Astronomy&Astrophysicsmanuscriptno.avalcarceM3_2016v2 (cid:13)cESO2016 May3,2016 Level of helium enhancement among M3’s horizontal branch stars A.A.R.Valcarce1,2,3,M.Catelan1,2,3,J.Alonso-García4,3,R.ContrerasRamos1,3,andS.Alves1,3 1 PontificiaUniversidadCatólicadeChile,InstitutodeAstrofísica,FacultaddeFísica,Av.VicuñaMackenna4860,782-0436Macul, Santiago,Chile 2 PontificiaUniversidadCatólicadeChile,CentrodeAstroingeniería,Av.VicuñaMackenna4860,782-0436Macul,Santiago,Chile 3 MillenniumInstituteofAstrophysics,Santiago,Chile 6 4 UniversidaddeAntofagasta,UnidaddeAstronomía,FacultadCs.Básicas,Av.U.deAntofagasta02800,Antofagasta,Chile 1 0 ReceivedApril22,2015;acceptedJanuary25,2016 2 y ABSTRACT a M Context.Thecolorandluminositydistributionofhorizontalbranch(HB)starsinglobularclusters(GCs)aresensitiveprobesofthe originalheliumabundances ofthoseclusters.Inthissense,recentlythedistributionsofHBstarsinGCcolor-magnitude diagrams 2 (CMDs)havebeenextensivelyusedasindicatorsofpossiblevariationsintheheliumcontentY amongthedifferentgenerationsof starswithinindividual GCs. However, recent analyses based onvisual and near-ultraviolet(UV) CMDshaveprovided conflicting ] results. R Aims.Toclarifythesituation,weaddresstheoptimumrangesofapplicability(intermsoftheT rangecoveredbytheHBstars)for eff S visualandnear-UVCMDs,asfarasapplicationofthis“HBYtest”goes. . Methods.WeconsideredbothStrömgrenandHubbleSpaceTelescope(HST)bandpasses.Inparticular,wefocusontheF336Wfilter h of the HST, but also discuss several bluer UV bandpasses, such as F160BW, F255W, and F300W. Using the Princeton-Goddard- p PUC(PGPUC)code,wecomputedalargesetofzero-ageHB(ZAHB)lociandHBevolutionarymodelsformassesrangingfrom - o MHB = 0.582to0.800M⊙,assuminganinitialheliumabundanceY = 0.246,0.256,and0.266,withaglobalmetallicityZ = 0.001. r TheresultsofthesecalculationswerecomparedagainsttheobservationsofM3(NGC5272),withspecialattentionontheyvs.(b−y) t andF336Wvs.(F336W−F555W)CMDs. s a Results.Ourresultsindicatethat,fromanevolutionaryperspective,thedistributionsofHBstarsintheyvs.(b−y)planecanbea [ reliableindicatoroftheHecontentincoolblueHB(BHB)stars,particularlywhenadifferentialcomparisonbetweenblueandred HBstarsiscarriedoutintherangeTeff .8300K.Conversely,wedemonstratethatCMDsusingtheF336Wfilterhaveamuchless 2 straightforwardinterpretationatthecoolendoftheBHBbecausethedistributionsofHBstarsintheF336Wvs.(F336W−F555W) v plane,forinstance,areaffectedbyatripledegeneracyeffect.Inotherwords,thepositionofanHBstarinsuchaCMDisexactlythe 7 sameforagivenchemicalcompositionformultiplecombinationsoftheparametersY,M ,andagealongtheHBevolutionarytrack. HB 4 OtherHSTUVfiltersdonotappeartobeasseverelyaffectedbythisdegeneracyeffect,towhichvisualbandpassesarealsoimmune. 7 Ontheotherhand,suchnear-UVCMDscanbeextremelyusefulforthehotteststarsalongthecoolBHBend. 6 Conclusions.BasedonareanalysisofthedistributionofHBstarsintheyvs.(b−y)plane,wefindthatthecoolestBHBstarsinM3 0 (i.e.,thosewithT <8300K)areverylikelyenhancedinheliumby∆Y ≈0.01,comparedwiththeredHBstarsinthesamecluster. eff 1. Using near-UV HSTphotometry, on the other hand, we findevidence of a progressive increase inY withincreasing temperature, 0 reaching∆Y ≈0.02atTeff ≈10900K. 6 Keywords. globularclusters:general-–globularclusters:individual(M3=NGC5272)-–Hertzsprung–RussellandC–Mdiagrams- 1 –stars:abundances-–stars:evolution-–stars:horizontal-branch : v i X 1. Introduction NGC2808showsthreediscreteMScomponents,stronglysug- r gestinglargeHevariationsfromonesuchcomponenttothenext aGlobularstarclusters(GCs)arefarfrombeingthesimplestellar (Piottoetal.2007),eventhoughtheclusterdoesnothavealarge populationsthattheywereoncethoughtto be.Thepresenceof spread in metallicity (Grattonetal. 2011). In like vein, a bluer multiplepopulations,correspondingtodifferentepisodesofstar MScomponentinωCenalsoappearstorequireamuchhigher formation within individual GCs, is now routinely revealed by Heabundancethanwouldhavebeenimpliedbytheirdifference photometry and spectroscopy alike. One important open prob- inmetallicity(Piottoetal.2005).Eventhoughitisbelievedthat lemisthedegreetowhichthedifferentpopulationsmaydifferin mostofthestellarpopulationsofGCsfollowahelium-to-metal theirheliumcontent,andhowanyinternalspreadinthehelium enrichment law (Y ≈ Y + ∆Y/∆Z × Z, with ∆Y/∆Z ≈ 2.0; p abundanceY maydifferfromoneclustertothenext. Renzini1994;Salarisetal.2004;Valcarceetal. 2013),thedis- The discovery of multiple main sequences (MSs) in crete main sequences and the different chemical compositions NGC 2808 (Norris 2004; D’Antonaetal. 2005; Piottoetal. observedamongstarsinthesameGCsuggestedtheexistenceof 2007) and ω Centauri (NGC 5139; Bedinetal. 2004; helium-richstarsineveryMilkyWayGC(Grattonetal.2012). Bellinietal. 