredbysmall HD 93129Ahas. strong SiivPCygniprofiles,whichneitherR136anor 04 and05starshavewelldevelopedNivPCygni He iiprofilesofthesestarshaveemissiononlyonthe (04 If+),fPupIf),andHD14947(05If+)show profile revealswhatmightbeanarrowemissionfeature, most earlyOstars(seeFig.4).Henshowsabroadflat mediate inappearancebetweenthoseofHD303308and does appearinstarsoftype05andlater.Aswiththe dispersion. CmX1247.4isnotdetectedinR136,whileit effect ontheFevlinestrengths.TheCmmultipletnear depth orevolutionarystatemayhaveanimportant HD 269698(04If+)hasanFevspectrumapproxi- not theresultofcontamination.Thenonsaturation R136a spectruminFigure3wasobtainedwiththeIUE He ilXI640emission.However,likeHD93129A,the early Ostars,includingHD15570(04If+),16691 emission. InadditiontoHD93129A,otherevolved emission componentnotseeninanyoftheOstar early OstarspectraarefoundintheCivandHenline HD 93129A. 93129A. TheNivPCygniprofileofR136aisinter- evidence ofSiivstellarwindabsorption.Photospheric Milky Way03Vand04stars,R136ashowsno and 04Vstars,barely,ifatall,visibleinlow Figure 4. structure seeninFigure3isintrinsictoR136aitselfand the Heiiprofile,however,possiblyindicatingthat spectra doshowakinkinthelong-wavelengthsideof aperture, thecontaminationlevelisstillabout25%of spectra examined.AcloseexaminationoftheHen stars, buttheabsorptiondoesnotsaturatelikethatin similar tothoseoftheOstars.Theabsorptionedge stellar linearecomparableinstrengthtothoseHD the small-aperture,high-dispersionprofileillustratedin the CivprofileisalsoshowntobeintrinsicR136aby the measuredsignal.Thelow-dispersion,small-aperture Si ivUnes.The“wings”superposedontheinter- Si ivisblendedwiththestronginterstellarandnebular earliest Ostars,illustrated by HD93129Aand 303308, andthegeneralsimilarity ofR136atotheearly O stars,weconcludeitispossible thattheobserved The majordifferencesbetweentheR136aandvery Before proceeding,weonceagaincautionthatthe Noting thediversityin spectral featuresofthe 611 1983ApJ. . .273. .5973 with theclusterhypothesisisinclusionofanumber bers ofevolvedstarstemperatureclass04andlateris early typeOstars.Spectralclasses04Vand03V-I velocity andcontinuumbrightness,couldprovidethe would provideemissionatHen(atbothpositiveand The simplestwayofreconcilingtheCivandHenUnes may berepresentedinsuchacomposite,butthenum- R136a spectrumisacompositeofnumbervery negative velocities)and,dependingontheirCivedge of Wolf-Rayetstars,specificallyWNstars.Suchstars severely restrictedbytheabsenceofPCygniSiivlines. (possibly theFevUnes). flux tofillintheCivabsorptionandalsoweakenany spectral featuresthatappearonlyintheOstarspectra 612 WN starswehaveexamined.ThenarrownessofitsHe better thantherelativelynarrowUneofR144.The in WS17.FortheWNstars toexplainthenonsatura- il UneimpUesthat,ifnormalWNstarsareresponsible predominantly duetotheOstarshasbeenmodifiedby early WNsubclasseswithHe ilUnesevenbroaderthan for theHenemission,principalcontributorsare spectrum ofR144isverysimilartothoseotherlate star matchestheobservedHenUneofR136amuch continuum level.Inbothcasesthespectrum,whichis required 13R122’sand1R144tomatchtheR136a required 15R122’sandWS17’s,compositeB the breadthofHeilUne.ThebroadUneWN3 semblance oftheR136aspectrum. composite spectraareshowninFigure7.CompositeA component wehavechosentwoLMCWolf-Rayetstars saturate ortheirterminalvelocities areontheorderof tion oftheCivUneprofile,either theirCivUnesdonot at nearlyoppositeendsoftheWNsequence,R144 group ofveryearlyOstars.TorepresenttheWolf-Rayet principal differencesseeninlowdispersionare the W-RstarsenoughtogiveaquaUtativelygoodre- spectra. ThusinacompositespectrumHenmightbe far thestrongestemissionfeaturesinultraviolet (WN7) andWS17(WN3)(seeFig.7).Thekeyaspectof the onlyWolf-Rayetfeatureclearlydetectable.Two the WNstarspectraisthattheirHenXI640Unesareby spectrum ofR122torepresentthecontributionfroma a compositespectrum,wewillusesomemultipleofthe in Figure6,thecontinuumlevelofR136aisequivalent ence ofstrongHenemissioninR136a.Inconstructing 7. Thereisaremarkablesimilaritybetweenthetwo.The He iiemissionUne.Asnotedin§VIanddemonstrated of WNstarsrequiredbythestrengthandbreadth required tomatchthefluxlevelofR136aandtypes the shallowerCivabsorptioninR136a,andpres- the weaknessofOvX1371inR136arelativetoR122, spectra ofthesetwostarsareillustratedagaininFigure star R122(03III(f*)).Thelow-dispersionultraviolet to about16timesthecontinuumlevelofLMC03 © American Astronomical Society • Provided by the NASA Astrophysics Data System The majordifferencebetweenthetwocompositesis We haveinvestigatedthenumbersofnormalstars SAVAGE, FITZPATRICK,CASSINELLI,ANDEBBETS 1 1 -1 base oftheCivUne,WNstarsneedtoprovide WN