1970AJ 75. . 933H THE ASTRONOMICALJOURNAL of theGalaxyandwithregardtophysicalprocesses demonstrated, bothwithregardtothelocalstructure THE importanceofthestudystellarassociations involved instarformation(Gratton1963;Blaauw important forthesereasonsandofferthefurther advantage overthelocalobjectsofbeingsomewhat have beenstudiedindetailrecentpapers(Woolley and manyoftheMagellanicCloudstellarassociations system. ThelargerassociationsintheAndromeda easier tosampleandstudyasmembersofacomplete 1964). Stellarassociationsinothergalaxiesarealso catalogue ofsuchobjectspublishedbyLuckeand Galaxy havebeenexaminedbyvandenBergh(1964) Hodge (1970).Wehavestudiedthephysicalcharac- tions intheLargeMagellanicCloudisbasedon attempt toevaluatethephysicalpropertiesof teristics andthespatialdistributionofobjectsinan 1960; Westerlund1961;HodgeBokand system ofassociationsasquantitativelypossible. 1962; Boketal.1964;WalkerandMorris The meandiameterwascalculated onthebasisof mean characteristics.Table I summarizestheresults. with thoseinothergalaxies,wehavecalculatedtheir 1968; Hodge1970a). major diametersofthestellar associations,asdelineated in thecatalogue(Luckeand Hodge1970),inorderto the stellarassociationsinLargeMagellanicCloud for theassociationsinM31.The quotedmeanprojected conform withtheprocedure adopted byvandenBergh The presentanalysisofthesystemstellarassocia- In ordertomakeacomparisonoftheproperties © American Astronomical Society • Provided by theNASA Astrophysics Data System in theneighborhoodofsunhasbeenclearly 3 projected squareparsecsintheassociationsis19,whilemeantotalpopulationofsuchstars18per stellar association.Bothofthesefiguresaresomewhatlargerthanthepopulationsandpopulation borhood. Themeannumberofstarswithabsolutevisualmagnitudesbrighterthanabout—4per10 the meandiameteris78pc,similartoofstellarassociationsrecognizedinsolarneigh- ciation andthetotalnumberofstars.Fifteen“starclouds”arerecognizedthesehaveameandiameter densities ofsuchstarsinstellarassociationsnearthesun.Thereisaconsiderablespread bright starsintheLMC’sassociations,butageneralover-allcorrelationexistsbetweensizeofasso- are moresimilartotheobjectsstudiedinM31byvandenBerghthanmajorityofstellarassocia- of 225pc,withtotalpopulationsaveraging32starsbrighterthanA/V=—4.2.Indimensionstheseobjects tions intheLMCorourGalaxy.Thereis,however,noobviousdiscontinuitybetweencharacteristics contain clustersthatareprobablycoexistentphysically,onlyafewofwhichmightbesuperimposedbychance considerably steeperthantheradialdistributionofstarsorHiinLMC.Sixteenassociations of thetwokindsobjects.TheradialdistributionassociationsisshowninFig.2.Itappearstobe H iiregionsarerelatedtothemeanstellardensitiesofassociations. on theassociation.Theseclustersseemtobesimilarnucleifoundinsomestellarassociationsofour Galaxy. Atotalof96the122recognizedstellarassociationscontainHnregions.TheHafluxesthese From astudyofthe122recognizedstellarOassociationsinLargeMagellanicClouditisfoundthat The SystemofStellarAssociationstheLargeMagellanicCloud II. MEANCHARACTERISTICS I. INTRODUCTION Astronomy Department,UniversityofWashington,Seattle,Washington (Received 13May1970;revised10July1970) Paul W.HodgeandPeterB.Lucke VOLUME 75,NUMBER8 933 from theoriginalidentificationchartsandcounts published inthecatalogue.Themeantotalpopulations densities arebasedoncalculationsoftheareasmade were takendirectlyfromthepublishedcounts.Inall more than1000stellarassociationsestimatedtobein factor ofapproximately10inthedifferencetotal M31 showsseveralsimilaritiesanddifferences.First, cases, thedistancemodulustoLMCwastakenas masses