2009, 2010) has reopened the question about the Nevertheless,thefractionoftheseHe-richstarsinGCsandthe initial helium abundanceof the GC stars (vandenBergh 1965, 1967; Sandage&Wildey 1967; Hartwick 1968). Remarkably, Articlenumber,page1of13 A&Aproofs:manuscriptno.avalcarceM3_2016v2 amountofheliumenrichmentineachclusterarestillamatterof andnear-UVregimes,forsettingconstraintsontheinitialhelium debate. abundancesofthedifferentstellarpopulationsinGCs. Because the surface helium abundance can only be mea- sured spectroscopically in stars with an effective temperature T & 8000 K, only GCs presenting relatively blue horizontal eff branch (HB) extensions can harbor stars with sufficiently high 2. Estimating∆Y fromHBpopulations temperatures to enable direct spectroscopic estimations of the helium abundances. However, these stars have passed through Since the discovery of multiple populations in GCs, two main the whole MS, red giant branch (RGB), and pre-HB phases, approacheshavebeenusedto estimatedifferencesinthe initial whichhasmodifiedtheirsurfacechemicalcompositionthrough helium abundances in the HB population. The first approach the effects of internal mixing (e.g., Sweigart&Mengel 1979; involves the computation of synthetic HBs, which can then Vandenberg&Smith 1988), and they might also have been af- be compared to the distribution of stars observed along the fectedbystellarrotation(e.g.,Mengel&Gross1976),massloss HB (e.g., Rood 1973; Catelanetal. 1998; D’Antonaetal. (e.g.,Peterson1982;DeLaRezaetal.1996),and/orinteraction 2002; Caloi&D’Antona 2008; D’Antona&Caloi 2008; with other bodies (planets or stars, Soker 1998; Siess&Livio Dalessandroetal. 2011, 2013; Salarisetal. 2013; Leietal. 1999),amongothereffects(seeCatelan2009). 2013a,b).Inthesecondapproach,referenceevolutionarylociare EvenforthesehotHBstars,spectroscopicmeasurementsof directly overplotted on the empirical CMD data (Catelanetal. helium unfortunately remain complex because these lines are 2009, hereafter C09; Brownetal. 2010; Valcarceetal. 2014, weak and a high signal-to-noise ratio (S/N) is required to de- hereafterV14). rive accurate results. Moreover, it is well established that GC Bothapproachesarebasedonextensivecalculationsofevo- HB stars with T & 11500 K show higher metal and lower eff lutionarymodelsforGCstars,calculatedaccordingtothechem- helium abundances than all the other stars of the same GC. ical composition of the GC under study. More specifically, the This is caused by the effects of metal levitation and He sed- complete evolution of a star from the main sequence to the imentation (Grundahletal. 1999; Behr 2003), which limit the ZAHBiscalculated,mostoftenwithoutmassloss,with there- range of temperatures for measuring helium to a window of sultingmaximumZAHBmassdependingontheGCproperties ∆T ≈ 3500 K. Only recently, Villanovaetal. (2012) and eff (e.g., for 13 Gyr, Z = 0.001, and Y = 0.245, the maximum Marinoetal.(2013)haveobtainedhigh-precisionspectralmea- ZAHBmassis 0.810M ).ThisreferenceZAHBmodelis then surementsofheliumabundancesinthestarsofM4(NGC6121) ⊙ usedasastartingpointtocreatei)theZAHBlocusforallstars and NGC 2808, respectively. We still lack a complete under- with the same M and initial chemical composition (i.e., all standing, however, of how these photospheric abundances are cHe starsbornatthesametime),andii)theinitialmodelforHBstars relatedtotheinitialabundancesafterthewholepreviousevolu- atagiventotalmass(M )withthesameM anddifferenten- tion, given in particular their dependenceon details of the first HB cHe velope mass (i.e., due to mass loss during the RGB evolution; dredge-upepisodeatthebaseoftheRGB.Thisphenomenonis Serenelli&Weiss2005).Thenwecomputeforeach M value onlypoorlydescribedbycanonicalevolutionarymodelswithout HB theevolutionfromtheZAHBtotheheliumcoreexhaustion(also extramixing(e.g.,Grattonetal.2000).Otherspectroscopiche- called terminal-ageHB, or TAHB). The modelsare then trans- liumabundancestudiesincludethoseofDupreeetal.(2011)and formedtotheempiricalCMDplanes,usingsuitablebolometric Pasquinietal. (2011) for RGB stars in ω Cen and NGC 2808, correctionsandcolortransformations. respectively. These authors used chromospheric helium lines (whichhavemoretraditionallybeenusedasdiagnosticsofmass InthecaseofsyntheticHBpopulations,thecolorandabso- lossinRGBstars)thatrequireacarefultreatmentofthestellar lutemagnitudedistributionsofstarsalongtheHBarethensim- atmosphere(Dupree&Avrett2013). Thesedifficultieshavefa- ulated, using Monte Carlo methods. This requires an assump- voredindirectmethodstobeproposedasprobesoftheHeabun- tiononthemass-lossdistributionontheRGB,asthisdefinesthe danceinGCstars. resulting distribution in masses on the HB phase proper. Most Forthispurpose,theeffectsofdifferentinitialheliumabun- commonly, mass-loss efficiency is treated as a free parameter thatis constraineda posteriorito match the observations.Stars dancesoncolor-magnitudediagrams(CMDs)havebeenknown arethusfedintothe evolutionarytracksofdifferentmassesac- since the late 1960s (Simoda&Iben 1968; Aizenmanetal. 1969; Iben 1974; Simoda&Iben 1970; see also Valcarceetal. cordingtotheexpectedevolutionaryspeedalongtheHBphase. Photometric errors are then added to the photometric data for 2012, for a recent summary).These studies revealed that, even though these effects are present in all the evolutionary se- the “synthetic stars” to more realistically compare them with quencesalongtheCMD,theeffectofHeisamplifiedintheHB observationaldatasets. Dozensofsimulationsaretypicallyrun, for which some or all of the free parameters are varied to find phaseandis alsoeasierto detectbecauseofthehighluminosi- the distribution that most closely describes the empirical data. tiesofHBstars(Faulkner&Iben1966;Iben&Faulkner1968; Iben&Rood1970;Sweigart1987).Specifically,anincreasein Someofthesecomparisonsaremadeusingcolorandmagnitude Y increases the luminosity of all HB stars with T . 