1986AJ 92. .HIT be madefromzero-agemain-sequence(ZAMS)fitting,(iii) been madeintheprogramtoavoidproblemsoftype The ultimateaimofthesestudiesistoresolvesomethe metric UBVstudiesofclusterscontainingCepheidvariables. 4349, NGC1647andC2128+ 488,theclustercontaining members areobservedtoallowareliabledistanceestimate tied tothestandardsystem,(ii)enoughfaintcluster noted abovebyensuringthat:(i)observationsaresecurely reddenings oftheseclusters.Tothisend,someefforthas current uncertaintiesinourknowledgeofthedistancesand other problemswhichwillbediscussedhere. try. Thenarrowbandstudiesoftheseclustersareaffectedby VI726 Cygni. ing NGC129,7790,Berkeley 58,NGC5662, in closeproximitytotheCepheid. Thepresentstudyisanew ter member,and(iv)aconcentrationismadeonstarslying times theyareaffectedbysystematicerrorsinthephotome- a numberofotherclustersassociated withCepheids,includ- Cepheid SNormae.Futurepapers inthisserieswillexamine investigation ofNGC6087,the clustercontainingthe9f75 several independentobservationsareavailableforeachclus- members usedtodeterminetheclusterdistance,andsome- band UBVstudiesofthecalibratingclusterstendtoberather incomplete duetoundersamplingofthefaintcluster than isgenerallyrecognized.Manyoftheoriginalbroad- reddening. Regrettably,thenumberofCepheidsthatliein the immediatevicinityofCepheidtodetermineitsspace well-studied clustersofthistypeisprobablymuchsmaller ambiguously, and(iii)thereareenoughclustermembersin members aresufficienttodeterminetheclusterdistanceun- quantity ofthephotometricobservationsforcluster the Cepheidisindeedaclustermember,(ii)qualityand clusters providedthatineachcase:(i)itcanbeshown tance canbeobtainedforCepheidsthataremembersofopen weight indistancedeterminationsfornearbyspiralandir- color (PLC)relation,whichinturnbearsmuchofthe regular galaxies.Reliableestimatesofreddeninganddis- tance forthecalibrationofCepheidperiod-luminosity- galactic PopulationICepheidsisoffundamentalimpor- THE ASTRONOMICALJOURNALVOLUME92,NUMBER1JULY1986 GALACTIC CLUSTERSWITHASSOCIATEDCEPHEIDVARIABLES.I.NGC6087ANDSNORMAE 111 Astron.J.92 (1),July1986 0004-6256/86/010111-08S00.90 © 1986Am.Astron. Soc.111 This paperisthefirstinaseriesofnewbroadbandphoto- A knowledgeoftheintrinsiccolorsandluminositiesfor © American Astronomical Society • Provided by the NASA Astrophysics Data System technique. Thederiveddistancemodulus,V—M=9.78+0.03(internalerror),+0.13(totalesti- redetermine theclusterreddeninganddistancebymeansofacarefulapplicationZAMSfitting Cepheid SNormae.Thedataareusedwithpublishedphotoelectricobservationsforclusterstarsto New photoelectricUBVphotometryispresentedforstarsinNGC6087,theclustercontaining9^75 from WalravenphotometryandtheanalysisofStrömgrenH/?forclusterstars. mated uncertainty),isshowntobeinverygoodagreementwiththeclusterdistancemodulusobtained various luminositycalibrationsusedintheanalysis.Internalconsistencyofthistypehastendedtobe for NGC6087presentedinthisstudyresultsfromthedemonstrablyidenticalzeropointssharedby introduced bytheeffectsofstellarrotation.Theexcellentagreementvariousdistanceestimates corrections areignoredandreplacedbydueconsiderationofthecosmicscatterinH/?distancemoduli However, thehydrogen-linedataarefoundtoproducemoreaccurateandpreciseresultsifevolutionary disregarded insomerecentanalysesoftheCepheidcalibrationclusters. 0v I. INTRODUCTION Department ofAstronomy,SaintMary’sUniversity,Halifax,NovaScotiaB3H3C3,Canada Received 25February1986;revised2April1986 David G.Turner ABSTRACT and Breger(1966).However,noneofthesestudiesstrictly 6087 havebeenpublishedbyIrwin(1958),Fernie(1961), tion isparticularlyimportantsinceitraisesthequestionof satisfies allofthecriteriaforlineartransformationto how closetheobservedcolors(mostnotablyU—B)for is 18%smallerthanthatinferredfromZAMSfittingofthe Schmidt (1980)hasfoundadistancetoNGC6087using standard photometricanalyses,andmightexplainwhy the reddeninganddistanceofclusterasobtainedfrom photometry cancontributetowardserroneousestimatesfor cluster starsaretotheirtruecolors.Systematicerrorsinthe detail thequestionofclustermembershipformanyindivid- dening anddistanceofthecluster,investigateinsome order todeterminetheclusterreddeninglaw,derivered- that havebeenpublishedseparatelybyvariousauthorsin this studyandalsocombinesallusefuldataforclusterstars detail. Thispaperpresentsthenewobservationaldatafrom here wasinitiatedmainlytoinvestigatethissituationinmore mag indistancemodulus. Strömgren andHßphotometryforbrightclusterstarsthat both theslopesandzeropoints ofthecolortransformations ual stars.AmuchearlierinvestigationofthistypebyLan- need forsecond-orderterms.Popper (1982)hasnotedthat the Johnsonsystemwasquitelinear withnoindicationofa the transformationofinstrumental