1992ApJ. . .392. .519B covered inNGC5466byMateo etal.(1990),evolvebytrans- complex. Starsinfullcontact (WUMastars),likethosedis- with largerheliumabundances thanotherstarsinthecluster. mergers beginlifeaszero-age main-sequence(ZAMS)stars distributed throughoutthemerged remnant.Thuscollisional produced inthecoresofcolliding starswillendupfairlyevenly lisional mergersdifferfrombinaryinthatthehelium dynamical evolutionofthecluster(McMillan,Hut,&Makino density ofthecluster(Verbunt&Hut1987),whilepresence of primordialbinarysystemswillhavegreatinfluenceon the rate ofstellarcollisionsisproportionaltothesquare the glers willbeusefulasaprobeofglobularclusterdynamics. The 47 Tue(Paresceetal.1991). cores ofNGC6397(Auriere,Ortolani,&Lauzarel1990) and have resultedinthediscoveryofbluestragglersdense clusters orregionsofwhicharerelativelysparseand 1990, 1991).Benz&Hills(1987)havepointedoutthat col- advances ingroundbasedphotometry,andtheadventofHST, thus unlikelytocontaincollisionalmergers.However,recent blue stragglersequencescanordinarilyonlybecarriedoutin blue stragglersinNGC5466.Unfortunately,detailedstudiesof was showntobeconsistentwithabinaryoriginforallofthe contact binarysystemsfromthreebluestragglers.Thisresult NGC 5466hasbeenfoundbyMateoetal.(1990),intheform of variablelightcurvesindicativemassexchangingand evidence forbinarymergersintherelativelysparsecluster the coresofdenserclusters(Leonard1989).Convincing large (>50%),collisionalmergersarelikelytobefoundonlyin those createdbysinglestar-binaryencounters,willbereferred to as“collisionalmergers.”Unlessthebinaryfractionisvery stellar collisions(Hills&Day1976).Suchstragglers,including stragglers maybeformedfromthedebrisleftbehindbydirect will bereferredtoas“binarymergers.”Alternatively,blue from boundbinarysystemsinwhichthetwocomponents gradually spiraltogether(Mateoetal.1990).Suchstragglers (see Leonard1989forareview)ofwhichonethemost popular istherecentmergeroftwostars.Thishypothesishas two variants.Ontheonehand,bluestragglersmaybeformed explanations fortheoriginofthesestarshasbeenproposed populations foralmost4decades(Sandage1953).Various The AstrophysicalJournal,392:519-521,1992June20 © 1992.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. The situationwithbinary mergers issomewhatmore Distinguishing betweencollisionalandbinarybluestrag- The problemofbluestragglershasplaguedstudiesstellar NGC 6397and47Tueshowcharacteristicswhichmaybemoreeasilyexplainediftheyareformedfrom Subject headings:globularclusters:individual(NGC6397,47Tue)—stars:evolution collisional mergers. made frombinarymergers.Thiseffectmaybeobservable,particularlyinultravioletcolors.Mostbluestrag- evidence forbinarityinseveralcases.However,thenewlydiscoveredbluestragglersequencescoresof glers havebeendiscoveredinclusterssufficientlysparsethatcollisionsareimprobable,andthereisdirect © American Astronomical Society • Provided by the NASA Astrophysics Data System Blue stragglersmadefromcollisionsmayhaveexcessHeabundanceoutsidethecorecomparedtothose ARE THERETWOKINDSOFBLUESTRAGGLERSINGLOBULARCLUSTERS? Astronomy Department,YaleUniversity,P.O.Box6666,NewHaven,CT06511 Received 1991November7;acceptedDecember27 Charles D.Bailyn ABSTRACT 519 have createdamassofhelium equalto bluer andmoreluminousthan itsmasswouldsuggest. associated withthelargeHe abundance wouldresultinastar dance relativetotherestofcluster.Thedecreasedopacity