1997AJ 114.2556T THE ASTRONOMICALJOURNAL particularly theolderclusters,spurredoninrecentyearsby the needtostudyanextendedregionofgalaxyobtain metal content,andageproducedbythecollectivestellar in theimprovementaccuracyfordistancedetermination, ters haveproventobevaluablebutlimited.Theirvaluelies Adler (1982),Friel(1989),Janes&Phelps(1994), the analysisofitscompositepropertiesasdetailedinJanes& the availabilityofCCDcamerasontelescopesmodest there hasbeenincreasinginterestintheclusterpopulation, age restrictionsimposedprimarilybytidaldisruption,and understanding oftheclustersamplehaschangedlittlein a statisticallysignificantsample.Despitetheselimitations, arisen becauseoftheiroftensparsepopulationmembers, sample whichsharestheseproperties.Thelimitationshave in [Fe/H]existsamongclusters olderthan1Gyr,asfirst last 15years.Amongopenclustersthereisnoevidencefor et al.(1994),Carraro&ChiosiFriel(1995),and detailed inHirshfeldetal.(1978). WJiencombinedwiththe an age-metallicityrelation(AMR), thoughasignificantrange Scott etal.(1995),amongothers. size. Thesamplegrowthhasledtoacomparablein 2556 Astron.J.114 (6),December1997 0004-6256/97/114(6)/2556/30/$10.00 ©1997Am.Astron. Soc.2556 seminal studyoftheclustergalactic abundancegradientby As probesofgalacticstructureandevolution,openclus- Despite theimprovements,therearetwoareaswhereour © American Astronomical Society • Provided by theNASA Astrophysics Data System A setof76openclusterswithabundancesbaseduponDDOphotometryand/ormoderatedispersion by thedifferencesinchemicalhistoryoneithersideofdiscontinuityhasbeencarriedthroughto 4.6 Gyrago.Itissuggestedthatthediscontinuityareflectionofedgeinitialgalacticdiskas results implythat,contrarytoearlierclaims,thesunisnotatypicalofstarsformedinsolarcircle metallicity rangeisfoundtobeapproximatelyhalfthatofthefieldstardistribution.Whencoupledwith clusters interiorto10kpcasarepresentativesampleofthegalacticdiskoverlast7Gyr,cluster correcting forthediscontinuity,noevidenceisfoundagradientperpendiculartoplane.Adopting over thisdistancerange.BeyondR=10kpc,themetallicitydistributionhasadispersionbetween0.10 described bytwodistinctzones.BetweenRqq—6.5and10kpc,themetallicitydistributionhasamean evolution ofthegalacticdisk.Themetallicitydistributionclusterswithgalactocentricdistanceisbest spectroscopy hasbeentransformedtoacommonmetallicityscaleandusedstudythelocalstructure discontinuity inthegalactocentricgradient,discrepancymetallicitydistributionisinterpretedas and 0.15dex,butwithamean[Fe/H]=-0.3,implyingsharpdiscontinuityat/?=10kpc.After [Fe/H]=0.0 andadispersionof0.1dex;thereis,atbest,weakevidenceforshallowabundancegradient could maketheseparationoffieldstarsonbasisgalactocentricorigindifficult,ifnotimpossible. current stageofgalacticevolution.Ifcorrect,diffusioncoupledwiththeabsenceanabundancegradient defined bythediskglobularclustersystemandso-calledthickdisk;initialoffsetin[Fe/H]created an indicationofsignificantdiffusionfieldstarsintothesolarneighborhoodfrombeyond10kpc.These © 1997TheAmericanAstronomicalSociety.[80004-6256(97)02912-9] GC GC SOME REVISEDOBSERVATIONALCONSTRAINTSONTHEFORMATIONANDEVOLUTION Electronic mail:[email protected],[email protected],[email protected] Bruce A.Twarog,KeithM.Ashman,andBarbaraJ.Anthony-Twarog 1. INTRODUCTION Department ofPhysicsandAstronomy,UniversityKansas,Lawrence,Kansas66045-2151 Received 1997July7;revisedSeptemberaccepted9 OF THEGALACTICDISK VOLUME 114,NUMBER6 ABSTRACT = than age.Thoughsubsequentestimatesfortheclusterabun- plays amorecriticalroleindefiningcluster’sproperties Janes (1979),itisapparentthatpositionwithinthegalaxy ion byFriel&Janes(1993),supplementedThogersen Z-gradient, whereZreferstothedirectionperpendicular DDO system(Twarog&Anthony-Twarog1996),thephoto- plane, contradictingtheconclusionsofFJ.Theuniquenature lactocentric abundancegradientand,forthefirsttime,an Piatti etal.(1995;hereinafterPGA)toderiveasimilarga- confirmed manytimessince,inparticularlyconclusivefash- galactic diskismetal-poorrelativetotheinnerhasbeen original Janes(1979)value,thebasicresultthatouter dance gradienthaveoftenbeensomewhatsteeperthanthe the plane.Wefindthatcommonlyacceptedpictureofa metric systemadoptedbyPCA,suggestedtheneedfora of thelatterresultcombinedwitharevisedcalibration open clusterabundancegradientperpendiculartothegalactic et al.(1993,hereaftercollectivelyreferredtoasFJ). paper aretoexplainthechanges intheclusteranalysiswhich Instead, thediskbreaksupinto twodistinctpopulationsde- the solarneighborhoodfailsto account fortheobservations. coupled withalargespreadinabundanceatgivenage linear abundancegradientwithgalactocentricdistance(Z?c) closer lookattheclustersampleandrealityof fined byasharpboundaryatRqc 10kpc.Thegoalsofthis G More recentlyanevenlargersamplehasbeenusedby DECEMBER 1997 1997AJ 114.2556T tions ofthisphenomenoncanbefoundinHartwick&Mc- proximately 10%.Oneofthefewdiscussionsandapplica- Femie (1963)hascalculatedthatachangeintheintrinsic main sequencestarwhenobscuredbythesamedustlayer. has beenknownsincetheworkofLindholm(1957)and plicable toalargefraction.Wehavesurveyedtheliterature tion ofclusterreddening.Nosingletechniqueforreddening be unreliable. FJ andFriel(1995)becausemanyoftheclusterscontained We haveexcludedtheunpublishedabundanceslistedinFriel reddening, andabundancecalibrations.Simplemergerofall hood, andtoattemptanexplanationofthesource reality ofthetwoclusterpopulationsandgalacticabun- their relativecolors.Thetypical differenceinE(B—V)lies Final valuesareadoptedindividually forthemainsequence erally, aredgiantexhibitssmallerE{B—V)thanhotter enced byastarisdependentuponthecolorofstar.Gen- tive useofthereddeningdeterminationswehaveincludedan of thepublishedvalues.Inendwehaveadoptedarepre- for eachcluster,attemptingtoassesstherangeandreliability though theDDO—{B—V)techniqueofJanes(1977b)isap- estimation hasbeenappliedtoalltheclustersinsample, cases, theparametersfoundinLyngâ(1987)haveprovento in thesupplementalsamplehaveDDOdataand,key within theclusterpopulationunlessitisdonewithextreme cluster abundancescanwashoutordestroydetailedstructure timates involveanarrayoftechniquesusedbydifferentob- tallicity estimatesexistiswelloverahundred,butsuches- the distance.Giventheseandgalacticposition,onecancon- evolution ofthediskaresummarizedinSec.5. population withthatofthefieldstarsinsolarneighbor- tails fortheindividualclusters may befoundintheAppen- majority ofclusterswhichhave E(B—V)below0.10.De- in the5%to10%range,leading toaminordifferenceforthe mate isbasedonthegiantsorhotterstarsnearturnoff. cluster, wehavetakenintoaccountwhetherornottheesti- Clure (1972).Inderivingthereddeningappropriatefora adjustment whichisoftenneglectedinclusteranalyses.It care. Wewillmakeuseofonlytwosources,theDDO global characteristics.Thenumberofclustersforwhichme- apparent discrepancybetweenthefieldstarsandclusters dance gradient(Sec.3),tocompareandcontrastthecluster 2557 TWAROGETAL.:FORMATIONANDEVOLUTION and thegiantbranch,fixedtoensure consistencybasedupon (B-V) of1.0maglowerstheeffectiveE(B~V)byap- Schmidt-Kaler (1961)thatthedegreeofreddeningexperi- sentative estimateforeachcluster.Tomakethemosteffec- sample describedinPCAandthespectroscopicFJ. are thefoundationofnewsample(Sec.2),toquantify servers withdifferingchoicesforclustermembership, struct aspatialmapoftheclusterpopulationandstudyits (1995) andthesupplementaldataofLyngá(1987)usedby (Sec. 4).Ourconclusionsandaqualitativescenarioforthe The firststepintheclustercompilationisdetermina- Two keyclusterparametersofinterestaremetallicityand © American Astronomical Society • Provided by theNASA Astrophysics Data System 2.1 ReddeningandDistanceEstimation 2. THEDATA have beenused. 49 clusters. tie-in topastwork,thelocationofmainsequencefora tances havebeenderivedviamainsequencefitting.Asa the distance.Forcurrentinvestigation,threeapproaches photometric andthespectroscopic(Friel1997)approaches. metallicity techniquesandoneofthekeyreasonstheywere merger ofabundances.However,onethestrengthsboth rounds anumberofkeyobjects,e.g.,NGC6791. taken fromTable1.AsillustratedinFig.1(a),thereisa ters withwell-definedmainsequencesandredgiantclumps, is notalwayspossible.Mainsequencefittingwasappliedto cooler, unevolvedmainsequencetoderivedistances,butthis younger openclusters.Ideally,onewouldprefertousethe turnoff, Arapidlyincreases,reachingabove10astheturnoff main sequence,Aisbetween5and6.Fromthebaseof modulus is(A—3.!)*£(£-V).Fortheunevolved,cooler make thetruemodulussmaller.Thenetchangein main sequencefitting,twofactorscompete.Increasedred- fect ondistancedependsthetechniqueusedinobtaining of theweaksensitivitytochangesinE(B—V),canhavea is largerthanthishavebeenexcludedfromthediscussion. Clusters forwhichtheuncertaintyinadoptedreddening changes. ForachangeinE(B—V)of0.1mag,thecorre- appear toviolatetheconcernexpressedaboveregarding those listedinPCAandFJ,thoughcontroversystillsur- dix. Inamajorityofthecasesourresultsdifferlittlefrom troversy whichoftensurrounds suchchoices,wehavein- the mostmetal-richclusterin sample,NGC6791.InFig. weak dependenceonmetallicity, predominantlycausedby we havederivedthetypicalMforclumpusingmain adequate methodofestimatingthedistance.Using13clus- it canbeidentified,however,theredgiantclumpprovidesan quence istoofaintorill-definedtopermitreliablefits.If approaches vertical,orforthehottermainsequenceof corrections tothedistancemodulusforincreasedreddening apparent modulusisincreasedbyX*E(B—V).However, If theslopeofmainsequencewith(B—V)isA, dening correctionmovesthemainsequencetowardblue. described inVandenBerg&Poll(1989). chrone setareappliedusingthemetallicitydependenceas ences betweenthecluster[Fe/H]andthatofnearestiso- cluster ofagiven[Fe/H]istakenfromtheVandenBerg significant impactontheclusterdistance.However,ef- sponding changein[Fe/H]isbetween0.10and0.15forthe selected forthissampleistheirweaksensitivitytoreddening adopt anabsoluteagescaleand becomeinvolvedinthecon- sequence fitting.Assumedreddeningsandabundancesare (B —V)of0.65andM=4.$(Twarog&Anthony-Twarog star ofage4.6Gyrwithsolarcompositionhastheadopted stead usedanagerankingbased uponthemorphologicalcmd (1985) isochrones,adjustedtoguaranteethatasolarmass 1989). Differentialcorrectionstothedistanceduediffer- 1(b), thedependenceonage is investigated.Ratherthan v v First, wheneveradequateBVphotometryisavailable,dis- For theeffectofreddeningondistancedeterminationvia Reddening values,thoughnotcriticalto[Fe/H]because The grab-bagapproachtoreddeningmay,atfirstglance, Second, forsomeolder(>1.0Gyr)clusters,themainse- 2557 1997AJ 114.2556T 2558 TWAROGETAL.:FORMATIONANDEVOLUTION NGC 2112 NGC 2099 NGC 1817 NGC 1662 NGC 1545 NGC 1193 NGC 752 IC 166 NGC 188 TOM 2 NGC 2204 NGC 2168 BE 21 BE 21 NGC 2141 HYADES NGC 2420 NGC 2360 NGC 2335 NGC 2301 NGC 2287 NGC 2251 NGC 2243 NGC 2158 KING 8 Cluster PI 4 NGC 2548 MEL 66 NGC 2506 NGC 2489 NGC 2482 NGC 2477 NGC 2437 NGC 2423 RUP 18 BE 39 NGC 3960 NGC 3680 NGC 3114 NGC 2972 NGC 2682 NGC 2660 NGC 2632 NGC 2567 NGC 2547 NGC 2546 NGC 2539 NGC 2527 NGC 2516 HAF 8 NGC 5316 NGC 5138 LO 807 HAR 5 NGC 4349 IC 2714 NGC 3532 © American Astronomical Society • Provided by theNASA Astrophysics Data System 205.92 212.55 232.90 225.95 239.94 229.80 223.47 223.62 231.10 203.59 239.50 227.93 233.73 273.94 230.57 246.71 241.63 253.60 231.87 230.48 227.52 299.96 299.76 294.42 292.44 286.77 289.64 283.34 274.73 215.58 265.86 262.74 205.91 249.81 264.45 254.91 246.13 259.61 177.65 187.70 146.80 130.07 122.78 310.23 307.55 306.79 186.84 186.84 198.08 176.40 186.13 180.05 153.36 137.18 186.58 186.64 198.11 -12.18 -12.60 -13.13 -21.11 -22.40 -23.35 -17.97 -16.07 -10.24 -15.88 -14.29 -2.51 -2.51 -5.79 -0.19 -1.42 -6.80 -4.92 -1.26 -1.98 -0.78 -5.80 -2.37 -8.53 -3.83 -3.03 -1.76 22.47 31.72 32.48 19.65 10.09 15.38 11.13 16.93 0.17 3.09 4.07 0.29 0.13 2.19 3.12 9.91 2.98 3.54 0.12 0.20 0.81 6.17 3.55 1.34 1.79 1.89 1.96 1.78 1.97 1.46 0.96 0.75 0.63 0.29 0.28 0.32 0.00 0.36 0.20 0.04 0.86 0.10 0.45 0.21 0.10 0.40 0.04 0.04 0.22 0.32 0.06 0.09 0.21 0.44 0.35 0.55 0.35 0.22 0.26 0.21 0.36 0.31 0.38 0.05 0.04 0.09 0.36 0.04 0.38 0.03 0.00 0.13 0.05 0.06 0.16 0.10 0.10 0.11 0.16 0.05 0.10 0.25 0.10 0.40 0.05 0.68 0.12 Table 1.Clusterabundanceparameters. E{B-V) n 0.70 0.27 0.26 0.29 0.33 0.19 0.04 0.80 0.09 0.30 0.06 0.08 0.40 0.90 0.33 0.60 0.51 0.00 0.04 0.20 0.19 0.09 0.11 0.36 0.04 0.03 0.05 0.09 0.14 0.05 0.41 0.09 0.20 0.09 0.36 0.05 0.62 0.29 0.05 0.08 0.34 0.04 0.35 0.00 0.12 0.06 0.14 0.09 0.10 0.31 0.20 0.24 0.19 0.33 0.35 0.04 var [Fe/H] Code DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DTR DTR DSP DSP DSP DSP DSP DSP DSP SPE SPE DSP DSP SPE SPE SPE SPE SPE DSP SPE SPE SPE SPE -0.026 -0.835 -0.268 -0.095 -0.060 -0.293 -0.088 -0.160 -0.178 -0.046 -0.338 -0.238 -0.535 -0.262 -0.110 -0.460 -0.080 -0.360 -0.440 -0.480 -0.160 -0.266 -0.150 -0.154 -0.030 -0.368 -0.281 -0.010 -0.177 -0.055 -0.080 -0.354 -0.349 -0.376 -0.170 -0.121 -0.073 -0.181 -0.030 -0.160 -0.130 -0.090 -0.060 -0.175 -0.011 -0.097 -0.022 [Fe/H] 0.089 0.175 0.060 0.040 0.080 0.019 0.059 0.143 0.060 0.080 0.120 0.137 0.060 0.120 0.022 0.002 0.000 0.142 0.128 0.120 ^[Fe/H] 0.120 0.101 0.101 0.090 0.146 0.040 0.015 0.090 0.178 0.071 0.040 0.204 0.098 0.060 0.130 0.164 0.160 0.250 0.090 0.064 0.048 0.095 0.034 0.074 0.098 0.113 0.090 0.060 0.022 0.020 0.130 0.062 0.128 0.145 0.108 0.010 0.026 0.115 0.090 0.092 0.040 0.073 0.090 0.079 0.131 0.051 0.070 0.021 0.086 0.092 0.118 0.095 0.069 0.120 0.090 0.090 0.030 0.087 0.127 0.050 0.090 0.090 0.123 0.103 0.116 0.062 0.088 Num. 25 38 16 10 11 19 10 15 11 11 10 11 12 4 4 4 4 4 4 4 5 5 2 2 6 2 5 5 5 8 7 2 3 4 2 7 2 4 6 9 5 5 2 2 3 2 6 2 7 5 3 3 2 6 8 5 3 1 1 1 1 1 1 1 1 1 1 s.e.m. 0.015 0.118 0.040 0.130 0.120 0.032 0.058 0.058 0.024 0.090 0.073 0.095 0.023 0.020 0.090 0.103 0.018 0.018 0.032 0.012 0.042 0.052 0.080 0.090 0.036 0.090 0.060 0.028 0.040 0.059 0.054 0.010 0.047 0.090 0.040 0.017 0.024 0.090 0.026 0.032 0.015 0.014 0.059 0.095 0.040 0.120 0.014 0.090 0.130 0.090 0.030 0.029 0.019 0.065 0.090 0.033 0.053 0.019 0.019 0.044 0.035 0.019 0.073 0.050 0.090 0.055 0.041 2558 1997AJ 114.2556T My. reddest orbluestpoint.FromMEL66toNGC6791(ap- have taggedafewoftheextremeclusters.Theyoungest reddening dependenceonstellarcolor.Toallowreadersto removing themetallicitysensitivityofcmdmorphology. both parametersenhancestheagesensitivitywhilepartially red giantclumpandthebrightestpointatturnoffto parameter, MAR,asdefinedinAnthony-Twarog&Twarog proximately 5Gyrto9Gyr),theMdeclinesfrom0.61.2 is typicaloftheaveragestarinclump,ratherthan the exactdefinitionofclump.Wehaveattemptedto mately 1to5Gyr),themeanMis0.6±0.1;scatter M67, MEL66,NGC188,BE39,and6791. place thetrendinFig.1(b)ontheirpreferredagescale,we The ratioshouldalsobereddening-independent,butthisas- the clumpandbluestpointofturnoff.Combining color differencebetweentheredgiantbranchatlevelof The MARisaratioofthemagnitudedifferencebetween clusters, thoughitdoesappearlikelycomparingFigs.1(a) the effectsofageandmetallicityonMamongoldest for NGC6791.Thefactthat6791hasboththelargest choose thelatterparameterbaseduponmagnitudewhich explained bytheuncertaintiesindistancemoduliand objects areNGC7789and3680,whiletheoldest 2559 TWAROGETAL.:FORMATIONANDEVOLUTION Appendix. Wenoteinclosing that Janes&Phelps(1994) 0.7 and1.0.Thespecificsforeach clustercanbefoundinthe will adoptM=0.6fortheclump. Formetal-richand/or and 1(b)thatageisthepredominanteffectinincrease age andthelargest[Fe/H]makesitimpossibletodecouple sumption failsforclusterswithlargeE(B—V)duetothe significantly olderclusters,we willadoptavaluebetween (1985) andrevisedinTwarog&Anthony-Twarog(1989). v v v v NGC 7789 NGC 7209 NGC 7142 NGC 6940 NGC 6939 NGC 6819 NGC 6791 IC 4756 NGC 6705 NGC 6633 NGC 6494 NGC 6475 NGC 6425 NGC 6281 NGC 6259 NGC 6208 IC 4651 NGC 6134 NGC 6067 NGC 5822 For mostclustersintheagerange of1Gyrto5Gyr,we Cluster Over theagerangefromNGC7789toMEL66(approxi- © American Astronomical Society • Provided by theNASA Astrophysics Data System 355.86 357.97 347.81 341.98 333.69 340.07 334.92 329.76 321.71 115.49 105.42 95.98 73.98 70.01 95.51 69.90 36.39 27.30 36.09 9.84 -2.78 -4.53 -1.59 -1.52 -5.83 -7.88 -0.19 -2.20 -5.36 -7.34 -7.16 12.30 10.96 9.45 5.26 2.86 2.00 8.28 3.58 8.48 E{B-V) n 0.30 0.17 0.43 0.20 0.49 0.30 0.16 0.23 0.44 0.17 0.39 0.04 0.29 0.15 0.68 0.35 0.12 0.39 0.35 0.15 Table 1.(continued) E{B-V) n 0.27 0.15 0.40 0.45 0.28 0.15 0.40 0.16 0.36 0.04 0.27 0.14 0.61 0.32 0.11 0.35 0.31 0.14 var var with thepoints. metallicity and(b)age.For(b),cluster identificationsarealignedvertically Fig. 1.Theabsolutemagnitudeofthe red giantclumpasafunctionof(a) (a) [Fe/H] Code DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DDO DTR DTR DTR DTR DSP DSP DSP DSP SPE SPE SPE SPE SPE -0.083 -0.089 -0.060 -0.012 -0.028 -0.020 [Fe/H] 0.070 0.040 0.013 0.074 0.150 0.000 0.070 0.070 0.005 0.020 0.000 0.095 0.182 0.138 0.026 0.094 0.136 0.090 [Fe/H] V [Fe/H] 0.120 0.152 0.050 0.058 0.135 0.112 0.123 0.161 0.141 0.102 0.072 0.086 0.050 0.117 0.090 0.090 0.095 0.090 0.060 0.053 0.087 0.064 0.090 0.086 Num. 20 35 11 13 15 13 11 10 17 14 4 4 7 2 6 6 6 2 2 5 3 3 1 1 s.e.m. 0.021 0.035 0.050 0.018 0.060 0.065 0.035 0.041 0.028 0.032 0.039 0.035 0.064 0.090 0.090 0.095 0.090 0.042 0.029 0.022 0.023 0.058 0.017 0.032 2559 1997AJ 114.2556T , M calibrationusingintermediateorbroad-bandphotometry E(B —V)growslarger. brightness oftheclump.ChangingE(B—V)alterstrue the abundancesforclustersdiscussedinFigs.1(a)and by Janes&Phelps(1994)isacompositefromwidevariety The differenceiseasilyexplained.clustersampleused have attemptedasimilarestimateoftheclumpluminosity, projects intoasmallerchangeingalactocentricdistance,for based distances.Fortheweakermainsequencedataand trivial. Evenforagivenapproach,notalldistancesderived this impliesadistancedeterminationtiedtophotometric modulus byA=3.1*£(Z?