colorsandmagnitudesto UBV systemspecifiedbyJohnson(1963).Thisconsidera- metric systemusedfortheseobservations, andasexpected listed byJohnson(1963)wereclosely matchedbythephoto- a standardUBVfilterset.Theprecepts forUBVphotometry ter equippedwithadry-ice-cooled1P21photomultiplierand were observedwiththeUniversityofToronto’s0.6mtele- scope atCerroLasCampanas,Chile,onthreenightsin stars inNGC6087lyingcloseproximitytoSNor.They UBV data.Thisdiscrepancyamountstoadifferenceof0.4 draw upon. dolt (1964)didnothavenearlyasmuchpublisheddatato 1981 Mayusingasingle-channelpulse-countingphotome- Previous broadbandUBVphotoelectricstudiesofNGC The newphotoelectricUßFstudyofNGC6087described The objectsobservedinthisstudycompriseasampleof31 II. OBSERVATIONS 1986AJ 92. .HIT been published.Therefore,acomparisonofthepresentpho- identified accordingtothenumberingsystemsofLandolt This againisindicativeofafairlygoodmatchtotheJohnson point correctionsof—1.0and+1.1,respectively,were tively goodmatchtotheJohnsonsystemforwhichzero- correction of-h0.72.Thesevaluesseemtoindicatearela- present observationstheB—Vtransformationhadaslope to thedataofFemie(1961)andBreger1966).Acompari- tometry withpreviouslypublishedobservationsisrestricted confusion. plemented byBreger)areprovidedwherepossibletoavoid his tablesofphotometricdata,sothestaridentifications system. dard-star observationswereplottedasfunctionsofcolor. of anintrinsicscatterabout+0.01magwhenthestan- There werealsonoobvioustrendsintheresidualsexcess adopted withtheinstrumentalcolors(cf.Popper1982). of 0.97andazero-pointcorrection—1.13,whilethe strumental systemtothestandardUBVsystem.For give importantinformationconcerningthematchofin- son withthephotometryofFemieyieldsfollowingdif- identifications withthenumberingsystemofFemie(sup- used hererefertothosespecifiedbyFig.2ofhispaper.Cross numbered byLandolthaveaccidentallybeentransposedin ferences (Turner—Femie):AF=+0.0080.007, 112 DAVIDG.TURNER:NGC6087ANDSNORMAE U —Btransformationhadaslopeof1.02andzero-point (1964), Femie(1961),andBreger(1966).Afewstars a III-2-5N The UBVdataofIrwin(1958)forclusterstarshavenever The dataarepresentedinTableI,wherethestars Landolt Fernie/BregerVB-VU-B II-2-5 II-2-3 II-2-2 II-2-1 II-3-1 II-2-15 II-2-14 II-2-6 © American Astronomical Society • Provided by the NASA Astrophysics Data System II-3-2 II-2-16 IV-3-1 IV-2-1 II-6-1 II-3-3 IV-3-2 IV-3-lWb IV-3-1SW IV-2-1S IV-3-lWa 1-4-2 1-2-2 1-2-1 1-5-4 1-4-7 1-4-3 1-4-1 1-3-4 1-3-2 1-3-1 Table II. Close companiontoSNormae.Data contaminatedbystray 1-5-3 light fromSNormae.Notincluded incombineddataof Star Identification Table I.NewphotoelectricdataforNGC6087. 128 129 157 127 130 64 68 66 65 92 34 35 69 25 70 11 10 36 39 12 9 7 8 (12.78:) 11.10 10.01 13.10: 12.43 12.49 12.11: 12.11: 10.08 12.08: 11.48 12.20 12.65 11.97 12.80 12.56 10.38 11.32 10.60: 11.50 10.19 10.75 9.44 8.28 7.95 9.72 9.39 8.71 9.15 8.97 7.46 (+0.49) +0.29 +0.06 +0.24 +0.25 +0.39 +0.23 +0.14 +0.11 +0.07 +0.44: +0.79 +0.06 +0.34: +0.04 +0.27 +0.05 +0.29 + 1.53 +0.19 +0.29 +0.07 +0.15 +0.04 +0.09 +0.84 + 1.17 + 1.32 +0.40: + 0.04 +0.34 (+0.18) +0.24: +0.24 +0.17 +0.25 +0.20 +0.22 +0.08 +0.43 +0.23: +0.26 +0.22 +0.24 +1.91: +0.17 -0.26 -0.34 -0.25 -0.17 +1.36: -0.34 -0.40 +0.97 +0.04 +0.23 +0.22 -0.29 +0.46 -0.37 -0.37 -0.23 and photometricequipmentusedbyFemiedonotsatisfythe differences seemtobebothmagnitudeandcolordependent, magnitude andcolorwithrespecttotheTableIdata.These that Fernie’sobservationsexhibitsystematicdifferencesin fied byJohnson(1963),anditisprobablynotsurprising criteria forlineartransformationtotheUBVsystemspeci- A(B-V)= -0.030+0.004andA(U-5)=+0.067 as wasfoundpreviouslybyLandolt(1964)fromacompari- photometric data. following differences(Turner—Breger):AV=—0.005 son withIrwin’sunpublisheddata,andbyBreger(1966). upon theradialvelocity(Feast1957),propermotion,and sions regardingtheclustermembershipofeachstarbased tions forclusterstarsinthistablearefromFeast(1957), compendium ofthecombinedphotometryforNGC6087 obtain amorecompletesampleofphotoelectricUBVdata.A The linearcolorterminÎ7—2?isatypicalproblemencoun- the dataarecloselytiedtostandardsystem. photometry. Inthiscase,theresultsappeartoconfirmthat vides informationaboutpossiblesystematicerrorsinthe exist. Theanalysisoftheclusterreddeningrelationalsopro- photometry ofclusterstarsforcomparison,butthesedonot would requireanumberofindependentsetsclassicalUBV from King(1982).ThefinalcolumnofTableIIlistsconclu- while proper-motionmembershipprobabilitiesaretaken using Landolt’s(1964)relations.Thespectralclassifica- ysis; their2?FdatahavebeentransformedtoIrwin’ssystem Femie arelistedherebecausetheyimportanttotheanal- pected formultiplephotoelectricobservationsofthesame tered