subtypescouldmeetbothrestrictions.Intermediate provide boththeobservedR136aHenUnestrengthand The WN3starslikeWS17haveHenUnestoostrongto principal contributionstoacompositespectrum.The WN subtypeswouldbetterexplainthenitrogenemis- mediate WNsubtypes,WN4orWN5,amixtureof 2000 kms,orboth.Toreproducethefluxlevelin with thosederivedfromotherspectralfeatures,namely, velocities downtovaluescharacteristicofthelaterO 20% oftheobservedcontinuumlevel.Perhapsinter- about 20%ofthetotalobservedcontinuumintensity. dominating thespectrumhaveCivedgevelocitiesbe- and Conti1982;Walbom1977«). sion-Une ratiosobservedatvisualwavelengths(Ebbets very representativeofitsclass,beingsimilartothestars, stars, 2000kms~,wouldyieldamoreroundedabsorp- tween 3000and3600kms.Asmoothdistributionof steep blueedgeoftheprofilerequiresthatstars would berequired. bly thetwomostluminousW-RstarsinLMC,with The continuumlevelofR144,however,isdecidedlynot have examined,includingHD269692,268847, that inacompositespectruminterpretation,03and04 tion profilethanisseen.Thisconstraintconsistent length ultravioletspectmmis showninFigure7.Assum- respectively. Thustoproducethesamespectrumasin M~ —8.0and-7.7,respectively.Theultravioletcon- representative oftheaveragelateWNstarsinLMC. R145 (WN7),HD36063WS18 objects wereincluded.TheUnespectrumofR144isalso composite asWS17longapproximately15ofthe 24783, WS37,alltypeWN3,HD270149(WN3-4), number ofearlyWNstarsintheLMCwhosespectrawe levels. WS17isquitesimilar,bothinspectralUne stars providemostoftheemergentfluxR136a. ing, aswehaveforR122, that itisonlyaffectedby composite B,correspondingnumbersofthosestars It andR145,bothinthe30Doradusnebula,areproba- tral classificationsaretakenfromBreysacher(1981). 269015 (WN4),andHD269549(WN4).Anyofthese classes ofobjects,particularlyintheirabsoluteflux how representativethosestarsareoftheirrespective one other03starisknownin theLMC.ItisSk22-67°, flux anddictatesthecontinuumshape.Atpresentonly 33133 (WN8),andWS27(WN8).AllWolf-Rayetspec- stars, oranycombination,wouldyieldnearlythesame strengths andinultravioletcontinuumlevel,toalarge composite spectrainFigure7,itisreasonabletoask 03 If*/WN6-A(Walbom 1982), anditsshort-wave- tra showninFigure7becauseitproducesmostofthe those ofHD36063,WS18,33133,and27, tinuum ofR144is6,8,11,and14timesbrighterthan v The CivUnealsoprovidesaconstraintonthe R122 isparticularlyimportanttothecompositespec- Because onlytwostarswereusedinconstructingthe 1983ApJ. . .273. .5973 21-0 21- provide theHeilXI640emission.ThedataforR136aandR144were dereddenedasdiscussedinthetext.R122,WS17,andSk22-67were have beenconstructedfromtheR122,R144,andWS17datatosimulate thespectrumofaclustercomposedearlyOstarsandWNstars. dereddened assumingonlyforegroundgalacticextinctionwithE(B —V)=0.04.ThedatafortheMilkyWaystarsHD93129Aand the R136adataaremarkedabovespectrum.InterstellarlineidentificationscanbefoundbyreferringtoFig.2.Simplecompositespectra prepare thefigureinclude:R136a-smallaperturespectra-SWP13976+15006+16605, LWR106248+11561;R144-SWP2767+13981, determined fromhighdispersionWEspectra.Theyare:R136a,8X 10cm;R144,4XR122,9XHD93128A, interstellar HILyaabsorptionhavebeenremovedfromallthespectra bydividingthedataexp[—oN(Hi)]convolvedwithWE galactic earlyOstars.Allsuchstarsarestronglyaffectedby extinctionwhosewavelengthdependenceisuncertain.Theeffectsof In thecompositespectra,bulkofultravioletradiationcomes fromtheearlyOstarsrepresentedbyR122.TheWolf-Rayet spectral smearingfunction,whereaistheLyalinecrosssectionand N(Hi)isthehydrogencolumndensity.ThevaluesofA(Hwere 303308 arenotcorrectedforextinction.Thesedataillustratetheproblem ofcomparingtheenergydistributionR136awiththose 3 X10cm;HD303308,2.3.ForWS17andSk22-67 thevalueofA(Hi)forR122wasadopted.TheWEimagesusedto LWR 2494+10629;R122-SWP6969+9315,8076;WS17-SWP 9169+10730,LWR7916;Sk22-67-SWP10730;HD93129A- SWP 8312,LWR7258;HD303308-SWP11225,9841. x x Fig. 7.—WEultravioletspectraareillustratedforR136aandvariousLMCMilkyWayhotstars.Linesofprimarilystellaroriginin © American Astronomical Society • Provided by the NASA Astrophysics Data System 1983ApJ. . .273. .5973 difficult forfurtherevolution toproducetheearlytype higher stillthanforthelateWNstars. M, mayevolveintolatetype WNstars,butthatitis proposed R136aclusterwouldsuggestinitialmasses suggest thatthemostmassive stars,35MCarina nebula(Walbom1982).ThreeWNstarsarealso number of03starsrequiredwouldincreasebyroughly tions fromWalbom1974).ThespectrumofHD93162 might beloweredifonlythemostluminousOandWN reproduce theobservedR136aspectrum.Thisnumber construction ofthecompositespectrainFigure7, other thanR122orHD93129A,wereusedinthe finds thattheaveragevalueofMfor03Vstarsis Galaxy (ContiandBumichon1975).Walbom(1982) as brightHD93129A(M=-6.6;Walbom1982), R122 hasanabsolutemagnitude(M=-6.44)nearly Galactic foregroundreddening,Sk22-67°isonly30%as stars knownwereincluded. cluster containingabout3003and15WNstarscould morphology andcontinuumintensityofR136aisthata 22-67° inFig.7orHD93129Aand303308Figs. a factorof2,to~30objects. 