would beamassiveZAMSstarwithanenhancedHeabun- of Hburninginoneorboththecollidingstars),result mass ofthetwocomponents,andinwhichmaterial has been thoroughlymixed.Iftherehasasignificantamount (appropriate forglobularclusters)alikelyoutcomeissingle object whichhaslostsignificantlylessthan10%oftheinitial stars, andshowedthatforencounterswithlowinitialvelocities namics simulationsofcollidingequalmassmain-sequence teristics whichmayindicateacollisionalorigin. (Auriere etal.1990)andpossiblythenewlydiscoveredblue straggler sequencein47Tue(Paresceetal.1991)havecharac- that theanomalousbluestragglersincoreofNGC6397 tially observable,particularlyinultravioletbandpasses,and diagram. Thispapershowsthatthesedifferencesarepoten- positions inandevolutionthroughthecolor-magnitude create productswithdifferentchemistry,andhencedifference lent collisionproduct.Thusthesetwomergermechanismswill helium distributedthroughtheouterregionsthanequiva- processed materialfromtheprimarywillremainincenter of themergedremnant,andthusabinarymergerwillhaveless massive envelopedescribedbyBodenheimerandTaam(1984). remain intheinteriorofmergerproduct.Ineithercase, In thiscaseprocessedmaterialfromthesecondarywould Alternatively, thecentralregionsoftwostarscouldspiral together inamannersimilartothedoublecoreevolution case onewouldexpectprocessedmaterialfromthesecondary to bemixedthroughouttheenvelopeofmergerproduct. speeded upversionofwhatoccursinshallowcontact.Inthis refers toas“deepcontact”isachieved,atwhichpointthetwo ferring massfromthelessmassivetomore either thesecondaryiscompletelyabsorbed,orwhatWebbink component (Webbink1979).Thisprocesscontinuesuntil occurs isnotclear.Itmaybethatrapidcoalescencemerelya stars coalesceonafairlyshorttimescale.Exactlyhowthis A starwhichhasbeenonthe mainsequencefortimetwill Benz &Hills(1987)performedsmoothparticlehydrody- 1992ApJ. . .392. .519B respectively. Misindicatedonabscissa. Theinitialheliumabundanceof Y =0.23isindicatedbydottedline. 2 then turnsrapidlybacktotheblue.Mostbluestragglerswould redder inU—Bforstarsbetween0.9Mand1.4, particularly strikingintheU—Bcolorforlow-metallicity which cannotbereachedbybinarymergers.Thisresultis main-sequence stars,andatleast0.06Mmoremassivethan stars. DuetotheBalmerdiscontinuity,ZAMSbecomes partially enhancedbinarymergers.Thus,thereareregionsof the color-magnitudediagramaccessibletocollisionalmergers The enhancedHeabundanceofthecollisionalmergersmakes them appearmorethan0.1Mmassiveordinary enhanced ZAMSarethesameforproductswithM<1.3. secondary onlyhasbeenmixedin.SecondarieswithM<0.5 appropriate forcollisionswithstarsof0.8M.Thepluses location ofZAMSstarswithheliumabundancesaugmentedas Mq produceessentiallynoHe,sothenormalandpartially show positionsforstarsinwhichprocessedmaterialfromthe to 1.6M;crossesshowthelocationsofstarsmasses1.0, and thedottedlineindicatescontinuationofZAMSup ucts. ThesolidlineindicatestheZAMSforstarsupto0.8M, and U—Bcolor-magnitudediagramsforsuchcollisionprod- initial heliumabundanceofY=0.23producesthe (Green, Demarque,&King1987)todetermineappropriate abundances formergedstarsshowninFigure1. values of<1^)andisthemeanluminosityofstarduringitsmain- sequence lifetimesofar,and€=0.007istheefficiencyof 520 0 0 0 q 0 2 01 Fig. 1.