16000 histograms,butin othercases, onlya visualinspectionis used, eff K, but it also decreases the luminosity of these stars for T & whichcansometimesleadtomisinterpretations. eff 16000 K. These effects are due to different efficiencies of the Whilethereferenceevolutionarylociusedinthesecondap- H-burningshell and to the differencesin the helium-coremass proachcanalsobeobtainedfromsyntheticHBmodels,theyare (M ;Valcarceetal.2012)withchangingY. moredirectlyretrievedfromtheHBevolutionarytracksproper. cHe However, depending on the set of filters used to transform We have already encountered two such reference loci, namely fromthetheoreticaltotheobservationalplane,theseeffectscan the ZAHB and TAHB. Additional loci may easily be defined. become more or less evident in the HB part of the CMD, and They represent the fraction of the entire HB lifetime that a thusmoreorlesseasilydetectable.Inthispaper,weexplorethe given HB has already gone through.For instance, C09 defined effects on the HB of using two different sets of filters, Ström- the middle-age HB (MAHB) locus that closely corresponds to grenandHubbleSpaceTelescope(WFPC2),coveringthevisual starsthatarehalf-waythroughtheirHBevolution,andalsothe Articlenumber,page2of13 A.A.R.Valcarceetal.:HeenhancementamongM3’sHBstars 90AHB,whichcorrespondstostarsthathavecompleted90%of consensusas yeton whetherthereis a He spread,andif so, by theirlivesasHBstars.1 howmuch.One of ourmain goalsis to explainthe reasonsfor IfagivenGCcontainsasinglepopulationofstars,onewould seemingly conflicting results based on similar samples of HB then expect to find ≈ 50% of all HB stars between the ZAHB starsinthisonecluster. and MAHB, ≈ 40% between the MAHB and 90AHB, ≈ 10% In the following, we assume that there is no difference between the 90AHB and TAHB, etc. (see also C09). However, in either Z or C+N+O among stars in M3 (σ = 0.03; [Fe/H] if there are several populations of HB stars with similar tem- Snedenetal. 2004) so that we can compare the expected and peraturesbutdifferentinitialheliumabundances,thesepercent- observedluminositydifferencesusingdifferentsetsoffilters. ages should change as a result of the increase in luminosity of Figure 1 showsthe HB populationof M3 on a CMD based HB stars with the increase of the initial helium abundance. In ontheyvs.y−bfiltersoftheStrömgrensystem.Similarly,Fig.2 particular, if there is a spread in the initial helium abundance showsthe F336W vs. F336W −F555W CMD, usingthe HST ∆Y = 0.01in a GC andmodelsfora single Y are comparedto Wide-FieldandPlanetaryCamera2(WFPC2)visible(F555W) photometric data, the percentages of stars located between the and near-UV (F336W) filters, respectively. M3 observational MAHB and 90AHB locishould be largerthan expectedon the data in the Strömgren filters are taken from Grundahletal. basisofsingle-Ymodels.Thustheevolutionarylocimethod,like (1998,1999)asdescribedinC09withatotalof138non-variable the synthetic HB populationsmethod,can be a powerfulprobe HB stars, while data in the HST filters were kindly provided ofthepresenceofpopulationswithdifferentHeabundancesthat byE.Dalessandro(2014,priv.comm.)withatotalof155non- co-existinaGC. variable HB stars. In these figures are also plotted theoretical While HB stars with different Y values can coex- ZAHB loci for a chemical composition according to the M3 ist over a given temperature range, theoretical arguments metallicity (Z = 0.001 and [α/Fe] = 0.30;see C09) and three (Sweigart&Gross1976)aswellasrecentspectroscopicobser- initial helium abundances Y = 0.246, 0.256, and 0.266 as red, vations(Marinoetal. 2011; Villanovaetal. 2012; Marinoetal. blue, and green lines, respectively. HB evolutionary tracks for 2013) alike suggest a tendency for blue HB (BHB) stars to be Y = 0.246 are also plotted as red dot-dashed lines for some preferentiallyHe-enhanced,comparedwithredHB(RHB)stars masses (M = 0.582, 0.596, 0.609, 0.632, 0.652, 0.665, and HB in the same cluster.2 As a consequence,if a spread in He is in- 0.800M ).Thesearemarkedwithcirclesatthreestagesoftheir ⊙ deed present within a GC with both blue and red HB compo- evolution–ZAHB,MAHB,and90AHB–withdifferentcolors nents,onewouldexpecttheBHBtobenoticeablybrighterthan beingusedfordifferentM values.Thesefiguresareexplained HB theRHBinthesamecluster.Thesizeoftheeffect,overtheoth- inmoredetailbelow. erwise ratherflat partofthe HB, is about0.03mag in Johnson V orStrömgrenyforadifferenceinY of0.01.Wecallthiscom- 3.1.Previousstudies parisonbetweenthebrightnessofRHBandBHBstarsusingthe ZAHBandotherreferencelocitheHBY test. ThemorphologyoftheHBpopulationofM3([Fe/H]= −1.50) It is worth mentioning that stars with high helium abun- wasstudiedinD13togetherwiththatoftwootherGCs,namely dances could in principle also have suffered an increase in Z M13=NGC6205andM79=NGC1904,whichhaveverysim- or C+N+O, as suggestedrecently by Jangetal. (2014) in their ilar [Fe/H] ratios (−1.53 and −1.60, respectively; Harris 