198 9ApJ. . .347. .835G 1213 1213 12134 12 1314 tion. Astronomy Observatories,operated bytheAssociationofUniversitiesfor Research inAstronomy,undercontract withtheNationalScienceFounda- only ifthemassofstar is knownfairlywell.Thisan accurate comparisonofobservations withtheorycanbemade Tinsley, andSchramm1978fortheC/ratios).Hence, Dearborn, Eggleton,andSchramm1976 mass (see,e.g.,Iben1965,1966,1967fortheLiabundance and the theoreticalpredictions,bothforC/ratiosand the Li abundances,areusuallycomputedasafunctionofstellar that aremuchlowerthanthetheoreticalpredictions.However, considerable fractionofthestarsexhibitcarbonisotoperatios with standardtheoreticalpredictionsshowsthatinallcases, a for fieldPopulationIIgiants.Acomparisonoftheseresults disk giants,andSneden,Pilachowski,VandenBerg(1986) field PopulationIgiants,CottrellandSneden(1986)for old Tomkin, Lambert,andLuck(1975);Luck, field giantshavebeenundertakenbyseveralauthors,e.g., Lambert andRies(1977,1981);Kjaegaardetal(1982) for red giants. provide avaluableinsighttotheextentofconvectivemixingin Lambert (1976);Dearborn,Lambert,andTomkin(1975); the lightelements.Hence,astudyofthesequantitieswould the surfaceC/andNratiosabundancesof elements likeLi,Be,andB,fromthesurfacetointerior.The net resultofthisso-calledfirstdredge-upphaseistodecrease mixing alsotransportstheprimordialCandfragile,light occurs inthehydrogenshelloutsidecore.Theconvective surface areCandN,bothproductsoftheCNcyclewhich envelope expandsinwardandmixesapproximatelytheouter important indicatorsofnuclearprocessingthatoccursinsidea 50% ofthestarbymass.Themajorelementstobemixed star. Asthestarevolvesupgiantbranch,itsconvective 1 The AstrophysicalJournal,347:835-848,1989December15 © 1989.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. 12134 1213 VisitingAstronomer,KittPeakNational Observatory,NationalOptical Studies ofC,N,OandtheC/Nratiosin C, N,OabundancesandC/ratiosinredgiantsare 1213 1213 1213 masses, andtheLiabundancesingiantsare,general,lowerthantheoreticallypredictedvalues.Various Subject headings:clusters:open—stars:abundancesevolution theories toexplaintheobservedabundancetrendsarediscussed. standard predictions;and(3)nostrongcorrelationexistsbetweentheLiabundancesclusterturn-off the theoreticallypredictedvalueswhilethosewithlargerturn-offmasses(M>2.2M)showratioscloseto mass ofapproximately2.2Mwhentheratiolevelsoffabruptlytoavaluenear26;(2)olderclusterswith turn-off masseslowerthanabout2.2MingeneralexhibitC/ratiosthatareconsiderably turn-off massesindicatesthefollowing:(1)C/ratioincreasessteeplywithmassuntila the clustercolor-magnitudediagrams.CorrelationofisotoperatioandLiabundanceswith from 1Mtoabout6.Theagesandturn-offmassesweredeterminedbyfittingtheoreticalisochrones The agesoftheclustersvaryfromabout50millionyearsto5billionyears,andtheirturn-offmasses giants mabout20GalacticopenclustersandC/ratiosLiabundancesdeterminedforthesestars. 0 0 0 0 © American Astronomical Society • Provided by the NASA Astrophysics Data System New high-resolution,highS/NspectraofCNlinesat8000ÂandLi6707havebeenobtainedfor CARBON ISOTOPERATIOSANDLITHIUMABUNDANCESINOPENCLUSTERGIANTS I. INTRODUCTION Department ofAstronomyandMcDonaldObservatory,UniversityTexas,Austin Received 1989February21;acceptedJune13 1 K ALPANAKrISHNASWAMYGiLROY ABSTRACT 835 1213 1213 the behaviorof stellarmixingwithmass.We chosetostudythe predictions shouldhelpprovide amorerealisticdescriptionof this kind,andacomparisonof theseresultswiththetheoretical dances withtheclusterturn-off masses.Thisisthefirststudyof giants inclustersofvaryingages andtocorrelatetheseabun- and Liabundancesinacomprehensive sampleoflate-type explain theanomalousabundancesobservedinthesegiants. one mustgobeyondstandardstellarevolutiontheories to also obtainsvalueslowerthanpredicted.Hence,wesee that published theC/ratiosfortwogiantsinM67,and he (Pilachowski, Saha,andHobbs1988).Brown(1987) has to beconsiderablylowerthanthetheoreticalpredictions values. LiabundancesinbothM67andNGC752werefound abundance somewhathigherinthesestarsthanthepredicted results indicatedthattheisotoperatiowaslowerand Li dance inthisclusterwasobtainedbyPilachowski(1986).Their predictions. TheC/ratioingiantsNGC7789 was obtained bySnedenandPilachowski(1986),theLiabun- showed abundancesthatgenerallyagreedwiththetheoretical and Lambert(1976)fortheircarbonisotoperatioby cluster giants.TheHyadesgiantsstudiedbyTomkin,Luck, Lambert, Dominy,andSivertsen(1980)fortheirLiabundnace recent studiesofcarbonisotoperatiosandLiabundancein mation ofthegiantmass. This method,althoughnotperfect,providesafairlygoodesti- a fairrepresentationofthestellarmassongiantbranch. theoretical evolutionarytracks.Thismasscanthenbeusedas the clusterturn-offmasscanbefoundbycomparisonwith theoretical isochronestothecolor-magnitudediagrams,and the sametime,clusteragecanbedeterminedbyfitting all thestarsinclusterareassumedtohavebeenformedat tainty istoobservegiantsinopenandglobularclusters.Since range ofages. extremely difficulttaskinmostfieldgiants,especiallythe Population Igiantssincethesearespreadoverafairlywide 1213 The aimofthisprojectwasto determinetheC/ratios A literaturesurveyshowsthattherehavebeenonlyafew An idealwayofgettingaroundtheproblemmassuncer- 198 9ApJ. . .347. .835G 12 1213 1213 1213 1213 general, notaffectedbyuncertaintiesinmodelatmosphere lines aresaturated,theuncertaintiesinatmosphereparam- parameters, unlikethelatter.Thisisbecause,unlessC eters affectboththeCandlinessameway,leaving the isotoperatiounaffected.HenceC/canbe important toknowthemassesandevolutionarystagesof determined withgreateraccuracythantheC/Nratio. ages wouldhelpustestthesetheoriesmoreaccurately. and observationsofclustergiantsspanningawiderange giants. Onceagain,thisisnotaneasytaskforthefieldgiants, number