—Heliumabundance(F)ofa collisionproduct.Toplineindicates Figures 2and3showthepositionsonVversusB—V = 0.8Mq,lowerlines0.75 M, =0.7Mj0.6 0 © American Astronomical Society • Provided by the NASA Astrophysics Data System Y = T Yo +io M +Lq 12 0.10 M+ q12 M 2 BAILYN recently discoveredinthecore of47Tuemayalsohavea in binarybluestragglers.The morethan~20bluestragglers short-term variabilityofthekind foundbyMateoetal.(1990) these starsshouldbeanomalously brightintheUV,andlack from thesestars,butHSTobservations shouldyielddetailed UV information.Thecollisional hypothesiswouldsuggestthat mergers shouldbereconsideredasanoriginforthesestars. Unfortunately, Auriereetal.(1990)presentnot/-banddata (Chernoff &Weinberg1990)thisimpliesthatcollisional duced bymasssegregationinthecollapsedclustercores this constraint.Togetherwiththeinvertedmassfunctionpro- close totheturnoffmassasappearedberequired.Thepossi- bility ofheliumenrichmentallowsanextra0.1Mslack in core resultinginbluestragglerswouldbebetweenstars as dismissed thecollisionalmergerhypothesisforthesestars on the groundsthatitwouldbeunlikelyallcollisionsin the the bluestandbrightestincluster.Auriereetal.(1990) Auriere etal.(1990)within5"ofthecenterNGC6397 are straggler massfunctionthanothercolors. M/Mq <1.6mayallowmoredetaileddiagnosticsoftheblue mergers, butmightbetheresultofcollisions.Ingeneral, color; brightbluestarscouldnotbeproducedbybinary rapid colorchangesinU—Bforstarswithmasses0.8< therefore beredderthanthemain-sequenceturnoffinthis 0 M <0.5Mq,thatisfor1.3,essentiallynoheliumproducedby masses, withheliumenhancementfromthesecondaryonly.Notethatfor the secondary. sions involvinga0.8Mstar.Plusesindicatepositionsforstarsofsimilar to thecrosses,butwithheliumabundancesenhancedasappropriateforcolli- to M=1.6Mq.Crossesindicatesmassesof1.0,1.1,1.2,1.3,1.4,1.5,1.55,and 6397. SolidlineterminatesatM=0.8forY0.23,dottedcontinues 1.6 timessolar.Circlesindicatezero-agepositionsforstarswithsimilarmasses 20 0 0 It ishighlysuggestivethatthebluestragglersfoundby Fig. 2.—Zero-agemain-sequencesforZ=0.0002,appropriateNGC Vol. 392 1992ApJ. . .392. .519B Hills, J.G.,&Day,C.A.1976,Astrophys. Lett.,17,87 Green, E.M.,Demarque,P.,&King, C.R.1987,TheRevisedYaleIsochrones Mateo, M.,Harris,H.C,Nemec,J.,& Olszewski,E.W.1990,AJ,100,469 Leonard, P.J.T.1989,AJ,98,217 Djorgovski, S,&King,1.1986,ApJ, 267, L85 Chernoff, D.F.,&Weinberg,M.1990, ApJ,351,121 Bodenheimer, P.,&Taam,R.E.1984,ApJ,280,771 Auriere, M.,Ortolani,S.,&Lauzarel,G.1990,Nature,344,638 Benz, W.,&Hills,J.G.1987,ApJ,323,614 there aremorethan~20bluestragglersinthecoreof47 Tue the coreofNGC6397.NewHSTdatahavedemonstrated that that thecollisionrateincoreof47Tueis^25times in parameters foundinWebbink’s(1985)compendium,onefinds encounter ratesindifferenceclusters.Usingthephysical tionally, andusethisinformationtoconstrainthedistribution of binarystars.Itishowever,interesttocomparetherelative try toestablishthenumberofcollisionalmergersobserva- mate ofthenumbercollisionalmergerstobeexpectedina given clusterishighlysuspect.Itmaybemoreproductiveto parameters inglobularclustercoresareunknown,anyesti- with binarystars,ratherthanfromencountersbetweensingle stars. Sincethebinaryfractionanddistributionoforbital globular clustersarelikelytoarisefromresonanceencounters presumably belesspronouncedthaninNGC6397. colors occursathighermass(seeFig.3),soanyUVexcesswill metallicity clusterlike47Tue,theturnbacktowardbluerUV collisional origin.Ifso,thecomparisonwithanempiricalmain Paresce etal.