—V),makingitsmalleras to afixedM,theapparentmodulusisby tances for4clusters. larger [Fe/H]leadstoamodulusforgivenmain- the clusters,particularlythoseingalacticanticenter.A isochrone fitsfromdifferenttheoreticalmodels,isaprimary normalization amongthevariousobservers,particularlyin licity, andtheadoptedmainsequenceforfit.Thelackof 2560 TWAROGETAL:FORMATIONANDEVOLUTION photometric onacommonsystem, DDO,andonespectro- based upononetechniqueand obtainedbyoneobserver. certainty willhavelittleimpactonourconclusions. clusters within2kpcofthesundoublingestimatedun- change intheabsolutedistanceanderror uncertainty inthedistancemodulusproducesapercentage metric, techniques,theuncertaintiesarecloserto±0.3mag, majority ofthedistancesbaseduponother,primarilyphoto- estimated toprovidemoduliwithanuncertaintybetween quence fitting,inadifferentialsense,thebestclusterdataare dependent uponuncertaintiesinthemoduli.Formainse- moduli, theconclusionsofinvestigationareonlyweakly way outandusedthesameuncertainty,±0.2,forall gam ofdatasources.Thoughwehavetakenthecoward’s with thatmethodwillhaveequalerrorswhenusinganamal- estimation oftheuncertaintiesindistancemoduliisnot tem. Thisapproachwasadoptedfor23clusters. changing thereddeningisuniquetoeachphotometricsys- of thegiants,asinDDO,ormainsequencestars, technique availableintheliteratureisused.Inmostcases, clump matchareplausiblemeansofderiving(m—M),any of sources.Distancemodulidependuponreddening,metal- deriving M=0.90±0.40from23clustersolderthan1Gyr. techniques, itisstillimportantthat theabundancesfromeach and maybelargerinextremecases.However,becausethe uvbyYiß. Theexactimpactonthedistanceestimateof sequence fit.Theredgiantclumpwasusedtoderivedis- source ofboththelargescatterandfainterM.Second, Since thisisimpossiblewemake useoftwosamples,one scopic butobservedbyonlyone group.Evengivenonlytwo 1(b) aregenerallyhigherthanfoundinpastdiscussionsof ±0.1 and±0.2mag;thesamenumberappliestoclump- v y y v y Third, ifneithermainsequencefittingnoraredgiant For clusterdistancesderivedbyfittingtheredgiantclump As apreludetothediscussioninSec.3,wepointoutthat Ideally, onewouldpreferto discuss onlyabundances © American Astronomical Society • Provided by theNASA Astrophysics Data System 2.2 MetallicityEstimation terms ofACN,thetechniqueappliedtoopenclustersin the slopeandzero-point.Piattietal.(1993)revised the DDOsystemfordiskgiantswasfirstproposedbyJanes the metallicityscaleandmeansofstandardizingre- revised calibrationisapplicablefrom[Fe/H]=+0.25to photometric errorsaside,tolessthan±0.05forthebright be emphasizedthattheTaylor(1991)andTwarog& 438 giants,Twarog&Anthony-Twarog(1996)confirmed traditional SCNindexto[Fe/H]requiredbothaslopeand tem. Taylor(1991)concludedthatthetransformationof the comprehensiveapproachofTaylor(1991),usingover dances isthelackofconsistencyamongsourcesfor ing oftheearlierrecalibrationstiedtospectroscopicabun- PC A.Beyondthemodestsamplesize,primaryshortcom- G andKgiantstoredefinetheDDOmetallicitycalibrationin definition oftheCN-index,usinganexpandedsample82 changes inthezero-pointhavebeenmadebyDemingetal are onacommonscale.Thefollowingsectionswilldefine tallicity estimateforclusterswith partialinformationplaces nary classifications.Inclusionof thesestarsinthemeanme- clusters lackdefinitivemembership informationandnobi- Appendix. Thereasonforthis is thatalargefractionofthe Definite non-memberswereeliminatedfromthesample,but reddening, radial-velocitydata,andproper-motionstudies. of theliterature.Ananalogousapproachwasappliedto tometry wascollectedusingthereferencescitedbyPCA, Anthony-Twarog 1996). extrapolated. TheSCNindexincreasinglyoverestimatesthe internally consistentspectroscopicsampledoeshalvethe Anthony-Twarog (1996)calibrations,exceptforasmall the colordependenceofcalibrationzero-point,butfound field giantsreducedandanalyzedinauniformway.From tied solelytothedataofMeWilliam(1990),asample671 log ofspectroscopicabundances,theDDOcalibrationwas Anthony-Twarog (1996).Ratherthanuseacompositecata- zero-point whichweretemperature-dependent. 300 fieldgiantsadjustedtoawell-defined,standardizedsys- spectroscopic abundances.Thisweaknesswascorrectedin sults. all theclustersonanequalfooting. Moreover,itwasfound were starsclassedasbinaries,unless otherwisenotedinthe and expandedwheneverpossiblebydoinganupdatedsearch abundance ofmoremetal-deficientgiants(Twarog& an expandedsampleshowsthatthecalibrationcannotbe [Fe/H] =+0.12,giveverysimilarresults.Theuseofamore zero-point shifttoapproximateaHyadesabundanceof a constantandshallowerslopethanJanes(1975).Itshould (1977) andTwarog(1981),whileLuck(1991)revisedboth (1975) basedupon44stars.Sincethattime,suggested stars withnomembershipinformationwereincluded,as star photometricsampleofMcClure&Forrester(1981).The scatter intheresidualsattributabletoDDOcalibration, (1990) isinadequatetoallowareliablecalibration,butuseof —0.5. Atlowerabundances,thesampleofMcWilliam The commonlyadoptedcalibrationofthe£CNindex For eachclusterinthefinalsampleofPCA,DDOpho- The calibrationadoptedhereisdetailedinTwarog& 2.2.1 DDOphotometry 2560 1997AJ 114.2556T the revisedcalibrationusing followingrelations: be acommonoffsetofabout0.1dexbetweenthetwome- that foundontherevisedscale. BaseduponFig.2,wecan ity rangeamongclustersonthe scaleofPCAislargerthan ear trendwithincreasing[Fe/H], implyingthatthemetallic- higher metallicity,theresidualsshowanapproximatelylin- tallicity scales,withthatofPCAbeingmoremetal-poor.At circles). Thescatteratagiven[Fe/H]isencouraginglysmall parison wehaveadoptedthesameE(B—V)asPCAforeach isolate theimpactofdifferentcalibrations,forthiscom- cussed aboveandcomparedwiththatlistedbyPCA.To metallicity hasbeencalculatedusingthecalibrationdis- in thefollowingmanner.Forclusterswherenumberof based exclusivelyuponunpublishedphotometry.Ratherthan radial-velocity dataindecidingmembership. problem iscompoundedbytheinclusionofunpublished ber ofstarsincludedintheclusteraverage.Thesedifferences transform anabundancesofPCA totheapproximatescaleof tions asafunctionofthecolorgiant.Forclusterswith are plottedinFig.2asafunctionofthe[Fe/H]PCA(open cluster. Theresidualsin[Fe/H]thesense(PCA-TAT) difference existsinthesampleofgiantsincluded,DDO cluded havelittleimpactontheclustermeanorwhereno giants islargeenoughthatsmalldifferencesinthetotalin- drop theseclustersfromthesample,wehaveattempteda we havenomeansofmakinganexactcomparison.This membership classification.BecausePCAdoesnotprovide the twotechniques,butoftenarisebecauseofdifferencesin those ofPCAwilloccasionallyshowdifferencesinthenum- upon themeanabundanceordispersionforacluster. that exclusionofbinariesrarelyhadasignificantimpact data ofFJhavebeenshiftedby+0.05in[Fe/H]toplacethemonthePCA [Fe/H] below—0.2onthesystemofPCA,thereappearsto and isprimarilycausedbysmalldifferencesinthecalibra- are sometimesattributabletodifferentcalibrationlimitsfor those ofPCA(opencircles)andFJ(crosses),inthesense(REF-TAT).The Fig. 2.MetallicitydifferencesbetweentherevisedDDOabundancesand simple transformationofthePCAabundancestooursystem specific detailsonwhichstarsineachclusterareexcluded, scale. 2561 TWAROGETAL:FORMATIONANDEVOLUTION In afewextremecases,theclusterabundancesinPCAare [Fe/H]p^ -0.15,[Fe/H]= [Fe/H]pA+0.09, Comparison oftheresultsforindividualclusterswith CATAT C © American Astronomical Society • Provided by theNASA Astrophysics Data System -.8 -.6-.4-.20.2.4 [Fe/HjpcA Table 1.Columns1through3 give theclusteridentification metallicity isconstantandwell-determined;itexhibitsno below —0.3,theabundanceisbaseduponeitherspectro- been appliedindividuallytoeachgiantinFJ,andtheresults Within theuncertainties,residualsfollowtrendcon- DDO dataalone. NGC 2204at[Fe/H]=—0.34,wellabovethesensitivity evidence forchangingsensitivityovertherangeofinterest. ing thedispersion.Foreveryclusterexceptonewith[Fe/H] mean oftheabundancedistributionandartificiallydecreas- for thespectroscopicandphotometricapproachesmergedto the mean[Fe/H]estimateforeachclusterofFJhasbeen mon E(B—V)andashiftof+0.05in[Fe/H]toplacethem FJ agreewellovertherangein[Fe/H]exceptforasmall techniques ofPCAandFJmakeusemanythesame cluster hasbeenreviewedtoeliminatenon-membersandre- the faintnessofgiants. tances, themajorityofwhichhavenoDDOestimatedueto indication ofmembershipviaradialvelocitiesmodestac- cutoff, istheonlymetal-poorclusterwithametallicityfrom dances. AsdiscussedaboveandillustratedinFig.2,theoff- could occurinapurelyDDO-basedsample,shiftingthe analysis. BecauseoftheinsensitivityDDObelow[Fe/H] analysis forthecluster. define themeanandstandarddeviationusedinfinal adjusted inthismanner.Forclusterscommontothetwo vise thereddeningestimatewhenevernecessary.Because with FJ.AstheDDOsample,literatureoneach clusters, arelatedbyproductisinclusionofthelargest curacy. Thoughtheemphasisofsampleisonolderdisk the approachisthatinadditiontometallicity,oneobtainsan last decade;theresultsaresummarizedinFJ.Thebeautyof collected andcalibratedbyFrielhercoworkersoverthe Twarog (1996)hasbeenappliedindividuallytothegiants. other casestheDDOcalibrationofTwarog&Anthony- where theDDOphotometryremainsunpublished.Inall and itsgalacticcoordinates.Columns 4and5listtheadopted set betweentheDDOandspectroscopicscalesatlower scopic dataorthecombinedDDOandspectroscopicabun- scopic sampleremovesonepotentialsourceofbiasfromthe samples, thetransformationbaseduponclustermeanhas sistent withthetransformationdefinedbydataofPCA; [Fe/H] inthesense(FJ-TAT),plottedFig.2ascrosses. on thesamesystemasPCA,onederivesresidualsin zero-point shift.Afteradjustingtheabundancesforacom- field giantsintheircalibrations,theabundancesofPCAand sample ofabundancesforclustersatlargegalactocentricdis- = —0.5,itispossiblethatanapparentmetallicitycutoff The abundanceresultsfortheclusters aresummarizedin It shouldbeemphasizedthattheinclusionofspectro- The currentDDOsamplehasanoverlapof14clusters Moderate-resolution spectraofclustergiantshavebeen These transformationshavebeenusedonlyforclusters [Fe/H]pA> -0.15,[Fe/H]AT=0.55[Fe/H]p+0.2. CTCA 2.3 TheClusterParameters 2.2.2 Spectroscopy 2561 1997AJ 114.2556T plains thesourceofmetallicityestimate:DDOmeans reddening E(B—V)asdefinedbythestarsatcluster Twarog &Anthony-Twarog(1996);DDTmeanstheabun- turnoff andbytheredgiants,respectively.Column6ex- 2562 TWAROGETAL.\FORMATIONANDEVOLUTION DSP. Forthisline,theabundancelistedisaverageabun- the relationsdiscussedabove;andSPEimpliesspectro- dance ofPCAadjustedforreddeningandtransformedusing tween thestars.Forclusterswithonlyonegiant,errors mean havebeensettoone-halfthedifferencein[Fe/H]be- giants, thestandarddeviationanderrorof error ofthemeanfor[Fe/H].Forclusterswithonlytwo the numberofstarsincludedinaverage,andstandard column 7,followedbythestandarddeviationofsample, In thediscussionswhichfollow,thisissourceof dance usingtheDDOsampleandSPEtogether. abundances, anadditionallineisincludedandtaggedwith above. Forthe14clustersthathavebothDDOandSPE scopic abundancesofFJ,adjustedforreddening,shiftedby [Fe/H] fromtheredgiantsbaseduponcalibrationof the readerisreferredtoAppendix.Column5lists upon theturnoffstars;RGCisassumedredgiantclump main sequencefittingwiththereddeningbasedprimarily tifies themeansofestimatingdistancemodulus:MSFis give theidentification,mean[Fe/H],andstandard galactic propertiesoftheclustersystem.Columns1,2,and3 quoted aretheaveragestandarddeviationin[Fe/H]fora is 8.5kpc. tified, andMfortheclump,derivedifMSFislistedbut with thereddeningbasedprimarilyupongiants;and error ofthemeanfromTable1,respectively.Column4iden- or moregiants. [Fe/H] adoptedfortheseclusters.Theispresentedin OTH iswhateveradditionalmethodavailable.Fordetails, single starbaseduponclusterswithDDOabundancesfrom3 points joinedbyalinearethe limiting valuesforBE21as bined spectroscopicandphotometricmethods.Thetwo transformed directlyfromthesystemofPCAtorevised galactic plane,respectively,onascalewhereRforthesun distances inkpcfromthesun,galacticcenter,and adopted apparentmagnitudeoftheclump,ifitcanbeiden- troscopic data,andfilledtrianglesareabundancesfromcom- of individualstars,squaresareDDOclusterabundances open circlesrepresentrederivedabundancesfromDDOdata and inFig.3(b)withouterrorbarsforclarity.In assumed ifRGCispresent.Columns8,9,and10listthe apparent modulus,followedincolumns6and7bythe this isprobablyanoverestimate forclusterswhosedistances errors ofthemean.Theerrorbars inRhavebeenderived discussed intheAppendix.Errors in[Fe/H]arethestandard scale, opentrianglesareclusterswithabundancesfromspec- +0.05 in[Fe/H],andtransformedusingtherelationscited assuming thattheuncertaintyin theapparentmodulusis same forallclusters,±0.2mag. Thoughonarelativescale v GC GC In Table2onecanfindtheinformationrelatingto The datainTable2areplottedFig.3(a)witherrorbars 3. THECLUSTERABUNDANCEPATTERNWITHPOSITION © American Astronomical Society • Provided by theNASA Astrophysics Data System 3.1 TheGalactocentricAbundanceGradient - 1 -1 -1 by determiningthegalactocentricdistanceoverrangein remaining clusters.Again,forspecificclustersthereaderis red giantclump,itislikelytobeanunderestimateforthe referred totheAppendix.Theuncertaintyinisderived licity doesoccuroverthegalactocentricrangefrom6.5kpc the directionofgalacticcenterandanticenter. Moreover, theerrorinRisminimizedforclusterswith are baseduponeithermainsequencefittingortheMof that thegalacticdiskcontainsarelativelyabruptdiscontinu- they arelocatedexclusivelybeyond10kpc,whilenot that thechangeislinear.Notonlydoclustersbelow to 15kpc.Whatisnotclearthevalidityofassumption crease withincreasingdistancefromthesunforclustersin galactic longitudenear90°and270°.Thus,theerrorsin- gion. Ratherthanalineartransitionwithdistance,itappears (m —M).Clearly,thecloserclusteristosun, high [Fe/H].TheresultsaresummarizedinTable3. BE 21withthelow[Fe/H],andinclusionof under threecircumstances:exclusionofBE21,inclusion we firstattemptthetraditionalapproachofsimplyfittinga ity near10kpc. a singleclusterwith[Fe/H]>—0.15occupiesthesamere- [Fe/H] =-0.2liepreferentiallyinthegalacticanticenter, smaller theabsoluteerroringalactocentricdistance. pletely. Inallthreecases,theuncertaintyinslopeis ent rangesfrom-0.077kpc\ifthelow[Fe/H]dataforBE line throughthedata.Wehavederivedleast-squaresfit mean in[Fe/H]toenhancetheimpactofclusterswith probability ofcomingfromapurelyrandomsample.Ifwe the correlationcoefficientisreducedto0.22,whichhasa9% yond (14clusters)Rqq^10kpc?Fortheinnergroup,asmall this size(P)iswellbelowonepartintenthousand. dance gradient.Moreover,theprobabilitythatacorrelation 21 areused,to—0.067kpcifBEisexcludedcom- producing asignificantgradientandeventhisvalueismar- ing BE21withthelow[Fe/H]parameterscomescloseto inner group.Ofthethreepossiblecases,onlyoneinclud- more reliableabundances,thegradientweakensinsizeand weight thedatausinginverseofstandarderror gradient persists,butitsstatisticalsignificanceismarginal; groups basedupontheirlocationwithin(62clusters)orbe- coefficient near0.75canarisefromatrulyrandomsampleof lished spectroscopicabundances ofFriel(1995)becausethey limitation, thoughtherangeinRisgreaterthanthatfor derived gradientof—0.023kpccomesfromarandom Fig. 7ofFriel(1995)areinexcellent agreementwiththe ginal. Wepointoutthatwhilewehaveexcludedtheunpub- sample. Fortheoutergroup,smallsamplesizeisasevere included. Onlythemostdistant cluster,BE20,fallsjust above interpretationandwould onlyenhancetheresultif as reddeningestimatesanderror bars,thestarredsymbolsin statistical significance;thereisa19%probabilitythatthe are preliminaryinnatureandlack fundamentaldetailssuch ±0.008, smallerthananypreviousclusterstudyoftheabun- G v R G It isclearfromFig.3thatasignificantchangeinmetal- To begintheevaluationofsignificancethisfeature, What happensifweartificiallybreakthesampleintotwo In thefirstthreecases,slopeofabundancegradi- 2562 1997AJ 114.2556T 2563 TWAROGETAL.:FORMATIONANDEVOLUTION NGC 1662 NGC 1545 NGC 1193 NGC 188 NGC 2547 NGC 2539 NGC 2527 NGC 2506 NGC 2482 NGC 2477 NGC 2437 NGC 2423 NGC 2420 NGC 2360 BE 39 NGC 2335 NGC 2301 NGC 2287 NGC 2251 TOM 2 NGC 2243 NGC 2204 NGC 2168 NGC 2158 BE 21 BE 21 NGC 2141 NGC 2112 NGC 2099 NGC 1817 HYADES NGC 752 IC 166 NGC 4349 NGC 3960 NGC 3680 NGC 3532 NGC 3114 NGC 2972 NGC 2682 NGC 2660 PI 4 NGC 2632 NGC 2567 NGC 2548 NGC 2546 NGC 2516 MEL 66 NGC 2489 HAF 8 RUP 18 KING 8 NGC 6208 NGC 6134 NGC 6067 NGC 5822 NGC 5316 NGC 5138 LO 807 HAR 5 IC 2714 Cluster NGC 6475 NGC 6425 NGC 6281 NGC 6259 IC 4651 © American Astronomical Society • Provided by theNASA Astrophysics Data System [Fe/H] -0.11 -0.46 -0.27 -0.09 -0.06 -0.29 -0.09 -0.18 -0.03 -0.34 -0.24 -0.54 -0.83 -0.26 -0.07 -0.03 -0.16 -0.08 -0.35 -0.38 -0.27 -0.01 -0.15 -0.18 -0.03 -0.08 -0.36 -0.44 -0.16 -0.13 -0.09 -0.06 -0.17 -0.01 -0.10 -0.02 -0.18 -0.05 -0.03 0.12 0.14 0.08 0.12 0.02 0.06 0.06 0.06 0.04 0.09 0.17 0.02 0.00 0.14 0.08 0.06 0.14 0.09 0.18 0.14 0.13 0.12 0.07 0.07 0.00 0.02 0.00 s.e.m. 0.05 0.09 0.06 0.01 0.04 0.13 0.05 0.12 0.09 0.03 0.06 0.06 0.02 0.09 0.07 0.09 0.02 0.01 0.02 0.09 0.10 0.02 0.12 0.04 0.04 0.02 0.04 0.04 0.01 0.01 0.06 0.09 0.04 0.12 0.01 0.09 0.13 0.03 0.09 0.03 0.04 0.03 0.01 0.02 0.06 0.04 0.02 0.09 0.03 0.03 0.09 0.03 0.02 0.07 0.05 0.09 0.09 0.05 0.03 0.09 0.09 0.09 0.09 0.04 0.02 0.03 (m —M)Code OTH RGC RGC RGC RGC OTH OTH OTH OTH MSF MSF OTH MSF OTH MSF MSF MSF OTH OTH OTH OTH OTH MSF MSF OTH OTH OTH OTH OTH OTH OTH OTH MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF MSF Table 2.Clusterpositionalparameters. (m-M) 11.60 16.20 14.30 15.30 12.15 10.35 13.50 11.55 15.60 13.45 13.30 10.30 14.50 16.20 11.80 11.55 10.70 14.40 16.00 11.35 10.45 11.45 13.35 11.45 10.65 10.75 13.95 12.60 12.50 10.30 11.55 11.35 10.30 12.45 12.10 12.20 10.00 11.70 12.20 10.10 10.85 12.40 12.15 12.00 10.25 10.35 13.90 11.20 10.25 11.10 12.60 9.80 9.60 9.00 9.80 9.20 9.00 9.30 3.40 8.35 8.50 6.40 8.25 8.70 7.40 9.35 V(RGC) 14.40 16.20 13.85 15.10 16.80 16.80 14.90 12.35 10.55 14.50 13.20 12.60 10.90 10.90 9.00 Mu(RGC) phn0.65 0.90 0.60 0.55 0.60 0.60 0.60 0.60 0.65 0.55 0.60 0.50 0.65 0.75 1.00 4.22 4.50 4.08 4.49 4.81 4.52 4.91 0.86 0.79 0.85 0.93 0.40 0.05 0.83 0.44 0.86 0.66 0.19 0.59 0.41 0.63 0.47 6.40 5.24 5.70 5.06 0.47 2.72 3.08 2.45 8.59 0.81 0.74 0.29 0.80 0.60 0.95 0.95 2.01 2.01 2.28 1.10 1.22 1.43 1.38 1.00 1.15 1.57 1.35 1.80 1.61 1.04 1.08 1.20 1.62 1.66 1.00 1.85 1.14 1.02 1.41 1.13 1.89 1.73 1.46 1.10 dr. o 11.87 15.25 11.29 11.58 12.97 13.28 14.87 12.86 13.72 10.25 13.51 12.37 10.47 10.69 10.72 R, 9.37 9.24 9.01 9.96 9.35 9.33 9.85 9.24 9.39 9.11 9.11 9.19 9.26 9.29 9.02 9.01 9.41 9.17 9.85 9.12 9.19 8.87 8.54 8.80 7.89 7.67 7.45 7.74 7.95 8.32 8.49 8.61 8.65 8.89 8.55 8.85 8.77 8.48 7.92 7.54 7.63 7.65 6.84 8.00 8.06 8.27 8.35 7.70 6.37 8.08 8.22 GC -0.03 -0.14 -1.02 -1.39 -1.13 -0.21 -0.28 -0.46 -0.20 -0.41 -0.14 -0.02 -1.20 -0.18 -0.02 -0.02 -0.02 -0.06 -0.11 -0.13 -0.08 -0.04 -0.07 -0.14 -0.03 -0.06 -0.04 -0.13 -1.21 -0.02 -0.14 -0.10 -0.03 0.00 0.07 0.10 0.08 0.16 0.24 0.02 0.53 0.03 0.10 0.06 0.04 0.82 0.74 0.00 0.03 0.14 0.28 0.00 0.62 0.30 0.04 0.45 0.04 0.03 0.19 0.01 0.05 0.00 0.12 0.00 0.00 0.02 2563 1997AJ 114.2556T rederived DDOabundances,opentriangles arespectroscopicabundances, Fig. 3.