1996, proposed explanation of the Oosterhoff dichotomy observedin 2010edition).D13comparedtheresultsofsyntheticHBcalcu- Milky Way GCs (Oosterhoff 1939; see also Catelan 2009, for lations to HST near-UV observations that were obtained with a recent review and references). This can also favor helium- WFPC2. These simulations require four free parameters (the normal and helium-rich stars sharing similar colors on the HB minimum Y value, the range of helium abundances, the mean ofa givenGC,andthereforeservesasawarningagainstsolely value of the mass lost along the RGB, and the spread around usingcolorinformationasanindicatorofHespreadsamongGC thismeanvalue)foran assumedageof12Gyr.Theamountof stars.3 In particular, the percentages between ZAHB, MAHB, mass lost on the RGB is also expected to be a function of Y 90AHB,andTAHBlociareexpectedtochange,whencompared (e.g., Catelan&deFreitasPacheco 1993). Clearly, the number totheexpectationsforasinglestellarpopulationbecauseofthe of possible parameter combinationsand the resulting synthetic differenceinluminositythatisbroughtaboutbythepresenceof distributionsis verylarge,even thoughin D13 it is onlypossi- aheliumspread. bletoexaminethebest-fittingsimulationforeachGC.ForM3, thesesyntheticHBcalculationssuggest,accordingtoD13,that HBstarswith0.2.(F336W−F555W).0.5haveadispersion 3. TheM3caserevisited inY of0.02,butthedispersioninY advocatedbyD13forM13 HereweconsiderM3(NGC5272)becausethisclusterhasbeen isonly0.01overthesamecolorrange.Althoughsmall,thepres- the subject of numerous recent studies of the HB population enceofsuchadifferenceisnoteworthy,giventhesimilarbright- (C09; Dalessandroetal. 2013, hereafter D13), without a clear nessextensionsinF336WoftheHBpopulationsinbothclusters. Thisdifferencemightplausiblybetracedbacktobothclusters’ 1 C09point out thattheMAHB (originallydefined astheHB ridge- strikinglydifferentHBmorphologies:M3andM13constitutea line in the CMD) and 50AHB (defined in complete analogy with the “classical”second-parametercase,andoneofthefirstforwhich 90AHB)locimaydifferslightly,butherewedefinetheMAHBasbe- chemical anomalies, as opposed to age, were suggested as the ingsynonymouswith50AHB. explanation(Catelan&deFreitasPacheco1995). 2 BHB and RHB stars are terms that are often used to refer to stars The population of HB stars was studied by C09 using the bluer and redder, respectively, than the RR Lyrae instability strip, re- visualfilters of the Strömgrensystem, andthe resultingCMDs spectively. are compared to different reference evolutionary loci (ZAHB, 3 Thereareother possibilitiesforHe-richandHe-poor starstoshare MAHB,90AHB,andTAHB;seeSect.2).Ascanbeseenfrom similarcolorsontheHB.Forexample,starswiththeexactsameZand C+N+Obutdifferentmass-losshistoriesmayverywellhaveexactlythe Fig.2inC09,theevolutionofHBstarsinthesefilters(My, Mb, sameZAHBtemperature,eventhoughtheirluminositiesmaybevastly and Mv) implies that, in the canonical scenario, brighter stars different. (at a given temperature) are also more evolved stars, at least Articlenumber,page3of13 A&Aproofs:manuscriptno.avalcarceM3_2016v2 15.55 15.6 15.65 PGPUC (Y=0.246) 14.5 PGPUC (Y=0.256) PGPUC (Y=0.266) 15 15.5 16 16.5 0 0.2 0.4 (b-y) Fig.1.M3HBstarscomparedtotheoreticalHBmodelswithZ=0.001intheStrömgrenyvs.(b−y)plane.Theupperpanelisazoom-inaround theZAHBlocus.PGPUCZAHBlociareshownforheliumabundancesY =0.246(redline),0.256(blueline,shownintheupperpanelonly,for clarity),and0.266(greenline).C09andBaSTIZAHBlociforY =0.246arealsoshownasmagentaandblacklines,respectively,withdifferent distancemoduliinordertomatchthePGPUCZAHBlocuswithY =0.246.TheoreticalHBevolutionarytracksforY =0.246areshownasred dot-dashedlinesfor M =0.582,0.596,0.609,0.632,0.652,0.665,and0.800M ,whichhavebeenmarkedwithcirclesattheZAHB,MAHB, HB ⊙ and90AHB(colorsdependontheHBmass,forclarity).AnadditionalHBtrackforY = 0.256and M = 0.582M isalsoshowninthetop HB ⊙ panel(bluedot-dashedlineswithpinkcircles).Verticaldot-dashedlinesindicatesomeT valuesattheZAHBlocusforY =0.246.Thedistance eff moduluswasselectedasdescribedinSect.3.3andFig.6.ThedatacomefromC09. for T . 9500 K. However, since an increase in Y also in- vationaldatasetsusedbytheseauthors.Figure3isazoom-inof eff ducesan increasein the HB luminosityat similar temperatures Fig.2inthecolorrange0.2.(F336W−F555W).0.8.Inthese (see,e.g.,Sweigart1987),anincreaseinY canbemimickedby figures we show HB evolutionarytracks (red dot-dashedlines) differentagesalongtheHBevolutionarytracks(t ),andvice- with masses ranging from M = 0.800 down to 0.582M , HB HB ⊙ versa. However, as mentioned before, theoretical evolutionary with a progenitor mass of 0.800M , Y = 0.246, Z = 0.001, ⊙ modelspredictthat50%ofallHBstarsmustbefoundbetween and [α/Fe] = +0.3. To show the different evolutionary stages, theZAHBandMAHBloci,or,similarly,betweentheZAHBlo- each HB track has been marked with coloredcircles at the po- cuswithprimordialY valueandtheZAHBlocuswithaY value sitions in their evolution correspondingto the ZAHB, MAHB, increasedby0.01,asthelatter’sZAHBcloselyoverlapsthefor- and 90AHB, the colors of the circles depending on the stellar mer’sMAHB.Basedonthis, themainresultinC09isthatany mass. The distance modulus (m − M) = 14.909 is selected to heliumspreadinM3mustaccordinglybe∆Y .0.01. reproduce the expected proportions of stars according to evo- In summary,the proposedlevel of He enhancementamong lutionarystage along the HB phase (see Sect. 3.3).4 These fig- M3 HB stars proposed by D13 is in conflict with the smaller ures also show the BaSTI (Pietrinfernietal. 