ofthegalaxies,thisdiscrepancyisroughly in theestimatedtotalnumberofassociations, not quite)asthoroughlyarestellarassociationsnear our Galaxy(Gratton1963).However,consideringthe stellar associationsaresampledalmost(butprobably accounted for.Itappearspossiblethen,thattheLMC shows averyconsiderabledifferencewithourGalaxy, the sun.ComparisonwithM31,onotherhand, 1969). supporting theconclusionofvandenBergh(1964) 122 LMCobjectstotalconsiderablyfewerthanthe figures givenbyHodge(1961). Mean totalpopulation(Mv<—4) Mean projecteddensities(number Mean diameter(pc) Estimated totalnumberof b 32 ° Thisfigureincludes,ontheaverage, onefieldstar,accordingtothe FromvandenBergh(1964). •From Gratton(1963)andBlaauw(1964). associations of starswithM7<—4per10pc) A comparisoninTableIwithourGalaxyand Table I.CharacteristicsofLMCOassociations, 18.90 (HodgeandWright1969;Gascoigne ab Feature LMCGalaxyM31 compared tothoseinothergalaxies. 34 122 10-10188 78 «60480 18 «7 19° «3 OCTOBER 1970 Associations 1970AJ 75. . 933H 2 2 number ofcountedstarsbrighterthanapparentVmag14.7. associations (themeanofthemajorandminoraxes) 934 per 1000pcforassociationnumber 111,thereisnever- near thesun(about60pc).Thisfurthersupports that hissurveyincludesonlytheverylargestobjects extent, thisrelationshipis possibly duetoselection and thesizeofassociations (Fig.1).Toacertain theless aclearcorrelationbetween thenumberofstars stars per1000pcforassociation number59to60stars much largermeandiameterof480pc. brighter than—4.2)variesover awiderange,from1.6 visual magnitudesgreaterthan—4.2arealsocompared LMC hasrecentlyexperiencedarelativelyaccelerated that mightbetermedstellarassociations. rate ofstarformationcomparedtoourGalaxy(Payne- have significantlymoresuchstarsthandothoseinthe examined. ThoseinM31,however,havethevery conclusion thatroughlysimilarobjectsarebeing the estimatedmeandiameterofstellarassociations in theLMCisconsistentwithotherevidencethat lations ofsuchluminousstarmembers,whicharemore young stellarassociationsintheLMCcomparedto in TableI,withtheresultthatLMCassociations supported bythecomparisonofmeantotalpopu- 78 pc,onlyslightlyandnotsignificantlylargerthan Gaposchkin 1969). Such anoverabundanceofyoungerstellarassociations than twiceasgreatintheLMClocalGalaxy. the localneighborhood.Thispossibilityisfurther effects andthatthereisarelativeoverabundanceof solar neighborhood.Itislikelythatthedifference too largetobedueentirelysystematicselection Fig. 1.Therelationshipbetweentheaveragediameterof The meandiameteroftheLMCassociationsis Although thedensityofsupergiantstars(withMy The meanprojecteddensityofstarswithabsolute © American Astronomical Society • Provided by theNASA Astrophysics Data System HODGE ANDLUCKE LMC, thecentroidofsystem wasfirstlocatedby associations andthatreportedbyvandenBergh(1964)for MFig. 2.AcomparisonofthesizedistributionLMCstellar in spaceofthesystem stellar associationsinthe LMC aresummarizedinTableII. making theseobjectsalmostidenticalinsizetothe for associationslargerthan150pcisthesame nation, theM31surveydidnotincludemanyassocia- M31. major dimensionsaverageabouttwicethisvalue, limit. AsnearasFig.2canshow,thesizedistribution tion andbecauseofadifferentsetrulesdiscrimi- M31 sample.Thecharacteristicsofthestarclouds tions thatmayexistwithdiameterssmallerthanthis fied byanSinthecatalogueofLuckeandHodge(1970). lanic Cloudarethosetermed“starclouds”andidenti- whereas almostalloftheM31associationsarelarger A totalof15theseobjectsareidentifiedintheLMC Since thisisapproximatelytheratioofmasses times asmanyassociationsinM31theLMC. two