(a)Clusterabundancesasafunction ofgalactocentricpositionas squares aretransformedDDOabundances fromunpublishedphotometry. closed trianglesarecombinedDDOand spectroscopicresults,andopen given inTable2.(b)Sameas(a)without errorbars.Opencirclesare tinct groupsofclusters.Withineachgroup,themetallicity tween R=6.5kpcand15kpc,thereareactuallytwodis- implies thatratherthanacontinuousdeclinein[Fe/H]be- groups dividedpurelyonthebasisofgalactocentricposition ent whenoneshiftsfromthecompletesampletotwosub- change thatoccursinthestatisticalsignificanceofgradi- outside themetallicityrangedefinedbyclustersbetween 2564 TWAROGETAL:FORMATIONANDEVOLUTION 10 and13kpc. gc NGC 7789 NGC 7209 NGC 7142 NGC 6940 NGC 6939 NGC 6819 NGC 6791 IC 4756 NGC 6705 NGC 6633 NGC 6494 Our interpretationoftheseresultsissimple.Thedramatic Cluster © American Astronomical Society • Provided by theNASA Astrophysics Data System [Fe/H] -0.08 -0.06 0.07 0.04 0.01 0.03 0.07 0.15 0.14 0.00 0.09 s.e.m. 0.02 0.05 0.02 0.06 0.06 0.05 0.04 0.03 0.04 0.04 0.06 (m —M)Code RGC MSF OTH MSF OTH MSF MSF OTH MSF MSF MSF Table 2.(continued) (m-M) 10.75 12.45 12.95 11.10 13.05 12.30 12.90 10.50 13.40 8.80 8.35 jections ofasingleGaussian,whilevaluesinthe0.05to0.10 rithm returnsaprobability,Pkmm»whichismeasureofthe pothesis. FulldetailsareprovidedbyAshmanetal.(1994). range indicatemarginalrejectionofthesingleGaussianhy- be tiedtocorrelatedstructureinthegalactocentricdistribu- ric and/orspectroscopicanalysiswerenotchosenbecause tempted: homoscedasticandheteroscedastic, low-[Fe/H]BE numbers ofclustersineachgroup forthefouroptionsat- metallicity distributionthanasingle Gaussian.Table4gives mine whethertwoGaussiansprovide abetterfittothecluster improvement ofamulti-groupfittothedataoversingle case) orunequalvariance(heteroscedasticcase).Thealgo- in thefitrelativetoasinglegroup.Thesegroupsareby tion. they hadpeculiarorextremeabundances.Ifarerepre- metallicity, i.e.,theclustersthatwereselectedforphotomet- these areolderthan1Gyr,incontrastwiththeinnerclusters ters atlargegalactocentricdistancesandthemajorityof bution. Itshouldbenotedthatthesampleisneitherrandom used intheanalysisishardlyobjective;itwaschosenspe- position. a summaryoftheprobabilities, means,dispersions,andthe Gaussian. Pkmmvaluesbelow0.05representsignificantre- Gaussians eitherwiththesamevariance(homoscedastic any structurefoundwithinthemetallicitydistributionmust while thesampleisnotideal,itbiasedonbasisof which includealargefractionofyoungerobjects.However, nor complete;allclustersofageshavenotbeenincluded cance ofmultiplepeakswithintheclustermetallicitydistri- man etal.1994)whichcanexplorethepresenceandsignifi- mixture-model algorithm(McLachlan&Basford1988;Ash- determining groupmembershipforaclusterisgalactocentric inner clusters.Thus,theonlycharacteristicofsignificancein clusters are,onaverage,0.3dexmoremetal-poorthanthe tween thetwogroupsisintheirmeanmetallicity;outer gradient isweaktononexistent.Theprimarydifferencebe- statistically-preferred groupsandquantifiestheimprovement sentative oftheclustersattheirgalactocentricdistance,then at allgalactocentricdistances.Thereareclearlyfewerclus- cifically becauseFig.3indicatedthatthegradientsoneither somewhat moreobjectively,wehaveemployedtheKMM side ofthebreakpointwereweak.Toapproachanalysis In thepresentcase,wehaveused thealgorithmtodeter- The KMMalgorithmobjectivelypartitionsadatasetinto Unfortunately, onecouldarguethatthesamplebreakpoint V(RGC) 12.95 13.05 14.60 13.65 Mv (RGC) 0.50 0.60 0.75 1.20 2.01 2.11 0.41 0.37 0.72 3.81 2.03 1.14 1.25 1.63 1.88 9.53 9.27 7.79 6.76 8.68 8.16 8.81 8.18 8.03 8.17 8.21 R GC -0.19 -0.15 -0.16 -0.10 0.35 0.35 0.28 0.72 0.04 0.05 0.04 2564 1997AJ 114.2556T provide similarnumbersofclustersineachbin,ratherthan binned theclustersampleintofivegroupspurelyonbasis of R.AslistedinTable5,thespacinghasbeenselectedto the factthatpartitionsinmetallicitycorrespondcloselyto apparent inFig.3. our divisionoftheclustersaboutcriticallocationatR certainty in[Fe/H]formanyoftheclustersunderdiscussion, from —0.54to—0.24;onlytwoouterclustersnear[Fe/H] tains 12clusters,allbeyond10kpcandinthe[Fe/H]range unaltered. Forthehomoscedasticcase,separationbypo- represent thefivemostmetal-richclustersinlowmetal- with [Fe/H]=-0.13isincluded.Themeansareessentially metallicity groupincreasesandanadditionalinnercluster changed fortheheteroscedasticcase.Thevarianceoflow tion ofthesampleintotwogroupsremainsalmostun- ity grouphave[Fe/H]intherangefrom-0.15to-0.18and ports thediscontinuitydiscussedpreviously.Noneof populate themetal-poorsample,rangingfrom sition isevenmoreapparent.Thelowmetallicitybincon- licity bin. five clusterswithR<\0kpcassignedtothelowmetallic- outer clustersisassignedtothehigh-metallicitycamp.The tic case,twogroupsareagainfound,butnow19clusters have identicalvariances.Inthelessrestrictiveheteroscedas- The smallnumberofmetal-poorclustersisaproductthe low [Fe/H]forBE21andtheconstraintthattwogroups the low-metallicitycamp,justoverhalfouterclusters. improvement alwaysoccurswiththeadoptionoftwo [Fe/H]=-0.15 to-0.83.Thebreakdownofthesamplesup- groups. Forthelow-[Fe/F[],homoscedasticcase,cluster all casesasingleGaussianfitcanberejected;significant 21, high-[Fe/H]BE21.ThederivedPkmmindicatethatin sample dividesintotwogroupswithonly8clustersin = 10kpcisanimpressiveconfirmationofwhatwebelieve 2565 = —0.2areclassedinthemetal-richcategory.Givenun- gc GC GC To emphasizetherealityofdiscontinuity,wehave If oneadoptsthehigh-[Fe/H]valueforBE21,parti- © American Astronomical Society • Provided by theNASA Astrophysics Data System TWAROG ETAL/.FORMATIONANDEVOLUTION > 10kpc > 10kpc > 10kpc < 10kpc < 10kpc R RangeN[Fe/H]atÆ=0 gcGC LO BE21 HI BE21 ALL ALL ALL 62 62 75 76 76 13 14 14 0.001 0.001 Table 4.KMManalysisoftheclustermetallicitydistribution. -0.26 -0.17 0.18 0.21 0.24 0.57 0.66 0.61 ([Fe/H]) -0.328 -0.401 -0.009 Table 3.Linearfitstoclusterdata. 0.007 0.22 0.32 0.25 0.15 0.13 0.07 0.07 0.07 Homoscedastic d[Fe/H]/dR 0.100 0.100 0.123 0.123 -0.004 0.018-0.07 -0.043 0.026-0.42 -0.013 -0.023 -0.028 -0.067 -0.076 -0.071 by anyrealgalactocentricabundancegradient,age includes thescattercausedbyabundancedeterminations, for allbins.Themetallicitydispersionisimportantbecauseit the binmean.Thisapparentlycontradictoryresultstems of the[Fe/H]distributioniseffectivelyconstantnear±0.10 case.) Whatisalsointriguingtheobservationthatscale weight tothemoreextremedataforBE21inlow-[Fe/H] from theuseofbiweightestimatorswhichgiveslower that useofeither[Fe/H]forBE21givesasimilarresult where themeanmetallicitydropsby0.30to0.35dex.(Note within theerrors.Thischangesradicallyforlastbin, the ROSTATpackage. Table 5are90%confidenceintervalsontheseparameters calculated withabootstrapresamplingtechniqueincludedin discussion). Ourconclusionsareunalteredifweusetheclas- mators arelesssensitivetooutliersthantheirclassical significant gradient.Themeanmetallicitiesareconsistent when dealingwithsamplesasmallnumberofpoints counterparts—a propertywhichcanbeparticularlyuseful classical meananddispersionofadistribution.Theseesti- lated thebiweightestimatorsoflocation(C)andscale Gaussian (P=0.070). the innerbinisskewandmarginallyinconsistentwitha consistent withaGaussian.Themetallicitydistributionof rior to10kpcisirrelevant;allthatmattersthepositioning We emphasizethatthespacingofbinsforclustersinte- sical meananddispersioninthisanalysis.Alsoincluded Gaussian; exceptfortheinnermostbin,alldistributionsare et al.1990;Bird&Beers1993)totestitsconsistencywitha each binhasbeenanalyzedwiththeROSTATpackage(Beers of thelastbinbeyond10kpc.Theabundancedistribution that alltheclustersbeyondR=10kpcfallwithinonebin. identical spacingingalactocentricdistance,withtheresult (S). Thesearerobustestimatorsanalogoustothefamiliar (see Beersetal.1990andBird&1993forafull BI GC BI It isapparentthatinside10kpc,therenoevidencefora For eachofthemetallicitydistributionswehavecalcu- 0.020 0.017 0.016 0.008 0.008 0.008 64 68 12 -0.18 -0.17 -0.22 -0.72 -0.76 -0.74 ([Fe/H]) -0.223 -0.225 0.029 0.022 <0.0001 <0.0001 <0.0001 0.82 0.13 0.54 0.19 0.09 Heteroscedastic 0.153 0.086 0.200 0.090 UNWEIGHTED WEIGHTED NO-BE21 NO-BE21 LO-BE21 LO-BE21 HI-BE21 HI-BE21 Notes 20 56 57 19 2565 1997AJ 114.2556T = removes thecontributiontodispersionofmetallicity have agradientconsistentwith zero, whiletheouterclusters divided intotwosamplesatRqc 9 kpc.Theinnerclusters intriguing istheresultforintermediate agegroupwhen termediate age,to—0.115±0.037 foroldclusters.Whatis gradients atdifferentagesfor the openclustersrangefrom Tosi (1981),Cameron(1985),andFriel(1995).Theoverall cent exampleofthiseffectisseenintheworkbyMolláetai the gradientwithdistance,ratherthanadiscontinuity.Are- Panagia &Tosi1981;FJ;Friel1995).However,theprevious that thefeaturehasbeennotedinpast(e.g.,Janes1979; mained hiddenforsolong.Thestraightforwardansweris discontinuous disk,onemightaskwhythisfeaturehasre- cepted viewofalinearabundancegradientandthat clusters inthesolarneighborhoodwillbefocusofSec.4. for asinglelineartrend.Themetallicityspreadamongthe distribution correctedforthisdiscontinuitythanacorrection will definitivelyshowalowerdispersionforthemetallicity this isthecase,wepredictthatanexpandedclustersample tric distanceisabetterinterpretationoftheclusterdata.If 90% confidencelimitsonthesetwovaluesoverlap,thisis tinuous weakgradientsis0.097(0.087,0.108).Whilethe whereas thescaleafteracorrectionbasedontwodiscon- corrected forasinglelineargradientis0.113(0.100,0.126), persion ratherthanS.)Thescaleofthedistributionwhen leads tosimilarresults,asdoestheuseofclassicaldis- used thehigh[Fe/H]valueforBE21:usinglow resent the90%confidencelimitsonthisquantity.(Wehave S= 0.156(0.128,0.186)dex,wherevaluesinbracketsrep- metallicity distribution. data willleadtoalowerdispersionoftheresultingcorrected tinuity. Further,oneexpectsthatthebetterdescriptionof distribution producedsimplybyeitheragradientordiscon- cluster basedonitsgalactocentricdistance.Suchacorrection gradients listedinTable3,wecorrectthemetallicityofeach two groupsinteriorandexteriortoR=10kpc.Usingthe weak gradientsfoundwhentheclustersaredividedinto of ametallicitydiscontinuityisprovidedbycorrectingthe given positionatthetimeofformationclusters. 2566 at thislevelorthefeaturewasinterpretedasasteepeningof samples wereinadequatetoguaranteetherealityofstructure sample forthebest-fitsinglelineargradientandtwo spread, andfinallybyanyintrinsicspreadin[Fe/H]ata suggestive thatadiscontinuityinmetallicitywithgalactocen- (1997) usingacompositesampleofclustersfromPanagia& —0.089±0.025 foryoungobjects, to—0.072±0.020forin- B1 BI GC A finalconsistencycheckthatsupportsourinterpretation In lightofthesharpcontrastbetweencommonlyac- The uncorrectedscaleofthetotalclustersampleis © American Astronomical Society • Provided by theNASA Astrophysics Data System TWAROG ETAL.:FORMATIONANDEVOLUTION Rcc Range 9.11-9.96 kpc 6.37-7.95 kpc 8.54-9.02 kpc 8.00-8.49 kpc >10.00 kpc >10.00 kpc 20 14 14 14 14 14 N Table 5.[Fe/H]asafunctionofgalactocentricdistance. -0.307 -0.317 -0.012 -0.009 0.025 0.062 Cu -0.356—0.267 -0.369—0.269 -0.052-0.026 -0.035-0.078 -0.049-0.027 0.009-0.117 90% Limits recalibration oftheDDOmetallicityscaleorcluster kpc intheanalysisofoutersample.Theclustersample have pointedoutthesharpdropinnumberofolderclus- relative contributionsofyoungandoldclustersindifferent remove thediscontinuity.Theouterdiskclusterswouldall the metallicityrankingofclusters.Asnotedearlier, plausible thatastepfunctioncouldbesmearedintolinear the internalconsistencyofmetallicityanddistance number ofclusters,therangeingalactocentricdistance,and Tables 1and2supersedesallpastanalysesintermsofthe radial abundancegradient. increase inthemean[Fe/H]forinnermostbinTable5. in eachgroup. for aclusterAMRamongeithertheinnerclustersor toward youngeropenclusters.IfthereisasignificantAMR The clusterdataofFJ,whichdominatestheanalysisin be systematicallylowerin[Fe/H]byabout0.1dex;thescat- troscopic data.DirectuseofthePCAorFJscalewillnot possibility ofabiasduetothedeclineinDDOsensitivityat the range,primarilyatmetal-richend,ithasnotchanged gradient; thereverseprocessseemshighlyimplausible. gradient reflectstheinclusionofclustersbetween9and10 in age,themean[Fe/H]shouldbe higher,independentofany ters interiortoR=7.5kpc.Thoughalargeincreasein van denBergh&McClure(1980)andJanesPhelps(1994) groups isthesame,thoughouterclustersamplelarger. outer clustersalone;therangeinagecoveredbytwo galactocentric regions.Thoughthesampleisnotaslarge actually enhanced. ter amongtheinnerclusterswillbroadenwhilemean lower [Fe/H]isremovedthroughtheinclusionofspec- show agradientof—0.083±0.027.Thepresencetheouter clusters intheinnermostbinare exclusivelylessthan1Gyr a changeof0.05to0.10dex over thelast5Gyr(Twarog are analyzed,thisconcernevaporates.Thereislittleevidence one mightwish,iftheclusterswithagesbeyond1Gyralone among clusters,thediscontinuitymightbeareflectionof anticenter, ispredominantlycomposedofclustersolderthan [Fe/H] shiftsdownbyabout0.05dex.Thediscontinuityis selection? ThoughtherevisedDDOscalehascompressed scales. Withsignificantscatteramongthedatapoints,itis [Fe/H] withdecreasingageisexcluded bytheobservations, Strangely enough,theoldestclustersintwogroups (NGC 6791andBE39)arealsoamongthemostmetal-rich 1980b; Meusingeretal1991)is wellwithintheerrors.If 1 Gyr.ThesampleinsideR=0kpcisheavilyweighted GC GC Is itpossiblethatthediscontinuityisaproductof The agedistributionmightplaysomeroleintheapparent As forclusterselection,thereisonesourceofconcern. 0.098 0.109 0.099 0.096 0.106 0.080 0.061-0.181 0.085-0.160 0.080-0.120 0.085-0.141 0.061-0.110 0.071-0.149 90% Limits LO-BE21 HI-BE21 Notes 2566 1997AJ 114.2556T -1 -1 = result. Again,wecautionabout overinterpretingtheabsolute d[0/H]/dÆ =-0.06±0.02kpcarefoundinallthenebular the apparentdiscrepancybetweengradientsidentified no gradientatall.Themean[O/H] is—0.06withadisper- interpretation iswellwithinthe errorsofthedata.The11B a gradientof—0.07±0.01kpc.CloserexaminationFig. attribute thelackofagradientinpreviousstudiestosmall dium, thispresentsaproblem.Smartt&Rolleston(1997) etary nebulae)andBstars.Abundancegradientsof through [O/H]measuresinnebularregions(Hnandplan- element andnottheother.ThefocusofBstaranalysisis different, itispossibleforagalacticgradienttooccurinone cussion. SincethenucleosyntheticoriginsofOandFeare and thefield.Theirmetallicityindicatoris[O/H],not[Fe/H], a homogeneousmannerspectraof21Bstarsinopenclusters over thesameregion. at agivenRq,areclearlyconsistentwiththeclusterdata the modestgalactocentricdistancerange,lackofamea- only 0.04.ThoughFry&Carney(1997)justifiablycaution gradient is—0.003±0.018withacorrelationcoefficientof distance range,ifoneCepheidatR=1.5kpcwithan probably impliesasmallzero-pointshiftrelativetoour The meanmetallicityofthesampleis[Fe/H]=—0.05,which were observedintwoclusterstotestfornon-LTEeffects. were observedatmorethanonepulsationalphasetotestfor copy of23CepheidsoverR=6\.o10kpc;18thestars clusters andassociationsbySmartt&Rolleston(1997). Cepheids byFry&Carney(1997)andofBstarsinyoung ture areofparticularrelevance,thespectroscopicanalyses lower massdwarfs.Twostellarsamplesintherecentlitera- ues derivedfromdifferenttechniquesforcoolergiantsor culty inestimatingmetallicityandcomparingitwithval- is topickthoseofhighmass,thoughthiscanleaddiffi- ent. Thebestwaytoguaranteeasampleoftrulyyoungstars move significantdistancesfromtheirplaceoforiginand the disk.Youngstars,however,havenothadenoughtimeto to diffuseoverlargedistancesonGyrtimescales(Widen recently formed.Unlikeclusters,fieldstarshavetheability exclude thestudiesbaseduponfieldstarsunlessare For reasonswhichwillbediscussedmorefullyinSec.4,we notion ofanabundancediscontinuityinthegalacticdisk? sion ofonly0.09dex,fortuitously similartotheCepheid small uncertaintiesinbothdistance andabundance,exhibit stars betweenRgc6and10kpc, themajorityofwhichhave samples, anderrorsindistancesabundances;theyderive stars shouldberepresentativeofthecurrentinterstellarme- studies, whileBstarsexhibitlittleornogradient.Since so someconcernexistsovertherelevancetocurrentdis- against acceptingthisresultasstatisticallysignificantgiven anomalously low[Fe/H]isexcluded,thederivedabundance any [Fe/H]dependenceonthetemperaturescaleanddwarfs 2567 TWAROGETAL.:FORMATIONANDEVOLUTION surable gradient,aswellthemodestdispersionin[Fe/H] scale. Whatismoreimportantthatoverthisgalactocentric should provideareliableindicatorofthecurrentdiskgradi- 1(b) ofSmartt&Rolleston(1997)showsthatanalternate 1977), distortingifnotdestroyingpotentialfinestructurein C GC GC Smartt &Rolleston(1997)havecompiledandanalyzedin Fry &Carney(1997)obtainedhighdispersionspectros- Aside fromclusters,isthereanyevidencetosupportthe © American Astronomical Society • Provided by theNASA Astrophysics Data System -1 -1 What mattersiswhetherornot onecan,giventhesample beyond thediscontinuityatR=10kpc.Thekeyquestion beyond 6kpcdoesnoteliminatethegradients,thoughthey Fich &Silkey(1991),Dinersteinetal.