ofmechanismstoexplaintheobservedlowC/ project. Theoristsoverthepastfewyearshavepresenteda C/ ratioratherthantheC/Nsinceformeris,in deal withtheanalysisanddiscussionofresults,respec- discuss theobservationsanddatareduction.SectionsVVI tively. and agedeterminationfortheclustergiants,in§IVwe ratios infieldgiants(e.g.,Dearborn,Eggleton,andSchramm ments. Oursamplelistconsistsofabout60starsin20Galactic due tolackofgoodphotometricandpropermotionmeasure- Schatzman 1984).Toconfirmorrejectthesetheories,itis Mengel 1979;Schatzman1977;Bienaymé,Maeder,and 836 years to5billionyears.Mostoftheyoungclustersinour ascertaining theirmembership.Membershipuncertaintiesarise massive andevolverapidlycomparedtothoseintheolder course duetothefactthatgiantsinyoungclustersare sample haveonlyonetofourgiantmemberseach,whilethe open clusters,whoseagesspanarangefromabout50million clusters. Henceitisdifficulttofindmanyredgiantmembersin older clustershavefourtosevenmemberseach.Thisisof 1976; Dearborn,Tinsley,andSchramm1978;Sweigart the youngclusters. paper byHarris(1976)andreferencestherein.Thispre- motions, radialvelocities,andspectraltypes.Wehaveused intermediate-aged openclusters.Membershipsofclusterstars younger clustersthatarefainterthanV=8.5mag.Hence for intermediate-aged clusters.Harris(1976),however,does not have beenconfirmedorrejectedonthebasisoftheirproper sents anextensivesurveyofevolvedstarsinyoungand discuss clustersthatareolderthantheHyadesorgiantsin the these resultstoselectoursamplegiantsintheyoung and the oldandfaintgiants,weusedotherpublishedphotometric membership probabilitiesweregreaterthan90%.Table1gives probabilities. Ingeneral,onlythosegiantswereincludedwhose and propermotionsurveystodeterminethemembership have includedafewstarswhosemembershipsmaybeques- the listofsampleclustergiantsselectedforthisproject. We isotope ratiosandLiabundances intheopenclusterswith cluster agesand turn-offmasses.Althoughthese twoquantities We havethereforeassignedalowerweighttotheresults tionable owingtoalackofstarsinthatparticularagegroup. obtained forthesestars. There wasoneotherimportantreasonforundertakingthis In §§IIandIIIwewillbrieflydescribetheselectioncriteria One ofthemainproblemsinobservingstarsanyclusteris A majorsourcefortheselectionofclusterstarswas One oftheaimsthisproject wastocorrelatethecarbon © American Astronomical Society • Provided by the NASA Astrophysics Data System II. SELECTIONOFSAMPLECLUSTERSTARS III. CLUSTERAGESANDTURN-OFF MASSES GILROY 13 12 1213 1213 13 isochrones totheclustercolor-magnitudediagram. denBerg 1985;etc.),thereisafairamountofdisagreement calculations arebasedinaHeabundance,Y,of0.25,anda,the best fitstotheopenclustercolor-magnitudediagrams.These chrones, thosepublishedbyVandenBerg(1985)providedthe an independentstudyoftheseparametersbyfittingtheoretical among thevariousresults.Wethereforedecidedtocarryout Barbaro, Dallaporta,andFabris1969;Patenaude1978;Van- masses (M/M)determinedforeachclusteralongwiththe cluster, andthemassatbluestpointonisochronewas each cluster.Thebestfittingisochronegavetheageof chrones ofBarbaro,Dallaporta,andFabris(1969)wereusedto licities from[Fe/H]=—1.0-0.0.Weusedtheseisochronesfor chrones spanarangeofagesfrom0.3to15Gyr,andmetal- ratio ofmixinglengthtopressurescaleheight1.6.Theiso- errors weredeterminedbythescatterincolor-magnitude reddening anddistancemoduliusedtofittheisochrones.The the clusterturn-offmass.Table2givesages(t)and reddening weretakenfromthelatestavailablephotometryfor determine theagesandturn-offmassesofyoungestclusters all ourclustersexceptthoseyoungerthan0.3Gyr.Theiso- diagrams, andthesizeoftheseerrorsdependedonages of oursample.Theabsolutemagnitudes,B—Vcolors,and the openclusters. and AM/M.Figure1showssampleisochronefitsfortwoof turn-off masses.WethereforepresentinTable2theratiosAt/t in question(e.g.,SandageandEggen1969;Harris1976; have beendeterminedbyvariousauthorsfortheopenclusters 0 q (F <6.0mag)wereobservedwithaReticondetector(Vogt, relatively uncrowdedspectralregionandtheCNlinesare Tull, andKelton1978),usingthe2.1mtelescopecoudé resolved fromtheCNlines.Thebrighteststarsinoursample sample stars.Wechosethe8000Âregionbecausethisisa obtain theC/andLiabundances,respectively,inour with aTICCD15x/mipixelsatthe2.1mtelescope cluster starswereobservedusinganRCACCDdetectorwith 0. 12Àwasobtainedwiththissetup.Abouthalfofthefainter trograph attheMcDonaldObservatory,andrestobserved spectrograph attheMcDonaldObservatory.Aresolutionof Typical S/Nratiosinallcaseswasbetween80and300. In Â, respectively,wereobtainedwitheachofthesesetups. Kitt PeakNationalObservatory.Resolutionsof0.25and 0.21 and coudéspectrographthefeedtelescopeat the dard GandKgiantsobtainedtheC/ratios Li addition totheclusterstarswealsoobservedabout10stan- 30 x/mipixelsusingthe2.1mtelescopeandcoudéspec- abundances inthese. the two-dimensionalspectratoonedimensionandforremoval ric lineswereremovedbydividing thesamplespectrumwith the programSPECTRE(FitzpatrickandSneden1987).Tellu- of cosmicrays.Thedispersioncurvesweredeterminedusing have nospectralfeaturesoftheir owninthisregion.Figures2 that ofahot,rapidlyrotating star,sincethesestarsingeneral ences inthestrengthsof 'CNandtheLilinesin regions. Thesespectrawere chosen soastoshowthediffer- and 3showsamplespectra in boththe8000Âand6707 different stars. Of alltheavailabletheoreticalevolutionarytracksandiso- CN linesat8000ÂandLi6707wereobservedto A standardreductionprogram,IRAF,wasusedtocollapse IV. OBSERVATIONSANDREDUCTIONS Vol. 347 198 9ApJ. . .347. .835G trend. Figures4aand4bshow thefinalplotsofFeabun- the plotofabundanceversus excitationpotentialshowedno dance andsimilarly,theeffective temperaturewasaltereduntil until boththestrongandweak Feilinesgavethesameabun- sphere parameters.Thestellar microturbulencewasadjusted lower excitationpotentialsin ordertodeterminetheatmo- dances werethenplottedagainstthelineequivalentwidths and computes abundanceswithrespecttotheSun.Theseabun- potential. Thus,iftheinputabundanceissolar,program equivalent width,oscillatorstrengthandlowerexcitation abundance andthecalculatedoutputforagiven program whichcomputesthedifferencebetweenagiveninput abundances wereobtainedusingastandardLTEfineanalysis both the6707Âand8000spectralregions.The Fe determined spectroscopicallyusingunblendedFeilines in (15) Roman1949;(16)Hiltner,Inarte,andJohnson1958;(17)Herzog,Sanders,Seggeweiss1975. Eggen 1968;(9)CiaríaandRosenzweig1978;(10)SmythNandy1962;(11)Williams1967;(12)Pesch1961;(13)KleinWassink1927;(14)Janes andSmith1984- No. 2,1989 The modelatmosphereparametersforourclusterstarswere References.