0 attempt toaccuratelydeterminetherelativenumbersof strengths of,say,OvX1371orivÀX1339,1343.Any stars couldbefoundthatwouldyieldtheobservedline 3 and7)thatitseemssomecombinationofthese 7. ThereissufficientrangeintheUnestrengthsamong the R136aspectruminourcompositespectraFigure 0 these featuresintoaccount. the R136aspectrumwouldofcoursehavetotakeall the varioustypesofstarswhichmightberesponsiblefor the earhestOstars(compare,forexample,R122andSk 614 v v v -5.4 ±0.2.Thusifalmostanyoftheknown03stars, Evolutionary modelsincluding theeffectsofmassloss © American Astronomical Society • Provided by the NASA Astrophysics Data System How likelyisitthataclustercontainingtherequired We havenotattemptedtoreproduceallthedetailsof Our conclusionfromanexaminationofthespectral SAVAGE, FITZPATRICK,CASSINELLI,ANDEBBETS 3- 53 7-3 -1 -1 -1 -3 10 10-3 bright component,R136astandsoutasmanyordersof has beencomparedtoR136abyWalbom(19736).HD NGC 3603,HD97950.Thissystem,withacomposite As afinalexample,wehavetheTrapezium-likecoreof magnitude moredensethananyotherknownregionin ever, itseemsclearthat,ifinterpretedasaclusterand certainly otherexampleswhichcouldbeinvoked;how- mass ofhotstarswithina0.1pccube,thederived parameters implyamassdensity,p«5.8X10Mpc. density ofhotstarsisp«3xl0Mpc“.Thereare (Walbom 19736).Takingavalueof300Masthetotal pc. Thefivestarsmakingupthe6Orisystemin the MilkyWayorLMC. 3" Xor0.1pcregionatadistanceof8.4kpc 97950 consistsofsixprincipalcomponentswithina spectral typeof05-6(n)+WN6-A(B)(Walbom19776), implications ofthesenumbersistocomparethem the speckleinterferometryiscorrectforsizeof and liewithinaregionofdiameterd«0.32pc.These Orion TrapeziumhaveatotalmassofM«100 (1971) reportsp«2.6xl0Mpcintheinner0.2 more familiarregions.FortheGalacticcenter,Oort WN stars(Chiosi,Nasi,andSreenivasan1978).The This averageisnotverydifferent fromthevalueinferred The averagevsinifor10Milky Way03and04stars by adispersioninvsiniamongtheclustermembers. M starsinacompactclusterwithdiameterof0.005 lobe overflow(Paczynski1967).Becausewehavea from theR136aFevdata. Thus,theobservedline observed byContiandEbbets (1977)is135kms. correspond tousin/«110kmsaremostlyproduced pc isapproximately40kms.Intheclustermodel, early WNstarsrequiredbyspectralconsiderationsmight then thedensityestimatewouldbereducedtoabout we taketheemittingregiontobea0.005pcdiameter necessary toproducetheR136aspectmmandphysi- calculate themassdensityofhypotheticalcluster. cal extentoftheregioncontainingR136a,wemay Milky WayexampleofearlyOstarsandlate-typeWN alternatively beproducedinbinarysystems,viaRoche this wouldimplythattheR136aFevlinewidthswhich WE smallaperturecomesfromthebrightpointsource, only one-halfoftheultravioletradiationdetectedin M pc.IftheactualspatialcharacteristicsofR136a cate M«2000Mq.Thissizeandmassthenimply From thespeckleinterferometryofMeabumetal(1982) the productionofearlyWNstarsfromeitherthese, stars coexisting,andbecausemechanismsmayexistfor are morecomplexasdiscussedbyWeigelt(1981)and an averagemassdensityofhotstars3x10 sphere including30veryearlyO-typestarswhichindi- types. the proposedclustercontainsaplausiblemixofstellar 0 0 0 1.5X10 Mpc.Thesimplestwaytojudgethe TOT0 0 0 0 tot 0 From thevirialtheoremrmsvelocityofforty50 Knowing thespectraltypesandnumbersofstars Vol. 273 1983ApJ. . .273. .5973 83 Ä53/211/2 No. 2,1983 presumably havethickerormoremassivewinds.Onthe The spectralpropertiesofR136aarewithinthebroad intermediate between03andWolf-Rayetstars,which range observedinothersinglestars.Thespeckleob- other hand,theCivabsorptionislikethatin03 emission thanin03stars,andthissenseR136ais stars. TheHen1640and4686Unesaremorestronglyin by theemissionfromasingle supermassivestar.We of thiscontaminationareunknown.Insectionwe star HD93250,awithneghgibleHenemission. istence ofasmallbrightcomponenttoR136a.Inaddi- important toconfirmthebasicspeckleresultsofWeigelt properties exists.Thisconclusionstronglydependson We consideritveryunlikelythataclusterwiththese presence ofabout30-40veryearlyOandWNstarsina pothesis. widths ofR136aarecompatiblewiththeclusterhy- inferred extreme-ultravioletradiationtobedominated choose toconsidertheobservedultravioletand from faintersources.However,theultravioletproperties that theIUEsmall-aperturedatawillincluderadiation servations ofWeigelt(1981)atvisualwavelengthimply known cluster.Itwoulddynamicallyevolveveryrapidly. (1981) andMeabumetal.