2004) and C09 onesuggestedin C09 forstars overthe same rangeineffective ZAHBloci (in blackand magentacolors, respectively),shifted temperature(i.e.,basicallyforthesamestars).Belowweaddress according to the indicated distance moduli in order to match somepossiblereasonsfortheseconflictingresults. the PGPUC ZAHB locus for Y = 0.246 (red line). Note that the different required shifts, hence distance moduli, are due to differences in the adopted chemical compositions (in C09, the 3.2.Comparingmodelsinthevisualandnear-UV Y = 0.230 ZAHB locus is used), bolometric corrections (C09 andPGPUCmodelsuseClemetal.2004,whereasBaSTImod- To look further into the reasons for the noted discrepancy between the results of C09 and D13, we show in Figs. 1 and 2 4 Eventhoughthedistancemodulusislowerthanreportedinthelitera- HB evolutionary tracks and reference loci in CMDs based on ture(e.g., VandenBergetal.2013),thisdoesnotaffectthemainresults thesamefiltersasusedinC09andD13togetherwiththeobser- ofourpaper. Articlenumber,page4of13 A.A.R.Valcarceetal.:HeenhancementamongM3’sHBstars 15.9 16 15.6 15.8 16 16.2 16.4 -0.5 0 0.5 1 Fig.2.SameasFig.1,butfortheHSTF336Wvs.(F336W−F555W)plane.Theoreticalmodelsfor M = 0.665and0.632 M arenotshown HB ⊙ hereforclarity(seeFig.3).TheobservationaldataarethesameasinD13,askindlyprovidedbyE.Dalessandro(2014,priv.comm.). elsuseCastelli&Kurucz2004),and/orinputphysics(morede- tails can be found in Sect. 2.7 of Valcarceetal. 2012). Blue andgreenlinesrepresentthePGPUCZAHBlociforY = 0.256 and Y = 0.266, respectively. All these ZAHB loci are based onthesamechemicalcompositionmentionedbefore,exceptfor the C09 model, whose higher metallicity (Z = 0.002)explains its further extension toward red colors, compared to the other ZAHB loci shown. Finally, vertical dash-dotted lines schemat- ically indicate some specific T values (based on the ZAHB eff loci), as were used to compare different sets of filters. For the sake of clarity, evolutionarytracks for 0.665 and 0.632M are ⊙ notshowninFig.2,andneitheristheC09ZAHBlocus. A quick inspection of these figures reveals that the Y-t HB degeneracy existing in the HB with the filters used in Fig. 1 (y vs. b − y plane) can induce a misinterpretation not larger than ∆Y = 0.01 (compare the blue line in the upper panel with the overplottedevolutionarytracks) because roughly50% of the HB stellar population with Y = 0.246 must be brighter thanthe ZAHB locuswith Y = 0.256in the temperaturerange 5700K.T .8300K,regardlessoftheexistenceofahelium eff spread. However, the filters used in Fig. 2 (F336W vs. F336W − F555Wplane)presentaY −t −M tripledegeneracyatthe HB HB HBlevel.Specifically,fromredtobluecolors,theZAHBlocus Fig. 3. Zoom-in of Fig. 2 around the loop shown by the ZAHB loci. startswith0.800M⊙atacoloraround0.6,andwhenthemassis HereareshownallthecomputedHBevolutionarytracks.Notethedif- decreasedto∼0.665M⊙,thecoloris0.4–,butforlowermasses, ficultydisentanglingavariationinY frommassandevolutionarystate, theZAHBlocusbeginstodevelopaloop.Inotherwords,afur- atcolorsaround(F336W−F555W)≈0.4. therdecreaseinmass,insteadofproducingabluerstructurewith roughlythe same brightnessas in Fig. 1, gives rise to a redder Articlenumber,page5of13 A&Aproofs:manuscriptno.avalcarceM3_2016v2 Fig.4.SyntheticCMDsattheHBlevelfrommodelswithZ=0.001inthefiltersofinterestoftheStrömgren(toppanels)andWFPC2(middleand bottompanels)systems.TheZAHBlocusforY = 0.246isrepresentedasablackline.Auniformmassdeviaterangingfrom0.595to0.705M ⊙ with2100starsisusedtorepresentahelium-normalpopulation(Y = 0.246,lightgraycircles).Helium-richpopulations(Y = 0.256,0.266)are createdwith1000starswithmassesrangingfrom0.595to0.645M (darkgraycircles).StarsidentifiedasRHBstarswithY =0.246areplotted ⊙ asredcircles.BHBstarsareplottedincyanforY =0.246,blueforY =0.256,andgreenforY =0.266.Inthesesimulations,photometricerrors are included, following thephotometric data, at the level of 0.01 mag ineach bandpass. Panelsg, h, and i arezoom-ins of panels d, e, and f, respectively.Eachpanelisaccompaniedbyhistogramsshowingthedistributionincolor(top)andmagnitude(right)ofthedifferentcomponents. andbrighterCMDposition.Apeakinbrightnessisreachedfor in and around the temperature range where the loop occurs, a ZAHB mass of 0.652M . When the mass is decreased even as indicated in Fig. 2 – that is, between about 5800K and ⊙ more, one finally returns to a color of 0.4, which occurs for 8300K. Unfortunately,many M3 HB stars live at this temper- M =0.632M .Finally,theusualmonotonicbehaviorisagain ature range, including all of its RR Lyrae and the redder of its HB ⊙ recoveredforevenlowermassesorhighertemperatures. BHB stars. Note that 72±7% of the M3 total HB population hasT < 8300K.Inaddition,asaresultofphotometricerrors eff This non-monotonic behavior and the resulting triple de- (see the error bars plotted in Fig. 3), this effect potentially af- generacy in HB parametersseriously hinders the interpretation fectsthe evolutionaryinterpretationof notonlythose few stars of photometricobservationswith the F336Wfilter, particularly Articlenumber,page6of13 A.A.R.Valcarceetal.:HeenhancementamongM3’sHBstars that are located within the exactboundariesof these loops, but devoidofsucha loopfeature(Fig.5). F300Wisnoteworthyin alsoofstarsintheirimmediateneighborhood,inthecolorrange thiscontext,astherelevantCMDsappeartoattempttodevelop 0.34.