(1993),andstillun- tic disk,confirmingtheearlierworkbyShaveretal.(1983). in theabundancesofanumberelementsacrossgalac- results. Mostrecentwork(e.g.,Fich&Silkey1991;Maciel Luck (1982),whofindsJ[Fe/H]/JR=-0.13±0.03kpc There isnoquestionthat,inthemean,[O/H]ofouter uncertainties. Theentiresourceofthegradientcomesfrom will returntoinSec.4. inconclusive. Additionalconcern comesfromtheabsolute is notifthedatacanbefitbyalinearrelation;theybe. exclude astepfunctionbecauseanalysisofonlyHnregions cluster sample,andcorrelationcoefficientsnear0.7.They ties intheslopescomparabletoourlinearfitsforentire mately -0.07kpcfor[N/H],[S/H],and[O/H],uncertain- tween 0and12kpc.Theyfindlineargradientsofapproxi- Afflerbach etal.(1997)using34compactHIIregionsbe- exist beyond10kpc. published resultstosuggestthataseconddiscontinuitydoes where ourdiscontinuitybegins,theypointtotheworkof that theirabundancedata,rangingfromR=0to10.5kpc, past attemptsatderivingfinestructurewithinthediskusing the rangeofgalactocentricdistancevariessignificantlyfrom However, thenumberofpoints,analyticalapproach,and from 50latetypesupergiantsoverR0(2=1.1to10.2kpc. very youngstarsisindirectcontradictionwiththeresultsof lack ofagradientbetweenR=6and10kpcfromthese mains determinedbytheeyeofbeholder.Incontrast, the 10pointsbeyondkpc,majorityofwhichhave that alltheBstarshavesameabundance,within terminations arecomparabletothedispersion,thisimplies abundances derivedfromthenebular samples,apointwe and alinearrelation.Thenebular datatodateare,atbest, are reduced.However,ofthe18regionsbeyond6kpc,4lie dances at6kpc,i.e.,thereisnoabundancegradientbetween without gradientsbutlinkedviaadiscontinuityintheabun- dance decreasingwithdistanceorequallywellbytwozones can bedescribedbyalineargradientwiththemeanabun- one isfacedwithmanyofthesameproblemsthatplagued & Koppen1994;Simpsonetal.1995)indicatesavariation gradual changein[O/H]withdistanceoradiscontinuityre- disk starsislowerbyabout0.4dex;whetherthisduetoa est dispersionisreal.Giventhattheerrorsin[O/H]de- size andtheerrorbars,distinguish betweenastepfunction 6 and10kpcfromthegalacticcenter.Thoughtheirdataend absent fromthenebularsurveys.Simpsonetal(1995)claim significantly largererrorsinbothdistanceandabundance. abundances giventhepossibilityofscaleshifts,butmod- spite theproblems,possibleevidenceforfinestructureisnot stars andclusters.Itisintriguingtofind,however,thatde- study tostudy,andnoneofthestudiesmeasureFe.Thus, 10 kpcaswellalinearrelationhasbeenchallengedby GC GC GC The conclusionthatastepfunctionfitsthedatainteriorto An equallytantalizingpictureispaintedbythenebular 2567 1997AJ 114.2556T Rqc~ 10kpcwhicharelocatedwellawayfromtheplane, radial trendandfoundthataresidualgradientstillremained recognition thattheclusterdistributionisapproximatelya by PCAthatagradientin[Fe/H]existedamongclusters Fig. 4.Absolutedistanceawayfromthegalacticplaneasafunctionof Fig. 5,wheretheabsoluteZpositionispresentedasafunc- the galactocentrictrendandaZ-gradientisobvious.PCA through thedataofFJ.Nocorrectionhasbeenappliedfor perpendicular totheplane,contradictingearliercluster metallicity fortheclustersusedbyPCA(opencircles)andadditional 2568 TWAROGETAL:FORMATIONANDEVOLUTION tric distance.SamesymbolsasFig.4. Fig. 5.Absolutedistanceawayfromthe planeasafunctionofgalactocen- tion ofRqq.Thoughtherearesomeolderclusterswithin next appliedalinearcorrectiontothesampleeliminate crosses areclustersaddedtothecurrentsampleprimarily of [Fe/H];opencirclesareclustersincludedinPCAwhile ward toshowthatthegradientperpendicularplaneis natunga &Freeman1989;Morrisonetal1990).Withthe et al1987;Sandage&FoutsYoshiiRat- away fromtheplane.Thesourceofproblemisseenin distance isplottedforalltheclustersinTable3asafunction an artifactofthePCAanalysis.InFig.4,absoluteZ analyses butconsistentwithfieldstarstudies(e.g.,Yoss sample ofFJ(crosses). step functionratherthanalineargradient,itisstraightfor- 3.2 TheAbundanceGradientPerpendiculartothePlane A primemotivationforthisinvestigationwastheclaim © American Astronomical Society • Provided by theNASA Astrophysics Data System 6 81012 14 16 Rgc = plane. Thus,thediscontinuityin[Fe/H]ispairedwithadis- Fig. 6.Sameas4afteradjustingtheclustersbeyondÆgc^IOkpcfor rected datahaveamean[Fe/H] of+0.010andarobustdis- been applied.Forourpurposesitisirrelevantwhetherthis For oursample,wewillonlyuseclustersbetween/?gc6 not affectedbyasignificantAMR,thatclustersareatypi- is realorsimplyaselectioneffectintheclustersample.Note continuity inZdistribution.Forpurposesofresolvingthe of thedisk.Incontrast,clustersbeyond10kpcarepre- the majorityofclustersareyoungerandliewithin200pc an offsetin[Fe/H]of0.32. cludes anyresidualtrendswith age orgalactocentricposition persion ofonly±0.096(0.085, 0.108).Thedistributionis or both.Ouronlyinterestisin the dispersionin[Fe/H]ata in themeanageofsamplewithgalactocentricdistance, gradient existsbecauseofanintrinsicwithposition dient amongtheinnerclusters(seeTable3,unweighted)has galactocentric positionandthederivedsmallabundancegra- among theclusterstosampledegreeofinhomogeneity lifetime ofthedisk,onecanusemetallicitydistribution interstellar mediumdoesnotchangesignificantlyoverthe formation, andthattheintrinsicmetallicityspreadwithin cal oftheinterstellarmediumindiskattimetheir cal historyofthegalacticdisk.Assumingthatclustersare outer clusters,onegetstherevisedversionofFig.4aspre- discontinuity. If,insteadofcorrectingforalineargradient, question athand,itmakesnodifferencewhetherthischange dominantly olderandarepositionedwellawayfromthe given locationatatime. From62clusters,thecor- at agivenage,becauseofanAMRconvolvedwithchange and 10kpc.Asmallcorrectiontoeach[Fe/H]basedupon among thestarsformingatarandomtimewithindisk. one merelyappliesazero-pointoffsetof0.32dextothe also thattheclustersampleofPCAextendsjustbeyond slightly boxierthanaGaussian. Becausethisdispersionin- sample allowsonetoplaceanotherconstraintonthechemi- sented inFig.6;theZ-gradientdisappears. 4.1 GalacticClustersandFieldStars:TheDiscrepancy Questions ofthegalactocentricgradientaside,cluster 4. THEMETALLICITYDISTRIBUTIONINSOLAR NEIGHBORHOOD 2568 1997AJ 114.2556T with theclusterdata.Venn(1995) derivesameanofsolar tallicity fromeitherOorFeis typically±0.10dexorless results foryoungstarsasdefinedbytheCepheidsandB than thedispersion,afactwhichshouldbekeptinmind abundance for13metalsfromspectroscopic analysisof22A with ameanabundancenearsolar, inexcellentagreement within thesolarneighborhood?InSec.3,wediscussed when comparingtheresultsfromdifferentinvestigators.) ters isbestrepresentedbythesumoftwoGaussians.As in [Fe/H]ismoreappropriatewhenaGaussiandistribution use themoreclassicalmeasureofdispersion.Therange observed above,therangeisabouttwotothreetimeslarger a poorrepresentation,e.g.,whenthesampleofallclus- viation aboutthemean.Ourconclusionsareunchangedifwe The dispersionsquotedforthesmallerstudiesdonotassume though thedistributionisnotaperfectmatchtoGaussian. tion ofclustersinside10kpcisarobustfittoGaussian the inhomogeneityin[Fe/H]amongstarsandclusters hood. Thoughthesampleissmall,abundanceestimates supergiants, thoughtheabundance dispersioniscloserto stars inside10kpc.Inbothinstances,thedispersioninme- a Gaussianprofileandrepresentthetraditionalstandardde- changed significantlyoverthelast5Gyr,andameannear found above:the[Fe/H]rangeamongopenclustersin have unusuallyhighprecisionforopenclusters.Boesgaard persion amongtheclustersofonly±0.08;nocorrelation rates theearlierinvestigationsofsmallersampleswithpre- only 0.087.Thesetwostudiesareconsistentwithwhatis tion; themeandifferencein[Fe/H]sense(NI-Table clusters inNissen(1988)arefoundthecurrentinvestiga- was foundbetween[Fe/H]andage.Forreference,9ofthe the metallicityspread.Themean[Fe/H]=+0.05withadis- nearby openclusterswithagesbetween0and2Gyrtostudy Nissen (1988)useduvbyYißphotometryofF-dwarfsin13 mately thesametime;itdoes.Thisresultmerelycorrobo- in [Fe/H]doesnotexistamongclustersformedatapproxi- metallicity scaleforthegiantshascompressedprevious the gradientisbasedsolelyuponinnerclustersandthere- the clustersbeyond10kpc,abundancecorrectiondueto solar neighborhoodvariesfromstudytostudy.Thedistribu- solar. (Anoteofclarification:theparameterusedtodescribe [Fe/H]=-0.2 to+0.2,withnoevidencethatthishas solar neighborhoodisbetween0.3and0.4dexwide,from Gyr, tostudythemetallicityspreadinsolarneighbor- analysis ofFdwarfsin6galacticclusters,allyoungerthan1 2) is0.00±0.10. cise abundancesbyNissen(1988)andBoesgaard(1989). fore actuallynarrowsthedispersion,andchangein metallicity dispersionwithintheinterstellarmedium. of theclusterdistribution(e.g.,Carraro&Chiosi1994;Friel mates, itshouldberegardedasanupperlimittotheintrinsic (1989) findsamean[Fe/H]of+0.015anddispersion scale fortheinnerclusters.Thatisnottosaythatatruerange and theobservationaluncertaintiesinabundanceesti- 2569 TWAROGETAL.:FORMATIONANDEVOLUTION ±0.2 duetothelargeruncertainties intheindividualabun- 1995), butitshouldberememberedthatwehaveexcluded How doesthiscomparewiththeresultsfromfieldstars Boesgaard (1989)usedhighdispersionspectroscopic This dispersionseemssmallcomparedtopastdiscussions © American Astronomical Society • Provided by theNASA Astrophysics Data System been underdiscussionforsometime,asevidencedbythe borhood. Potentialproblemswithnebularabundanceshave point uncertaintiesaside,thefield starpopulationinboth the curvesmoredistinguishable. Itisapparentthat,zero- ters (solidcurve)totheThinDisk (dash-dotcurve)andThin compare thenormalizedabundancedistributionofclus- matically whenoneexpandsthesampletoincludeanyand ture relativetotheyoungstarsandclusterschangesdra- investigators. yond thescopeofthisinvestigationandexpertise found withintheclouds,models,orbothisbe- However, whethertheoriginofproposeddeviationis work ofMathis(1995)andAlexander&Balick(1997). tent oftheHIIregionsbyabout0.3dexinsolarneigh- the sourceofdiscrepancylieswithnebularabun- that ifthesunisanomalouslymetal-rich,sotypicalstar is anomalouslymetal-richforitsageandlocation.Thedata monplace toexplainthisdiscrepancyasevidencethatthesun metal-deficient byabout[m/H]=-0.3.Ithasbecomecom- near thesun,anissueraisedinSec.3shouldbedealtwith. curve). Thefirsttwohistogramshavebeenoffsetby0.02in[Fe/H]fromthe curve), ThinDisk(dash-dotcurve)fromWG,andtheinnerclusters(solid Fig. 7.Metallicitydistributionforfieldstars,Thin+ThickDisk(dashed groups containsanexcessof starsbelow[Fe/H]=—0.2 Gilmore (1995,hereafterreferredtoasWG).InFig.7we of thelocalmetallicitydistributiontodateisthatWyse& all fieldstarsnearthesun.Themostcomprehensiveanalysis dances, i.e.,theysystematicallyunderestimatethemetalcon- formed at/?=8.5kpcoverthelast1Gyr.Wesuggestthat for theyoungstarsandopenclustersdemonstrate formed 4.6Gyrago.Despitetheextraordinaryagreement, mean metallicityisbasicallythesameaswhensun The clustersampleindicatesthatattheRofsun,this ity amongrecentlyformedobjectsissolarwithin±0.1dex. young starsdefinitivelydemonstratesthatthemeanmetallic- The metallicitydistributionoftheyoungclustersand dances. Beforediscussinganexpandedsampleoffieldstars scales ofthelattertwohistograms havebeenoffsettomake et al1997)consistentlyfindthattheISMnearsunis analyses ofgalacticnebularabundances(e.g.,Afflerbach solid curvetomakethecurvesmoredistinguishable. + ThickDisk(dashedcurve) samples ofWG.The[Fe/H] GC GC Returning tothefieldstarmetallicitydistribution,pic- 2569 1997AJ 114.2556T plain thediscrepancywith the clusters.Clusterswith representative oftheISMin solarneighborhoodoverthe If thestellarsamplesinE93and Jónch-Sórensen(1995)are persion in[Fe/H]near0.1dexforstarsyoungerthanthesun. Twarog (1980b)finds[Fe/H]=-0.05at4.6Gyrandadis- Due tothe[Fe/H]selectionbias,forE93dispersionisof More important,therangein[Fe/H]amongfieldstars4Gyr by starsinteriortoR=10kpc,iscloser[Fe/H]—0.35. rived intheearlierworkandranges[Fe/H]arelarge Nissen (1995),theAMRderivedbyCarlbergetal(1985)is last 10Gyr,adifferenceinthe agedistributioncannotex- the valuefoundbyJ^nch-Sorensen(1995)forstarsnear of thesunisbetween—0.15and—0.20forE93,similarto enough thatstarswith[Fe/H]wellbelow—0.2canbefound tallicities atagivenagearesystematicallylowerthande- Edvardsson etal.(1993,hereafterE93)andFónch-Sífrensen the dataselectionandanalysis,includingthosediscussedby isochrones arealteredand/orimproved.Becauseofbiasesin in qualitativeagreementwiththeanalysesoffieldstars much moregradualincreaseoverthelast8Gyr.Thistrendis nantly membersoftheolddisk,i.e.,theyfallinagerange than 9Gyr;thelong-livedfieldstarsprovideamorerepre- ters andfieldstars.Theclustersamplecontainsasignificant implies thattheydonotsamplethesamedistributionofga- bution isthatlistedbyWGasYoungDisk. ages greaterthan2Gyrandthe meanmetallicityforclusters questionable value;therangeisamorereliableindicator. old andolderforbothstudiesisbetween0.60.8dex. at anyagebeyond2Gyr.Infact,themean[Fe/H] discussion.] unreliable forolddiskstarsandshouldnotbeincludedinthe illustrate thechangesinrelationasanalysisand et al.(1985)isinvariablycitedindiscussionsoftheAMRto vised byMeusingeretal.(1991).[TherevisionCarlberg crease in[Fe/H]withintheinterstellarmediumduring tidally disrupted. which overlappedwiththisfieldstargroupbutithasbeen cluster population.Therewasanopenpopulation clining samplewithincreasingage,andnoclustersolder fraction ofclusterswithageslessthan1Gyr,arapidlyde- Young DiskbutnottheThinimpliesthatdiscrep- chemical historyofthediskandsuggestsastraightforward lactic populations.Thisdifferenceisanimportantcluetothe of WGwhichcomesclosetoreproducingtheclusterdistri- which isnotreflectedintheclustersample.Theonlysubset 2570 TWAROGETAL.:FORMATIONANDEVOLUTION [Fe/H] =—0.2orlowershould be typicalofthesamplefor sun forJcfcich-S^rensen(1995),asamplewhichisdominated first thirdofthelifetimegalacticdisk,followedbya of 8to13Gyranddonotoverlapinagewiththesurviving ancy willariseiffieldstarswith[Fe/H]<—0.2arepredomi- ancy arisesfromdifferencesintheagedistributionsofclus- solution. Thesimilarityoftheclusterpopulationto solar circle.Themeanfortheentiresampleatageof solar neighborhoodasdiscussedbyTwarog(1980b)andre- sentative distributionoftheentiredisklifetime.Thediscrep- (1995) generallyagreewiththeearlierwork,meanme- GC In contrast,thoughthequahtativeAMRtrendsfoundby The disagreementbetweenthefieldstarsandclusters Such anexplanationisconsistentwithasignificantin- © American Astronomical Society • Provided by theNASA Astrophysics Data System hood atallagesismissingthelowermetallicityportionof bution ofthesurvivingopenclustersinsolarneighbor- rich, olderstars.TheGdwarfsalsoproduceametallicity reference ontherelationdespite[Fe/H]biasinconstruct- ters beyondthislocationwhich brackettherequired[Fe/H] the metal-poorfieldstars,there isarichpopulationofclus- the analysisofFdwarfsbyTwarog(1979),assumma- may stillobserveastarofgiven[Fe/H]declinesas termine thestar’stemperature,probabilityoffindinga ture rangeandareeliminated.Thus,onlythelowestmassF few Gyr.However,whileyoungstarsofanymassthatfall cited butincorrectoptionshouldbeeliminated.InAMR range, theresolutionofdiscrepancymustresidewith ing thesample.Themeanabundanceswithagecannotbe included inthediscussionbecauseithasbecomeastandard mean AMRforthesolarneighborhoodasdefinedbyE93is large rangeinmetallicityderived amongthefieldstarsasin lative clustersampleisoften cited asconfirmationofthe there arenoclustersinteriorto 10kpcwhichoverlapwith the fieldstardistribution.TheworkofE93andJcSnch- the averageinnercluster. distribution heavilyweightedtowardmetallicitieslowerthan discussion ofWGwhouseonlylong-livedGdwarfsintheir that fortheobservedsample,AMRisnotsimplyare- completely eliminate[Fe/H]-dependentbiasinanF-star rized inTwarog(1980a,1980b).Whileitisimpossibleto tailed byKnude(1990)weremodelledandincorporatedin Tinsley (1976)werewellknownandminimizedinthe within theclustersample. Tinsley 1976;Knude1990)astheexplanationforexist- increases. Friel(1995)citesthisselectionbias(McClure& changes withage.Ingeneral,thelimitingageatwhichone cause themetallicityandmassofamainsequencestarde- dwarfs willsurviveovertheentirelifetimeofdisk.Be- within theF-startemperaturerangeareobservable,asa on timescalestypicalofthelifetimegalacticdisk,a field stars.Beforediscussingtheprobablesolution,anoften- of theISMinsolarneighborhood,even4to8Gyr considered reliable(Nissen1995).] greater than1Gyrinageshouldbewellbelowsolar.