(1982)whichimplytheex- exceeding bymanyordersofmagnitudethatany region speckleinterferometrysuggestsissmallerthan pc. Clusterevolutioniscomplex.However,thevarious yrs foraclusterwithA=40,m50M,andR0.002 valid forclusterswithA<100andignorethepossible to theexistenceofotherfaintercomponentsR136a. the discrepancybetweentwospeckleresultsrelating tion, attemptsshouldbemadetoresolvetheoriginof 7 X10~pc.Suchaclusterwouldhavemassdensity lifetime ofonly900yr. events proceedwithtimescalesthataresomemultiple existence ofnumerouslow-massstars,weobtain¿=60 short dynamicaltimescale.Clusterscales ultraviolet spectraandother stellarandnebulardata consider theconsistencyofthis interpretationwithour the validityofspecklemeasurements.Itwillbe cluster ofnormalstars,thiswouldimplyacorecollapse of t.Forexample,clustercorecollapseoccursafter about 15/(Spitzer1975).ForthehypotheticalR136a taining halfthemass(seeSpitzer1975;Lightmanand average stellarmass,andRistheclusterradiuscon- [log (0.4A)]whereNisthenumberofstars,m time, C(yr)9X10R*pc)AmM)" are oftenmeasuredintermsofthereferencerelaxation ter neededtoexplaintheR136adataisitsexceedingly Shapiro 1978).Ifweassumetheexpressionfortis that havebecomeavailablein thepastyear. 0h rh Th rh h h0 rh R136a hasmanyspectralfeatureshkethoseof03 In summary,theR136aultravioletdatarequire Perhaps themostrestrictivecharacteristicofclus- © American Astronomical Society • Provided by the NASA Astrophysics Data System c) TheSupermassiveSingleStarInterpretation NATURE OFR136a 51 40 15 + _l 40_1 240 photons persecondthatisrequiredtoproducethe30 ity, visualmagnitude,andmass)wasderivedbyassum- we herechoosetousethiscontinuumasafirmerbasis ing thatthestarR136aemits~5X10ionizing wavelength weseefromFigure6bthatXL=1X10 luminosity atX=2325A(orv1.2910Hz)asthe Doradus nebula.Nowwehaveamuchimprovedmea- cal modelswechoosetousethemonochromatic in theH,He,andHeionizingcontinua.Itisknown els witheffectivetemperaturesrangingfrom30,000to chromatic fluxesforplanetarynebulacentralstarmod- ergs s.HummerandMihalas(1970)presentmono- calibration pointforthecontinuumofR136a;atthis surement oftheUVcontinuumR136a(Fig.66),and properties ofR136a.ThefollowingusefulEmitsonthe This yieldsthemassgiveninlastcolumnofTable4. The stellarpropertiesUstedinTable4arederivedfor back scatteringbywindshasrecentlybeenstudied 200,000 K,andtheytabulatethephotonnumberfluxes lower Emitonthemassofstarcanbederivedfrom luminosity directlyfollowsfromR+and^eff>the is theemergentfluxofmodelat2325A.The various modelatmospheresusingthefactthatR136a This rangeshouldencompassallprobablemodelresults. of Cassinelli(1971),andblackbodyemergentfluxes. plane-parallel models,theextendedatmospheremodels sensitive toTathighfrequenciesandalsoblanketingby dent becausethethermalsourcefunctionB(T)isvery that theEUVradiationofstarsisrathermodeldepen- to deriveotherstellarproperties. more reaEsticestimateofthestellarmasscanbederived emergent fluxappearssomewhatmorelikethatofa Hummer (1982)andAbbott(1983).They R136a ofEbbetsandConti(1982) andChu(1982), sion measureofthestellarwind. Theopticalspectraof from considerationsoftheopticaldepthandemis- from thewell-knownempiricalproportionaEtybetween emits XL=10ergssat2325Â.Thestar’ssurface gray bodyorblackbody.Tocrudelyrepresenttheuncer- find thattheLymandiscontinuityissmallerand the windcanaffectemergentflux.Theeffectsof stellar temperatureandmasslossratecanbederived 3u, i.e., terminal speedandescape(Abbott1978),v^= the EddingtonEmit,M=aL/(47rcG).Ahigher, area isthengivenbyAirR=10/(7r\F),where7tF spheric absorptionEnes.The Eneprofileswillbebroad- the UVspectrafromour observations showphoto- tainties inmodels,wewillusetheHummerandMihalas x V x esc EDes x In CMSatableofstellarproperties(radius,luminos- For convenienceincomparisonwithvarioustheoreti- Table 4formsabasisforderivingconstraintsonthe 1/2 v =3600kms-'3[2<7(MAf)/Ä*]. xED (7.1) 615 1983ApJ. . .273. .5973 +2 +2 +2 2 616 wind. Fitzpatrick(1982)hasconsideredtheeffectsof noncoherent scatteringinWolf-Rayetwinds.Thefact wind opticaldepthisgiveninCassinelliandOlson ened bynoncoherentelectronscatteringinthestellar yields depth ofthewindinelectronscatteringmustbeless (1979), r=a(2M/(4ttR*1^))<1,whichforourcase that Unesarerecognizableindicatestheoptical than approximatelyunity(r<1).Anexpressionforthe where R+isthestellarradiusandMinunitsofsolar masses peryear.The30Doradusnebuladoesnotshow recombination ofHeinanebulaaroundveryhot He ilX4686emissionthatmightbeexpectedfromthe Assuming theHeStrömgrensphereiscontainedin star. ThismustmeanthattheHeionizingfluxfrom the photosphereisfullyabsorbedinwindofstar. The right-handsideisproportionaltothewindemission the stellarwind,wefind measure (VVol)whichinturnisproportionaltothe es mass lossrate,M,squared.Fromthisconditionand es Cassinelli andOlson(1979)wederive the expressionforawindemissionmeasuregivenin where V+2istheionizing photon numberfluxtabu- e He © American Astronomical Society • Provided by the NASA Astrophysics Data System v+ +2 * He r Ldv/hv=V(He)Va(HeVol.