(1991)maybemisleading.Forarelativelyhigh sequence ofstarsnormalHeabundances,asperformedby No. 2,1992 and LuminosityFunctions(NewHaven: YaleObs.Pub.) Leonard (1989)hasshownthatmoststellarcollisionsin Fig. 3.—Sameas2exceptforZ=0.0036,appropriate47Tue © American Astronomical Society • Provided by the NASA Astrophysics Data System B-V BLUE STRAGGLERSINGLOBULARCLUSTERS REFERENCES .1985,inIAUSymp.113,Dynamics ofStarClusters,ed.J.Goodman& McMillan, S.,Hut,P.,&Makino,J.1990,ApJ,362,522 .1991,ApJ,372,111 Webbink, R.F.1979,ApJ,227,178 Verbunt, F.,Hut,P.1987,inIAUSymp. 125,OriginandEvolutionofNeutron Paresce, F.,etal.1991,Nature,352,297 Sandage, A.1953,AJ,58,61 Sarajedini, aswellsupportfromNASAgrantNAGW-2469. Pierre Demarque,ClaytonHeller,MarcPinsonneault,and Ata details ofclusterstellarpopulations. the dynamicsofclustersaswelltothoseinterestedin the result whichwouldbeofconsiderableinteresttostudents of provide awayofdistinguishingthesestragglersfromothers, a enhanced heliumabundancesincollisionalmergerswill ible thatthesignificantstructuralchangescausedby not outsidetherangeofcurrenttechniques.Soitseemsplaus- these problemssomewhat.Incorporatingtheoreticaland colors ofthebluestragglers.ObservationsinU,whichwould observational details,whilebeyondthescopeofthispaper,is be themostsensitivetoeffectsdescribedhere,wouldreduce fitting toconstraintheseparametersbeperformedwiththe same datasetasisusedtodeterminethemagnitudesand large effects,soitwouldseemadvisablethatmain-sequence chrones. Smallchangesindistanceandreddeningmayhave tional difficultiesincomparingstragglerswiththeoreticaliso- important inbothbinaryandcollisionalmergerproducts. Benz &Hills(1987).Also,rotationaleffectsarelikelytobe show effectsnotfoundintheequalmasscollisionsstudiedby given here.Collisionsbetweenstarsofdifferentmassesmay color-magnitude diagramwilldiffersomewhatfromwhatis further increasesinHeabundancesifthemasslostisprimarily (Benz &Hills1987)andmaybepartiallycompensatedforby and theywillevolvequickly,sotheircurrentpositionsinthe unenriched. Collisionalmergerswillnotbeperfectlymixed, loss duringcollisionsmaymovethestarbacktowardsfainter many detailswhichhavebeenignoredhere.Forexample,mass isochrones forbluestragglerswouldrequireconsiderationof and reddercolors.However,suchmasslossmaybequitesmall binary populationsofthetwoclusters. significant differencesbetweenthestellarandprimordial licity differencebetweenthetwoclustersmaybereflectedin expanding core,inwhichcasethecollisionratemayhavebeen significantly higherintherecentpast.Andofcoursemetal- Auriere etal.(1990)suggestthatNGC6397mayhavean lower inthecore,sofewercollisionsoccur,andthatcolli- stars, andthusnotresultinrecognizablebluestragglers. which maymeanboththattherelativebinaryfrequencyis sions whichdooccuraremorelikelytobebetweenlow-mass far in47TueasthemoredynamicallyevolvedNGC6397, model (Djorgovski&King1986)soWebbink’snumbersmay not berelevant.Masssegregationmayhaveproceededas tions ofthisdiscrepancy.NGC6397isnotwellfittedbyaKing respect toNGC6397.Thereareanumberofpossibleexplana- (Paresce etal.1991),butthatisstill5timesfewerthanwould be expectedonthebasisofrelativecollisionalratewith P. Hut(Dordrecht:Reidel),541 Stars, ed.D.Helfand&J.-H.Huang(Dordrecht :Reidel),187 I ampleasedtoacknowledgeusefulconversationswith In additiontothesetheoreticalproblems,thereareobserva- A carefulcomparisonbetweenobservationsandtheoretical 521