(F336W−F555W).0.45,whichcomprisesasignificant asimilarloop,butdonotquitesucceedatthetask. fraction(28±5%)ofthenon-variableHBstarsinM3.Notethat Clearly, then, the loop feature affecting F336W is caused this fraction increases to 55± 6% when RR Lyrae stars occu- solely by the corresponding bolometric corrections. Over the pyingsimilarcolorsarealsoincluded(Catelanetal.2001,their corresponding temperature region, and as we have mentioned Table1forr<50′′). previously, the degeneracy involves a parameter in addition to The preceding analysis can be further explored using syn- Y and t , namely the mass M , as stars with different M HB HB HB theticHBmodels,asshowninFig.4,wheresyntheticCMDsat valuesbutthesame(F336W−F555W)canhavevastlydifferent theHBlevelareshownintheStrömgrenandWFPC2 systems. absolute magnitudes in F336W (Figs. 2-3). Close examination In each case, a uniform mass deviate was used with 200 HB of these figures, alongwith the synthetic HBs shownin Fig. 4, stars per0.01M , leadingto T andlogL valuesfor eachstar suggeststhatthistripledegeneracyphenomenoncanlead toan ⊙ eff which are then transformedinto the observationalplanesusing incorrect reading of the He abundance that may be as high as theClemetal.(2004)andCastelli&Kurucz(2004)bolometric ∆Y = 0.02, in the specific case under study. Note, in particu- correctionsforyvs.(b−y)andF336Wvs.(F336W−F555W), lar, that HB stars with Y = 0.246 starting their evolution close respectively. The photometric uncertainties, in all bandpasses, totheloopregionwillbecomeasbrightastheZAHBlocusfor are at the levelof 0.01magon averageand were also modeled Y = 0.266bythetimetheyreachtheMAHB(see evolutionary after normaldeviates. Theleft panels(a, d, andg) show a stel- tracksforMHB =0.665and0.652M⊙inFig.3). lar populationwith Y = 0.246with masses between 0.595and Based on this HB Y test of the presence of He enhance- 0.705M (lightgraycircles),andpanelsbande(candf)showa ment and internal spreads in GCs, our suggestion would be to ⊙ helium-enrichedstellarpopulationwithY = 0.256(Y = 0.266) avoid relying excessivelyon the F336Wfilter overthe temper- withmassesbetween0.595to0.645M (darkgraycircles). ature range 5800 . T [K] . 8300. Other near- and far-UV ⊙ eff To show how the high percentage of BHB and RHB stars filters can be safely used, as can F336W outside this tempera- around the aforementioned loop feature mix in magnitude and turerange.Naturally,fortemperaturesinexcessoftheGrundahl color for (F336W−F555W), RHB stars are cleanly selected in jumpatTeff ≃11500K,itisnecessarytoproperlytakeintoac- theStrömgrensystemasthosewith0.28≤ (b−y),andBHBas counttheeffectsofheliumsedimentationandradiativelevitation thosewith0.02≤(b−y)≤0.13andy≤15.7.Thesamestarsare ofheavierelements(Grundahletal.1999),asdone(albeitinan then accordinglylabeled in the CMDs that use the WFPC2 fil- approximateway)forNGC2808byDalessandroetal.(2011). ters.TheRHBpopulationswithY =0.246areshownasredcir- cles, whileBHBpopulationswithY = 0.246,0.256,and0.266 3.3.Onthe∆Y levelamongM3’sBHBstars areshownascyan,blue,andgreencircles,respectively.Asex- pected,intheStrömgrenyvs.(b−y)CMD,thecolordistribution Followingtheseresults,weusedthemethoddescribedinV14to separatesalmostcompletelyBHBstarswithY =0.256or0.266 obtainanewestimateofthedifferencein(initial)heliumcontent fromRHBstarswithY = 0.246,exceptforasmallfraction(of betweenRHB(0.28 90%).Theupperpanelsshowthattheyvs.(b−y)plane StrömgrenbandsasinC09,herewerestrictthebluestcolorfor canbeusedreliablyonlyfor(b−y) & 0.06(Teff . 8300K)be- BHB stars as described in the previous section, and study the cause of the observationalerrors. Thus, unless the photometric variation in the relative proportions of stars distributed in the errors are smaller than indicated and/or these errors are prop- CMD between the ZAHB, MAHB, 90AHB, and TAHB loci in erly taken into account in the analysis, no safe constraints can greaterquantitativedetailfordifferentadopteddistancemoduli. beplacedonthelevelofHeenhancementforsuchhotHBstars We allow for the possibility of a formal solution in which the basedsolely(asinC09)ontheyvs.(b−y)CMD. distancemodulus,asinferredonthebasisof theBHB starsfor For the F336W vs. (F336W − F555W) plane, we find, as anassumedY value,isdifferentfromtheonederivedusingthe expected, that the color distribution around the loop cannot be RHBstarsforthesameY.Sincephysicallybothdistancemod- used to separate BHB from RHB stars in the range 0.34 . uli shouldobviouslybe the same, any such differencesmay be (F336W−F555W) . 