[The and ageranges.Thelargespread in[Fe/H]withinthecumu- answer issuppliedbylookingattheclustersample.While stars comefromiftheydonotthelocaldisk?The as young2to4Gyr.Wheredothelowermetallicityfield S^rensen (1995)impliesthatthemetal-weakthindiskcanbe sample, therebyavoidingthepossibilityofexcludingmetal- flection ofselectionbias.Thisconclusionisconfirmedbythe sample, themodelsandanalysisinTwarog(1980a)show sample selectionprocedureofTwarog(1980a)andtheanaly- ence ofanAMRamongthefieldstarswhilenoneoccurs star ofagiven[Fe/H]intemperature-limitedsample sample ages,thehotterFdwarfsevolveoutoftempera- studies Fdwarfsarecommonlychosenbecausetheyevolve sis ofthesamplebyTwarog(1980b).Alleffectsde- The fundamentaldiscrepancyisthatthemetallicitydistri- If oneacceptsthatthesurvivingclustersarenotatypical However, theselectionbiasesoutlinedbyMcClure& 4.2 GalacticClustersandFieldStars:ASolution 2570 1997AJ 114.2556T Rqq havenobearingonthediscussionofISMnear E93. Buttheexistenceofadiscontinuity(orsteepgradient) R=\0 kpc.Thestarshavediffusedinwardontimescales property whichmakestheclustersdifferentfromfield in thediskguaranteesthatmetal-poorclustersbeyond bined withthespatialanalysisofSec.3,itisconcludedthat the cumulativeeffectsofsuchinteractions.Thus,whencom- massive gascloudswillbenegligible.Incontrastindividual over arangeingalactocentricdistanceastheygoaroundthe galactic orbitswhicharenon-circular,causingthemtomove 2571 TWAROGETAL.:FORMATIONANDEVOLUTION remainder ofthissectionwillfocusontheplausibility orbital perturbationscausedbythepassageofstarsand/or galaxy (see,e.g.,PCA),theircollectivemassensuresthat stars isnottheagebutmass.Thoughclusterscanhave regarding theclusterpopulationshouldbedealtwith. from thelocallyformedfieldstarsofcomparableage.The which makesitimpossibletodistinguishthemdynamically of afewGyr,mixingwiththelocalpopulationtodegree with [Fe/H]<—0.2areinterlopers.Thesestarsthefield a significantfractionofthestarsinsolarneighborhood sun andcannotbecombinedwiththelocaldistribution.The PCA atfacevalue,clustersnearthesuntypicallyrangeover takes theorbitalanalysisbyCarraro&Chiosi(1994)and the spreadininitialconditionsamongclusters?Ifone much diffusioningalactocentricdistancearisesbecauseof the galactocentricorbitsofclustersisunlikely,how Though onecanreadilyassumethatsignificantalterationin stars canexperienceconsiderableorbitalperturbationsfrom improbable. lactic abundancegradientasdefinedbytheclustersseems star counterpartstothemetal-poorclusterpopulationbeyond these kinematicresultsisavailablebycomparingtheorbits potential. Anindicationofthedifficultiesininterpreting tance, radial-velocity,propermotion,orassumedgalactic ties intheirorbitalparametersduetopotentialerrorsdis- Chiosi (1994)norPCAgiveanyindicationoftheuncertain- that theproblemlieswithorbits.NeitherCarraro& about 3kpcingalactocentricdistanceastheyorbit.Under star populationnearthesun. solution andhowitfitsinwiththecurrentpictureoffield remaining 18clustershaveamean eccentricityof0.19,im- between apogalacticonandperigalacticondividedbytheir to 4.8kpcforNGC2506.Evenmoresignificantisthedis- for thefiveclustersinCarraro&Chiosi(1994)with such conditions,survivalofasharpdiscontinuityinthega- for aclusterat/?=10kpc. No significantdifferenceis plying anaveragerangenear4kpc ingalactocentricdistance one excludestheextremecaseofNGC7789(¿=0.6), tribution inorbitaleccentricity,e,definedasthedifference cantly largerthanfoundbyCarraro&Chiosi(1994);the galactocentric distanceforclustersinPCAwereallsignifi- same objectsinthelargersampleofPCA.Theranges eccentricities atthislevel,they areimplausibleforanunbi- found comparingclusterssorted byage.Whilefieldstar increase inAi?wentfromalowof0.9kpcforNGC2420 sum. Ofthe19clustersinPCA,onlyonehasebelow0.1.If samples coveringalargerange inagecanexhibitaverage gc GC GC Before addressingthefieldstarquestion,apuzzlingpoint Our clearlybiasedinterpretationofthiscontradictionis © American Astronomical Society • Provided by theNASA Astrophysics Data System based upontheexpecteddistributionofeandexistence but notenoughtodestroyit. Even ifonechoosesatypicalvalueofe=0.1fortheclusters, i.e., closerto0.0than0.2.Turningthequestionaround, larger thanfoundamongnewlyformedclustersandstars, turbations shouldleadtoaverageeccentricitiesonlyslightly WED). ThegoalofWEDwastodeterminethegalactocentric Jong (1997).Outofthemanysuggestions,onepri- clusters diffuseoveramuchsmallerrangethanclaimed. orbits haveoverestimatedtheorbitaleccentricitiesandmost of theclusterdiscontinuity,recentderivations ased openclustersample.Theineffectivenessofper- mary relevanceisthatofWielenetal.(1996,hereinafter expended toexplaintheoriginoflargedispersionin at i?=10kpc,aclusterwillorbitbetweenRqq=9kpcand [Fe/H] withintheISMatagivenagegalactocen- done ifoneassumesthat:(a)theoriginaldispersionin Gyr ithasdiffusedawayfromitsinitiallocation.Thiscanbe origin ofthesunbaseduponassumptionthatover4.6 summary ofthemanyoptionsisgiveninvandenHoek&de [Fe/H] amongstarsofagivenagenearthesun;useful WFD, buthasbeenmodifiedtoaccountfortheresultsof i.e., therangein[Fe/H]foundE93isaproductofdiffu- the sunand,moreimportant,concludethatinitialdisper- proach, WFDderiveÆ=6.±09kpcforthebirthplaceof in [Fe/H]fromthemeanvalueforitsage,andmoving given agecanbederivedbymeasuringhowmuchitdeviates one knowstheAMR,galactocentricoriginofastar galactocentric gradientin[Fe/H]existswithinthedisk.If among starsmorethanafewGyrold;and(b)significant tric locationwassignificantlysmallerthanthatobserved numerous investigationsovertheyears(Mayor1976; amount whichaccountsforthedeviation.Followingthisap- both inspaceandtime,makethetracingofstellarorbits Binney (1981)andreiteratedinWFD,therandomizingef- vestigation whichattemptstosortstarsintobinsofcommon conclusions aresimplebut,ifcorrect,devastatingtoanyin- the mostimportantanalysisisthatbyWielen(1977).Its Grenon 1987;Francois&Matteucci1993;Fuchsetal.1996, observational roleofstellardiffusionhasbeenthefocus on theroleofstellardiffusionisinorder.Thetheoreticaland sion in[Fe/H]withintheISMatagivenRisrathersmall, star alongthegalactocentricabundancegradientbyan hopeless task.Ifthearguments of WFDarecorrect,thecon- imperfect knowledgeofthegalacticgravitationalpotential fects ofdiffusionontheorbitsstars,coupledwithan origin throughkinematics.AssummarizedinMihalas& among others).ForthecurrentdiscussionandthatofWFD, Sec. 3.Beforewediscussthosedetails,ageneralcomment sion. clusion ofE93thatdiffusion is inadequatetoexplainthe older starsbackintimetolocatetheirinitialbirthplacesa 11 kpc,enoughtoroundofftheedgesofdiscontinuity dispersion in[Fe/H]basedupon theapproachofGrenon questionable onkinematicgrounds andbecauseitdepends of diffusionbyvandenHoek & deJong(1997)isequally (1987) isflawed.Amorerecent attempttoconstraintherole GC GC GC Since theworkofE93,agreatdealefforthasbeen The solutionweproposeisquahtativelythesameas 2571 1997AJ 114.2556T rather thanalineargradient.Inside R=10kpcthereis,at tance forthediskclustersisbest describedbyastepfunction the clusterpopulation.Itisconcluded that: the spatialandchemicalproperties ofthedisksampledby compiled, standardized,andmerged topermitananalysisof metallicity ofthestarsdetectableinoldestagebins. tainty intheagedeterminationamongolderstars,and upon avarietyofpoorlyknownfactors:theratioinner between 9and12Gyrago,theobserveddispersiondepends outer diskoverthesameagerange,increasinguncer- disk toouterfieldstarsinthesample,AMR cussed inSec.5,theAMRinteriortoÆqc^IOkpcdrops E93, andJónch-S<¿rensen(1995).Amongtheoldeststars R=10 kpcisindependentofdistance,itmakesnodiffer- varying degreesinTwarog(1980b),Meusingeretal.(1991), selection biasdiscussedabovewhichplacesacaponthe effectively constantuptoabout8Gyr.Thiseffectisseen Gyr, butincreasedramaticallybeyondthisageandremain from thesolarsampleandshouldincreaseinnumberwith locally formedstarsofsimilarage,theycannotbeisolated Because thesestarsarekinematicallyindistinguishablefrom age. Thenetresultisthatthemetallicityrangeamongfield ence whetherastarcomesfromÆg^IO.5kpcor14.5kpc. fuse inward,reachingthesolarcircleontimescalesof2to3 tions withage.However,starsbeyondR=10kpcalsodif- tallicity distribution;theydochangethekinematicdistribu- toward ^(^=8.5kpc,buthavenoimpactontheoverallme- by E93.Becausenogradientin[m/H]exists,thesuncould 4.6 Gyrago,apointemphasizedinsignificantlymoredetail Gyr (Widen1977).Becausethe[Fe/H]distributionbeyond to —0.10formthere.Starsbetween6and10kpcdiffuse dispersion mightbelarger,i.e.,starsintherangefrom—0.60 lactic diskbetweenR=10and15kpc,withminormodi- the approachoutlinedbyWFDtoidentifybirthplaceof have formedanywhereinthe6to10kpcrange.Ingeneral, (>8 Gyr),theexacttrendisdifficulttopredict.If,asdis- stars shouldremainsmallamongyoungerthan2to3 galactocentric distancerange,theabundancegradientsin cally have[Fe/H]between-0.2and+0.2.Overthesame Beyond apossiblegradualchangein[Fe/H]withtime,the between 6and10kpchasbeensolarwithin±0.10dex. ters? First,overthelast8Gyrmean[Fe/H]ofISM fication. Themean[Fe/H]intheouterdiskis—0.35,and field starsbecomesmoot. ent. Thus,thesunisnotatypicalofsolarneighborhood various metalshaveremainedshallowtoflat,i.e.,nogradi- dispersion in[Fe/H]atallageshasremained±0.09dexor explain thediscrepancybetweenfieldstarsandclus- Fe. smaller. Inshort,starsformedinthe6to10kpcrangetypi- cales discussedbyWielen(1977)arevalid,howdoesone upon theexistenceofwell-definedradialgradientsinOand 2572 TWAROGETAL:FORMATIONANDEVOLUTION GC GC GC GC An extensivecollectionofopenclusterdatahasbeen (a) Theabundancedistribution withgalactocentricdis- Second, avirtuallyidenticaldescriptionappliestothega- Assuming thatstellardiffusiondoesoccurandthetimes- © American Astronomical Society • Provided by theNASA Astrophysics Data System 5. ASUMMARYANDSCENARIO large partbecauseofthelogarithmic natureof[m/H].The time passesthemeanmetallicity ofthegasoutwhich in eitherzonehaschangedsignificantly overthelast8Gyr ters isameanmetallicitywhichdiffersby0.3to0.4dex.As tion oftheorigindiscontinuityisneitherfeasiblenor regarding therealityofdiscontinuity,adetailedexplana- reality, thisquestioncanbebrokendownintotwoequally excluded. Inmostmodelsofgalactic chemicalevolution,as though, again,increasesonthe orderof0.1dexcannotbe noted earlier,thereisnoevidencethatthemeanabundance which fitstheobservationalevidenceatpresent,bothfor lies inunderstandingwhyR=10kpcissospecial.Given it maintainedover10to12Gyr?Thesolutionundoubtedly puzzle isstillmissing:whydoesadiscontinuityexist?In clusters andfieldstars. warranted. Wecan,however,offeraqualitativescenario the currentstateofdataandjustifiableuncertainty difficult problems:howisadiscontinuitycreated,and gradient. tallicity rangeamongthefieldstarsinabsenceofalinear continuity, however,providesameansofincreasingtheme- stars formwillapproachsome limiting valuein[m/H], ary scenarioareinternallyconsistent,onekeypieceofthe the natureofgalactocentricabundancegradient.Thedis- metallicity distributionatthesolarcircleisindependentof the productofdiffusionstarsfrombeyondR=10kpc.It ity rangefoundamongstarswithagesgreaterthan3Gyris persion in[Fe/H]amongthefieldstars,generalideaof indication thateitherthemeanordispersionisasignifi- When coupledwithatruediscontinuityin[Fe/H],linear diffusion astheprimaryculpritsurvives.Thelargemetallic- hood. Thoughthelackofanabundancegradientremoves cant functionofage.Thisimpliesthatthemetallicity clusters greaterthan1Gyrinageismodest,thereno ters atthesolarcirclereducesto±0.09dexwhileaver- the innerclusters,dispersionin[Fe/H]foropenclus- fect whichtranslatesintoaverticalgradient. pendicular tothegalacticplaneforopenclusters.Theclus- should beemphasizedthatthesmalldispersionincluster agrees withthatderivedforfieldstarsinthesolarneighbor- age [Fe/H]isapproximatelysolar.Thoughthesampleof clusters. Aftercorrectingforaslightabundancegradient correction withgalactocentricdistanceleavesaresidualef- on average,largerZdistancesthatthosewithin10kpc. ters beyondR=10kpcincludedinpastdiscussionshave, best, ashallowabundancegradientwhichmaybetheprod- specific solutionproposedbyWFDtoexplainthelargedis- sun isnotatypicalforitsage. scribed bytwoGaussiansidentifiedwiththeinnerandouter exists. Thediscontinuityat10kpcseparatesthecluster uct oftheagedistributionsample.Beyond10kpc, sample intotwogroupsthatdifferinmean[Fe/H]byabout sample istoosmalltodetermineifacomparablegradient GC —0.35 dex. GC GC The primarydifferencebetweentheinnerandouterclus- Though theempiricallyderivedtrendsandevolution- (d) Themetallicitydistributionfortheinnerclustersdis- (c) Themetallicitydistributionfortheclustersiswellde- (b) Thereisnoevidenceforagradientinabundanceper- 2572 1997AJ 114.2556T by thetransitionpopulation,andmergeslocallytobecome cale ofafewGyr.Yetthediscontinuity hassurviveduntilat ferent, onewouldexpectradial gas diffusiontoplayarolein the gasinouterandinner disk weresystematicallydif- tinuity, wehavenoexplanation. Iftheinitialmetallicitiesof bution ofthediffusedstarsisdominatedbyin what WGrefertoasthemetal-weakthindisk. mained constantforsuchalongtime,themetallicitydistri- disk. Becausethemean[ra/H]inouterzonehasre- the starformationrateduringthisperiod,neitherofwhich thick diskwith[ra/H]between—1.0and—0.5,thin smearing, ifnotwipingout,the discontinuity overthetimes- [ra/H] rangefrom—0.1to—0.6,exactlytheoccupied distorted bytheadditionalfactorofdiffusionfromouter are known.Thisratherclassicpictureofdiskformationis tion dependsuponthetimescaleoverwhichitevolvedand lation connectingthetwo.Thesizeoftransitionpopula- with [ra/H]between—0.3and+0.2,atransitionpopu- tallicity distributionnearthesuntoreflectthreepopulations: offset betweenthetypicalthickdiskstar([m/H]=—0.6to current [ra/H]oftheinnerandouterdiskswhichreflects inner disk.Inthelongrunthisproducedadifferencein thick diskphasethatdrovethechemicalenrichmentof follows asimilartrend.However,theinitialmetallicityof time scaletheouterdisktakesshape,andchemicalevolution outer diskremained[ra/H]1.0becauseofthelack disk. Howquicklythisoccurredremainsunknown,butby8 inner diskevolvedfromthethicktostandardthin increased rapidlyfrom-0.6toapproximately-0.1andthe tween 10and8Gyrago,themean[ra/H]ofinnerdisk rounding halointhatwhilegasmayinfallfromtheto R ce~10kpc.AsoutlinedbyWGbasedprimarilyuponthe Gyr ago,themean[m/H]wasclosetosolar.Oversame in thesurroundinghaloremainsrelativelyunchanged.Be- disk risesfrom[Fe/H]1.0to-0.6,themeanmetallicity the surroundinghalo.Whilemetallicitywithinthick the disk,evolutionofthickdiskhaslittleimpacton namically andchemicallyasaseparateentityfromthesur- when thetypicalmetallicityofgasnearR=10kpcwas data ofE93,thispopulationoriginatedabout12Gyrago ters, theexactradialboundaryofthickdiskissetat therein). Throughtheuseofrichpopulationopenclus- the solarcircle(Armandroff1993;Zinn1996,andreferences lation typifiedby47TueandM71,extendsto,atleast, thick diskisassociatedwiththeglobularclusterpopu- [Fe/H]~ —1.0.Itisassumedthatthethickdiskevolvesdy- ferred toasthethickdisk.Chemicallyandkinematically, original boundaryofthenewlyformeddisk,currentlyre- reason. Asanalternative,weofferthefollowingscenario: yield forthechemicalevolutionofouterdiskissimply lower thanthatoftheinnerdiskforsomecurrentlyunknown under consideration.Thus,itispossiblethattheeffective yields, theinitialmassfunction,gasflows,andelement the exactvalueoffractiondependsuponstellar limiting valueissomefractionofthenucleosyntheticyield; —0.7) andthehalotransition([ra/H]=—1.0). 