(7.3) vej8 18/2 /2 M> 8.4X10“(R+/Rof (V0',(7.4) He+ 40.000 60.000 90.000 75.000 50.000 94,681 60.000 50,555 50.000 BB =blackbodies. 90.000 75.000 Temperature Model b a EstimatedusingtheempiricalrelationforterminalvelocityversusescapespeedOstars. Three setsofModelsaretabulated:HM=HummerandMihalas1970;EXTCassinelli1971extendedmodels; Effective 3 (K) Atmosphere 5 M<1.4X10" (R*/R),(7.2) 0 EXT EXT HM HM HM HM HM BB BB BB BB SAVAGE, FITZPATRICK,CASSINELLI,ANDEBBETS Properties ofSingleStarswiththe2325ÀLuminosityR136a 25 25 25 24 24 26 25 25 24 25 25 (cm 2.57X 10 7.02 X10 3.44 X10 5.64 X10 6.25 X10 7.25 XlO 3.61 X10 1.33 X10 1.40 X10 1.31 X10 1.43 XlO Flux ofH Ionizing Photons 2 -s') + - 2! 24 19 20 20 24 2 21 23 18 23 20 Flux ofHe (cm s) 7.30 XlO 6.79X10 2.25 XlO 8.99 XlO 2.81 XlO 5.25 XlO 5.55 X10 5.51 XlO 1.56 X10 1.54X 10 1.03 XlO Ionizing Photons TABLE 4 31 + +2 41 lated incolumn(4)ofTable4,R*isthestellarradius, cannot belargerthan-10"Myr"becauseofthe of R*.Usingtheearlierrequirementthatstarshave emitted shortwardoftheHeionizationedgeat228À relation forMversus^effbecause,asthestellartempera- electron scatteringlimit.Therequirementthatallofthe and Misinunitsofsolarmassesperyear. boundary canbeplacedonFigure8.Illustratedbythe (54 eV).Thissteeprelationisalsoverymodeldepen- He radiationbeabsorbedinthewindgivesasteep the correct2325Acontinuumflux,wecanrelateR*to been derivedintworecentpapers. Theresultsdepend caused byEUVradiationfromthestarR136a,athird (or somefraction)ofthe30Doradusionizationis dent. IllustratedinFigure8withthecurvelabeledBBis ture increases,amuchlargerfractionoftheradiationis the maximumandminimummasslossrateasafunction did CMS,thatalloftheionizingfluxisfromR136a,we the Hummer-Mihalasfluxes.Ifwenowrequirethatall the resultobtainedinusingablackbodyfluxinsteadof find averynarrowrangeofallowedstellartemperatures stellar temperaturesthatcanproduceallorone-halfof arrowed bandsinthebaseoffigureisrange ^eff ’throughthenumbersinTable4. ferences byfactorsof2orso arenottobeconsidered somewhat ontheassumedstellar temperature,sodif- 2XlO" than about10.CassinelliandOlson(1979)carried trum. ThePCygniprofileofSiivisweakorabsent. and theoreticalmethods.FirstconsidertheUVspec- R136a appearstobeunsaturated, andagainthismight 50,000 K.Thisisconsistentwiththetemperatureof Q X 0 — 53,000KderivedbyHamann(1980)for03stars, Now letusseeifthestringenttemperaturelimits Fig. 8.—ConstraintsonthemasslossrateandtemperatureofR136ainterpretedasasinglestar.Anupperlimitis © American Astronomical Society • Provided by the NASA Astrophysics Data System NATURE OFR136a +2 34 51- lines atÀÀ1640,4686,5411,and10124canbeusedto Therefore theobservedHenrecombinationemission Another waytoestimatethestellartemperatureis equivalent widthisgivenby estimate thestellarfluxatenergiesbeyond54eV.The make useofourearlierargumentthattheHe dance of10'at58,000Kand10“66,000K. Combining equations(7.5)and(7.3),weobtain effective temperaturesdiscussedabove,andwededuce models. Theresultsappeartobefullyconsistentwith earlier: (a)Hummer-Mihalasmodelsand(b)blackbody Strömgren sphereisfullycontainedwithinthewind. greater than54eVisT(BB)«54,000K.Theionization used inFigure8thecondition thatthestaraccountsfor conditions inthe30Doradus nebulacanalsobeusedto that theblackbodybrightnesstemperatureforenergies the twoextremestellaratmospheremodelsdiscussed estimate thetemperatureof R136a.Wehavealready 5X10 ionizationss.Mathis (1982)hasdeveloped The observationsandresultsaregiveninTable5,for + 2 ^7 =hvN(ilz)a{n)Vol/LA,r.(7.5) ieA W = x + 2 ^(He )L(A„r x v ¿0.(11) + /oo ^He L dv/v. v (7.6) 617 1983ApJ. . .273. .5973 + caneuse + + + + jects discussedbyMaeder(1980).Theinteriorisfully we havebeenusingthroughoutthissectionand The ratiosdependontheparticularmodelatmosphere versus S/Sbdtoestimatethestellar•^eff• which haveformationionizationpotentialsof28eVand well asthestrengthsofArmX7136andNeniX3870 using theblackbodyandHummer-Mihalasmodelsthat Figure 4ofhispaperheshowsthataplot0/0 another reliablemethodtoderivethetemperaturesof be onthezero-agemainsequence,theyderivedfrom lines thatareslightlytoostrong.Together,theseresults predicts NemX3870andArX7136linesthataretoo ionizing 30Doradusisveryhot.Thelocationof Mathis hascomputedthe0/0andS/Sratiosas kindly carriedoutforusananalysisof30Doradus chosen toprovidethefluxesinLymancontinuum, stars fromaconsiderationofnebularlineratios.In polytropic theoryofHoyleandFowler(1963)that^eff be veryusefultohavemeasurementsofthenebularline weak. A75,000KblackbodyyieldsNeniandArin indicates thata60,000Kblackbodyistoocooland 41 eV,respectively.Theresultsindicatethatthesource and sooneshouldusearangeofmodels.Dr.Mathishas massive starweremadeinCMS.Assumingtheto emission lineofSmX9070tobetterdistinguishamong indicate asourcetemperatureofT(BB)«70,000K.The the LMCabundancesasgivenbyPageletal.