0.45(8300 & Teff[K] & 5800).In turn, indicativeofadifferenceinintrinsicluminosity,henceintheHe luminositydistributionscanbeusedtoidentifytheexistenceof abundance,betweenthetwoHBcomponents. a helium-enrichedBHB population,butonly if RHB and BHB The resultsare shownin Fig. 6, where theleft panelsshow populationshavesimilarnumbersofstarspopulatingtherelevant thevariationsinthenormalizedfractionofstarsatdifferentevo- color range and if the statistics is high. Since it is well known lutionarystagesforRHBstars(upperpanel)andBHBstars(bot- thathelium-enrichedpopulationsoftencompriseonlyafraction tompanel)asafunctionoftheadopteddistancemodulus.Since of the helium-normal population in GCs, a reliable characteri- photometric errors can induce variations in these fractions, we zation of the stars around the loop feature in terms of their He consideredatthesametimethenormalizedfractionofstarsthat enhancementlevelsisverychallenging. are(1)fainterthantheMAHB(thinblacklines),(2)betweenthe IstheloopfeatureonlyachallengeforF336W,orareother MAHB and the 90AHB (magenta lines), and (3) brighter than UVfilterssubjecttothesameproblem? the 90AHB (cyan lines). The corresponding normalized frac- To answer this question, we applied the same HB models tions are expected to be around 50%, 40%, and 10% for each as shown in Fig. 2–3 to other UV and optical filters in the ofthesethreeranges,respectively. HST set, using bolometric corrections from Castelli&Kurucz TherightpanelsinthesamefigureshowtheCMDsofRHB (2004). The filters included in this analysis are the following: (reddots) and BHB (blue dots)stars using the distance moduli in the far- and near-UV, F122W, F160BW, F170W, F185W, thatbestreproducetheexpectedevolutionaryproportionsshown F218W, F255W, F300W, and F380W; in the visible, F439W, intheleftpanels(dottedanddashedverticallines,respectively). F450W,F555W,F606W,F702W,F814W,andF814W.Interest- TheoreticalmodelsareshownforY = 0.246(HBtracksingray ingly, CMDs producedusing all of these filters are completely lines,ZAHBlocusasaredline,MAHBlocusasabluedashed Articlenumber,page7of13 A&Aproofs:manuscriptno.avalcarceM3_2016v2 Fig.5.ZAHBlociforY = 0.246andZ = 0.001fordifferentcombinationsofHST-WFPC2filters,asindicatedintheinsets.F336Wistheonly filterproducingtheloopalongtheZAHBlocus(thinblacklineintheupperrightpanel). line,90AHBlocusasadottedmagentaline,andTAHBlocusas errors,ofasmallpopulationwithlowerY,and/oroftheneedofa alonggreendashedline)andforY = 0.256(ZAHBlocusonly, smallincreaseinthemetallicityofPGPUCmodels.Incidentally, shownasablueline).InthebottomrightpaneltheZAHBlocus such a formal difference in distance moduli corresponds very forY = 0.256usesthedistancemodulusobtainedfortheRHB closelytowhatwouldbeexpectediftheBHBcomponentwere starsonlyasareferencelocus. moreHe-richthantheRHBcomponent,by∆Y ≈0.010±0.002 (wheretheindicatederrorbaristheformalvalueincludingonly Figure6suggeststhattheRHBandBHBstellarpopulations thePoissoniancomponent).AssumingthislevelofHeenhance- doindeedappeartobebestcharacterizedbydifferencedistance ment for the BHB component, the same distance modulus as moduli. Specifically, the formal distance moduli for the RHB obtainedfor the RHB providesan excellent match to the BHB andBHBpopulationsare14.909+0.002and14.872±0.002mag, −0.006 componentaswell,asshowninthebottomrightpanelofFig.6. respectively;moreexactly,thedifferenceamountsto0.037mag in y. Motivatedby the HB simulationsshown in the top panels Unfortunately, this method cannot be as straightforwardly ofFig.4,weattributethefewRHBstarsthatarefainterthanthe used with the F336W filter because of the difficulties that are ZAHB locus with Y = 0.246 as a consequenceof photometric broughtabout by the triple degeneracyeffect mentionedprevi- Articlenumber,page8of13 A.A.R.Valcarceetal.:HeenhancementamongM3’sHBstars 1 0.8 0.6 0.4 0.2 1 0.8 0.6 0.4 0.2 14.85 14.9 Fig.6.DistancemodulusdeterminationfortheStrömgremfiltersaccordingtothefractionofstarsineachevolutionarystagealongtheHB,for Z =0.001andY =0.246.Theupperandlowerpanelsontheleftshowthenormalizedfractionsforstarswith0.28<(b−y)<0.44(RHBstars) and0.060 < (b−y) < 0.135(BHBstars),respectively,forfiltery.Colorlinesrepresentthenormalizedfractionofstarsthatare(1)fainterthan theZAHBlocus(redlines),(2)betweentheZAHBandMAHBloci(bluelines),(3)betweentheMAHBand90AHBloci(magentalines),(4) betweenthe90AHBandTAHBloci(cyanlines),(5)fainterthantheMAHBlocus(thinblacklines),and(6)fainterthanthe90AHBlocus(thick blacklines).Verticaldottedlinesrepresentthedistancemodulususedintheupperrightpanel(andFig.1),andtheverticaldashedlineinthelower rightpanelrepresentsthedistancemodulusthatshouldbeselectedifonlyBHBstarsareconsidered.InthebottompaneltheZAHBlocuswith ∆Y =0.01isshiftedaccordingtothedistancemodulusobtainedfromtheRHBpopulationforcomparison. ously.ThisisnotonlyafeatureobservedintheF336Wfilter,but ZAHB locuswith Y = 0.256.This can plausibly be associated alsoinfilterssamplingthesamewavelengthrange(seeSect.4). withanM3componentwithasmallheliumenrichment,thatis, Inparticular,Fig.7showsM3HBstarsintheStrömgremuvs. with ∆Y ≈ 0.01, with ∆Y possibly increasing by an additional (u−y)planewithstarscolor-codedasinFig.6.Clearly,because 0.005−0.01asthebluelimitofthisrangeisapproached.Over RHBandBHBstarsoverlapat(u−y) ≈ 1.75,aninterpretation thiscolorrange17±4%ofthenon-variableHBstarsarelocated, oftheseCMDsisfarfrombeingstraightforward. whichcorrespondsto11±2%ofthetotalHBpopulationinM3 Note, however,that overthe colorrange0.10 . (F336W− (consideringRRLyraestars;Catelanetal.2001).Similarly,over F555W).0.30,Fig.2revealsalackofHBstarsfainterthanthe the color range −0.32 . (F336W−F555W) . 