—(1) Hardy1979;(2)KrzeminskiandSerkowski1967;(3)Harris1976;(4)Hoagetal1961;(5)vanBuren1952;(6)Alcaino1967-(7) Cox1954-(8) Cr 140: NGC 2360: NGC 2287: NGC 2281: NGC 1662: Hyades: NGC 1545: NGC1342: Tr 2: Stock 2: GC 752: HD 58535 12 107 Q L F 2 eTau yTau .... 0Tau .... <5Tau .... 1 3 4 43 1 295 ... 311 ... 213 ... 11a ... 97 21 75 77 ... 75 ... © American Astronomical Society • Provided by the NASA Astrophysics Data System 1 ... Star a) ModelAtmosphere V. ANALYSIS 10.34 5.36 7.78 7.80 7.44 6.91 8.31 7.30 8.87 8.83 8.34 4.04 3.63 3.86 3.98 6.74 9.33 7.38 7.60 8.30 9.01 9.26 9.30 8.99 9.43 8.90 -2.77 -0.20 -1.43 -1.41 -1.77 -2.30 -0.46 -1.47 -0.06 -0.55 -3.68 -0.14 -2.38 -0.82 -0.10 0.01 0.74 0.33 0.56 0.68 0.98 0.96 0.87 1.23 1.27 1.40 (B —V)References 0 1.05 1.02 0.89 1.14 1.15 1.25 0.89 1.50 0.93 1.06 0.96 1.26 0.99 0.98 1.01 0.87 0.98 1.38 1.04 0.94 0.97 1.52 0.99 0.94 1.02 1.01 Stellar ParametersfortheClusterGiants OPEN CLUSTERGIANTS 1 TABLE 1 to obtainthestellargravity. The bolometricmagnitudeswere tude ofthestarinrelation together withtheeffectivetemperature andbolometricmagni- turn-off massasagoodrepresentationofthestellar ii linesintheregionsweobserved.Instead,usedcluster the same.Wecouldnotusethismethodsincetherewereno Fe ii linesandadjustingthegravityuntiltwoabundances are involves comparisonofabundancesobtainedusingFeiand Fe nation ofthestellargravityusingfineanalysismethod dance wasconsequentlylargerbythisamount,andhence the Fe abundanceseeninFigures4aand4bisnear0.0.Determi- while computingtheatmosphereparameters,outputabun- abundance inthiscase,wasscaleddownbyloge(Fe)=-0.2 potential forthecaseofstar77inNGC752.Sinceinput Fe dance againstlineequivalentwidthandlowerexcitation IC 4756: NGC 6633: UMA Group: M67: Praesepe: NGC 2548: NGC 2451: NGC 2422: 314 .. 296 .. 249 .. 228 .. 176 .. 140 144 .. 134 116 HR 1327.. 6 CMi 2Aur F266 F223 F170 F164 F108 F84 428 283 253 212 HD 63032 15 8 7 87 .. 14 8 log (g/g)=(M/M+4 log(T/7;) e0 effff 0 Star + 0.4(M-M) bolbolo 10.55 10.58 10.55 10.59 9.64 9.33 9.07 9.40 9.41 9.25 8.79 9.40 8.96 8.31 8.32 4.55 4.77 5.27 9.69 9.72 6.90 6.44 6.39 6.59 9.48 8.43 3.58 8.02 7.80 K (B-V) -0.18 -0.90 -0.68 0 -0.69 -3.76 0.76 0.45 0.19 0.52 0.53 0.78 0.37 0.95 0.52 0.30 -0.62 -0.84 0.31 0.37 0.40 1.23 1.26 0.70 0.24 1.23 0.19 0.39 1.27 0.36 0.85 0.81 0.97 0.86 0.84 0.91 0.91 0.89 0.98 0.93 1.06 0.81 0.92 1.41 1.03 1.06 0.96 1.30 1.07 0.98 1.38 0.96 1.03 1.02 1.00 1.10 0.96 1.68 1.14 References 17 16 15 14 13 12 11 10 837 198 9ApJ. . .347. .835G -1 models ofBelletal.(1976).Typicaluncertaintiesintheatmo- cluster stars;themodelatmospheresusedwereinterpolated the rest,theywerecalculatedfromtheirabsolutemagnitudes taken fromHarris(1976)formostoftheclusterstars,and g =±0.3,A£±0.2kms,andA[Fe/H]±0.2. the finalmodelatmosphereparameterscomputedfor and thebolometriccorrectionsofJohnson(1966).Table3gives sphere parameterswereasfollows:AT=+150K,Alog 838 eff Fig. 1.—Sampleisochronefitsfortwo openclusters,M67andPraesepe.AlltheisochronesshownarefromVandenBerg (1985)forZ=0.169. © American Astronomical Society • Provided by the NASA Astrophysics Data System NGC 752 Cluster Tr 2 Stock 2 NGC 1342 NGC 1662 NGC 1545 NGC 2360 NGC 2287 Hyades M67 Praesepe NGC 2548 NGC 2451 NGC 2422 NGC 2281 UMa Group Cr 140 IC 4756 NGC 6633 NGC 7789 * Thesevalueswereconsideredindividuallyforeachclustermember. (m-M)o (mag.) 7.45 7.97 9.40 8.60 8.80 9.15 6.20 9.00 8.50 7.90 3.30 9.50 8.40 8.04 2.00 7.19 10.3 8.25 7.50 11.7 Cluster AgesandTurn-offMasses E(B-V) (mag.) 0.02 0.29 0.32 0.34 0.33 0.00 0.08 0.02 0.09 0.00 0.04 0.08 0.03 0.06 0.05 0.28 0.21 0.17 * * GILROY 8 7 9 8 8 8 8 8 8 7 7 8 8 7 TABLE 2 8 9 9 8 8 9 4.0 x10 9.0 x10 2.0 x10 4.0 x10 4.0 x10 3.0 x10 4.5 x10 2.2 x10 8.0 x10 6.5 x10 5.0 x10 8.0 x10 5.0 x10 7.0 x10 3.5 x10 5.0 x10 1.3 x10 8.0 x10 5.5 x10 Age (r) 1.5 x10 yrs. 12 13 isotope ratio.TheLifeatureisblendedwithanFeilineat were thelinesat8004Â.Sincethisfeatureisablendofthree synthesis techniqueshadtobeemployeddeterminethe 6707.441 ÂandCNlinesat6707.5296707.816Â. CN lines(28004.554,28004.728,and28004.781),spectrum 131213 The mainCfeaturesusedtodeterminethe/ratio (M/M)*.. At/tA(M/M) 0oe 4.5 2.8 2.8 3.1 1.6 4.8 6.0 2.8 2.2 3.8 2.7 2.2 2.6 5.0 2.9 1.9 2.5 2.2 1.2 1.8 0.13 0.20 0.33 0.17 0.13 0.06 0.25 0.23 0.11 0.21 0.30 0.32 0.04 0.20 0.31 0.29 0.16 0.19 0.13 0.25 b) SpectrumSynthesis 0.13 0.20 0.31 0.35 0.12 0.15 0.23 0.40 0.80 0.20 0.20 0.15 0.30 0.60 0.12 0.10 0.40 0.20 0.30 0.40 Vol. 347 LOo 00 No. 2, 1989 OPEN CLUSTER GIANTS 839

7995 8000 8005 8010 8015 Wavelength Fig. 2.—Sample spectra of the CN region at 8000 Á for two cluster giants, F108 in M67 and HD 58535 in Cr 140