withthephotometricsystemusedbyBreger.However, termined bythreeindependentmethods,namely:(i)from tied tothe£/i?Fsystem.Fullconfirmationofthisconclusion the discussioninSec.IIthatdataofTableareclosely stars ispresentedinTableII.Afewobservedonlyby stars, thusjustifyingacombinationofthetwodatasetsto the magnitudeandcolordifferencesarenolargerthanex- it seemsclearthathisphotometryagreeswellwiththeTable color excessescalculatedforearly-typeclusterstarsinTable dividual starspriortoaZAMSfit.Itcanbeassumedfrom of theinterstellar-reddeningrelationappropriatefor that thedataareonstandardsystem,(ii)determination U-B data);AF=+0.118±0.007andA(B-V) A2. Method(iii)isausefultechnique forclustersthatcon- served colorsofreddeneddwarf starsfromthetangentpor- NGC 6087forwhichbothUBVdata(KlareandNeckel excesses calculatedforearly-typestarslyingwithin3°of cluster, and(iii)correctionsforinterstellarreddeningofin- open clusterstars(Turner1981)include:(i)confirmation tain earlyAtypedwarfs,butstrictly providesonlyalower tion oftheintrinsiccolor-color relationnearspectraltype II withreasonablycertainspectraltypes,(ii)fromcolor I dataoncethisistakenintoaccount.Thestandarderrorsfor limit fortheslope ofthereddeningrelation.Its primaryuse- (Garrison etal1977)areavailable, and(iii)usingtheob- 1977; Schildetal.1983)and MK spectralclassifications + 0.059forfivestarswithV<10(onlythreeofthesehave = —0.055+0.008forsixstarswithV>10.Thetelescope = +0.055(+0.006)+0.077(U-Æ)for19stars. ±0.007, A(B—V)=+0.0050.003andA(U-B) Breger A comparisonwiththephotometryofBregeryields The reddeningrelationthatappliestoNGC6087wasde- The stepsinvolvedinadetailedanalysisofUBVdatafor III. ANALYSIS 112 1986AJ 92. .HIT fulness isinprovidingacheckontheconsistencyofother implied slopeE_/ofthereddeningrelationfor estimates. E_/ =0.84wasadopted intheanalysisofNGC s.e., and(iii)0.84+0.04s.e. (threestars).Avalueof three methodswere:(i)0.83 + 0.07s.e.,(ii)0.850.03 reddening slopesE_/ resultingfromeachofthe confirmed bythegoodagreement ofthethreeestimates.The NGC 6087isunusuallylargefor B-andA-typestars,butis 6087, andpossiblecurvature in therelationwasignored 113 DAVIDG.TURNER:NGC6087ANDSNORMAE UBV ubF UB v The resultsofthisanalysisareillustratedinFig.1. © American Astronomical Society • Provided by the NASA Astrophysics Data System C b Propermotionmembershipprobability(King1982). ^ Membershipstatusfromthisstudy. II1-4-1 III-3-2 III-3-1 III-2-5N III-2-1 Radialvelocitymembership(Feast1957);M=clustermember,NM =non-member. II1-5-1 III-8-1 11-2-1 S Normae6.41+0.95+0.64 II-2-16 11-2-14 11-2-6 11-2-5 II-2-3 II-2-2 F 23 F 22 F 18 I1-4-1 II-3-3 11-3-2 II-3-1 II-2-15 F 17 II-10-2 II-6-1 II-4-7 II-4-4 IV-2-1 II-9-1 II-6-7 11-6-2 IV-3-1 IV-2-1S IV-3-7 IV-3-lWb IV-3-lWa IV-3-1SW IV-3-8 IV-3-2 1-3-1 1-2-1 1-3-4 1-3-3 1-3-2 1-2-2 1-6-1 1-5-4 1-5-3 1-4-7 1-4-3 1-4-1 1-9-2 1-8-3 1-4-2 Identification fromFig.2ofLandolt(1984). Star 11.46 13.10: 12.43 12.49 11.10 12.11: 12.56 12.10 10.09 12.08 12.18 10.29 10.03 10.63 11.96 12.84 10.75 12.28 12.77 12.80 12.56 12.65 10.18 11.41 11.39 11.90 10.31 11.50 10.19 10.60: 11.45 11.02 11.32 10.38 9.44 8.28 8.72 9.74 9.40 7.95 7.67 9.29 9.70 9.89 8.71 9.82 9.70 8.99 8.31 8.47 7.48 9.92 9.18 8.79 V B-VU-BnMKSp.Type(B-V)EP^ oB Vq Table II.CompendiumofobservationaldataforNGC6087. +0.29 +0.06 +0.24 +0.37 +0.24 +0.39 +0.22 +0.15 +0.44: +0.79 +0.06 +0.34: +0.06 +0.03 +0.34 +0.28 +0.03 +0.09 +0.07 + 1.78 +0.01 + 1.50 +0.18 + 0.13 + 0.24 + 1.32 +0.29 +0.29 + 0.05 + 0.03 +0.18 + 0.09 + 0.84 +0.06 +0.03 +0.17 + 1.74 + 1.18 + 1.40 + 1.40 + 0.00 +0.04 +0.34 + 0.09 +0.02 + 0.14 + 0.05 +0.40: +0.05 + 0.03 +0.22 +0.13 +0.07 +0.15 +0.25 +0.24 +0.25 +0.18 +0.24 +0.17 +0.21 +0.08 +0.24: +0.43 -0.18 + 0.17 +0.12 +0.24 -0.40 -0.34 -0.18 + 2.39 -0.25 +0.26 + 1.90 -0.34 -0.25 -0.33 +0.12 +0.46 +0.13 + 1.36 + 0.24 +2.32: + 1.00 +0.21 + 1.79 +0.22 -0.02 -0.34 +0.18 + 0.23 -0.41 -0.38 -0.23 +0.04 -0.29 -0.36 -0.03 -0.24 -0.36 -0.38 Ml III AO III:(V?) B9 III: B5 V(e) M3 III: G4 III F8-G2 lb B8 VorIII B5 V B8 V(e) B9 V B8 V B8 V found inNGC6087. since itisunimportantforsmallrangesofreddeninglikethat individual starswasbasedupon their(i)radialvelocity members andlikelynonmembers aredepictedbydifferent reddening acrosstheregionof NGC6087.Likelycluster clearly demonstratesthelackofanysignificantdifferential colors, and(iv)implieddistance modulusfromtheirspec- reddening fromFig.2ordereddening theirStrömgren symbols, wheretheassignment ofclustermembershipfor (Feast 1957),(ii)propermotion (King1982),(iii)derived The clustercolor-colordiagramisshowninFig.2,and +0.19 +0.06 + 0.21 +0.06 +0.26 +0.04 +0.13 -0.09 -0.15 -0.09 -0.13 -0.11 -0.16 -0.13 +0.14 0.1512.1090 -0.14 0.17 -0.03 +0.78 0.175.8395 -0.02 -0.11 -0.06 -0.09 -0.13 -0.01 -0.16 + 0.00 -0.15 -0.15 -0.14 -0.16 -0.15 -0.15 -0.03 -0.10 -0.12 -0.04 -0.14 -0.06 0.17 0.15 0.18 0.15 0.16 0.18 0.18 0.18 0.15 0.19 0.22 0.22 0.15 0.14 0.18 0.20 0.19 0.15 0.16 0.18 0.18 0.18 0.20 0.16 0.19 0.17 0.15 0.19 0.19 0.20 0.23 0.18 0.19 0.22 0.19 12.03 11.55 12.06 11.54 11.52 10.90 12.54 11.72 10.17 10.79 10.80 11.34 10.77 10.43 10.73 8.13 9.35 7.27 9.86 8.91 7.82 9.59 9.08 9.43 9.32 95 9.48 8.73 7.97 9.11 0 9.39 91 8.32 0 9.79 8.56 8.43 7.75 96 94 90 93 96 96 88 96 95 90 96 95 95 87 NM 96 92 94 95 95 82 95 88 95 96 95 34 77 0 2 0 0 0 NM NM NM NM NM NM NM NM NM NM NM NM NM NM NM NM NM NM? NM NM M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M 113 1986AJ 92. .HIT tral type,ßindexorZAMSfit.Membershipstatuswas usually obviousfromthesetests.ExceptforSNor,noneof the late-typegiantsinclusterregionqualifiesasamem- 114 DAVIDG.TURNER:NGC6087ANDSNORMAE basis ofitsradialvelocity(Stibbs1955;Feast1957)and proper motion.Inaddition,theimpliedageofCepheid distance of902+10pc. stars isF—=9.78+0.03s.e.,whichcorrespondstoa objects. Theclusterdistancemodulusderivedfromthese served starsarelikelytobesingle,unevolved,main-sequence of Turner(1976a,1979)impliesthatonlyseventheob- gions oftheMilkyWay(Turner1976b).AfittoZAMS made usingR=A/E_=3.1asfoundforadjacentre- ber ofNGC6087onthebasisbothradialvelocityand dolt (1964)inhisearlierstudyofNGC6087. proper motion.DifferentconclusionswerereachedbyLan- 6087 isgiveninFig.3,whereextinctioncorrectionswere 0 VBF The CepheidSNorqualifiesasaclustermemberonthe A reddening-freecolor-magnitudediagramforNGC © American Astronomical Society • Provided by the NASA Astrophysics Data System intrinsic relationfor main-sequencestars. circles) andnonmembers(lightcircles). Thecontinuouslineisthe Fig. 2.Color-colordiagramforNGC 6087likelymembers(dark has slope_=0.84. NGC 6087(darkcircles)andintheclusterproper Fig. 1.Derivedcolorexcessesforstarsinthefieldof relation (plussigns).Theadoptedreddening stars lyingnearthetangentportionofcolor-color (light circles),andforlikelyclustermain-sequence v B-V 7 7 distance modulusof3.15(Turner1979),butisbasedupona of +0.15magtotheabsolutemagnitudesHyadesstars fit toaZAMSofsolarmetallicitybyincludingcorrection by Hanson(1980)indicatesthattheHyadesdistancemodu- adjust themtoPleiades(solar)metallicity.Theresulting pheid withaperiodof9f75. lus couldbeaslarge3.30,butitisworthremarkingthat position isappropriateforclusterstars.Arecentsummary makes itpossibletoaccommodateaHyadesdistancemodu- ZAMS isverysimilartoolderversionspublishedbyJohnson cation attheheartofNGC6087,thesevariouspieces from olderstellarevolutionarymodels(Tammann1970)is the astrometricdataquotedin thissummarytoconsidera there stillremainanumberofsources ofsystematicerrorin derived forNGC6087,assumingofcoursethatasolarcom- lus of3.21withnocorrespondingchangeinthedistance ades metallicityis0.21magratherthan0.15mag,which Pel 1985)indicatethatthecorrectionfromHyadestoPlei- and Blaauw(cf.Stothers1983)exceptfortheearliestB- ror), avaluethatisentirelyappropriateforclassicalCe- cluster memberis(My)=—3.95+0.04s.e.(internaler- has beenassumedbypreviousworkers.Itsluminosityasa evidence stronglysuggestthatSNorisaclustermember,as an intrinsiccolorrelation.CombinedwiththeCepheid’slo- turnoff (cf.MaederandMermilliod1981).Theaverage 4.1 (+0.8)X10yr,ingoodagreementwithaclusterageof clusters themselvesprovidesome evidenceinfavorofa pointed outinlaterpapers this seriesthattheCepheid Hyades distancemodulusof3.2 avalidpossibility.Itwillbe O-type stars.Morerecentfindings(Turner,unpublished; using CousinssystemVRIphotometryforthevariableand ofE_ y=0.18wasderivedforSNorbyDeangiû/.(1978) a starwiththecolorsofSNor.Anearlyidenticalreddening E*_(B0) =0.19+0.01s.e.,orE_=0.17for field reddeningofclustermembersnearSNoris about 5(+1)X10yrcorrespondingtoitsmain-sequence Hyades distancemodulusofless than3.3(seealsoStothers 1983; Schmidt1984). B f5F The clusterdistancemodulusof9.78istiedtoaHyades line representstheZAMSforV—M=9.78. ability forSNorareindicated,whilethecontinuous for NGC6087likelymembers.Theextremesofvari- Fig. 3.Reddening-correctedcolor-magnitudediagram 0y IV. THEZERO-POINTQUESTION 114 1986AJ 92. .HIT a Cepheid calibratingclusters. The lastofthesevalueswasadoptedforcomparisonby similar omissionwasmadebyFernieandMcGonegal accounted forbyhisneglectofthemetallicitycorrection.A least 0.2magofthe0.4discrepancyfoundbySchmidt for theHyadesclusteriscloseto3.2(cf.Stothers1983).At rection foranupwardsadjustmentoftheHyadesdistance licity. Suchacorrectionisnecessarysincethesolarheavy- place themonaZAMSdistancescaleforstarsofsolarmetal- to adjustitaHyadesdistancemodulusofabout3.3.As based uponZAMSfittingare9.8±0.1(Landolt1964),9.7 but thedifferencesareslightandonlyhalfaslarge could