2573 TWAROGETAL.:FORMATIONANDEVOLUTION GC For theadditionalquestionof survivalofthediscon- If nothingadditionaloccurred,oneshouldexpecttheme- The discontinuityatR=10kpcisareflectionofthe GC © American Astronomical Society • Provided by theNASA Astrophysics Data System ter disruptionismoreefficient inside R=10kpc.Ifeither implies thatthetruescaleheight oftheouterdiskisgreater probability oftidaldisruption at largergalactocentricdis- young diskintheouterzoneor,moreimportant,of than thatoftheinnerdisk;additionally, itislikelythatclus- tance, theexcessofhigh-Zclusters canbereduced.Thisstill Phelps (1994)forÆ^MOkpc.Ifthescaleheightof tion ofoldandyoungclusters(Figs.79)inJanes& beyond 10kpcbecausethereislittlereliableinformationon have norealinformationonthescaleheightofoldclusters plane. Beyond^^=10kpc,thescaleheightisunknown;we height near55pc,alteredbytidaldisruptionovertime 55 pc,whencombinedwithselectioneffectsandthelower at thetimeofformationolderclustersisgreaterthan of stars;thisisapparentifonecomparestheradialdistribu- disk remainhiddenbydustorwithinarichbackgroundfield the numberofclustersneargalacticplane.Whilethose through preferentialdestructionoftheclustersnear to R=10kpc,thetrueobservedscaleheightisintermedi- Fig. 5isreal,thescaleheightof325pcderivedbyJanes& suggestion canbeconfirmedobservationally, itwouldsup- away fromtheplaneareeasilydiscovered,thosewithin Phelps (1994)isacomposite.Fortheclustersampleinterior tric radii.Ifthediscontinuityat10kpcappliestoscaleheight Janes &Phelps(1994)analysisisthatalltheoldclusters tation comestomind.Afundamentalassumptionofthe ate to55pcand325pc,theproductofanoriginalscale as well,i.e.,thelargerZrangeforclustersbeyond10kpcin derived, itisapplicabletotheclustercountatallgalactocen- come fromthesamepopulation.Thus,whenascaleheightis the plane,evenaccountingfortidaldisruption.Thus,old the large-Zclustersashigh-Ztailofthindiskpopu- mergers toexplaintheapparentlylargescaleheight.The over thelast10Gyr. cluster Z-distributionimpliesevidenceforregularmergers lation overproducestheobservablenumberofclustersnear former explanationisrejectedbecauseanyattempttoexplain disruption ofclustersnearthegalacticplaneorsatellite direction willnotworkforclusters,oneisleftwithtidal older samplerangefrom1to9GyranddiffusionintheZ estimate fromyoungopenclusters.Becausetheagesof from thatintheinnerzone.Janes&Phelps(1994)have analyzed theZdistributionofoldclustersandderiveda of theoldclusterpopulationinouterzonemaydiffer distribution ofclustersinFig.5suggeststhatthescaleheight includes clustersyoungerthan1Gyrintheinnerzone,Z plied bythescaleheightofclusters. scale heightof325pc,significantlylargerthanthe55pc namical differencebetweentheinnerandouterzonesissup- the innerandouterzones.Potentialevidenceforsomedy- light theneedforadynamicalboundaryseparatinggasin no directbearingonwhathappensat10kpcexcepttohigh- mixing drivenbythepresenceofabar.Thissuggestionhas proposed bySimpsonetal(1995)islocatedattheouter arms; theysuggestthatthestepfunctionisduetoradial edge ofaringmolecularcloudsandassociatedspiral least 1Gyrago.ThepotentialdiscontinuityatÆ=6kpc GC g GC Given thediscontinuityindisk,analternateinterpre- Though thesampleisbynomeanscompleteandplot 2573 1997AJ 114.2556T tances arethesameasthosein PCA andnodetaileddiscus- the clusterpropertieswhichareofinterest.Insomein- handful ofrecentpaperstohundredsspreadoverthelasthalf prehensive. Individualclusterreferencesnumberfroma more completedescriptionofthedetailsbehinddata Friel, A.Fry,M.Gim,E.Hufnagel,R.D.McClure,andJ. ered controversialbyothers,our primarygoalinthisAppen- conclusions differsomuchfrom theirsandwillbeconsid- consistent aspossibleinarrivingatrepresentativevaluesfor century. Whatwehaveattemptedtodoisbeasinternally This researchhasmadeuseoftheSIMBADdatabase,operated improved thankstothethoughtfulcommentsofreferee. and/or ourchoicesdifferradically fromtheirs.Becauseour amounts ofadditionalinformation haveappearedsincePCA at CDS,Strasbourg,France. aided thisinvestigation.Theclarityofthepaperhasbeen circle tellusnothingaboutthelocalgalacticdisk. formed inthethickdiskhavelongsincebeendestroyedand the outerzonestarsmightexhibitabundanceratios,e.g., the diskatsolarcircleusingfieldstarsmayremainan yond R=10kpcfromthosewhichformedlocallyaspart lating themetal-weakthindiskstarswhichcomefrombe- result isthatunlessanalternativemethodderivedforiso- sion oftheclusterisnecessary. Inothercases,significant stances, thephotometricsamples,reddeningvalues,anddis- summarized inTables1and2.Ourintentisnottobecom- Shields whosuppliedinformationand/orcommentswhich any distantclustersinthisagerangewellbeyondthesolar exercise infutility.Onepossibilityisthatatagiven[Fe/H], of thetransitionbetweenthickandthindisk,any metallicity couldbeusedtodistinguishamongstarsformed istence ofalinearabundancegradient,WFDshowedthatthe individually intobinsofgalactocentricorigin.Withtheex- i.e., ithasbeenassumedthatstellardiffusiondidnotmix typifies theISMatsolarcircleoverthatsametimescale, premised ontheassumptionthatonecanisolateafieldstar tion overtime.AllthestudiesofAMRtodatehavebeen batable, note.Aprimarygoaloffieldstarstudiesistodelin- Phelps 1994). port thenotionofadynamicaldifferencebetweeninner sample. Clustersrepresenttheidealobjectbutanyclusters [O/Fe] or[Ca/Fe],whichseparatethemfromtheinner attempt tounderstandthechemicalanddynamicalhistoryof above iscorrect,thispositionaltageliminated.Thenet at variousgalactocentricdistances.Iftheclusteranalysis arguments ofWielen(1977)andWFDarecorrect,thisas- eate thechemicalevolutionofdiskataparticularloca- mergers asathickeningagentingalacticevolution(Janes& in theoutergalaxyisalsoconsistentwithexplanationof sumption failsandkinematicscannotbeusedtosortstars stars fromsignificantlydifferentgalactocentricorigins.Ifthe and outergalaxy.Notethatthesuggestionofathickerdisk 2574 TWAROGETAL.:FORMATIONANDEVOLUTION sample ranginginageoverthelifetimeofdiskwhich gc The purposeofthissectionistoprovidethereaderwitha It isapleasuretoacknowledgethehelpofB.Carney,E. We closethispaperonasomewhatpessimistic,butde- © American Astronomical Society • Provided by theNASA Astrophysics Data System APPENDIX A.THECLUSTERSAMPLE ing asbeingthemostlikely, fewdirectobservationsof Though indirectargumentspoint tothisvalueoftheredden- was derivedassuming[Fe/H]=0.0 andE(B—V)=0.12. Twarog &Anthony-Twarog(1989), where(m—M)=ll.5 without redoingtheentireanalysis. ized toaninternallyconsistentsystem,theonlychangesre- retical (LogT,M)planetoobservational[(B—V),My\ analysis willbeexplicitlynoted. of theexcludedstarsisimportant.Starsfrom quired areadjustmentsforchangesinE(B—V)and/or ery etal.(1993)]. distances amongvariousgroupswhencomparingsupposedly the observationalplane.Thecontrastintwofiguresde- The culminationofthisprocesscanbefoundintheexcellent ingly similarresultsasmoreconsistencywasestablished theoretical plane,butthetransformationstorealworld plane. Modelsareinvariablycheckedtomatchthesunin main sequencefittingwillbedifferentiallycorrect.Thene- bias inourselectionprocedure. believe thatthecollectiveresultscannotbeattributedtoa finitively identifiesthetransformationbetweentwoas different groupsarecomparedinthetheoreticalplaneand discussion byNordstrometal.(1997),whereisochronesof among thevariousgroupsproducingtheoreticalisochrones. using theisochronesfromdifferentgroupsledtoincreas- that a(B—V),Moffsetbaseduponthesunprovided upon specificchoicesoftheexactvaluesheliumabun- cessity ofdoingthisliesinthetransformationfromtheo- technique hastheadvantageofensuringthatdistancestiedto isochrones havealwaysbeenadjustedtoguaranteethata theoretical isochronesfromawidevarietyofauthors.The we havemaintainedaconsistentapproachtorenormalizing is simple.Inallofouranalysesclusterdistancesandages, significant impactontheoverallmetallicity,soidentification [Fe/H] andthesecanbeappliedinastraightforwardmanner similar models[see,e.g.,theanalysisofM67inMontgom- source ofmuchtheclaimeddiscrepanciesinagesand and metallicity.Ourcheckoftheresultsforagivencluster differ fromonegrouptoanother. dance, 7,andthemetalabundance,Z,veryoftenthese often differdramatically.Moreover,isochronesarebased argue aboutthespecificchoices,particularly(B—V), authors (AT&T),andmorerecentattemptsatreanalyzingthe cluster isanearlierdiscussionofthebytwo disagree withspecificdecisionsonindividualobjects,we we selectedandanalyzedoursample.Thoughsomemay dix istoprovidethereaderwithadetailedsummaryofhow standardize acompletesetofisochrones,irrespectiveage simplest, thoughnottechnicallythemostcorrect,meansto [(5-V) =0.65]andthesolarM=4.S4.Thoughonemay solar massstarat4.6Gyrhavethecolor same orsimilardatahavebeenexcluded.Thereasonforthis ebol v V NGC 188.Ourstartingpointfor thisoldopenclusteris Finally, insomeclustersexclusionofafewstarscanhave Because ourclusteranalyseshavealwaysbeenrenormal- Implicit inourrenormalizationapproachwasthebelief In anumberofcasesthestartingpointinanalysis 2574 1997AJ 114.2556T member, confirmingtheoriginalsuggestionofMcClure DDO photometricsampleisthatofMcClure(1974)alone. recent yearswasbyHobbsetai(1990),whousespectros- bership ofthegiantsareill-determined.Moreover,only tion oftheredgiantclumpinNGC188isaslisted is cutinhalf.Thogersenetal.(1993)include4giantstheir the Table1includesremaining8giants;ifIII-Jisex- 1-1, isaknownFKComae-typevariable(Harris&McClure et al.1996)andradialvelocities(McClure1997).Ofthe10 the cluster,leadingtoE(B—V)—0.09forgiants.Our from 7stars.Wehaveadoptedthisrepresentativevaluefor copy ofstarsattheturnofftoobtainV)=0.10±0.03 the clustergenerallyimply£(5-V)=0.06to0.10(Twarog E(B—V)=0.% forthegiants,whichimplies0.86 particularly NGC752.IC166isdefinitelymoremetal-poor published cmdisthephotographicstudybyBurkhead many ofthemoredistantobjects.Thereddeningandmem- more preciseCCDsurveysinrecentyears. in anextendedregionaroundtheclusternotincluded Anthony-Twarog &Twarog(1985),baseduponaphoto- ing andaslightlylowerabundance,theanalysisofTwarog excluded fromthediscussion.Adjustingforalowerredden- cluded, theaverage[Fe/H]risesto—0.02anddispersion giants, III-94isaproper-motionandradial-velocitynon- Cluster membershipisavailableviapropermotions(Dinescu 2575 TWAROGETAL:FORMATIONANDEVOLUTION by FJ. The distancemodulusisbaseduponadifferentialcompari- nated bystar2-15.Ifthisisdropped,thedispersion turnoff. Themean[Fe/H]hasasignificantuncertainty,domi- than NGC752and,baseduponcmdmorphology,interme- graphic surveyofthegiantswithproper-motionmembership abundance basedupononeline.Thesetwogiantshavebeen a proper-motionnon-member.III-58hasanimplausible spectroscopic sample.11-181isaradial-velocitymemberbut found inDanieletal.(1994).TheDDOphotometryof 752 andleadstoasubstantiallylargerdistancethanadopted cut inhalfandthemeanabundancedeclinesby0.10dex. diate inagebetweenNGC5822and752.Wewilluse of IC166withanumbernearbyintermediate-ageclusters, & Anthony-Twarog(1989)gives(m—M)=11.35.Theloca- with ourcalibrationendsupvirtually identicaltothevalue weighting systembaseduponthenumberofobservations. formed tothecompositesystemofJanes(1979)andHardy of thisclusterandthemembershipredgiantscanbe (1974) baseduponthephotometricanalysis.Asecondstar, metallicity andtheapparentmodulus. [Fe/H] ofFJ,whenmergedwith theDDO,raisesmean adopted inDanieletal.(1994). However,thetransformed spective ofmembershiporbinarity. TheDDOabundance Stars HI10andH208falloutsidethecalibrationrange,irre- Schmidt (1984)andClariàetal.(1996)hasbeentrans- son ofthegiantbranchesNGC5822,IC166,and (1969), whoderivesEX#—V)=0.8fromaroughcomparison (1979), asdefinedinTwarog(1983),andmergedusinga 1985) andhasbeenexcluded.TheDDOabundancelistedin 1978). Theonlysignificantdirectmeasurefortheclusterin IC 166.Thisclustersuffersfromproblemscommonto NGC 752.Thecomprehensivediscussionoftheproperties NGC 1193.Thisanticentercluster isincludedinthe © American Astronomical Society • Provided by theNASA Astrophysics Data System E(B—V) =0.25usingaredgiantclumpwhoseexistenceis the staridentifiedas2isnotphotoelectric have excludedthisclusterfromoursample,eventhoughitis questionable. Asacompromise,wewilladoptE(B—V) mitted. Noriega-Mendoza&Ruelas-Mayorga(1997)derive resents thelowerlimittorangeallowedbyKaluzny tainty comesfromthereddening.FJadoptedE{B—V) how lowitgoesisamatterofdebate.Theprimaryuncer- reddening estimatearethesameasinPCA.Theresulting range inE(B—V)alsoproducesasignificantscatterthe dex in[Fe/H]fromtheDDOdataamong3giants.The Adopting ameanvalueinE(B—V)leadstorangeof0.6 members. AuvbyRßstudyoftheclusterbyPenaetal. list. Theproper-motionstudybyFrancic(1989)showsthis found inPCA.Thereare4giantsJanes(1979).Ofthese, quence fitusesthecmdfromKaluzny(1988). E(B —V)=0.33forthegiant.Theonlyrefiablecmd star tobeanon-member;theotherthreestarsareprobable as 2inHoagetal.(1961),butastarfromthephotographic analysis ofFJ.Thoughitdefinitelyhassubsolarmetallicity, E{B —V)=0.29.Wewilluse0.32 forthemainsequencefit turnoff starstoobtainE{B—V)=0.32.FromDDOanalysis from Janes(1979)andClariáetal.(1996);botharemembers from binarystarobservationswithinthecluster(Torresetal. this [Fe/H]andmainsequencefittingleadsto{m—M) DDO photometry,butsmallerthanderivedbyPCA.Using of thespectroscopicabundancescaleusedtocalibrate mated at±0.2to0.3. of thescatterindata,uncertainty(m—M)isesti- derived frommainsequencefittingtothissample.Because cluster isthatofHoagetal.(1961)andthedistancehasbeen adopted here.Thisleadstoaslightlylowerreddeningof literature, 0.34to0.38,frommainsequencestarsandwillbe Clariá etal.(1996);ofthese,oneisaprobablenon-member data, thisobjecthasbeenexcludedfromtheanalysis. et al.(1961)photometry.Giventhelackofreliablecluster distance baseduponmainsequencefittingusingtheHoag (1994) leadstoarangeinE(B-V)from0.280.41. (1988) discussion.ValuesashighE(B—V)=0.24areper- that ofHoagetal.(1961). from radial-velocitywork.Lyngâ(1987)derivesE{B—V) stars intheareaofthisclusterfromSchmidt(1984)and Harris &(1977).From UBVanalysistheyfind of thegiants,Janes(1979)and Clariáetal.(1996)bothget [Fe/H] of+0.17isslightlyhigherthanthezero-pointvalue (1996), £(#—V)=0.36,istypicaloftherangefoundin (Clarià etal.1996).ThereddeningadoptedbyClariá and 0.29forthegiants.Thecmd forthemainsequencefitis slightly tobetween3.30and3.35. =0.19 forthegiantsand0.20turnoff.Themainse- =0.12 basedupontheworkofKaluzny(1988),butthisrep- 1997). Ifwerequire[Fe/H]=+0.12,{m—M)decreases =3.40, inexcellentagreementwiththerecentdetermination =0.34, whilePena&Peniche(1994)useuvbyUßofthe NGC 1342.Thereasonforthisnoteistoexplainwhywe NGC 1545.DDOphotometryisavailableforonlytwo Hyades. Thedatasources,membershipinformation,and NGC 1662.TheDDOdataareamergedsetfortwostars NGC 1817.Thedefinitivestudy ofthisclustertodateis 2575 1997AJ 114.2556T revised boththereddeningandmetallicityestimatefor £(Z? —V)=0.28fortheturnoffand0.23fromgiants;we bluer stars.WewilluseE(B— V)=0.33forthegiantsand E(B —V)=035,thesamewithinerrorsasoriginal near infraredobservationsofRosvick(1995)whofinds radial velocitiesforonly3.TheofMinniti E{B —V)=0.60forthegiantsand0.63turnoff.Rich- persion spectroscopy.TheyalsofindE(B—V)=0.6from transformed spectroscopicscale.Thisisinexcellentagree- binary memberanddeviatesthemostin[Fe/H]from E(B -V)=0.29.StarQ[485inMermilliodetal.(1996)]isa for thegiantsand0.55turnoff.Thecmdistakenfrom higher valueofthereddening.Wewilluse£(5—V)=0.51 cluster, loweringE(B—V)from0.7to0.55andincreasing is fromHarris&(1977). will adjustthereddeningofgiantsto0.26.Only2124 which canbeusedtoderivethe distance. for theclusterisinVI,but doeshavearichclump variable reddening.FJusedE(B —V)=0.30.Thebestcmd 0.35 fortheturnoff,notingthat theclustermaysufferfrom tler &Kaluzny(1989)providetheonlycmdofmain two giants,thesamevalueadoptedbyFJ.Wewilluse ment withtheresultsofBrownetal.(1996)fromhighdis- be totallyanomalousornon-members(Richtler&Kaluzny excluding 4-16.Stars3-16and3-17havevelocitiesconsis- in [Fe/H]andrecentworkbyBrownetal.(1996)leadsto the galacticcenter.However,giantsexhibitawiderange unreliable toallowanaccuratereddeningestimatefrom tographic workbyBrosterhus(1963)andWest(1967)istoo the dispersiondropsbyalmostafactorofthree.Earlierpho- the redgiantsbyMermilliodetal.(1996)produces ticenter clusters.FJprovidedataontwogiantsbutadoptthe Christian (1981)and(1984).Thelatterreference was excludedfromtheFJsampleasanon-member.Thecmd 2576 TWAROGETAL.:FORMATIONANDEVOLUTION 2142. ThebestreddeningestimatecomesfromtheVIand off theexpectedlocationofgiantbranchthattheymust tent withmembershipwithintheerrors,butarelocatedsofar velocity. Brownetal.(1996)find4-2tobeamemberwhile non-members. FJexclude2-16baseduponitsradial- the conclusionthatallbutoneofstarsobservedbyFJare ticenter cluster,butnearenoughthatitfallswithin10kpcof the mainsequencefit.ThecmdistakenfromMermilliod mean; ifdeletedfromtheaverage,[Fe/H]risesto0.15and Glushko va&Rastorguev(1991)andMermilliodetal. finds E(B—V)=0.26from5stars.Radial-velocitystudiesby Christian (1981). sequence; itexhibitslargescatter. etal. (1996). [Fe/H] toalevelcomparablewithNGC2420andotheran- spectroscopic dataofFJwouldmakethisametal-poor,an- (1995) add5003totheprobablememberlistwhileexcluding (1996) confirmmembershipofallfivestars.UBVanalysis UBV estimateof0.30byBurkhead etal.(1972)fromthe UBV. WewilluseE{B—V)=0.27forthegiantsand0.29 1989). Thisleavesonly4-2whichhas[Fe/H]=-0.11onthe NGC 2141.FJprovideabundancedatafor7giantsbut NGC 2112.Thisisanimportantbutdifficultobject.The NGC 2099.TheDDOdataarefromJanes(1979)who King 8.Theprimaryanalysisofthisclustercomesfrom © American Astronomical Society • Provided by theNASA Astrophysics Data System rived withthelowerreddeningiscrucialbecauseitcarriesso ing. AsthoroughlydiscussedbyFJ,thelatterparameterpre- the globularsM71and47Tue.Allfourgiantshaveradial plying E(B—V)=0.19forthegiant.Surprisingly,despiteits E(B—V)=0.ll. Wewilladopt0.21asacompromise,im- Janes (1979).Radial-velocitydata(Friel1989)implythat the clusterundertwodifferentassumptionsforE(B—V), uncertainty inthereddening,wewillincludeestimatesfor the truedistancedecreases.Theextremelylow[Fe/H]de- the one-sigmalimit,mean[Fe/H]risesby0.3dexwhile the largestapparentmodulusandsecondredden- est metallicityclusterinthesample,moremetal-poorthan by 0.09dexand,moreimportant, thedispersiondeclines.We probable membersareincluded, the[Fe/H]for5starsdrops the highsidewithalargedispersion among8stars;ifonly the choiceofDDOgiants;mostareselectedtobebrightbut the sample.Fieldstarcontaminationisnotunexpectedgiven bership ofthegiantsgivenwiderangein[Fe/H]among richness, proximity,andthepresenceofatleasttwowhite photographic dataofmainsequencestarswhileVidal E(B—V) =0.23fromacombinationofphotoelectricand Lyngà (1987).Oursurveyoftheliteraturefindsonlytwo photometry fromtheworkofSchmidt(1984).Thereddening ing giants.If3401isexcluded,theDDOreddening0.36, ter. Noadditionalinformationisavailableforthe3remain- 0.7 and0.9.Thedistancemodulusisbasedupontheposition much weightintheabundancegradient.Duetolarge velocities consistentwithmembership.Theclusteralsohas definite non-membersareexcluded, themean[Fe/H]ison definitive isolationofthemembers isimpossible.Ifonly dwarfs, theHoagetal.(1961)cmdremainsbest value of0.16adoptedbyPCAisbaseduponthecatalog clump todeterminethedistance.Forageandmetallicity convective overshootmakethisapproachunreliable.Wewill The normalizationproblemsoftheisochronesandlack Berg (1985)toderiveE(B~V)=0.55and[Fe/H]=-0.45. tain E(B—V)=0.43,withasignificantuncertainty.Christian of theclumpfromcmdinPhelpsetal.(1994). sents thegreatestdifficulty.FJadoptE(B—V)=0.10,but sample. ThespectroscopicabundanceofFJmakesitthelow- and Minniti(1995)providesome radial-velocitydata,but often arepositionedwelloffthegiantbranch.Friel(1989) available andhasbeenusedtoobtainthedistance. agree withLyngâ(1987).Hoagetai(1961)derive direct determinationsofthereddening,neitherwhich of NGC2158,M=+0.6isused. cluster, however,itismorerehabletousetheredgiant while Arp&Guffey(1962)useUBVoffielddwarfstoob- 3401 isanon-memberwhile4413and4436areintheclus- state thatthisisaprobablelowerlimit.Ifoneadopts0.90, adopt E(B—V)=0.40forthegiants,implying et al.