Hence this region also had to be synthesized to obtain the Li Once the line lists were generated and the model atmosphere abundance. This was done with the help of a standard LTE parameters determined, synthetic spectra were produced for spectrum synthesis program, MOOG (Sneden 1973). The different values of the 12C/13C ratio and Li abundance. One of wavelengths of the 12CN lines were taken from the compilation 13 the problems in obtaining the carbon isotope ratio using only of Davis and Phillips (1963) and those for the CN lines from the lines in the 8000 Â region is that the 12CN lines in this Wyller (1966). The lower excitation potentials of the CN lines region are usually saturated, and hence the 12C abundance were computed using the vibrational and rotational constants determined using these lines alone would be affected bv uncer- published by Kotlar et ah (1980) and the line oscillator tainties in the stellar microturbulence. Because 6707 A region strengths were determined by first computing the Hönl- contains a few unblended, unsaturated 12CN lines, we synthe- London factors and multiplying these by the band oscillator sized this region first to obtain an estimate of both the 12CN strengths published by Sneden and Lambert (1982). The con- and the Li abundances. We then used this 12CN abundance as taminating atomic lines were taken from the compendium of input and synthesized the 8000 Â region to obtain the 12C/13C atomic lines by Kurucz and Peytremann (1975). ratio. For each observed spectrum, synthetic spectra were gen- erated for several values of the Li abundance, the 12C abun- dance, and the 12C/13C ratio. The best fitting synthetic spectrum was chosen both by eye and by using a numerical fitting procedure. The two methods agreed to within 10%. The numerical fit was obtained by first computing the square of differences between the synthetic and observed spectral inten- sities at each observed wavelength and then for each synthetic spectrum, these values were summed. The spectrum with the minimum value of this sum was taken to be the best fit to the observed spectrum. There were two clusters, M67 and NGC 2360, for which the Li region was not observed. We used the results of Brown (1987) as a good estimate of the 12C abun- dance for the M67 giants and for NGC 2360 we assumed a solar 12C abundance scaled appropriately to the cluster metal- licity. Figures 5 and 6 show sample syntheses fits for the 12CN lines and the Li i lines in the 6707 Â region for one of our cluster stars. Figures 7 and 8 show fits for the 12C/13C ratios for two stars in the 8000 Â region. Synthetic spectra are shown for three values of 12CN, and Li abundance and four values of 12C/13C ratios.

VI. RESULTS AND DISCUSSION 12 13 6700 6702 6704 6706 6708 6710 6712 6714 Table 4 gives the C/ C ratios and Li abundances for all Wavelength our cluster stars. The second and fourth columns give the Fig. 3.—Sample spectra of the Li i region at 6707 Â for two giants in the abundances determined by fitting synthetic spectra to the cluster NGC 752. observed spectra using an estimate by eye. The third and fifth

© American Astronomical Society • Provided by the NASA Astrophysics Data System 198 9ApJ. . .347. .835G 1213 12 840 columns givetheresultsofnumericalfitssynthetic mass. Itwaspossibleingeneral,todothissincethesevalues each clusterandplottedthesevaluesagainsttheturnoff isotope ratiosandLiabundancesforallthesamplestarsin spectra totheobservedspectra.Weaveragedcarbon top left-handcorner.Seetextfordetails,{b)PlotofFeiabundanceagainstthelineequivalentwidthparameterlogW/L diagram. Therewas,however,onecluster,IC4756,whose these starsareprobablyinsimilarevolutionarystates,afact agreed witheachothertowithin10%-15%.Thisimpliesthat giant membersexhibited/Cratiosthatdifferedfromeach strengthened bytheirpositionsinthecolor-magnitude clumped togetherontheH-Rdiagram,theymaybeindiffering other byover15%.Wesuspectthatalthoughthesestarslie lines indicatesyntheticspectrafor[ CN] =—0.3,—0.1,0.1. star 77inNGC752.Thecrossesindicate theobservedspectrum,andsolid 12 Fig. 4.—(a)PlotoftheFeabundanceagainstlinelowerexcitationpotentialforstar77inNGC752.Theatmosphereparametersthis are giveninthe Fig. 5.—Spectrumsynthesisfitsforthe weakCNlinesaround6707Âfor © American Astronomical Society • Provided by the NASA Astrophysics Data System GILROY 1213 evolutionary stages.Thatis,theclumpmaybeamixtureof core burningstage.Weexcludedtheisotoperatiosthatwere lower limitsandaveragedtherest.Table5givesaverage stars onthefirstascentofgiantbranchandthoseinHe with theclusteragesandturn-offmasses.Thelastcolumnin this tablegivesthenumberofgiantsineachcluster.Table6 12 eight standardGandKfieldgiantstogetherwithpreviously gives theisotoperatiosandLiabundancesobtainedbyusfor determined valuesfromliterature(LambertandRies1981 sonably wellwiththepublishedvalues. Lambert, Dominy,andSivertsen1980).Ourresultsagreerea- C/ ratiosandLiabundancesforalltheclusterstogether shown forloge(Li)=1.2,1.4,1.6. [CN] abundanceof—0.1hasbeen assumed,andsyntheticspectraare Fig. 6.—SynthesesfitsfortheLiiline at6707Âforstar77inNGC752.A Vol. 347 198 9ApJ. . . 347 . .835G 121 31214 12 14 121314 14 13 NGG 752 NGC 2281 Hyades NGC 1545 4 51502.80 NGC 1342 2 47002.80 NGC 1662 7 Tau48002.70 43 47502.20 75 49002.85 Star TfLogg NGC 2287 F 48002.55 6 Tau48002.70 6 Tau48002.70 3 40001.50 311 49002.85 295 50002.90 213 50002.90 77 49002.85 1 50002.85 Q 48002.55 L 45002.00 € Tau48002.70 Tr 2 Stock 2 97 45002.00 21 41001.60 1 47002.80 1 40001.50 11a 47502.20 75 45002.00 NGC 2360 107 45002.00 HD 5853547502.20 Cr 140 12 48002.70 ef light elementabundances. decreasing thesurfaceC/ C and/Nratiosthe and Lifromthesurfaceto the hotinterior.Thisresultsin the interiortosurfaceand transportstheprimordialC expands inwardandmixesthese CN-processedelementsfrom star evolvesupthegiantbranch, itsconvectiveenvelope created butnotdestroyed.Thisoccursevenwhenthemajor is preservedthroughoutthemain-sequencelifetime.When the source ofnuclearenergyisthep-preaction.Thisconfiguration inside, wherethetemperaturesarehighenoughforNto