(1978). mixed andthemasslossratelarge,sothatdifferencesin (within ±20%),andthe75,00090,000Kmodelsare Hummer-Mihalas modelsof60,000,75,000,and90,000 Doradus inthe0/0versusS/Sdiagnosticplane 618 cause ofthequasi-homogeneousevolutiontheseob- give acceptablestrengthsfortheNeniandArmlines sequence, itstemperatureshouldincrease.Thisisbe- these veryhotsourcemodels. slightly toohotonthe(0/0,S/S)-plane.Itwould « 63,000K.Asaverymassivestarevolvesoffthemain Theoretical estimatesofthetemperatureavery © American Astronomical Society • Provided by the NASA Astrophysics Data System lines areproducedbythesameobjectproducingultravioletcontinuum. Ebbets andConti1982.(3)Measured byusfromtheinfrareddataofVreux, contaminated high-dispersiondata ofFig.3yieldIF(1640)«4.0Á.(2)From Dennefeld, andAndrillat1982. 10124 X 4686 5411 1640 c b a Using continuumofPanagiaetal. 1983extrapolatedto10124A. Source.—(1) Fromthelow-dispersionsmallaperturedataofFig. 2. The Thesetemperatureestimatesare,ofcourse,validonlyiftheHenemission SAVAGE, FITZPATRICK,CASSINELLI,ANDEBBETS 3 Estimates ofTiffforR136afromHenRecombinationLines ^x(A) 10.0: 6.0 3.2: 1.0 0 Source (3) (2) (1) (2) TABLE 5 6 c Blackbody of a3000Mstar.Againthelinearanalysisindicated (1982) haverecentlyconsideredthevibrationalstabihty radial pulsationscanbeconvertedtononradialoscilla- plitude whichdrivesmasslossfromthestar(Ziebarth ing starStothers(1974)findsalimitof5000M.For tions (Papaloizou1973û,b).Ledoux,Noels,andBoury “infinite.” (b)Thepulsationscanreachalimitingam- nonuniform rotationhefindsthecriticalmasscanbe case ofnorotation.Forexample,forauniformlyrotat- Jager (1980).Hementionsthreeeffectsthatcanincrease tions inarotatingstarissignificantlygreaterthanthe stability theory:(a)Thestabihtyagainstradialpulsa- vibrationally unstable,andthesecondisthattheyshould with massesgreaterthanabout100Mshouldbe the masslimitthatisderivedfromlinearvibrational years. Notethatthislifetimediffersconsiderablyfrom 1970, Appezeller1970).(c)Thepotentiallydisruptive not form. (1982). interior temperaturestructuresarenearlyindependent 0 the existenceofsupermassivestars.Thefirstisthatstars Furthermore, sinceLaM,thelifetimesofobjects of massforMgreaterthanafewhundredsolarmasses. directly proportionaltothemass,andsurface 0 the smallerbutincorrectvaluegivenbyMeabumetal are thesameasfora100Mstar,approximately2X10 Hoyle andFowler(1963),issuchthattheluminosity Thus ourestimateofatemperaturegreaterthan certain, however,becauseNomotoandSugimoto(1974) 63,000 Kmaybeanindicationthatthestarisevolving composition fromphotospheretocoreareeliminated. give specialconsiderationtosurfaceboundarycondi- for astarof1000Monthezero-agemainsequence. tions andderiveaneffectivetemperatureof84,000K toward theheliummainsequence.Thisisnotentirely 0 0 Q Emitter 51,200 57,100 55,000 55,300 The vibrationalstabilitymasslimitisreviewedbyde There aretwowell-knowntheoreticalobjectionsto The structureofsupermassivestars,asdiscussedby 7;f(K) f Hummer-Mihalas c 75,200 77,700 78,900 77,600 Model Vol. 273 1983ApJ. . .273. .5973 The cocoonconvertsstellarluminositytoinfraredemis- infalling materialfartherout.Iftheluminosityissuffi- during theformationprocess.Aroundacollapsingstar, outward radiativeaccelerationondustthatdevelops M shouldnotbeabletoformbecauseofastrong (1974). Hearguesthatstarswithmassesgreaterthan40 limit canbeincreasedtowellbeyond1000Mby mass ofthestar.Wolfire(1982)hasbeeninvestigating cient, theinfallcanbereversed,puttingalimiton been suggestedthattheextendedblueclusternear ways: The30Doradusdust-to-gasratioislower(de perature from3600Kto2000K.Furthermore,dust sion whichcanradiativelydeceleratethedustin a dustcocoondevelopsatthesubliminationradius. young cluster.BegelmanandRees(1978)havecon- law isdifferent(FitzpatrickandSavage1983). increase themasslimit. decreasing theassumedaveragedustsubliminationtem- The criticalvalueforthemaximummassofamain- center of30Dorresemblesablueglobularcluster Doradus regiondiffersfromMilkyWaydustinseveral destruction byphysicalandchemicalsputteringcan the