(1985)attemptamatchtotheisochronesofVanden- (1981). Theprimarydifficultywiththisclusteristhemem- (1973), usingsimilardataofmuchlowerquality,finds =0.44 forstarsattheturnoff.Becauseofage v NGC 2158.DDOdataforthisrichclustercomefrom BE 21.Thisclusterdefinesthetermextremefor NGC 2204.TheonlyDDOdataarethoseofDawson NGC 2168.ThereisonlyoneclustermemberwithDDO 2576 1997AJ 114.2556T binary sequenceisapparentineverycmdafterHawarden E(B —V)=0.06forthemainsequenceandgiants. Norris &Hawarden(1978).Star1219isaradial-velocity the CCDdataofKassisetal.(1997)whichisusedtosupply branch oftheredgiantclumpastrueclump,ourmain binary populationcoupledwithasparsegiantbranch.The in NGC2243,aneffectwhichmaybetheproductofrich The cmdofBonifazietal.(1990)hasbeenusedinthemain ing andthecolordifferencebetweengiantsturn- constrain itbetween0.00and0.08.Fortunately,theredden- warden 1975).Kaluznyetal.(1996)useafieldRRLyr ing fromE(B-V)=0.0l(vandenBergh1977)to0.06(Ha- ture ofdwarfsandgiantsleadstoreddeningestimatesrang- nonmember (FJ)andfourothersareprobablemembers.Only the cmd. main sequencefitting.Thisdeficiencyhasbeenremediedby either didn’tgofaintenough(Hawarden1976a)orweretoo will adoptforthegiants.Untilrecently,clustercmd’s can befoundinHoudesheltetal(1992).Reddeningfrom will usethelattersamplewhichincludes1129,1133,1320, rones andNGC2420toderiveE(B-V)=0A0,whichis below 0.1,butthiscanbeexcludedgiventhecolorof have radialvelocitiesconsistentwithmembershipandthe metal-poor clustersinthesample,implyingMlessthanor close bynotingthatwhileNGC2243isoneofthemost off aresmallnomatterwhatthechoice.Wewilladopt variable andVIdatatoplaceanupperlimitofE(B—V) 2308 lacksmembershipinformation.UBVanalysisofamix- 2120, and4137.Additionaldiscussionofthemembership 2577 TWAROGETAE:FORMATIONANDEVOLUTION than attemptamainsequencefittotheCCDcmdinKubiak consistent withthespectroscopicanalysisofBrownetal. giant branchandtheredclump,asillustratedinKubiak with thereddening.Adler&Janes(1982)chooseavalue an ageintermediatetoM67andNGC188forthederived equal to+0.6fortheclump,itisalsoamongoldest,with et al.(1996).Thoughmostanalyseshavetaggedthebrighter sparsely populated(Frogel&Twarog1983)toallowreliable giants. Becauseofthescatterinmainsequence,rather et al.(1992).Kubiak(1992)usecomparisonstoisoch- [Fe/H] (Bergbuschetal.1991). sequence fitismoreconsistentwiththefainterbranch.We sequence fit,producing(m—M)=13.40.Bonifazietal. PCA. Aratherscatteredcmdis availablefromHoagetal. this cluster,sowewilladoptthe reddeningvaluegivenby is unpublished.Littledefinitive informationisavailablefor lies outsidethecalibrationrange. Theremainingphotometry et al.(1992),wewillusethewell-definedredgiantclumpat sample ofBrownetal.(1996).Thegreatestuncertaintylies (1975), butisexceptionallyvisibleintheVIplotofKaluzny (1990) havediscussedthedichotomyofredgiantclump Schmidt (1984),butitisclassified asanon-member;italso (1996). Asacompromise,wewilluseE(B—V)=0.3forthe UBV andDDOisconsistentwithE(B—V)=0.08,whichwe =0.08. Bonifazietal.(1990)useisochronemorphologyto (1961) andmain sequencefittinggivesadistance modulus V=16.2 toestimatethedistance. v NGC 2243.TheDDOphotometryfor6starsisthatof NGC 2251.Onestarinthisclusterhasbeenobservedby TOM 2.TwogiantshavebeenstudiedbyFJ;theyboth © American Astronomical Society • Provided by theNASA Astrophysics Data System reddening; wewilluse£'(5-V)=0.04forthemainse- The cmdusedinthemainsequencefitcomesfromHarris exclude star87.Noinformationisavailablefor204butits E(B-V) between0.10and0.12.Kassisetal.(1997)discuss radial-velocity members.Reddeningestimateshavebeenob- nonmember, star9,fallsoutsidetheDDOcalibration.The lieved tobeamember(Clariá1985);theprobable from DDOdata.Nissen(1988)getsE(B-V)=0.04 et al.(1993). will beexcluded.Star92fallsoutsidethecalibrationrange. quence andthegiants.Proper-motiondata(lannaetal.1987) ety ofsourcesandrangefrom0.0(Hoagetal.1961)with is undoubtedlylargerthan±0.2mag. very similartothatusedbyPCA.However,theuncertainty DDO iswell-determinedatE(B-V)=0.09from10giants by McClure(1972).Radial-velocitydataareavailableforall main sequencefittingwasdoneusingthecmdofKaluzny tained bycomparisonofthecmdtotheoreticalisochrones the uncertaintyin(m—M)isatleast±0.3mag. questionable, butwewillusethemodulusderivedbyClariá of Hoagetal.(1961). main sequenceandthegiants.Theonlycmdavailableisthat grounds (Clariáetal.1989)andproduceE(B—V)=0.0T 2 giants,bothofwhichappeartobemembersonphotometric abundance deviatessomuchfromtheclustermeanthatit et al.(1978).Reddeningestimatesareavailablefromavari- but contradictstheestimateofE(B—V)=0.0TfromEggen the giantsand0.12forturnoff.FJadopted0.12.The ants. OnlyoneofthetwogiantswithDDO,star4,isbe- uvbyHß of10Fstars.Wewilladoptthelattervaluefor the clusterisintermediatein agebetweenNGC5822 morphology (Eggen1968;Murrayetal.1988)indicatesthat consistent withallbeingmembers.Thereddeningfrom et al.(1993),thoughtherecentdiagramofKassis a newdeterminationof£(£—U)=0.11.Wewilluse0.11for scatter intheturnoffregionmakesanymainsequencefit sis oftheturnoffstars,implyingE(B—V)=0.36forgi- same asindicatedinPCA.E(B—V)=0.40fromUBVanaly- (1993) providesconvincingevidenceforasmallbutnonzero (1984), Pastoriza&Röpke(1983),andFeinsteinetal. NGC 5822to 2360toNGC752.Moreover, theintrin- luminosity ofthegiantsandturnoff starsgrowslargerfrom virtually identicalforallthree clustersbutthedifferential color differencebetweenthegiant branchandtheturnoffis sample. FJobserved7giants,allofwhichappeartobe (1973), adjustedforaHyadesvalueof3.4.Itisprobablethat (Pastoriza &Röpke1983).ThediscussionbyHarrisetal. (1978); thedataweretransferredtosystemofFeinstein stars exceptstar2fromMermilhod&Mayor(1990)anditis (Kaluzny &Richtler1989;Carraroetal.1994),leadingto (1997) worksequallywell. (Twarog etal.1993)andNGC 752 (Danieletal.1994).The (1968) usingUBVanalysisofmainsequencestars.Thecmd UBV ofthemainsequenceto0.07fromDDOgiants NGC 2301.Thissparselypopulatedclustercontainsonly NGC 2287.DDOphotometrycomesfromSchmidt NGC 2335.Essentiallyallclusterparametersremainthe NGC 2360.TheDDOphotometrycomesfromthesurvey BE 39.Thisclusterfallswithintheoldestgroupin 2577 1997AJ 114.2556T E(B —V)=0.62isconsistentwithintheerrorswithUBV relative toNGC5822and752. has thesameageasNGC752;cmdmorphologyismore reddening valueofEggen(1968)wouldinferthatNGC2360 member ofthissparsecluster.TheDDOreddening Eggen (1968)givesaveryuncertainvalueof(m—M) Main sequencefittingusingthesparselypopulatedcmdof consistent withE{B~V)=0Wto0.10forthegiants.We 2578 TWAROGETAL.:FORMATIONANDEVOLUTION reliable estimateof£(#—V)=0.10isavailablebasedupon reddening estimateandmodulus. the laterdiscussionandimplyimplausibleclusterparam- the average[Fe/H]slightly.Thehigherincreases to excludestarZasamember,theremaining10giantspro- membership assummarizedinAnthony-Twarogetal.(1990) Twarog etal.(1990),weadopt£'(B-V)=0.05forthegi- modulus isbasedupontheadjustedDDOestimate. find E(B-V)=0.61;wewilluse0.62forthegiants.The analysis oftheturnoffstarsbyMoffat&Vogt(1975),who will use0.09forthegiantsand0.10mainsequence. sic colorofthegiantbranchgrowsredderwithage.The E(B —V)=0.21.Thisisconsistent withintheerrorswith The resultingDDOmetallicityissimilartotheHyades,in upon theoriginalsurveybySmyth&Nandy(1962).Amore Mayor 1990).E(B-V)=0.13wasadoptedbyPCAbased eters. Forthetwoclustergiants,wewilluseonlyDDO cluster, buttheresultsfromearlierstudydisagreewith giants inthissparselypopulatedcluster.Fenkartetal.(1972) duce [Fe/H]=—0.28.MergerwiththedataofFJincreases et al.(1974)andusingradial-velocityproper-motion ants andthemainsequence.WithDDOdataofMcClure Hartwick etal.(1972)werecombined. Theprimarydiffi- rived fromtheintermediate-band photometry. Hyades modulusof3.4leadsto(m—M)=10.3withthe from intermediate-bandandRIphotometry.Adjustingtoa estimate. ThebestavailablemodulusisthatofEggen(1983) clusters byEggen(1983).Thecmdforthecluster(Hassan excellent agreementwiththedirectcomparisonoftwo off (Eggen1983),indicatingE(B—V)=0.09forthegiants. Clariá etal.(1989)hasbeenpublished.Ofthe4DDOgiants, and Moffat&Vogt(1975)havepublishedUBVdataforthe apparent distancemodulus. 0.20 forthegiantsaslistedinPCA. Noreliablecmdisavail- estimate fromanalysisofphotoelectricandphotographic etry forthisclusterremainsunpublished.Anearlyreddening adopted reddening. uvbyüß photometryof19AandFstarsattheclusterturn- star SN1isaradial-velocitynon-member(Mermilliod& able forthecluster,soweadopt thedistancemodulusde- stars byStetson(1981)givesamorerehablevalueof = 10.35,consistentwiththeluminosityofgiantbranch UBV databySmyth&Nandy(1962)producedE(B~V) 1976) containstoomuchscattertoprovideareliabledistance =0.14, butuvbyüßphotoelectricphotometryoftheturnoff NGC 2420.BaseduponthediscussioninAnthony- RUP 18.Clariá(1985)givesphotometryforoneprobable NGC 2423.OfthephotometryreferencedinPCA,only HAF 8.Clariáetal.(1989)provideDDOdatafortwo NGC 2477.DDOdatafromHesser &Smith(1987)and NGC 2437.ThoughincludedinPCA,theDDOphotom- © American Astronomical Society • Provided by theNASA Astrophysics Data System 4067 areprobablemembersfromtheradial-velocitydataof We willincludeit,butuseonlytheresultsfromFJsinceall DDO metallicityfromthefourgiantsisessentiallysame FJ; theothergiantshavenomembershipinformation.The mates areavailablefromIRphotometryinHoudasheltetal. ing acrossthefaceofcluster.Individualreddeningesti- culty withtheclusteristhatitsuffersfromvariableredden- based uponcmdmorphology.Thedistancemodulusob- NGC 2360and5822whichbracket2477inage For thedistance,wehaveusedcmdofKassisetal. troscopic abundancesareweaklydependentuponreddening. their starsappeartoberadial-velocitymembersandthespec- culties withthiscluster,PCAexcludeditfromtheiranalysis. only 4giantsremain(1044,4067,8019,andk).1044 tained isthesamewithin±0.1mag. check usingacomparisonofthegiantbranchtothose cluster ofE(B—V)=0.25.Wehaveperformedasecondary as theHyades,withsignificantscatter.Becauseofdiffi- E(B —V)=0.09forthegiantsand0.10mainse- than thepreliminaryestimateof0.04byMoffat&Vogt bers. Usingthehotterstars,theyfind£(5-V)=0.09while Johansson (1968)wasestimatedatE(B-V)=0.39,butthe based uponunpublishedradial-velocityinformation.Red- the calibrationrange.Allstarsexceptstar3aremembers uncertain (m—M)=10.3. modulus adjustedfortheHyadesdistance,onegetsavery to permitareliabledistancedetermination.UsingtheDDO DDO ofthegiantsproduces0.11,bothsignificantlylarger (1992). Ifstarsfromthissampleareselectedfortheanalysis, The mainsequencefitwasaccomphshedusingthecmdof member giantsusingDDOphotometry(Clariáetal.1996). Clariá etal.(1996).Allstarsexcept14and37falloutside quence. ThecmdofMoffat&Vogt(1975)istooill-defined (1997) whichimpliesamainsequencereddeningforthe centration ofafewbrightstars.Thismayexplainwhythe two probablemembers,acmdstudybyCarraro&Patat Ramsay &Pollaco(1992). giants. Thisisthesamewithinerrorsas0.40from2 CCD dataindicateE(B~V)=0A5,leadingto0.41forthe ous problemswiththephotographicUmagnitudes.The CCD workofRamsay&Pollaco(1992)demonstratesseri- dening fromUBVanalysisoftheturnoffstarsbyLindoff& (1983) whoidentify3probablegiantsasphotometricmem- of thisclusterisfoundinthe analysisofNGC2420 DDO abundancelistedinTable 1. Thespectroscopicresultis tive membershipinformation is included,oneobtainsthe calibration. Ofthese,fouraredefinitememberswithamean Clure etal.1981),only5fallwithintherangeofDDO each otherby0.28dex. virtually identicaltothephotometric. Theprimarydiscussion [Fe/H] estimatesfromDDOforthetwogiantsdisagreewith (1975). GiventhelargersampleofAstars,wewilluse (Anthony-Twarog etal.1990). If oneadoptsthesamered- [Fe/H] =-0.415±0.018(s.e.m.). Ifthegiantlackingdefini- (1995) concludesthatthereisnocluster,justarandomcon- NGC 2482.TheprimarysourceisClariá&Lapasset NGC 2489.TheDDOphotometryof6giantsisgivenin NGC 2506.Ofthe7giantswithDDOphotometry(Mc- RUP 46.ThoughincludedintheanalysisofPCAwith 2578 1997AJ 114.2556T but thosethatexistareconsistent. Pesch(1961)usedUBVof The cmdistakenfromDoddet al.(1977). E{B —V)is0.09to0.13usingdatadominatedbystarsatthe from themainsequencestarsfound byDoddetal.(1977). for thisgiant,[Fe/H]=-0.08,astheclustervalue.The overall distributionforclustersatitslocation.PCAreferto members ofthecluster(Clariá1985),butdifferencein main sequencefitisbaseduponthecmdprovidedbyMeynet In ajudgementcall,wewillexcludetheanomalousstar.Our turnoff; wewilladopt0.11fortheturnoffand0.10 radial-velocity andproper-motionworkquotedinPCA. produces ananomalouslyfaintclumpatM=1.8Kassis adopted reddeningforthegiantsisE(B—V)=0.09,slightly fied, thelargedifferencein[Fe/H]clearlyimpliesthatstar one starintheirdiscussion.Thoughnotspecificallyidenti- unpublished radial-velocityobservationsandincludeonly value ofthetwois[Fe/H]=+0.07,placingitwellwithin et al.(1993),whichissimilartothatofEggen(1972). the othersandallotherabundanceestimatesforcluster. appears tohaveananomalouslylowabundancecompared lem: twogiantsareinexcellentagreementwhilethethird Combining thesummaryinDachs&Kabus(1989)with modulus to13.95.FJandPCAadopt(m-M)=12.7,which Twarog etal.(1995)wherethesamereddeningadoptedhere referenced inGeisler&Smith(1984)indicatethat4326isa smaller thanPCAbutmoreconsistent with£(#—V)=0.08 giants. TheDDOabundancespresentsomethingofaprob- with [Fe/H]=—0.5. was used.However,thehigher[Fe/H]increasesapparent membership for1242,4151,and4266.Unpublishedresults Twarog etal.(1995).Radial-velocitydatafromFJimply Hawarden (1976b)fromUBV,derivesE{B-V)=0.14 The onlyDDOdataarethoseofDawson(1978)who,like tance hasbeensystematicallyunderestimatedinpastat- tempts toplaceitwithinthegalacticabundancegradient. tion (Anthony-Twarogetal1994).Mostimportant,itsdis- the turnoffwhichcannotbeexplainedbybinarycontamina- disk. Itisrichlypopulated,butexhibitsaspreadincolorat Twarog etal.1979)thatitisoneoftheolderclustersin has beenknownforsometime(Hawarden1976b;Anthony- NGC 2506requiresanadditionaladjustment. 0.4 magsmallerthanthatofNGC2420.Thelower[Fe/H] fitting requiresthatthedistancemodulusofNGC2506be (1986). Clusterreddeningestimates arefewandfarbetween, [Fe/H] forthetwogiantsis0.30dex.Notethatmean uvbyHß workofNissen(1988),therangeinderived et al.(1997)derive(m—M)=13.7,andalargeuncertainty, giants. Thecmdinthemainsequencefitisthatfound dance. Noinformationisavailableforthefourremaining field star,consistentwithitsanomalouslyhighDDOabun- the giants.ThisimpliesE(B—V)=0.16forturnoff,con- (1989). Allthreestarsappeartobemembersbasedupon sistent withintheuncertaintywithturnoffanalysisof dening andabundanceforthetwoclusters,mainsequence 2579 TWAROGETAL.:FORMATIONANDEVOLUTION 124 istheclustermember.Wehaveincludedabundance y NGC 2527.DDOdataexistfortwoprobablephotometric NGC 2516.TheDDOdataarethoseinClariàetal. NGC 2539.TheDDOdataarethose ofClariá&Lapasset MEL 66.Thisisakeyclusterfornumberofreasons.It © American Astronomical Society • Provided by theNASA Astrophysics Data System though barelyreachingtheunevolvedmainsequence,iscon- the giants.Stars16,54,and104falloutsidecalibration. E(B —V)=0.\3,whichimplies0.12forthegiants.Thisis E(B —V)=0.06,buttheturnoffstarsimply0.05(Pesch refer tounpublishedphotometry.Wewillincludeboth99 The metallicityiscomparableto thatoftheHyades,resulting cluster giantsfallwithintheDDO calibrationrange.Our & Pollaco(1992). limited cmdistakenfromClariá&Lapasset(1986)which, giants classedasphotometricmembers;wewilluse0.12for consistent withintheerrorswithDDOvalueforthree Clariá &Lapasset(1986).Fromthemainsequencestars, tency withtheHyades. DDO luminositiesofthegiants,modifiedtomaintainconsis- the mainsequenceexists,sodistanceisbasedupon tion range.Thereddeningforthethreeremaininggiantsis Clariá (1985).Proper-motionmembership(Ebbighausen the turnoffconsistentlygivenE(B-V)near0.06(Eggen in Clariá(1982).UBVanduvbyRßanalysesofthestarsat mate. We willusetheDDOmodulusadjustedtoHyadesesti- tively unimportantwhenderivingthegalactocentricdistance. upon oneofthetwogiantswithpublishedDDOphotometry, the mainsequencestarsandClariá(1985)derives0.14from sample isthesameasPCAexcept fortheunpublisheddata. Stars 37,61,and177arephotometricnon-members.The the clustermakesalargeuncertaintyinmodulusrela- we areunabletoascertain.Wewilluse0.14forthegiants average [Fe/H].Lindoff(1968)findsE(B—V)=0.1Tfrom to Harris(1976),butPCAincludeonlyoneasamemberand tial modulusrelativetoNGC2632usingtheredgiantbranch classified asfieldstars.ThesmallersamplefoundinPCAis siderably morepopulatedthanthe CCDdiagramofRamsay sequence fittingcomesfromClariá(1982). as amember.Aphotometricanalysisoftheclusterisgiven and 0.16fortheturnoff.Fortunately,galacticpositionof a singlegiant,356.PCAadopt0.11.Whetherthisisbased and 356eventhoughthisleadsalargeuncertaintyinthe Clariá (1985).Bothgiantsareprobablemembersaccording due totheexclusionofbinaries.Noreliablecmdmain observed byClariá&Lapasset(1986).Oftheremaining5 from Mermilliod&Mayor(1989)for6ofthe13giants the turnoffstars.Radial-velocitymembershipisavailable for thegiants.Wewilladopt0.09giantsand0.10 hotter starstogetE(B—V)=0.10,whileDDOimplies0.08 (m-M) =10.75. as discussedinMermilliod&Mayor(1989),leadingto sequence isavailableforthecluster.Weadoptdifferen- 29, 47)liewelloffthegiantbranchofmembersandare stars withintheDDOcalibrationrange,all(stars15,16,22, 1961). Wewilluse0.05forboth.Noreliablecmdreaching 1939) excludesstar108,while66fallsoutsidethecalibra- 1980; Clariá1982;Shobbrook1986).Thecmdusedinmain NGC 2632.AlsoknownasPraesepe, onlyfourofthe NGC 2567.Thedataforthisclusteraresummarizedin NGC 2548.TheDDOdatafor5giantscanbefoundin NGC 2547.Thisclustercontainsonlyoneprobablegiant NGC 2546.DDOdataaretakenfromSchmidt(1984)and 2579 1997AJ 114.2556T timates ratherthantheDDO and restrictthemetallicity we willbaseourreddeningfor thegiantsonturnoffes- velocities consistentwithmembership, asdiscussedinClariá especially ifonerestrictsthesampletothosestarswithradial the DDOestimateforreddeningissignificantlyhigher, Mermilliod 1981;Schmidt1982),leadingtoE(B—V) sample onlytothosestarswith radial velocities.Stars2and et al.