be high enoughtoburnCbutnotN(Dearborn, Eggleton, andSchramm1976).ANplateauexistsdeeper theory, veryearlyinthemain-sequencephaseofevolution, a dard stellarevolutionpredictions.Accordingtostandard ratios andLiabundancesintheopenclustergiantswithstan- C peakisbuiltupinsidethestar,wheretemperatures are No. 2,1989 1213 Our interesthereliesincomparingtheobservedC/ C © American Astronomical Society • Provided by the NASA Astrophysics Data System K a) StandardTheory Model AtmosphereParametersfortheClusterStars [Fe/H] 0.1 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.2 0.1 0.1 0.0 0.1 0.0 0.3 0.3 0.1 0.2 0.1 0.2 0.1 0.1 0.0 0.0 0.2 0.0 OPEN CLUSTERGIANTS -1 2.0 2.0 2.5 km s 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 1,8 1.9 7 2.0 1.9 1.9 8 1.8 1.8 2.0 2.0 2.0 1.8 £ Star 1.8 TABLE 3 13 1213 1213 F266 F223 F170 F164 F141 F108 M67 main-sequence phase(seediscussion inSneden,Pilachowski, F84 428 NGC 6633 6 CMi ÚMA Group 87 IC 4756 283 253 212 Praesepe NGC 2548 toward thesurfacethaninlow-mass starsattheendof theory, inhigh-massstarsthe Cpeakispushedfurtherout dependent (albeitweakly), because accordingtostandard low-mass stars.Thedecrease intheisotoperatioismass 296 249 228 HR 1327 2 Aur 8 NGC 2451 NGC 2422 314 HD 63032 be between25and30,thehigher valueonceagainbeingforthe 144 140 26 fora1Mstarandtoabout203-5star.Sneden, 176 134 116 14 15 Pilachowski, andVandenBerg(1986)predictthisreduction to ratios andheobtainsareductionofC/ratiofrom89 to The C/Nratioispredictedtodecreasebyabout0.4 dex. Dearborn (1988)hasrecomputedthepostdredgeupisotope and fora5Mstartheratioisfurtherreducedtoabout 25. about 30asaresultofthefirstdredge-upongiantbranch, assumed tobesolar,theC/ratiodecreasesfrom89 to surface compositionattheendofmain-sequencephase is Tinsley, andSchramm(1978)predictthatfora1Mstar,if the 0 0 0 Dearborn, Eggleton,andSchramm(1976) Te// 4800 4800 4250 4800 4800 4250 4800 4800 4800 4800 4800 4500 4000 4800 5200 5200 5000 5000 5300 4700 4600 4000 5000 5000 5000 5000 5000 4600 5000 5000 K b) CarbonIsotopeRatio Log g[Fe/H] 2.70 2.70 2.70 2.70 1.90 2.70 1.90 2.55 2.55 2.90 2.55 2.55 2.90 2.90 3.00 3.20 2.90 2.90 2.70 2.90 2.60 2.00 2.20 2.80 0.50 3.00 2.90 2.20 2.80 2.50 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.2 0.0 0.0 0.1 0.0 0.2 0.0 0.0 1 1.7 1.7 1.7 2.0 1.7 km s 2.0 1.7 1.7 1.7 1.8 1.8 1.8 1.6 1.6 1.8 1.8 1.8 1.8 1.8 1.8 1.8 2.0 2.0 1.8 1.8 1.8 1.8 2.3 2.1 1.9 £ 841 LOo 00 842 GILROY Vol. 347

Wavelength

Fig. 7.—Syntheses fits for the CN lines at 8000 Â for star NGC 77 in NGC Fig. 8.—Same12 13 as Fig. 7 for star F108 in M67. The synthetic spectra in this 752. A [12CN] abundance of —0.1 was assumed, and synthetic spectra for case are for C/ C ratios of 10,14,17, and 200. 12C/13C ratios of 12,15,20, and 200 are shown.

and VandenBerg 1986). Consequently, when first dredge-up to agree quite well with the abundance trend shown by our occurs on the giant branch, more 13C is brought to the surface sample clusters. 12 13 of high-mass stars, hence reducing the surface isotope ratios to Although we have plotted C/ C ratios for halo giants and lower values in these stars. old disk giants along with the Population I giants, we have to How do our results compare with these predictions? Figure be careful while interpreting these results. The Population II 9 shows a plot of the averaged 12C/13C ratios as a function of giants are in general very metal-poor and their low isotope the cluster turn-off mass. The following are apparent in Figure ratios may be due to low primordial 12C abundance rather 9: than mixing of excess 13C during stellar evolution. Hence, it 1. There is a definite change in the isotope ratio with the may not be entirely legitimate to combine results for different cluster turn-off mass for older clusters having turn-off masses population stars and give the same interpretation. The impor- less than about 2.2 M0. tant point here, however, is that even without including these 2. The isotope ratio increases steeply with increasing turn- off mass until this critical mass is reached, when the ratio levels off to a value of about 26. This is contrary to the theoretical predictions which indicate that the 12C/13C ratio should decrease very gradually with the stellar mass and then level off. 3. The isotope ratios in the low-mass stars are considerably lower than the predicted values while the intermediate- and high-mass stars exhibit ratios close to the theoretical predic- tions. This implies that the low-mass stars have somehow either mixed more 13C to the surface than the intermediate mass stars or depleted more surface 12C or both. Figure 9 shows also the 12C/13C ratios determined for field giants and cluster giants by other authors. These include NGC 7789 by Sneden and Pilachowski (1986), old disk giants by Cottrell and Sneden (1986), Population II giant studies by Sneden, Pilachowski, and VandenBerg (1986), and the globular cluster M4 by Smith and Suntzeff (1989). Once again we aver- aged the isotope ratios for all the stars in each study and plotted these averages. The mass estimate for the old disk giants was taken from the published results of Scalo and Miller Fig. 9.—Plot of the average 12C/13C ratios in each open cluster as a func- (1979) and that for the Population II giants from Sneden, Pila- tion of the turn-oif mass. Also plotted are the theoretical predictions of Dear- chowski, and VandenBerg (1986). We fitted isochrones to the born, Eggleton, and Schramm (1976) and Dearborn (1988). Uncertainties in color-magnitude diagram of NGC 7789 and hence determined the 12C/13C ratios at 13 and 25 are shown as bars on the left-hand side. See its turn-off mass. These results of the various authors are seen text for details of the plot.