resultinnonetcorrectionifthetruedistancemodulus modulus isinadditiontothismetallicitycorrection,but galactic neighborhood(vandenBergh1962,1977).Acor- element abundanceappearstobetypicalofstarsintheSun’s cluster distancemodulibyroughly—0.2maginorderto noted above,however,itisnecessarytocorrectallprevious Schmidt (1980),butwascorrectedby+0.26maginorder discrepancy of0.4maginferredfromSchmidt’sstudy. for thedistancemodulusofNGC6087liesclosertoPel’s estimate thantothoseofSchmidtorBalonaandShobbrook, and Turner(1979).Thevalueof9.78+0.03obtainedhere dulus thatisverysimilartofoundbyCrawford(1978) for thecluster.Pel’sresultistiedtoaPleiadesdistancemo- with theirrevisedM{ß)calibration.Incontrast,Pel dulus of9.64+0.09forNGC6087usingSchmidt’sdata bration forB-typestars.Crawford’sM(ß)scaleiscalibrat- 6087 frommain-sequencefittingofWalravenphotometry and Shobbrook(1984)derivedacomparabledistancemo- point nearlyidenticaltothatquotedaboveaccordingthe not usingstarsintheHyadescluster,yetitmusthaveazero ed primarilyusingtrigonometricparallaxesforFstarsand as clustermembers)usingCrawford’s(1978)M(ß)cali- tometry for16brightclusterstars(11ofwhichwereadopted obtained byCrawford(1978)andTurner(1979).Balona close similarityindistancemoduliforthePleiadescluster 9.60 +0.09forNGC6087fromStrömgrenandH/?pho- ( 1983)intheirrecentcompilationofdistancemoduliforthe (1980) inthedistancemoduliforNGC6087istherefore (Breger 1966),and9.76(SandageTammann1969). ( 1985)obtainedadistancemodulusof9.84+0.10forNGC v v v 115 DAVIDG.TURNER:NGC6087ANDSNORMAE k AbsolutemagnitudefromMillwardand Walker (1985)usingß-index. AbsolutemagnitudefromCrawford(1978) calibration, Older estimatesforthedistancemodulusofNGC6087 Star Landolt Schmidt (1980)derivedadistancemodulusof © American Astronomical Society • Provided by the NASA Astrophysics Data System 25 II-2-2 23 22 20 18 14 13 11 19 17 15 Table III.AnalysisofSchmidt’sdataforNGC6087. II-10-2 III-5-1 II- 2-1 I- 4-7 1-4-1 II-9-1 II- 2-14 IV- 3-1 III- 8-1 III-3-1 III- 4- 1 0.133 0.18 0.120 0.138 0.132 0.169 0.117 0.129 0.131 0.142 0.114 0.106 0.125 0.110 0.129 0.138 0.134 b-y 0.44 0.93 0.75 0.08 0.13 0.66 0.11 0.40 0.24 0.39 0.43 0.46 0.68 1 .44 1.52 9.35 9.32 9.11 9.43 9.39 8.32 9.59 9.08 8.87 8.91 8.43 7.82 7.75 7.97 8.73 7.27 +0.01 +0.16 +0.18 -0.50 -2.88 -2.19 -0.82 -4.46 -1.77 -0.47 -0.24 -0.46 -0.64 -0.92 -5.26 -1.20 vHaMb 10.14 12.53e 10.70 13.57 o-v V’TV 9.79 9.41 9.63 9.94 9.34 9.74 9.83 9.54 9.37 9.27 9.21 8.79 -0.17 -0.03 -0.80 -0.78 -3.61 -1.43 -0.12 -0.17 -0.53 -0.44 -0.95 -3.83 -0.46 -1.16 -1.77 -1.91 ll.lOe 10.10 12.72 10.12 9.52 9.82 9.59 9.52 9.40 9.55 9.42 8.49 9.53 9.37 9.59 9.66 is atbest±0.29magfortheH¿?data,butonly0.08 ß indexiscontaminatedbyweakemission;italsohasacir- ation foronestaraboutthederivedmeandistancemodulus increase inthecalculateduncertainty.Thestandarddevi- M (/?)calibration.Theresultingreddening-correcteddis- tance modulusto9.69±0.11s.e.,butwithacorresponding sis). Inclusionofthedataforstar7increasesmeandis- ty (possiblyaccountedforbythedifferentreddeninganaly- obtained bySchmidtexceptforaslightlysmalleruncertain- for M(ß).Ofthe16clusterstarsobservedbySchmidt,two the parameterusedinCrawford’sevolutionarycorrections tance moduliareplottedinFig.4(a)asafunctionof10A/?, net differenceincolorexcesses(Turner—Schmidt) patible withareddeningslopeoŒ_/'E—0.84.The dulus of9.60±0.08s.e.,whichisidenticaltothevalue ing stars(excludingstar10)resultinaclusterdistancemo- data. Theremaining14starshaveproper-motionmember- each star,whichareobtainedfromCrawford’s(1978) mag intheclusterdistancemodulus. small toberesponsiblefortheresidualdiscrepancyof0.2 amounts toonly+0.004maginE¿,_3,,whichismuchtoo reddening relationsfortheStrömgrenindicesthatarecom- lus comparedtootherclustermembers.Datafortheremain- emission-line staraccordingtoFeast(thespectraltype have novelocitymeasuresavailable.Star10isaprobable ambiguous (Feast1957);threeothers(stars20,23,and25) this fortenofthestars,whileone(star7)resultsare are actualclustermembers.Radial-velocitydataconfirm ship probabilitiesthataverage93%,anditappearsmost tently reducedusing“standard”reddeningrelations.Table data, onthebasisofitssystematicallylargedistancemodu- for star14,althoughithasaspectralclassificationsimilarto cumstellar componenttoitsreddeningwhichlikewisear- quoted bySchmidtisforanotherstar),anditappearsthatits HI thereforepresentsareanalysisofSchmidt’sdatausing star 7,aspermittedbytheproper-motionandradial-velocity star 10. gues thatitisaBestar.Thisproblemdoesnotseemtoexist Strömgren systemdataforclusterstars,whichwereconsis- Sec. Illraisessuspicionsaboutpreviousanalysesofthe v (stars 17and22)arenonmembersfromtheproper-motion v uBv Column 6ofTableIIIliststheabsolutemagnitudesfor An