protostellarmaterial.Forexample,Kahn’s(1974) sensitively onassumptionsconcerningdustpropertiesin this limitingprocessandhasfoundthatitdependsvery pulsation studies. physical environmentsintheouterregionsofgalaxies central massiveobject.Thesameprocessmightproduce runaway coalescencemayleadtothebuildupofa (McGregor andHyland1981).PerhapsR136aformed Boer, Koomneef,andSavage1980)thereddening rectly applicabletoR136asincethedustin30 growth ofthefirstharmonictowardsurface that radialpulsationsshouldgrow,buttheyalsofinda sequence stardoesnotappeartobedeterminedbythese star whichisconsideredastronglystabilizingfactor. The UVspectraanalyzedinthispaperprovideauseful regions areknowntoexist. mation masslimitsofstars,especiallywithregardto sidered thefatesofdenseclustersandsuggestthat as aresultofeventsoccurringinthecoredense sive starsthatcouldhaveoccurredforR136a.Ithas a compactsystemofsupermassiveobjects. No. 2,1983 cal constaintsonthesingle-starinterpretationofR136a. such astheLMCandM101wheresuperbrightHn of 16higherthanforanyother Ostar.Thelinefeatures foundation. Themonochromaticluminosityisafactor tional resultswhichhavebeenstudiedtoderiveempiri- 0 The estimatesoftheextreme UV(X<912)luminosity suggest thebolometriccorrection shouldbeverylarge. 0 The formationmasslimitisconsideredbyKahn There areothermodesfortheformationofsupermas- It ispossiblethatKahn’s(1974)analysisnotdi- Table 6andFigure9summarizethemanyobserva- © American Astronomical Society • Provided by the NASA Astrophysics Data System Further theoreticalworkisneededtostudythefor- NATURE OFRI36a 51 +2 +2 +2 line observationsofHuchtmeierandChurchwell(1974) line ratiosindicatethattheionizingsourceof30Dora- value observedintheUV.Nebularlinestrengthsand ionizing photonspersecond.Theradiorecombination on thewindandnebulasurroundingstar.The30 composed ofacluster“ordinary” OandWNstars ble X4686Henlineemission.Thisrequiresthatthe indicate thattheradioemissionisthermalratherthan Doradus nebularequiresasourceproducing5x10 must beinferredfromtheeffectsofstellarradiation wind. Therecombinationradiationisdetected,andit leads totheresultthatradiationfieldatX<228 dus isunusuallyhot.However,thenebulashowsnegligi- ergy distributioninthesourcemustrisewellbeyond (E> 54eV)islikethatofa54,000Kblackbody(nor- If so,theradiationshouldbeindirectlydetectable He ionizingradiationisabsorbedbythestellarwind. 30 Doradusionizationconstraintrequiresthattheen- that fromasupernovaremnant.Figure9showsthe The extended94,000KmodelofCassinelli(1971)fits plained inpartaseffectsoftheextensivestellarwind. is equivalenttothatfromastarwith^effmuchlarger malized tomatchtheUVfluxat2325À).Modelatmo- through therecombinationofHethatoccursin of 40,000K.Theinfraredfluxmeasurementsweremade is alsoconsistentwiththeconstraintsderivedinFigure plotted versusv.AHummer-MihalasmodelwithT= He edge,andsoabrightnesstemperatureof54,000K spheres forhotstarsshowlargedecreasesacrossthe Panagia etal.(1983)havefoundthattheshapeofIR flux canbecharacterizedashavingacolortemperature observations orinferencesfromobtained somewhat highbecauseofcontamination.Nevertheless, through aratherlargeaperture(10"),sotheymaybe the UVfluxverywell,eventhoughaccessible at theshortwardsideofWEobservations,and 75,000 Kfitstheresultsverywell.Notethistemperature distribution isshowninFigure9withvL(orXL) Mihalas model). explaining allofthesedata. flux distributionislikethatexpectedfromthewindofa the infraredwavelengths.Thesedifferencescanbeex- than 54,000K(75,000inthecaseofaHummer- UV. Thesingle-starhypothesisisseentobecapableof at frequenciesextendingfromradiothroughtheextreme fundamental stellarpropertiesthatarederivedfrom supermassive star. 8. Thereissomediscrepancyfromthe75,000Kmodel px We haveconsideredthepossibilities thatR136ais The observationallyandempiricallyderivedenergy At thebottomofTable6wegivearangevaluesfor d) OtherInterpretationsofR136a 619 a

TABLE 6 © American Astronomical Society • Provided by the NASA Astrophysics Data System A. Constraints on a Single Star Model for R136a 5 ^ H 2 § ^ m o — rj- V .o c 3 e •S o ^ ■G ^ 53 — CJ ^ G- ^ ca55 ^00^33 2 >^w*2’G a c1.2á^ o< (u oc v o r- X ? = « ^ cc2 •e go •S S 3 .2G*G .2 £° ►2 .2u x XK¿ § ■-S“! o w I §8 «2 > f^-3 : «E crsr « A I 2« S ° 8 o IT3 (N X n 5 c^ '2^ Il X) 3 C/5 UJ • û i ro s -3 -C5 5T v O X, v g- § Q? K CK V V (K o r- g i-j ^ 2 o oo - o « ^ 2 ^ 6 x & X G V ^ v ^ O O G v T vn wß o° O § 8^ m \or- (N —'— - °o o^ ^ V XS^ ^r, v ^8 ^ 0V S £ s a 8« ON - 5 U 1 ^ w Tí Pi á g s 5/3 V i ffig ^ où 111; S ^<2 ffl c^O 42 13.3vh « 2^o 'S -G «ic^x •3 3 3 g 3 g C eu 2 I« X ^Ph '3 S2'G tl-H OXCA Il ë .. üx 'S 8-- *•^1 2 g O>»H ''3 X 3 'C g.2G o S^ü G D^ H"* 3, o Su« 3 o« ^úñ •g^ië g °- Z2S — G .2 3 C/5 3 ÖD-G 2 ag g hS 0 3 2 w ^3g A) —.9g^ G >2 O ’Cw ^ -g1^I 2 42 |îS « S «|.