© American Astronomical Society • Provided by the NASA Astrophysics Data System 198 9ApJ. . .347. .835G 1213 13 sented hereexisted.Wewilldescribe brieflythesuggestedtheo- older andfainterones),hence nocorrelationsuchaspre- isotope ratiosinfieldgiants. Thesecondquestionwasnever obtain high-resolutionspectra ofclusterstars(especiallythe raised beforebecauseuntilrecently, itwasextremelydifficultto several authorsovertheyears, inanattempttoexplainlow at about2.2M?Thefirstquestionhasbeenaddressed by and (2)whyisthereanabruptlevellingoffintheC/ratio anomalously isotoperatioscomparedtostandardpredictions, require explanation:(1)whydothelow-massstarsexhibit stars. TherearetwomajorresultsshowninFigure9 that this isindeedtruesinceallourC-richstarsarelowmass low-mass main-sequencestars.WehaveshowninFigure9 that are sufficientlynumerousthattheymusthaveevolvedfrom hypothesized thatstarsexhibitinglowcarbonisotoperatios cluster turn-offmassforthemoremassivestars. metal-poor giants,thereisstilladefinitetrendvisibleinFigure and noobviouscorrelationbetweentheisotoperatio and 9 forthePopulationIstarswithmasseslessthanabout2.2 M HD 58535 NGC 2360 97 NGC 2287 0 21 Cr 140 75 NGC 2281 12 107 L F Q NGC 1662 2 Hyades 0 6 Tau 7 Tau NGC 1545 1 3 4 e Tau 6 Tau NGC 1342 43 NGC 752 Tr 2 311 295 1 Stock 2 213 75 11a 77 Star 1 No. 2,1989 Based ontheresultsforfieldgiants,ScaloandMiller(1978) © American Astronomical Society • Provided by the NASA Astrophysics Data System -0.20 <0.30 Log e(Li)eiLi)* -0.60 <0.20 0.20 0.70 0.65 0.60 0.20 1.05 0.50 0.40 0.90 0.85 1.05 0.80 1.00 0.10 0.20 0.70 0.30 0.80 0.40 0.50 1.40 <0.20 -0.95 <0.10 0.10 0.70 0.50 0.85 0.65 0.10 0.20 0.80 0.40 0.80 0.80 1.00 1.00 0.00 0.30 1.00 0.80 0.75 0.20 0.40 0.40 1.40 13 >14.0 2/c 30.0 22.0 22.0 23.0 26.0 13.0 26.0 22.0 30.0 29.0 23.0 28.0 25.0 24.0 26.0 25.0 27.0 C 30.0 25.0 30.0 17.0 13.0 15.0 13.0 18.0 Abundance ResultsfortheOpenClusterGiants OPEN CLUSTERGIANTS 213 C/ n 29.5 23.0 22.0 28.0 26.5 24.5 21.0 30.0 14.5 31.0 26.0 27.0 26.0 24.0 26.0 27.0 26.0 27.5 28.0 28.5 18.0 15.0 14.0 16.0 13.0 16.0 TABLE 4 13 F266 F223 UMa Group F170 6 CMi 2 Aur F164 NGC 6633 F141 HR 1327 F108 M67 IC 4756 F84 87 428 228 134 116 14 249 140 283 253 Praesepe 296 176 212 314 144 8 NGC 2548 NGC 2451 NGC 2422 Star HD 63032 8 15 7 stars thep-preactionincore setsupa//-barrierthatinhi- Dearborn, Eggleton,andSchramm 1976)sinceinthelow-mass stars withmassesbetween 2 and5M(Paczynski1973; dicted bystandardevolution theories. envelope andtheisotoperatios reducedbelowthevaluespre- giants. the mainsequence.ExcessCwouldbeproducedin the concluded thatCNprocessingofthestellarenvelopewould take placeasaresultofthismechanismwhilethestarisstill on leton, andSchramm(1976)studiedthisprocessindetail and they canexplaintheabundancetrendsobservedincluster ries forthelowisotoperatiosingiantsanddiscusswhether between thestellarlayers(Paczynski1973).Dearborn,Egg- circulation currentsaresetupthatcauseaslowmixing observed infieldgiants.Inrapidlyrotatingstars,meridional suggested toexplaintheanomalouslylowisotoperatios 0 This mixingprocesshowever, ispredictedtooccuronlyin Two possiblemain-sequencemixingmechanismshavebeen Log e(Li) n -0.40 0.80 0.20 0.70 0.25 0.55 1.70 0.30 0.45 0.30 1.20 0.95 0.85 0.40 1.00 0.70 0.85 0.50 0.85 0.70 0.60 1.00 i) MixingontheMainSequence -0.60 0.80 0.40 0.80 0.00 0.40 1.20 1.85 0.40 0.40 0.60 0.50 1.00 0.80 0.40 1.00 1.00 0.70 0.80 0.60 1.10 1.00 213 >30.0 >30.0 >30.0 C/ 26.0 15.0 27.0 23.0 13.0 15.0 25.0 23.0 12.0 23.0 11.0 14.0 11.0 23.0 22.0 27.0 25.0 19.0 25.0 30.0 21.0 18.0 14.0 25.0 20.0 24.0 27.0 213 >29.0 >33.0 C/ n 14.5 25.5 32.0 14.5 26.0 30.0 25.0 22.0 27.0 16.5 10.5 10.5 21.0 13.5 26.0 25.0 11.5 27.0 12.0 28.0 26.0 18.0 18.0 31.5 29.0 22.0 25.0 31.0 843 198 9ApJ. . .347. .835G 13 mass starsshownormalratios.Hence,unlessthemixing bits thecirculationcurrents.Ourlowisotoperatiostarsareall, main-sequence stars,asaresultofdifferentialrotation.The in theradiativezonebelowbaseofconvective meridional circulationtheorycannotaccountfortheexcess however, low-massstars(M<2M)whiletheintermediate- meridional mixingmechanism,thediffusiontheorydoes not changes whilethestarisstillonmainsequence.When first ratios arereducedbelowthepredictedvaluesinthesestars. CNO-processed elementsoutsidethecorearepushedfarther According tothismechanism,aslowturbulentdiffusionoccurs by Schatzman(1977)andGenovaSchatzmann(1979). reaction tookoverasthemajornuclearenergysource, occurred veryearlyinthemainsequencephase,beforep-p 844 dredge-up occursonthegiantbranch,surfaceisotope out towardthesurface,andhenceenvelopecomposition C onthesurfaceoftheselow-massgiants. 0 <5Crt 11.0 13.0 <-0.5<-0.55 ß Gem16.0 16.0 0.50.44 a Boo7.0 7.00 <-0.7<-1.50 \J/ UMa28.032.0 1.5 X Hya18.5 17.0 ...0.78 6 Get12.0 12.0 ... 0Cen 8.50 10.0 <-0.2<-0.39 aCas 12.013.0<-0.5<-0.60 There aretwomainadvantagestothistheory.Unlikethe A secondmain-sequencemixingmechanismwassuggested 123 Star (C/)C/[loge(Li)]£(Li)] newlitnewlit Abundance ResultsforStandardFieldGiants © American Astronomical Society • Provided by the NASA Astrophysics Data System TABLE 6 NGC 752 Cluster Tr 2 NGC 1545 NGC 1342 Stock 2 Hyades M67 NGC 2287 NGC 2281 NGC 1662 NGC 7789 IC 4756 NGC 6633 UMa Group Praesepe NGC 2548 NGC 2451 NGC 2422 NGC 2360 Cr 140 : 1 FromPilachowski1986forclumpgiants. ’ FromSnedenandPilachowski1986. FromPilachowskietal1988. 