galaxies.Overthatinterval,therearenearly10 M31 andthatfortheLMC(Fig.2).Thevastmajority and theirmeanaveragedimensionis12.8arcmin,or two ispossiblyverynearlythesame. two galaxies,therelativenumberofassociationsin than this.Possiblybecauseofthelimitationresolu- of theLMCassociationsaresmallerthan150pc, between thesizedistributionofstellarassociationsin 225 pc.Asmanyofthestarcloudsarehighlyelongated, region inthediagramwouldmostlikelybeidentified in thelowerright-handportionofFig.1,dueprobably as starclustersratherthanstellarassociations. to thefactthatanyobjectsmightoccupy the sharpcutoffthatseemstoallowvirtuallynoobjects effects. Particularly,sucheffectsprobablyaccountfor In ordertoanalyzetheprojected radialdistribution The largestcomplexesofOstarsintheLargeMagel- Selection effectsmayalsoexplainthedifferences III. SPATIALDISTRIBUTION 1970AJ 75. . 933H hm position (1975)a=524,ô=—68°39'.Thisdiffers catalogued objects.Thecentroidsodefinedhasthe averaging thepositionsinanX-Yplaneofall122 The threeprincipalfeaturesofthisplotare:(i)alow discussed bydeVaucouleurs(1955,1960). Magellanic Cloud,suchasthosebasedonstarcounts somewhat fromother“centers”foundfortheLarge all stellarassociationpositions. LMC, plottedwiththeadoptedcenterchosentobemeanof gradually tailingoffbeyond1500pctozerodensityat structure andtherotationofLMChavebeen and surfacephotometry(deVaucouleurs1955,1957; Bergh 1964,Fig.6)andthedearthofHnregionsin at 40min(700pc)and80(1400pc),(iii)a density atthecenter,(ii)adoublehumpwithmaxima stellar associationswiththedensityplottedasafunc- choosing differentcentersontheinterpretationof de VaucouleursandFreeman1969).Theeffectsof cant featurethatisperhapsanalogoustothesimilar tion ofdistancefromthecentroidassociations. due toamischoiceofthecentersystem.Thelatter lack ofassociationsinthecenterM31(vanden of associationscouldbelookeduponeitherasasignifi- 230 min(4kpc)fromthecentroid. which isclearlyataconsiderable distancefromany density ofassociationsoccurs nearassociationNo.93, seems unlikely,becauseasaglanceatthedistribution but probablynotbasicallyimportant. Itisapparently center ofthesystem. of theobjectsshows,only clearmaximuminthe the centersoflate-typespiralsandirregulargalaxies (Hodge 1969a,b),oritcanbeconsideredaspossibly Fig. 3.Theradialdistributionofstellarassociationsinthe The lackofHnregionsatthecentersystem Figure 3showsaplotoftheradialdistribution The doublehumpthatisvisible inFig.3issignificant, © American Astronomical Society • Provided by theNASA Astrophysics Data System ASSOCIATIONS OFTHELMC determined bydeVaucouleurs(1955). parts ofthesystemassociations (solidline,least-squares solution) andthegeneralradial distribution ofbrightstarsas inner maximumbeingduetothegroupingaround due totheveryspottydistributionofassociations, ings aroundassociation93,that78, grouping aroundassociation27,allofwhichhave outer humpduetothecombinedpresenceofgroup- association 64andthatincluding27,48,49the plotted intervalisgood.However,whenthisslope distances ofabout80minarcfromthecentroid, and thatsurroundingassociation34,thelooser has beenestablishedthatthedistributionoflight of thesystem. though theyareapproximatelyatfourdifferentcorners siderable spreadabouttheline,agreementover nential outwards.Figure4comparesthedistribution exponential decreaseoutwards.Althoughthereiscon- bution ofstarsintheLMC(deVaucouleurs1955).It Furthermore, itisalsosimilarinshapetothedistri- 80 minofarcissimilartothatobservedforM31. of associationswithalinethatwouldrepresentan spiral andirregulargalaxiesisusuallynearlyexpo- (from photometry)andofstarsstarcounts)in » Excludingembeddedclusters. Fig. 4.Acomparisonoftheradial distributionintheouter The gradualdecreaseindensityoutwardsbeyond a Total number15 Mean dimension Mean totalpopulation(Ily<—4)32 (pc) 225 (arc min)12.8 Table II.CharacteristicsofLMCstarclouds. 