initialanalysisofthedataforclusterstarswouldomit The steepreddeningrelationforNGC6087obtainedin by Schmidt,withadoptedclustermembers depictedby analysis (a)andforanewtypeof described dark circles.Resultsareshownfora standard-type versus distancemodulusforNGC6087 starsobserved here (b).Thelinein(a)hasunitslope. Fig. 4.PlotofCrawford’sevolutionary correction term 115 1986AJ 92. .HIT -1 - try forBtypestarsisthatitshouldnotintroducesystematic ing theeffectsofstellarrotationonhydrogen-linephotome- treme youthofOrionstarscontrasts noticeablywiththeages members aregenerallysmall. More importantly,theex- tion, particularlysincetherotational velocitiesofOriOBI km s,duetothecompensatingeffectofrandomaxialin- errors indistancemodulus,exceptperhapsforVsini>250 ation couldmaskanyobservable effectduetostellarrota- tions. Itisconceivablethatthedistance spreadinthisassoci- the lackofanysystematiceffectsonßindexforV clinations inalargesampleofstars.StudiesbyWarren form thebasisforinvestigationbyAnthony-Twarog sin i<250kms^However,itisnoteworthythatverydif- OBI havesubstantiatedthisconclusionbydemonstrating Schmidt (1984),andMillwardWalker1985)concern- line luminositiesthathavebeenpreviouslyascribedtoevolu- responsible forsomeofthesystematiceffectsinhydrogen- below, therearereasonsforbelievingthatstellarrotationis from hydrogen-linephotometry(Schmidt1984).Ofthese, important sourcesofexternalerrorinluminositiesderived rived fromdi{U—B)versusM(ZAMS)relation.Avar- to differincharacterfromtheevolutionarytrendsobtained mental calibrators.AsimilartrendappearsintheM(Hy) adopted asanadjustmenttoM{ß)inordercorrectfora specifically addressedinWarren’s(1976)study,butdid tional influencesondistancemoduliforOrionstarswasnot earlier studyoftheOrionassociation.Thequestionrota- tion. rington 1966;HardorpandStrittmatter1968).Asdiscussed line profilesofBtypestars(Guthrie1963;CollinsandHar- rotation anditsdirecteffectsonthecontinuahydrogen- the mostimportantinourviewseemslikelytobestellar iety ofeffectshavebeenconsideredatonetimeoranotheras for theseclustersbyCrawford(1978)usingluminositiesde- studies arenotidenticalforthesameclusters,andalsoseem tionary effects.However,thetrendsderivedintheseseparate cluster members,whereithasagainbeenattributedtoevolu- data ofMillwardandWalker(1985)forPleiadesaPer est starsinthePleiadesandaPerseiclustersusedasfunda- ferent conclusionswerereachedbyGuthrie(1963)inan dence canberemovedbyignoringtheevolutionarycorrec- systematic trendevidentinthedistancemoduliofbright- question ofwhyevolutionarycorrectionsareneededatall. tion term[cf.Fig.4(b)],butitspresenceraisesthenatural lutionary correctionforabsolutemagnitude.Thisdepen- tions ontheStrömgrenandH/7systemsfromcomparisons apparent lineardependenceofF—Mon10A/?,theevo- such asthis. is difficulttofindanyadvantagebegainedfromobserva- would expectfromphotometricerrorsalone,andmustre- from StrömgrenandH/3photometryislargerthanone tance modulusinherenttoabsolutemagnitudesderived sult fromlargeexternalerrorsintheluminosities.Infact,it toelectric data.Thefourfoldgreateruncertaintyinthedis- diagrams fromusingcarefullyanalyzed,goodquality,pho- which arisesindereddenedopenclustercolor-magnitude star, whichisactuallyfairlytypicaloftheinternalscatter uncertainty ofabout+0.015magin(i?—F)foracluster (1976) andAnthony-Twarog(1982)formembersofOri mag fortheZAMSfit.Thislastvaluecorrespondstoan (1982), albeitincludingCrawford’s evolutionarycorrec- 0v v v 0v 116 DAVIDG.TURNER:NGC6087ANDSNORMAE 0 The implicationfromdiscussionsbyCrawford(1978), A disturbingfeatureofthedataplottedinFig.4(a)is In Crawford’s(1978)calibrationthe10A/?termwas © American Astronomical Society • Provided by the NASA Astrophysics Data System 2 -1 2 cluster starswereobtainedfromtheliterature. to ouradoptedZAMS.PublishedestimatesofVsin/for tested. Itszeropointappearstobeidenticalthatapplying when plottedasfunctionsofthevariousparametersbeing applied tothoseobtainedfromtheCrawfordcalibration partially filledbyemissionforrapidlyrotatingstars(Craw- roughly asFsini(CollinsandHarrington1966;Hardorp relation between^andW(H/).Thiscalibrationwasadopt- of thisscattermayarisefromthetendencyforH/?tobecome tional effectsonF—Mappeartohave“saturated”fora least forFsin/<250kms.Abovethisvaluetherota- stellar distancemodulidependlinearlyuponFsin/,at without theevolutionarycorrectionterm. lar plotresultsfromusingCrawford’s(1978)calibration fore plottedasafunctionofthisparameterinFig.5.Asimi- moduli forstarsinthePleiadesandaPerclustersarethere- and Strittmatter1968;Warren1976).Thederiveddistance gravity fordifferentanglesofinclinationispredictedtovary tance modulithatexhibitedsmallerinternaldispersionthan tionary correctionssinceitwasfoundtogeneratestellardis- ed