8 7 9 Average AbundanceResultsforEachCluster 8 8 8 8 8 8 7 8 8 8 9 8 8 7 7 9 9 4.0 x10 9.0 x10 2.0 x10 4.0 x10 4.5 x10 4.0 x10 3.0 x10 8.0 x10 8.0 x10 6.5 x10 2.2 x10 5.5 x10 3.5 x10 5.0 x10 8.0 x10 5.0 x10 5.0 x10 7.0 x10 1.3 x10 1.5 x10 Age yrs 1213 (M/M)¿.o. C/Loge(Li)N 0 GILROY TABLE 5 4.5 2.8 3.1 2.2 2.8 1.6 4.8 6.0 2.7 2.8 2.9 2.2 2.6 3.8 2.2 2.5 5.0 1.2 1.9 1.8 1213 (1979) obtainisotoperatiosof8.9,18.2,and24.5fora1,1.6, been made,andthesevaluesagreereasonablywellwiththe require thestartoberapidlyrotating.Also,actualpredictions the trendseeninFigure10forlow-massstars.Also,a and 2Mstar,respectively.Thesevaluesagreequitewellwith observed abundancesinfieldgiants.GenovaandSchatzmann of thecarbonisotoperatioasafunctionstellarmasshave slope inbothcurvesoccursatapproximately thesamemass. observed intheclustergiantsasafunction ofmass.Notethatthechangein stellar masstakenfromIben(1967).The lowergraphshowstheC/Cratios 0 Fig. 10.—Theuppergraphshowsthe main-sequenceagesasafunctionof 6 16.9 27.9 28.8 27.6 25.8 26.0 26.5 24.3 27.0 15.2 27.3 26.5 28.9 26.8 29.8 23.8 21.0 19.4 13.0 13.8 a C -0.50 -0.78 luminosities,therearetwo cated. Ascanbeseen,mostoftheselow-massgiantshaveclose However, thereisstillachancethattheymaybefirst-ascent to clumpluminositiesandareprobablygiants. giants inHyadesandPraesepe.Theclumpposition(from evolutionary tracksfortheappropriatemassesalongwith the Cannon 1970),boththeluminosityandtemperatureisindi- M. Figure12showstheCrichgiantsplottedontheoretical the sameforallclusterswithturnoff*massesbetween1and 2.5 luminosities forclumpgiantsinalargenumberofopenclus- ters ofvaryingagesandconcludesthattheclumpposition is factors tobeconsidered.ArealltheC-richstarsinour sample post-Hecoreflashstars?Cannon(1970)givesmean indicate thattheHecoreflashmayhavebeenresponsiblefor the observedtrendinisotoperatios. discontinuity intheC/ratioappearsFigure9seemsto flash occursisverynearlythesameasmassatwhich mass stars.ThefactthattheabovewhichnoHecore would accountforthelowisotoperatiosobservedinlow- mixed withthesurfaceduringthisphenomenon,then occurs quiescently.IfexcessCfromtheinteriorweretobe burning onthegiantbranch.Inhighermassstars,He develop degenerateHecoresandundergoexplosive gested byseveralauthors(Dearborn,Eggleton,andSchramm 0 mixing onthegiantbranchasbeingcauseoflow isotope ratios.Henceitseemswecannoteliminateexcess occurring atabout2Mwhichagreeswiththetrendin from Sweigart,Greggio,andRenzini(1989),whiletheloweronceagainshows two curvesareseentobesimilar. the C/Cratiosinclustergiantsasafunctionofmass.Thetrends 1976; Scalo1979).Starswithmasseslowerthanabout2.2M C/ ratios,basedontheavailableinformation. 0 0 In Figure12theM67giants areindicatedasfilledcircles. However temptingthistheorymaybe,thereareseveral A secondmixingprocessonthegiantbranchhasbeensug- Fig. 11.—Theupperplotisthatofthegiantbranchagesvs.stellarmass 0 1234567 M/Mo 845 198 9ApJ. . . 347 . .835G 1213 12 13 12 12 1213 12 13 846 eliminate theHecoreflashasbeingcauseoftheirlow isotope ratiosaresimilartothosefortheclumpgiantswould tracks ofIben(1967)for1,1.5,and2.25M.Thepositiontheclumpallthesemassesisshownbyhorizontalverticalbars. ratios. Brown(1987)observedM67giantsfortheirC,N,O ascent andhadluminositiesmuchlowerthantheclump.These standard predictionsinstarsthatwereclearlyontheirfirst abundances, andheobtainedlowC/Nratioscomparedto lar clusterM4.Thestarsobservedinthispaper(Smithand plotted, alongwithourresults,theisotoperatioforglobu- the causeoflowC/Nratioingiants.InFigure9wehave observations wouldagainruleouttheHecoreflashasbeing caused bytheHecoreflash. the verylowisotoperatiosobservedcouldnothavebeen the giantbranch.Theseonceagain,arefirst-ascentstarsand dances attheendofcoreHeflash,andtheyconcludethat Suntzeff* 1989)werethebrightestgiantsorstarsattipof mixing didoccurduringthisevent,theC/ratiowould the majorelementmixedduringthisoccurrenceisC,and increase andnotdecreaseaccordingtotheresultsofthese result mightbetheformationofacarbonstar.Thus,evenif authors sinceitwouldbeveryhardtomixenoughC the will helpusdeterminewhethermixingbeyondthatpredicted less evolvedthanthoseinoursample(e.g.,clustersubgiants) vations ofthecarbonisotoperatioingiantsthataredefinitely surface withoutalsomixinglargeamountsofC.Moreobser- 0 mass stars. by standardtheoriesoccurredbeforetheHecoreflashin low- cause oftheobservedlowcarbonisotoperatiosinfieldgiants. born, Eggleton,andSchramm(1976)rejectedasbeing the would haveifnomasslosshad occurred.Thiswouldresultin from theinteriorismixedwith asmalleramountofCthanit lated thattoobtainisotoperatios aslow10,seeninsomeof values. Dearborn,Eggleton, and Schramm(1974)havecalcu- the finalC/ratiobeing reducedbelowthepredicted Mass lossonthemainsequencewoulddecreasesurface the clustergiants,50%mass lossisrequiredbeforethefirst C abundance.Hence,during thefirstdredge-upphase,C 1213 Deuprée andWallace(1987)havecalculatedsurfaceabun- There aretwoothertheoriesthatwereputforthbyDear- Fig. 12.—GiantswithabnormallylowC/Cratios(<20)intheolderopenclustersandgiantsHyadesPraesepeareplottedon evolutionary © American Astronomical Society • Provided by the NASA Astrophysics Data System GILROY Log T eff 13 1213 6 if suchanenormousmasslossdidoccur,itwouldcompletely dredge-up phase.Theydismissthisasbeingunlikely,buteven deplete thesurfaceLiabundancesincecanexistonlyin certainly exists,andthiswouldruleoutthemass-losstheory. low carbonisotoperatiogiants,however,althoughdepleted top 2%-3%,approximately,bymassofastar.Lithiuminour much lowerratiosthantheyoungerones,thenthatwould perhaps theC-richgiantswerebornwithamuchlowerratio than observedintheSun.Ifourolderclusterstartedoutwith medium isabout43(HawkinsandJura1987),whileinthe explain thelowratiosinoldgiants.Theproblemwiththis theory, however,isthattheC/ratiosininterstellar clusters thatexhibitthelowerratios. values fortheinitialcomposition,itshouldbeyounger solar systemitisabout90.Hence,ifindeedtherearangeof giant branchtheconvectiveenvelopeexpandsinward,it not top 2%-3%,approximately,bymassofthestar.Whenon the the extentofconvectivemixinginastar.Liisdestroyed at ports Liandotherlightelementsfromthesurfacetointe- temperatures around2x10Kandhencecanexistonlyin the factor of28while,ina3or5Mstar,thisreduction