935 1970AJ 75. . 933H 936 brighter thanabsolutemag—4.2andthemeanvalueon taken fromDohertyetal.(1956):Tstandsfor“trace,”and0 only) ofimbeddedHnregions,theHaintensitieswhichare Michigan scaleofHaintensity(dataforwholeemissionregions, * signifies noHadetectedbythemfortheregion. Ö from hisstarcounts,aconsiderabledifferenceisfound. compared withthatderivedbydeVaucouleurs(1955) 2 him forthedistributionofbrightstars,clearlynotin very muchmoresteep. Figure 4showsalinedrawnwithslopedeterminedby agreement withtheassociationdistribution,whichis in Fig.4mightresultfromseveraldifferentalternative may befortuitous,primarilytheresultofchoice radial distanceofthefarthestdetectedassociation.It coverage, asoursearchextendedoutto3timesthe explanations. Itcannotbeduetoadifferenceinareal fact, however,inspectionofdeVaucouleurs’Fig.4 discussion, thedistributionofstarsasdeterminedusing c hydrogen inlate-typespiral galaxies(Hodge1969a). neutral hydrogen.) not sufficientlysteeptobringitintoagreementwith of centers,whichweredifferentinthetwocases.In c the centerofbarratherthanhisadopted Physically, thedifferenceshown inFig.4couldbe of Hiiregionsandthedistribution ofstarsandneutral to asimilardifferencefoundbetweenthedistribution center. (HisFig.4clearlyshowsthatthedifferenceis of stardistributionhasaslightlysteeperslope,but tion oftheassociationshasbeen mostprevalentinthe explained bytheassumption thatcurrentlytheforma- apparent alsobetweenthestellarassociationsand that thedifferenceinslopeisduetochoiceof the associationdata.Itthereforeappearsimprobable central areasoftheLMCto theexclusionofouter (1955) suggestthatovertheintervalofradiusunder Fig. 5.Therelationshipbetweentheaveragedensityofstars The differencebetweentheslopesoftwocurves If theslopedifferenceisreal,thenitmayberelated T 4 0 3 © American Astronomical Society • Provided by theNASA Astrophysics Data System 5 678 Averagep ofAssociation HODGE ANDLUCKE íí, highest densitywhilestarsintheouterareasform The clumpingofstellarassociationsisquitecon- relatively greaterabundance.Alternativelyitmaybe areas, wherestarsofonlyslightlygreaterageexistin range between500and1000kpc,wherethereissignifi- jected distributioninspacetoseeiftherearestatistic- stellar associations,wehavealsoexaminedtheirpro- smaller groupsorindividually. that starassociationspreferentiallyforminareasof cant deviationfromaPoissondistributioninspace. ally significantgroupingsoftheassociations.Wefind spicuous totheeyeinpreviouslypublishedidentifi- that thereisatendencyforgroupstooccurinthesize cation chart.Asimilardumpinessappearsalsotooccur of theLMC,whenonlyclustersasingleageare on thesamesizescaleindistributionofstarclusters in differentpartsofthegalaxyattimes, averaging 25clustersorsopergroup,weredetected examined (Hodge1970b).Clumpsofstarclusters, years. extending asfarbackintimetensofmillions within thebordersofstellarassociations,butonly identify possibleclustermembershipoftheassociations conspicuous starclusters,anattempthasbeenmadeto stellar associations.Theseclustersarenotalwaysin brightest starssufficientlyluminoustosuggestthat symmetrically positioned.Clearly,notallofthestar as theclustersingalacticassociationsarenotusually the centersofassociations,butthatisnotacriterion, those clustersthatliewithinassociationsandhave a fewofthesearepossiblemembers.TableIIIlists of theLMC.Hundredsstarclustersarefoundtolie their agesarenotverydifferentfromtheof 1 FromHodgeetal.