0.10, the same Articlenumber,page9of13 A&Aproofs:manuscriptno.avalcarceM3_2016v2 Phase T range[K] ∆Y %non-variables %totalHB Maindiagnostic eff RHB+V <7500 <0.01 32±5% 58±6% Strömgren(visual) coolBHB 7500↔ 8300 0.01 21±4% 13±3% Strömgren(visual) BHB 8300↔ 9300 <0.02 25±4% 15±3% WFPC2(nearUV) hotBHB 9300↔10900 0.02 16±3% 10±2% WFPC2(nearUV) eBHB >10900 ? 6±2% 4±1% Table1.AmountofHeenhancementofM3BHBstarscomparedtoRHBstars.Fractionsarecalculatedconsideringonlynon-variablestarsand thatthetotalHBpopulationfollowsarelationB : V : R = 0.47 : 0.37 : 0.16,whereB,V,Rstandforthenumberofblue,variable(RRLyrae), andredHBstars,respectively(Catelanetal.2001). figureshowsalackofHBstarsfainterthantheZAHBlocuswith Y = 0.266thatcanbeassociatedwithanM3componentwitha largerheliumenrichment(∆Y ≈ 0.02),comprising16±3%of thenon-variableHBstars(10±2%ofthetotalHBpopulation). Based on these results, we cannot rule out at present that M3 HBstarsthatarehotterthantheGrundahljumpmayhavebeen subject to a higher level of He enhancement than their cooler BHBcounterparts–butthesecompriseonlyabout4±1%ofthe totalHBpopulationinM3(C09). In Table 1 we summarize the level of He enhancement in- ferred from Figs. 1 and 2, separating the different M3 compo- nents according to their temperatures and colors. The number fractions were determined assuming that the non-variable HB starscomprise63%ofthewholeHBpopulations(Catelanetal. 2001). Note that, even though multiple populations show different radial distribution in some GCs (e.g., Lardoetal. 2011; Miloneetal. 2012; Bellinietal. 2013) that may be reflected in different BHB-to-RHB number ratios (Catelanetal. 2001; Iannicolaetal. 2009), we did not observe a large difference in theseratioswhentheStrömgrenphotometry(withafieldofview of3′.75×3′.75Grundahletal.1999)wascomparedwiththeHST WFPC2 photometry(with afield ofviewof 2′.5×2′.5),asboth samplebasicallycoverthesame(central)regionofthecluster. This means that for the range of temperatures covered by Fig.7.M3HBstarscomparedtotheoreticalHBmodelswithZ=0.001 theHBY test, asappliedinC09,in V14,andinthispaper,the intheStrömgrenuvs.(u−y)plane.Lineshavethesamemeaningasin visual bandpasses of the Strömgren and Johnson filter systems Fig.1.RedandbluepointsrepresenttheRHBandBHBstars,respec- clearlyhaveamuchmorestraightforwardinterpretationattem- tively,thatareusedtoobtainthedistancemodulusinFig.6. peraturesbelow∼ 8300K,moredirectlyconstrainingthelevel ofHeenhancementamongM3(andM4;seeV14)starsalikeat ∆Y .0.01,whichconfirmstheC09resultsforthebulkoftheHB arepredictedbyourmodels.LoopsalongtheZAHBarefound inmostofthecaseswherefilterscoveringthesamewavelength starsinthecluster.Athighertemperatures,however,thesesame regimeareusedtodefinecolorindices. bandpasseslosesensitivity,anditbecomesnecessarytoresortto InFig.8weshowtheZAHBlociforY = 0.246,Z = 0.001 filters coveringthe UV regime,some of which(F336Win particular)leadtoCMDsthataredifficulttointerpretatlowertem- as black lines in the theoretical plane (panel a) and in empirical planes associated with these four different filter systems, peratures.UsingdataobtainedwiththeWFPCF336W,wewere namely:Johnson-CousinsU vs. U −V (panel b); Strömgren u abletosuccessfullyidentifysmallpopulationsofhotterHBstars vs.u−y (panelc);HST/WFPC2 F336Wvs. F336W−F555W in M3 thatverylikely haveslightly enhancedhelium,reaching (panel d); Sloan u vs. u − g (panel e). To compare the range upto0.02in∆Y. Thissupportssimilarconclusionsreachedby of effective temperatures restricting the loop in the F336W vs. D13forthishotHBcomponent. F336W−F555Wplanewithothersetsoffilters,weshowlines representing T = 6125K (red dashed lines), 6600K (green eff 4. OntheZAHBloopinUbandsofadditionalfilter dottedlines)and7800K(bluedashedline)inthese panels.As canbeseen, theSloanu vs. u−g planealsoshowsa loopfea- sets turealongtheZAHBlocus,butinthiscaseitisevenmoreim- Since thisloopfeatureisobservedin the HST/WFPC2 F336W pressive,extendingasitdoesupto∼ 9500K.FortheJohnson- vs. (F336W− F555W) plane (Fig. 2) and in the Strömgrem u Cousinssystem,theU−V colorindexmonotonicallydecreases vs. (u − y) plane (Fig. 7), we now address its possible pres- alongtheZAHBasT increases,whichmakesthesefiltersim- eff ence in two other filter systems equipped with bandpasses that mune to the aforementioned triple degeneracy effect, and thus cover a similar wavelength regime as F336W or Strömgren u, affordsacleanerseparationbetweenBHBandRHBstarsthanis namely Johnson-Cousins (Johnson 1955; Cousins 1976) and thecasewiththeotherthreefiltersystemsthatwestudied. Sloan(Fukugitaetal.1996).However,weemphasizethatthese What is the origin of this loop phenomenon, and why is arenottheonlyfiltercombinationsforwhichloop-likefeatures it seen more prominently in some filter systems than in oth- Articlenumber,page10of13

Level of helium enhancement among M3's horizontal-branch stars

Similar Level of helium enhancement among M3's horizontal-branch stars