is rior thusreducingthesurfaceabundanceoftheseelements. only bringsupprocessedmaterialfromtheinteriorbuttrans- dance attheendoffirstdredge-upphasedecreasesby a Iben (1966,1967)predictsthatina1Mstar,theLiabun- was about±0.2.Wehaveincluded theresultsofPilachowski, about 60fromitsmain-sequencevalue. chowski (1986)forNGC7789. Thefollowingcanbeseenin are showninFigure13.ThemeanerrortheLiabundance dance andtheturn-offmass. this figure: Saha, andHobbs(1988)for M67giantsandthoseofPila- 0 0 Dearborn, Eggleton,andSchramm(1976)alsosuggestthat The Liabundanceinastarprovidesanimportantcheckon The mainresultsfortheLiabundanceinopenclustergiants 1. Norealcorrelationisvisible betweentheclusterLiabun- c) LithiumAbundance Vol. 347 198 9ApJ. . .347. .835G 1213 12 Claria, J.J.,andRosenzweig,P.1978, A.J.,83,278. Cannon, R.D.1970,M.N.R.A.S.,150, 111. Alcaino, G.1967,Ap.J.,147,112. Brown, J.A.1987,Ap.J.,317,701. Boesgaard, A.M.1976,Pub.A.S.P.,88, 353. Bienaymé, O.,Maeder,A.,andSchatzman, E.1984,Astr.Ap.,131,316. Bell, R.A.,Ericksson,K.,Gustafsson, B.,andNordlund,A.1976,Astr.Ap. Barbaro, G.,Dallaporta,N.,andFabris, G.1969,Ap.Space.Sei.,3,123. Baglin, A.,Morel,P.J.,andSchatzmann,E.1985,Astr.Ap.,149,309. mechanism causedtheLidepletiondidnotdecrease the each oftheseclusters,andbothstarsaresupergiants. isotope ratio.Rapidrotationonthemainsequencecouldhave Their C/ratioshowever,arehighandhencewhatever ments likeCbyasignificantamount. main sequenceordilutionofLifromtheinside.Massloss abundances asseeninFigure13.Thereisonlyonemember in the surfaceLiwithoutchangingabundancesofother ele- intermediate-mass starswereleftwithathinnerlayeroflithium any magnitudeonthegiant-branchascentwouldthendeplete top 2%-3%.Thiscouldoccureitherthroughmasslossonthe on thesurface,i.e.,Liexistedintop1%only,insteadof turn-off andthepresentevolutionarystageofgiant. depletion hasbeenobservedintheseintermediate-massstars, lower thantheoreticallypredicted.Sincenomain-sequence masses >2Malsoshowanabundancethatisabout3times main-sequence starsandtheLiabundancedecreaseswith Perhaps attheendofmain-sequencephase,these Li destructionmusthaveoccurredbetweenmain-sequence due todepletiononthemainsequence,butstarswith tion atthelow-massendcanthereforebeexplainedasbeing at theendofmainsequence.InFigure13excessdeple- stellar mass,withstars>2Mshowingvirtuallynodepletion ing thatexcessdepletionofLihasoccurredatsomepointin the star’slifetime. theoretically predictedbyIben(1966,1967),(solidline),indicat- left-hand topcorner. discussed inthetext.ErrorsLiabundanceisshownbybarat clusters NGC2451and1545havingabnormallylowLiabundanceare function oftheturn-offmass.Thesolidlinerepresentstheoreticalpredictions of Iben(1967)forgiantsattheendfirstdredge-upphase.Thetwo 0 0 No. 2,1989 Suppl.,23,37. The clustersNGC2451and1545exhibitverylow Li Boesgaard (1976)hasshownthatLidepletiondoesoccurin 2. AlltheclustersexhibitLiabundancesthatarelowerthan Fig. 13.—PlotoftheaverageLiabundancesinopenclustersasa © American Astronomical Society M/M 0 OPEN CLUSTERGIANTS REFERENCES Provided bythe NASA Astrophysics Data System 2 13 1213 Duncan, D.K.1981,Ap.J.,248,651. Deuprée, R.G.,andWallace,K.1987, Ap.J.,317,724. Dearborn, D.S.P.,Tinsley,B.M.,and Schramm,D.N.Í978,Ap.J.,223,557. Dearborn, D.S.P.,Lambert,L.,and Tomkin,J.1975,Ap.J.,200,675. Dearborn, D.S.P.,Eggleton,P.and Schramm,D.N.1976,Ap.J.,203,455. Dearborn, D.S.P.1989,preprint. Davis, S.P.,andPhillips,J.G.1963, TheRedSystem(AU-X£)oftheCN Cox, A.N1954ApJ119188 Cottrell, P.L.,andSneden,C.1986,Astr.Ap.,161,314. isotope ratiosintheoldclusters. better insighttotheprocesscausinganomalously low ratios andLiabundancesinclustersubgiantswouldgiveusa with theturn-offmass.Observationsofcarbonisotope could nothavecausedthelowisotoperatios. isotope ratiosthatagreewiththoseforclumpgiantsandsince proved veryuseful. cussions withChrisSneden,CatyPilachowski,David the first-ascentgiantsarepre-coreflashstars,thismechanism back withHecoreflashtheoryisthatfirst-ascentgiantsexhibit Dearborn andAlanSweigartforunpublishedresults that undergo farmoremixingthana2-5M©star.Themajordraw- longer thanthatin2-5M©stars.Hence,a1starwould either onthemainsequenceorgiantbranch,then Lambert, JohnScalo,andDonVandenBergthanksDavid amount oftimeithastooperateina1Mstarisconsiderably explain thetrendseenincarbonisotoperatioswithmass.If there existsamixingmechanismthatoccursatconstantrate sequence lifetimeorduringthegiant-branchphasemay nated. MixingofCN-processedelementsduringmain- mixing eitheronthemainsequenceorgiantbranch. the differencesininitialcompositiontheorycanbesafelyelimi- Based ontheavailableinformation,mass-losstheoryand by thelow-massstarsmayhaveoriginatedasaresultofexcess higher massstars.Theabnormallylowisotoperatiosexhibited than about2.2Mandareindependentofstellarmassfor strong correlationwiththeclusterturn-offmassforstarsless theory. dances inclustersubgiantsisnecessarytoconfirmorrejectany by Duncan(1981).However,moreinformationontheseabun- the Hyadesmain-sequencestars,andfieldstarsobserved predicted Liabundancesthatresultfromthismechanism,and interior totheenvelopealsodilutessurfaceLi.Theyhave their resultsagreewiththeabundancesobservedinSun, main sequence.ThesameprocessthatbringsCfromthe giant branch,butitcanalsoaccountforLidepletiononthe can notonlyexplainthelowisotoperatiosobservedon 0 mann (1977)andBaglin,Morel,Schatzmann(1985) cantly. suggest thatturbulentdiffusionmixingonthemainsequence to haveundergoneLidepletiononthemainsequence.Schatz- main sequenceoronthegiantbranchcouldconceivablyalter the LiabundancewithoutaffectingC/ratiosignifi- without alteringthecarbonisotoperatio.Masslosson caused thisexcessdepletionalthoughitisdifficulttodo 0 Molecule (Berkeley:UniversityofCalifornia Press). 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