§ 2 AJä»h 0 X(NO o 4îàCS~ ex . cS Tî^ r a§ 1 2^1 c 21 (U XO 5c) 2g > OGc c/5 »3*3^4£ 3 GOÛXS isos’! > .2^X 2SUJ® g ’JaeS 2 ogpx &H 3 C 3oo . (U '«£ oo x « ^ Ê 3 2 S O i I 1983ApJ. . .273. .5973 _I 7 51- violet. Thestellarparametersassociatedwiththe75,000Kfitare with velocityamplitudesofthebrightestcomponent (1971) thatfitstherelativelyflatenergydistributioninultra- pirical data,and(fr)anextended94,000KmodelofCassinelli Hummer andMihalas(1970)thatfitstheobservationalem- energy distributionsoftwomodels:{a)a75,000Kmodel shock-heated nebulargas.Thesolidanddashedlinesshowthe be continued.Themostextensive opticaldatabaseis exceeding 15kms.Sincetherangeofpossibilitiesis relatively short-periodbinaryormultiplestarsystems Fe vabsorptionspectrumwouldappeartoeliminate view thantheinterpretationthatitisasinglestar.The very massivestarsinacompactgroup.Thepossibility since thedatamaybedominatedbydiffuseX-rayemissionfrom using thehigh-resolutionimagerhasbeencorrectedforinterstellar blackbody. X-raydatafromtheHEAO2satellite(Long1982) multiple supermassivestar interpretation.Optical enormous, wehavenotconsidered ingreaterdetailthe However, theabsenceofradialvelocitychangesin data availabledonotruleoutthisalternatepossibility. exciting fromatheoreticalorobservationalpointof mass ofaround2000M(§Vile).Thereisaninter- L =6.4X10,tf*48RandA/*2100M. absorption. TheX-raymeasurementisplottedasanupperlimit nebula. TheHenionizingradiationisabsorbedbythewind,and hydrogen ionizingcontinuumisderivedfromtherequirementthat of nebularandwinddata.Theinfrared,visual,WEenergy mediate caseinwhichtherecouldbeafew,say2-10, (§ VII6),orthatitisasingleobjectwhichmusthave reference pointinluminosityat2325Á.Thethe distributions arerepeatedfromFig.6.Thelargecircleshowsthe searches forradialvelocityvariability ofR136ashould that R136aisagroupofsupermassivestarsnoless the luminosityisdeducedinTable6tobethatofa54,000K y (Hz),obtainedfromdirectobservationsandinterpretations the starproduce5X10ionizationssin30Doradus 0 0 -1 Fig. 9.—vL(ergss)forR136aasafunctionoffrequency, © American Astronomical Society • Provided by the NASA Astrophysics Data System v NATURE OFR136a 3 23 ble multiplicityofR136a. have beenconsideredtoexplainquasarsandthecores ple, R136amightberelatedtothevariousobjectsthat more exoticthanasinglesupermassivemain-sequence varying degrees)toexplaintheobservationaldatafor of ordinaryandSeyfertgalaxies.Themodelsinvolve data mightprovideimportantconstraintsonthepossi- star orasmallgroupofsupermassivestars.Forexam- that ofMoffat(1982).Acarefulconsiderationthose needed toexplainR136a.Theinferredrotationspeedof X-ray emission—theirpropertiesarenotlikewhatis rotators, etc.Whilesomeofthesemodelsseemable(to objects suchasaccretiondisks,blackholes,magnetic R136a issmall,theenergydistributionratherwell quasars—large luminosities,powerlawcontinua,and processing theultravioletdata. WethankM.R.Meade bright componentisasingleobject,theobservational is eitherasinglesupermassivehydrogen-burningstaror would havedynamicalevolutionarytimescalesmea- less than1000AU.Suchaclusterwouldhavestellar luminous objectwithspectralcharacteristicssimilarto quasar models. compelling reasontogivestrongconsiderationthe described bymodelstellaratmospheres,andtheline mass densityexceedingbyafactorof10thein ultraviolet emissionfromR136aisdominatedbythat Madison. Y.-H.Chu,D.Massa, J.S.Mathis,M.Slovak, Center fortheirexpertassistanceinacquiringand vibrationally stableisnotknownfromtheoryor limit tothemassofastarthatcanformandbe ments canbemadethatformationoccurbyordinary data implyamassthatexceedsbyfactorof20 the nucleusofMilkyWay.Thehypotheticalcluster from thebrightcomponentdetectedinvisiblewith early WNstarswithina3"diameterregion.Ifthe spectrum lookslikethatofordinarystars.Weseeno and W.Sorrellprovidedmany helpfulcommentsre- for herassistancewiththeultraviolet datahandlingin observations. collapse inaregionwithpeculiardustpropertiesorby concerning stellarformationandstability.Argu- a compactmultiplesystemofsupermassivestars.Ifthe sured inintervalsfrom10toyears. speckle techniques,theclusterwouldhaveadiameterof that of03stars.IfR136aisacompactclusternormal coalescence inthecoreofacompactcluster.Theupper the mostmassivestarsknown. stars, theclustermustcontainapproximately3003and One mightimaginethatR136acouldbesomething We thanktheIUEstaffatGoddardSpaceFlight It ismorelikelythatthebrightcomponentofR136a Ultraviolet datarevealthatR136aisanexceedingly Such astarraisesimportanttheoreticalproblems VIII. 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