inpreferencetoCrawford’scalibrationwithoutevolu- lute magnitudesfromthepublishedßindicesusingtheir Walker’s MyiYLy)calibrationwasnextusedtoderiveabso- derived foreachclusterbyTurner(1976b).Millwardand ing relationscompatiblewiththeUBVreddening which hindersthissimpleinterpretationoftheresults.Part Per stars.Unfortunately,thereissomescatterinthediagram scribed previouslyforNGC6087starsusingspecificredden- bles. tometry forthePleiadesandaPerclusters,takenfrom made hereinthefollowingmanner.StrömgrenandHßpho- mination ofstellarluminositiesfromH/7photometrywas Crawford andPerry(1976)Barnes and aPerclusters,whicharecloserinagetoCepheidvaria- therefore bemoreclearlyevidentinclusterslikethePleiades are similartothoseobservedintheCepheidclustersshould of starsinclusterscontainingCepheidvariables.Effectsthat 0v (1974), respectively,werereanalyzedinthemannerde- 2 denote apparenttrendsinthedata. from Hßphotometryusingthetechnique describedinthetext.Lines recognized spectroscopicbinaries.The distance moduliwereobtained ades (darkcircles)andaPer(light circles) clusters.Barsdenote Fig. 5.Plotofdistancemodulusversus Vsin/forstarsinthePlei- The effectofrotationaldistortiononastar’seffective There isastrongsuggestioninthedataofFig.5that A simpletestoftheimportancerotationtodeter- 116 1986AJ 92. .HIT 2 be consistentwithclustermembership,itfallswellaway been excludedasaprobablenonmember.Althoughitsprop- of thisseries. discussion ofthedataforthiscluster isdeferredtoPaperII permitted bythestatisticstobeanonmember,andstar18 diagram (Fig.3)andalsoliesintheouterpartofcluster. from otherstarswhenplottedintheclustercolor-magnitude er motion,radialvelocity,anddistancemodulusappearto considered tobeaclustermemberhere,whilestar18has ward andWalker’s(1985)luminosityscale.Star7hasbeen the StrömgrenandYLßphotometry,analyzedusingMill- ed forstarsinNGC129,another Cepheidcluster.Afull field omitted.Similarlyreduced datahavealsobeeninclud- in Fig.4,butwithlikelybackgroundstarstheaPercluster appears tobethemostlikelycandidate. One ofthehigh-membership-probabilitystarsinTableIIIis ary correctionterm. properly forthesystematiceffectsresultingfromarangeof data ofFigs.4(a)and5,itdoesnotseempossibletoaccount derived forBstarsfromHßphotometry.Asindicatedbythe The limitingaccuracyindistancemodulusifVsiniisnot serious problemslimitingtheaccuracyofdistancemoduli calibration clusters.Itdoesillustrate,however,oneofthe known forastarappearstobeabout0.6magfromthiseffect. gen-line photometry,asarguedpreviouslybyCollinsand does affecttheabsolutemagnitudesobtainedfromhydro- voke stellarbinarityasapossibleexplanationforthe pendence onFsin/asdosinglestars,itisdifficulttoin- mag largerthantrueaPerclustermembers.Sincerecog- clusters isthereforeoflimitedvalueforstudyingtheCepheid approach adoptedinFig.5tostudythePleiadesandaPer generally lackingfordistantclusterssuchasNGC6087.The Harrington (1966)andHardorpStrittmatter(1968). observed trends.Quitesimply,itappearsthatstellarrotation nized binariesintheseclustersexhibitnearlythesamede- Fig. 5asaseparate“cluster”withdistancemoduli—0.6 photometry, informationonstellarrotationalvelocitiesis of errorinabsolutemagnitudesderivedfromhydrogen-line radial-velocity studies(cf.Turner1976b).Theyappearin have notbeeneliminatedfromclustermembershipbythe rotators. Inaddition,theextensiveareaofskyoccupiedby line luminosities. internal andexternalsourcesoferroraffectthehydrogen- Fig. 4(b)],whichagainmustbeduetothemannerin tions inVsin/forclusterstars.Theobservedrange matic spreadindistancemoduliresultingfromcosmicvaria- the aPerclustercontainsseveralbackgroundBstarsthat tance moduliforclustermemberswhichareextremelyrapid random photometricerrorsintheluminositiesonsyste- 0.78 mag,whichcanbeaccountedforbythesuperpositionof ford 1978).Thiswouldresultinsystematicallylargedis- V —Masafunctionofthe “evolutionary”parameter internal photometricerrors.Infact,thetotalrangein trends isabout+0.13mag,whichprobablygovernedby An inherentuncertaintyofabout0.5magwasfoundbyHar- dorp andStrittmatterintheirstudy. V sinivaluesforclusterstarsbyusinganartificialevolution- 117 DAVIDG.TURNER:NGC6087ANDSNORMAE V —MforNGC6087starsisverysimilar[0.75magfrom F —MforstarsinboththePleiadesandaPerclustersis Qv 0v 0v The distributionofunreddened stellardistancemoduli Figure 6presentsthedataofFig.5insameformused Figure 4(b)presentsthedataofTableII,obtainedfrom Although itisessentialtoaccountforallpossiblesources The residualscattersuperposeduponthestellarrotation © American Astronomical Society • Provided by the NASA Astrophysics Data System distance modulusforall 11clusterstarsis lus hasbeenrecalculatedfrom the dataofFig.6.Themean the variousmethods,providedthatStrömgrenandH/7 dence foradiscrepancyintheclusterdistanceasobtainedby