(1989).Giventheinternal consistencyoftheresults, from themainsequencestars(Jankowitz&McCosh1963; A numberofreddeningestimatesareavailable,primarily modulus. tric distancerelativelyinsensitivetochangesinthe the galacticcoordinatesofclustermakegalactocen- choose thesamephotometricmodulusasPCA.Fortunately, value forthegiants.Thelackofareliablecmdforcesusto the giantsgiveE(B—V)=0.34.Wewilladoptlatter possible non-memberdoesnotaltertheaveragesignificantly; cmd andestimateE(B—V)=0.35fromthebluerstarswhile we willincludeit.Vogt&Moffat(1973)publishascattered dances forallthreestarsaresosimilarthatinclusionofthe member baseduponphotometriccriteria.TheDDOabun- the discussionbyPCA.DDOdataareavailableforthree proper-motion datafromGirardetal.(1989)allowustoex- E(B—V) =0.04forthemainsequenceand0.035gi- giants (Clariáetal.1989),oneofwhichmaybeanon- clude IV-169,T574,andT654.Thecmdusedinthedistance ing arangefromE(B-V)=0.03to0.06.Wewilladopt been determinedfromawidevarietyoftechniques,produc- determination isthatofMontgomeryetal.(1993). ants. Radial-velocityworkfromMathieuetal.(1986)and of Janes&Smith(1984)alone.Reddeningestimateshave certainty in(m—M)isundoubtedlylargerthanaverage. turnoff andgiantbranchsimilarcompositions.Theun- tance viaacomparisonofthegiantbranchesNGC2360 produce areliablemodulus,sowehaveestimatedthedis- The DDOreddeningofE(B-V)=0.35istotallyconsistent Vogt (1975),adjustedforanincreasedHyadesestimate,and and NGC2660.Bothclustershaveverysimilarcmd’satthe from thediscussion.Themainsequenceistooill-definedto unpublished radial-velocitydataisgiveninHoudesheltetal. with thebluestarestimateof0.38(Hartwick&Messer is ofquestionableaccuracy. distance isbaseduponthemainsequencefitbyMoffat& Vogt 1975);weadoptthesamevalueusedbyPCA.The tometric grounds,butitsinclusionwouldonlyraisethemean in (m—M)=6.40usingthedataofJohnson(1952),same 170 lieoutsidethecalibrationrange. Thefivemembersare whether oneusesthegiantsorturnoffstars(Moffat& changed fromClariàetal.(1989).Star1isexcludedonpho- [Fe/H] by0.03dex.Theclusterreddeningisclearlysmall, 2580 TWAROGETAL.:FORMATIONANDEVOLUTION (1992). Additionally,theCNstrongstar,4224,isexcluded sample foundinVandenBerg(1985). =0.09 fortheturnoffand0.08giants.Unfortunately, 1973). Membershipinformationissparse,butadiscussionof NGC 3114.TheDDOdataarefromClariáetal.(1989). NGC 2972.Nothinghasbeenaddedforthisclustersince NGC 2682.BetterknownasM67,theDDOsampleisthat NGC 2660.DDOdataarefromMesser&Smith(1987). PI 4.MembershipcriteriaandDDOdataremainun- © American Astronomical Society • Provided by theNASA Astrophysics Data System averages. upon itsradialvelocity.Wewill includeitinthederived E(B—V) =0.36fromtheturnoffstarsand0.37 Clariá etal.(1994)classify34 asapossiblebinarybased ity risesto+0.02andthedispersion declinesto0.07dex. with adispersionof0.12;ifstar 34isdropped,themetallic- fall outsidethecalibrationrange.Ofremaining10,206is a radial-velocitynonmember. The mean[Fe/H]is—0.01 turnoff. Ofthe15giantswithDDO,87,105,199,and206 giants. Wewilladopt0.35forthegiantsand0.38 able reddening,butClariáetal.(1994)findinthemeanthat higher metallicity,butproducesaredgiantclumpwhichis the adoptedreddeningandmetallicity.TheresultfromNor- Platais etal.(1997)whenoneadjustsforthedifferencesin known toFJandexcludedfromtheiraverage.Removalof age. anomalously brightcomparedtootherclustersofcomparable dstrom etal.(1997)isalsoconsistentwithasignificantly modulus derivedhereagreeswellwiththatofKozhurina- naries leavesasampleof5giantswiththesamemean[Fe/H] E(B —V)=0.095,somewhatlargerthangivenbytheoriginal summary byClariáetal.(1994).Theclusterexhibitsvari- comes fromKozhurina-Plataisetal.(1997).Theapparent as theoriginalsample.Thecmdformainsequencefitting nation failsfortheDDOphotometry.Removalof3bi- most thesame[Fe/H]asfoundbyFJ.Moreover,thisexpla- non-members andbinariesleavesonlytwogiantswithal- binaries. Notethattwoofthethreenon-memberswere ter. ThisissignificantlydifferentfromtheuvbyHßresults metallicity from8giantsis[Fe/H]=-0.12withasmallscat- E(B—V) =0.04to0.05.Asacompromise,wewilladopt proper-motion (Kozhurina-Plataisetal.1995)non-members DDO ofthegiantsleadsto0.07.WewilladoptE(B—V) Lapasset (1988).Theonediscordantnoteinthediscussionof 0.05 forboththegiantsandturnoff.TheresultingDDO one removestheradial-velocity(Nordstrometal.1997)and quence fitisthatofFernandez&Salgado(1980)aspre- tometry oftheredgiants.TheconsensusfromUBVand the clusterishighE(B—V)foundfromDDOpho- as causedbytheinclusionofthreenon-membersandtwo a low[Fe/H],butNordstrometal.(1997)dismissthisresult which typicallyindicatethatNGC3680hasametallicity uvbyRß analysesofNissen(1988),Anthony-Twarogetal. Clariá &Lapasset(1983);theyareinexcellentagreement.If sented inMeynetetal.(1993). assumed tobenon-members.Thecmdusedinthemainse- signed thesamevalue.Stars122,157,236,522,and649are uvbyHß studiesisE(B-V)around0.03to0.04,while membership determinationareallsummarizedinClariá& analysis bySagar&Sharpies(1991). 59, 76,78,119,and184.ThecmdistakenfromtheCCD similar totheHyades,near+0.17inthisstudy.FJalsofind (stars 40,50,54,55),theDDOreddeningestimateis (1989b), andNordstrometal.(1997)whichproduce UBV analysisofhotterstarsbyEggen(1969)andthe =0.04, smallenoughthatthegiantsanddwarfscanbeas- IC 2714.Alldataforthisclustercomefromtheexcellent NGC 3680.DDOdatacomefromMcClure(1972)and NGC 3532.TheDDOdata,reddeningestimates,and 2580 1997AJ 114.2556T Lapasset (1989)for6stars.Based uponDDOphotometry E{B—V) =0.20forthegiantsand0.22turnoff.Main lower reddening. the moduluscorrectedforahigher Hyadesdistanceandthe E{B—V) =0.25fromtheturnoffstarsbut0.19gi- assumed tobenon-members. Reddeningestimates fromthe and thehighlydiscrepantabundances, stars1,3,and135are proper-motion informationisavailable.Clariá(1980)derives ants. Sinceourprimaryinterestisinthegiants,wewilluse work ofLindoff(1972a).OfthefourDDOgiants,only22 that ofClariá(1980),supersedingtheearlierphotographic Moffat &Vogt(1973). E{B-V)=0.24. Moffat&Vogt(1973)deriveE{B-V) sequence fittingusesthelimitedcmdofClariá(1980)with able foregroundstarfromphotometry.Noradial-velocityor and 151fallwithinthecalibrationrange;star63isaprob- dening isadoptedasis. listing inPCAsincetheDDOremainsunpublished.Thered- the turnoff.Thedistancemodulusisbaseduponfitby is large.Wewilluse0.19forthegiants,implying0.21 radial-velocity information.Itisassumedthatthisexcludes member, thereddeningisreducedtothatofstar1alone,or PCA includeonlyonegiant,star1,andrefertounpublished average fromtheDDOgiants.Ifoneeliminatesstar3asa tainty in{m—M)isprobablyatleast±0.3mag. We haveusedtheDDOestimateinClariá&Lapasset star 3fromthecluster.ThereddeningadoptedbyPCAis giants, bothofwhichareclassifiedasphotometricmembers. by lessthan0.02dex.Distancedeterminationfrommain from thederivation.Ifincluded,itlowersmean[Fe/H] lously high[Fe/H].Since185hasananomalousreddening consistent withmembership;onlystar174hasananoma- ric reasons.Ourmetallicityestimateforthisstarmakesit that star185appearstobeaprobablefieldforphotomet- radial-velocity dataareavailable,butbothgroupsindicate initially derivedbyLohmann(1961).Noproper-motionor E(B —V)between0.30and0.34,comparablewiththevalue modulus becauseofthescatterinmainsequence. bership informationareexcludedintheaverage,mean reddening estimateistheDDOvalueofE(B—V)=0.29;we sequence fittingisunreliablegiventhelackofaqualitycmd. and ananomalouspositioninthecmd,itwillbeexcluded of Janes(1981)whichproducesamoderatelyuncertain with theestimateofFJtransformedtooursystemandincon- will use0.31fortheturnoff.Iftwogiantswithoutmem- is Janes(1981)whogivesDDOfor6starsinthefield.Four (1989), adjustedforaHyades(m—M)of3.4.Theuncer- of thegiantshaveradialvelocitiesconsistentwithmember- sistent withthevalueinPCA.Theonlycmdavailableisthat [Fe/H] increasesto—0.15.Thederivedmeanisconsistent 2581 TWAROGETAL.:FORMATIONANDEVOLUTION (1978) andClariá&Lapasset(1989).Bothstudiesproduce ship (FJ);noinformationisavailablefor43or44.Theonly =0.19 fromUBVanalysisoftheturnoff,butuncertainty NGC 5316.TheonlyDDOavailable isthatofClariá& NGC 5138.Thedefinitivestudyofthisclustertodateis LO 807.Thedataforthisclusteraremodifiedfromthe HAR 5.Clariáetal.(1989)supplyDDOdatafortwo NGC 4349.DDOdataareamergedsamplefromDawson NGC 3960.Theprimarysourceforalldataonthiscluster © American Astronomical Society • Provided by theNASA Astrophysics Data System DDO analysis(Smith1982)produces —V)=0.15forthe radial-velocity members(Mermilliodetal.1995).Star23is giants. Wewill adopt 0.12fortheturnoffand 0.11forthe giant; itwillbeexcludedfrom the analysis.uvbyYißanaly- a binaryand,moreimportant,rareopenclusterCN-strong large scattermaketheuncertaintybiggerthanaverage. from Frandsenetal.(1996)butthesparsepopulationand tallicity; iftheclusterisverymetal-rich,colorshift sis oftheturnoff(Nissen1988; Anthony-Twarog&Twarog sample comesfromSmith(1982)andallthegiantsare sion anddatainAnthony-Twarogetal.(1988).TheDDO sequence fit.Thecmdusedinthemainfitistaken adopt E{B-V)=035forthegiantsand0.39main caused by[Fe/H]mightproduceanexcessiveE{B~V).We estimate inUBVisbaseduponanapproximatelysolarme- nique used.Onepossibleexplanationisthatthereddening of theturnoffalone,largerthanexpectedE{B—V) mined atE{B—V)=035forthegiants.Kjeldsen&Frand- binaries included.TheDDOreddeningisalsowelldeter- branch inthiscluster,producingDDOdatafor10members, tailed photometricandradial-velocitystudyofthegiant rare exampleswherethereddeningdifferentialbetweenhot imply E{B—V)=035forthehotterstarsand0.31 turnoff sample[seethediscussionbyWalker(1985)]and by Mermilliodetal.(1987).Stars232and236arenon- the presenceofcepheids.DDOdataarefromSchmidt(1984) sen (1991)findE{B~V)=0A6frombroad-bandUBVdata and coolstarsisexplicitlyincluded.Thecmdusedformain giants. ItshouldbenotedthatWalker(1985)isoneofthe members, while276isacomposite.Reddeningestimatesare and Clariáetal.(1989);radial-velocitymembershipisgiven creases theprevious{m—M). from thecombinedDDOandspectroscopicabundancesin- main sequenceis0.155.Theresultinghigher[Fe/H]found to(m —M)=11.7.Theuncertaintyinthedistancemodulusis Hyades modulusandthehigher[Fe/H]ofNGC5316leads E{B —V)=0.34.Wewilluse0.31forthegiantsand0.35 sequence fittingisthatofWalker(1985). available fromavarietyofstudies,heavilyweightedbythe giants isassumedtobe£(#—V)=0.140whilethatforthe cluster isfoundinTwarogetal.(1993).Reddeningforthe distance inClariá&Lapasset(1989).Adjustingforalarger is inreasonableagreementwiththehighlyuncertainDDO tempted byPedreros(1987),leadingto(m-M)=11.4.This ferential comparisontoNGC2516andthePleiadeswasat- the turnoff.ThecmdofPedreros(1987)isgenerallyinad- From UBVanalysisofthehotterstars,Pedreros(1987)finds at least±0.3mag. equate toprovideareliablemainsequencefit,thoughdif- =0.10, implyingthegiantsshould haveE{B—V)=0.09\ 1987) givesextremelyconsistent reddeningofE{B—V) giants includeJennens&Heifer(1975)withE{B-V) =0.39 to0.40,butwithintheuncertaintiesofCCDtech- =0.31 andClariá&Lapasset(1989)withE{B—V)=03?>. IC 4651.Thestartingpointforthisclusteristhediscus- NGC 6134.Clariá&Mermilliod(1992)havedoneade- NGC 6067.Thisisarich,wellstudiedclusterthanksto NGC 5822.Thecompletesummaryofthedataforthis 2581 1997AJ 114.2556T between 0.40and0.45,with the originalvalueof0.42 this cluster’sparametersmaybefoundinLeeetal.(1989) E(B—V)=0.11 fromtheturnoffstars.uvbyUßanalysesby lished DDOphotometryofthe cluster, derivingametallicity Hiltner etal.(1958). the giants.Themainsequencefitisbaseduponcmdby photoelectric UBVdataofHiltneretal.(1958)indicate unpublished. Thereisgoodagreementonthereddening.The comparable totheHyadesfrom 7giantsandE(B—V) turnoff. Themainsequencefitisbaseduponthecmdof to 0.36.Wewilladopt0.36forthegiantsand0.39 Lapasset (1989);starAfallsoutsidethecalibrationbutisa and Brocatoetal.(1993).Janes (1979)discussesunpub- four remaininggiantsgiveE(B—V)=0.40fromDDOanaly- proper-motion non-member(Sanders&Schroder1980).The remains unchangedfromPCA.ThereddeningofE(B-V) (Johnson etal. 1956) beingchosenmostoften, asinthe sult. WeadoptE(B-V)=0.1Tfortheturnoffand0.16 Schmidt (1976)andbyEggen(1989)producethesamere- Sanders (1990). cmd usedinthemainsequencefitistakenfromProsseretal. with theuvbyUßvalueof0.05fromNissen(1988).The the modulusandreddeningofclusterasgivenbyPCA. £'(5 —V)=0.16to0.17.Giventhelargersampleofturnoff bership, areasdescribedinPCA.TheDDOphotometryof3 sis whileUBVdataforthehotterstars(Sanders1990)leads cluster comesfromSchmidt(1984);themembershipstatus published DDOdataforonegiant.Littleinformationofcon- The cmdistakenfromMeynetetal.(1993). DDO ofthetwoprobablemembersgiantbranchgives (1995). sequence, notincludingacmd,isavailable.Wewilladopt stars, wewilluse0.15fortheturnoffand0.14giants. & Forte(1974)deriveE(B—V)=0.15fortheturnoffstars; field starfromthephotometry.FromUBVanalysis,Feinstein et al.(1989a)andthecurrentinvestigation. with thecomparisonclustersincludedinAnthony-Twarog apparent modulushasbeenadjustedtomaintainconsistency the giants,slightlylowerthanvaluechosenbyPCA.The metallicity, E(#-V)=0.68,implyingE(B-V)=0.61for of themainsequencetootherclusterscomparableageand cussed inAnthony-Twarogetal.(1989a).Fromcomparison lished. Theclusterreddeningandcmdarethoroughlydis- PCA. the metallicityforthiscluster.TheonlycmdisthatofLind- stars isfromClariáetal.(1989);star10classifiedasa off (1972a).Wehaveusedthesamereddeninggivenin giants. Withtheincreasedreddening,above-solarabun- =0.40. Theconsensusonthereddening isthatE(B—V)lies dance producesarevised(m—M)=10.25. 2582 TWAROGETAL:FORMATIONANDEVOLUTION =0.04 fromBecker&Fenkart(1971)isingoodagreement NGC 6705.AlsoknownasMil,detaileddiscussionsof NGC 6633.AllDDOphotometryforthisclusterremains NGC 6494.DDOdataforfivegiantsaregiveninClariá& NGC 6475.TheDDOphotometryforonegiantinthis NGC 6425.PCAincludeanabundancebaseduponun- NGC 6281.Thebasicclusterdata,reddeningandmem- NGC 6259.DDOphotometryforthisclusterisunpub- NGC 6208.PCAusesunpublishedDDOdatatoobtain © American Astronomical Society • Provided by theNASA Astrophysics Data System to derivetheabundances is.Theiranalysisuses theworkof liod etal.(1994).Whilemembership forthegiantsisun- main sequencefitmakesuseofthecmdAuner(1974). reddening remainsaprimaryuncertaintyforthiscluster.The questioned, thephotometryused byThogersenetal.(1993) understanding oftheclustergiants canbefoundinMermil- from Thogersenetal.(1993).An excellentsummaryofour for thegiantsand0.30turnoff.Weemphasizethat 0.3 (Auner1974).Thisagreeswellwiththephotographic of E(B—V)=0.12,butthemorphologycmdexcludes ter. Burkhead(1971)originallyfoundaratherlowreddening E(B-V), increasingfrom[Fe/H]=0.08atE{B-V)=0.10 the fiduciallinesofKaluzny&Rucinski(1995). Anthony-Twarog 1997)showsthatallthegiantsinJanes for theturnoff.Proper-motionanalysis(Cudworth& nosity, wewilladoptE(B—V)=0.15forthegiantsand0.16 consistency withtheclustermetallicityandclumplumi- is closeenoughtothesunthatthiswillnotaffectsignifi- tometric estimateofSchmidt(1978),roundedtothenearest Mayor (1990)derivearadialvelocitysoclosetothecluster because itsuffersfromvariablereddening(Schmidt1978). analysis byLindoff(1972b).WewilladoptE(B-V)=0.2$ a valuethislowandismoreconsistentwithE(B~V)near analysis ofthisrich,butgenerallyoverlooked,oldopenclus- so discrepant,itwillbeexcluded.Themainsequencefituses clump, evenforanoldcluster.Toapproachsomedegreeof the lowreddeningleadstoananomalouslyfaintredgiant to [Fe/H]=0.11atE(B-V)=0.11.However,adoptionof tallicity oftheclusterisrelativelyinsensitivetochangesin 0.03 (Anthony-Twarog&Twarog1985;KaluznyRucin- tend tofallintoeitherthehighcategory,E(B—V)=0.1T± mains akey,butcontroversial,object.Whilemostrecent cantly theestimatedgalactocentricdistance. 0.1 mag.Theuncertaintyin(m—M)islarge,butthecluster value fordistancedetermination.Wehaveadoptedthepho- mean thatthisstarisincludedasamember.Acmdpre- fall outsidethecalibrationrange.Star350isanon-member. ing correctionstothegiantsbaseduponuvbyUßanaly- Following Smith(1983),wehaveappliedindividualredden- This clusterhasoccasionallybeenexcludedfromdiscussions Thogersen etal.(1993)havedoneaspectroscopicanalysis. (1984) samplearemembers,buttheabundancefor3012is sion byKaluzny&Rucinski(1995)],thereddeningestimates upon thecmdinBrocatoetal.(1993). formed spectroscopicabundanceisconsistentwithametal- ski 1995),orthelowgroup,£(#-F)=0.10±0.03(Janes studies concurthattheclusterismetal-rich[seediscus- sented byHerzogetal.(1975),butthescatterweakensits Star 6isaproper-motionnon-member,butMermilliod& licity nearthatoftheHyades.Themainsequencefitisbased work ofThogersenetal.(1993).WewilluseE{B—V) sis bySchmidt(1978).Stars35,85,144,170,176,and314 1984; Montgomeryetal.1994).Surprisingly,themeanme- =0.44 fortheturnoffand0.40giants.Thetrans- NGC 6939.Thespectroscopic data forthisclustercome NGC 6819.FJincluded7redgiantmembersintheir NGC 6791.Becauseofitsextremeage,thisclusterre- IC 4756.DDOdatacomefromSmith(1983)while 2582 1997AJ 114.2556T resemblance toNGC7789,discussedbelow.Onthatbasis, £(£-V)=0.50. 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