(1970). In additiontostudyingtheradialdistributionof As someoftheOassociationsinourGalaxycontain Assoc. No. 103 100 111 105 41 35 69 61 94 27 24 90 87 84 18 10 Table III.ClustersinLMCOassociations. IV. STARCLUSTERSINASSOCIATIONS NGC 1876 NGC 1823 NGC 1763 NGC 1837 NGC 2018 NGC 1983 NGC 1910 NGC 2077 NGC 2070 NGC 2055 NGC 2044 NGC 2048 NGC 2027 NGC 2100 NGC 2084 NGC 2079 Cluster (s) SL 237 SL 360 mv (brighteststar a in cluster) 13.8 13.9 13.5 12.6 13.3 14.0 13.5 14.1 11.7 13.5 11.7 12.7 11.7 11.7 12.7 14.0 1970AJ 75. . 933H clusters listedinTableIIIarephysicalmembersof superimposed. Fromacomparisonoftheareasub- the associations;severalmustbeonlyapparently tended bytheassociationswithaveragedensity of starclustersthebrighteststarswhicharebrighter clusters listedinthetablearechancesuperimpositions than —14,takenfromthephotometryofHodgeetal. lations includedallavailableparametersandtheonly associations coincidewithatleastoneHnregion contain clustercondensations. on theassociations.Weconclude,therefore,thatover regions toseewhatextenttheirphysicalproperties have beenexpectedconsideringthesourceofluminosity associations mightbecorrelated.Ourtestsforcorre- density (forstarswithify<—4.2),aresultthatmight star densityandtheHaintensity.Itwasfoundthat significant andnontrivialcorrelationwasbetweenthe minute ofarctomorethan20starspersquare wider thanthesystematiceffect.Therangeinstar the Haintensityisgreaterforassociationsoflargerstar for thosewhichcontainHnregionshavingatotalHa given Haintensityamounttoonlyafewstarsper densities extendsfromafractionofstarpersquare 10% oftheassociationsLargeMagellanicCloud intensity of4,ontheMichiganscale,is9.0starsper intensities. Thereasonforthecorrelationhasclearly square minuteofarc. stars persquareminuteofarc,whereasthemeandensity associations withnodetectableHaintensityis4.7 square minuteofarc.Themeanstellardensity of theHiiregions.Theactualrangeisverymuch (1970), weconcludethatapproximatelythreeofthe of arc,whilethedifferencesinmeandensitiesfora would benecessarytodeterminewhetherthemore study ofeachindividualassociationandHnregion dense associationstendtohavemoreintenseHn and theOBstarsinassociations.Detailed to dowiththephysicalinteractionsbetweengas (taken fromDohertyetal.1956)andthoseofthestellar (Lucke andHodge1970).WehaveexaminedtheseHn The majority(96of122)thecataloguedLMC Figure 5showsmeandensitiesasafunctionofHa © American Astronomical Society • Provided by theNASA Astrophysics Data System V. HIIREGIONSINASSOCIATIONS ASSOCIATIONS OFTHELMC regions imbeddedinthembecauseofthehigherdensity most oftheseobjects. have begunaphotometricprogramdesignedtoprovide properties oftheassociationsinLMCitself,we Magellanic Clouds.Inordertoexplorefurtherthe stars, orbecauseofahigherdensitygas. of excitingstars,becauseasystematicdifferencein .1969a,Astrophys.J.155,417. systems oftheseobjectsinnearbygalaxiessuchasthe the maximumluminositiesandtemperaturesofthese ——. 1970b(inpreparation). .1969b,ibid.156,847. Bok, B.J.,P.F.,andBasinski1962,ibid.123,487. Bok, B.L,andP.F.1962,MonthlyNoticesRoy.Astron.Soc. Bok, B.J.1964,inTheGalaxyandtheMagellanicClouds(Aus- Blaauw, A.1964Ann.Rev.Astron.Astrophys.2,213. Bergh, S.vanden1964,Astrophys.J.Suppl.9,65. color-magnitude diagramsandluminosityfunctionsfor associations ofstarscanbedeterminedbythestudy .1957,ibid.62,69. .1970a,SmithsonianSpecialReports,No.319. Hodge, P.W.1961,Astrophys.J.Suppl.6,235. M. L.Driscollhelpedinsomeofthecalculations. 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