distance modulusforthecluster.Thereisthereforenoevi- photometries aretreatedinthe mannerdescribedhere. removes almostalloftheremaining0.18magdiscrepancyin ally identicaltotheresultobtainedfromZAMSfitting,and photometry forNGC6087isV—M~9.15Thisvirtu- tion ofVsin/valuesisskewed,asitappearstobeforNGC which isclosesttothetruevalue,particularlyifdistribu- lus obtainedfromsuchdatamaythereforebethevalue 0.8 magisexpectedinthedistancemoduliforclusterstars derived fromH/?photometry.Themediandistancemodu- to thedataofFig.5,acosmicscatterroughly0.7mag 6087. ThemediandistancemodulusobtainedfromtheHß values forclusterstars.Itisworthremarkingthat,according these valuesisclearlylimitedbythedistributionofVsin/ dulus of9.78+0.03presentedearlier.Closeragreement tendency towardsdecreasingdistancemoduluswithincreas- cluster toanother,althoughsomesimilaritiesareapparent. weak emissionduetorapidrotationinafewofthestars.The trends intheoppositesense,possiblyasaconsequenceof ing A/?.NGG129andaPer,ontheotherhand,exhibit For example,NGC6087issimilartothePleiadesin 9.84 +0.10andwiththeclusterZAMS-fitteddistancemo- uncertainty bothwithPel’s(1985)distancemodulusof mal evolutionorrotationaldisplacementfromtheZAMS, principle ofconservationangularmomentumdictates say for10A/?<0.5. appropriate ifdeterminedusingstarsexhibitingonlymini- fect. Meandistancemodulifortheseclustersmightbemore the Pleiadesareincorrectsenseforanevolutionaryef- creasing stellarradiusasstarsevolveawayfromtheZAMS. that stellarrotationalvelocitiesshoulddecreasewithin- V —My=5.49±0.08s.e., butisV—M stars inFig.4(b)consideredasclustermembersis uli accordingtoFig.5,thetrendsevidentforNGC6087and Since thiswouldresultinsystematicallysmalldistancemod- 0v 10A¿? inFigs.4(b)and6iscuriouslydifferentfromone 10A¿?>0.5 areexcludedfromthemean,resultis V —My9.68+0.09s.e.Thislastvalueagreeswithinits V —My=9.65+0.07s.e.Ifthethreestarswith = 5.57+0.08s.e.whenrestricted tothesixstars(random- Q 0v 0 0 10 Aß As afinalstepintheanalysis, Pleiadesdistancemodu- tance modulusforstarsinthePleiades,aPer,andNGC129clusters. Fig. 6.PlotofCrawford’sevolutionarycorrectiontermversusdis- The distancemodulusofNGC6087obtainedfromthe12 —i r Pleiades 5 6 ••if • —i 1 Alpha Per • —! r NGC 129 11 12 VrNL 117 1986AJ 92. .HIT Klare, G.,andNeckel,T.(1977).Astron.Astrophys.Suppl.27,215. Johnson, H.L.(1963).InBasicAstronomicalData,editedbyK.A.Strand King, D.S.(1982).J.Proc.R.Soc.NewSouthWales115,1. Hardorp, J.,andStrittmatter,P.A.(1968).Astrophys.J.153,465. Irwin, J.B.(1958).Astron.63,197. Hanson, R.B.(1980).InStarClusters,IAUSymposiumNo.85,editedby Guthrie, B.N.G.(1963).Publ.R.Obs.Edinburgh3,81. Garrison, R.F.,Hiltner,W.A.,andSchild,E.(1977).Astrophys.J. Fernie, J.D.,andMcGonegal,R.(1983).Astrophys.275,732. Fernie, J.D.(1961).Astrophys.133,64. Feast, M.W.(1957).Mon.Not.R.Astron.Soc.117,193. Dean, J.F.,Warren,P.R.,andCousins,A.W.(1978).Mon.Not.R. Balona, L.A.,andShobbrook,R.(1984).Mon.Not.Astron.Soc. Anthony-Twarog, B.J.(1982).Astron.87,1213. Crawford, D.L.,andPerry,C.L.(1976).Astron.J.81,419. Crawford, D.L.,andBarnes,J.V.(1974).Astron.79,687. Crawford, D.L.(1978).Astron.J.83,48. Collins II,G.W.,andHarrington,J.P.(1966).Astrophys.146,152. Breger, M.(1966).Publ.Astron.Soc.Pac.78,293. mann (1969).However,itdoesrepresentasignificantim- 118 DAVIDG.TURNER:NGC6087ANDSNORMAE result isvirtuallyidenticaltotheoldervaluesderivedby better than+0.10atthepresenttime,amorereasonable been usedtoderivethedistanceNGC6087.Sinceitis Landolt (1964),Breger(1966),andSandageTam- value forthedistancemodulusofNGC6087is probably unrealistictoexpectthiszeropointbeknown agreement inzeropointofthevariouscalibrationsthathave and providesanindependentargumentfortheexcellent This lastvalueisidenticaltothePleiadesdistancemodulus obtained byTurner(1979)andthatadoptedPel1985), ly distributedinVsini)lyingwithinthe10A/?<0.5limit. V —M=9.78+0.13(totalestimateduncertainty).This 0v J. E.Hesser(Reidel,Dordrecht),p.71. Suppl. 35,111. (University ofChicago,Chicago),p.204. Astron. 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Landolt, A.U.(1964).Astrophys.J.Suppl.8,329. acknowledged. with theunpublishedlistingbyJohnIrwinismostgratefully tion ofpublishednumberingschemesforNGC6087stars during thetenureofaNSERCUniversityResearchFellow- search CouncilofCanada(NSERC),andwascompleted ship. TheassistanceofArloLandoltinprovidingacorrela- awarded throughtheNaturalSciencesandEngineeringRe- ago. what waspossiblewiththetechniquesinusetwentyyears provement ingeneralaccuracyandinternalconsistencyover Dordrecht), p.236. Symposium No.38,editedbyW.BeckerandG.Contopoulos(Reidel, Suppl. 51,321. p. 1. No. 82,editedbyB.F.Madore(CambridgeUniversity,Cambridge), This investigationwassupportedbyresearchfunds 118