1985AJ 90.22210 a) 9 Foundation. NO AOoperatedbyAURA,Inc.,under contractwiththeNationalScience Visting AstronomeratKittPeakNational Observatory,adivisionof 2221 Astron.J.90 (11),November1985 0004-6256/85/112221-18$00.90 © 1985Am. Astron. Soc.2221 spheroidal thestarsexhibitadispersion in[Fe/H]continues ences therein).Furthermore,evidence thatwithinagiven Aaronson andMould1985;Buonanno etal.1985,andrefer- absolute magnituderelationhasbeenshowntoexist(see but aconsiderableagerangemaybepresentwithinsomeof branches inthespheroidalsystems.Furthermore,thesesys- spheroidals arenowalsoavailable,andameanmetallicity- tems notonlyappeartodifferfromeachotherinmeanage, level ofthemain-sequenceturnoff.Suchageeffectsseemto onson andMould1985;Buonannoetal.1985). appear tospananagefrom~3X1015X10yr(Aar- them. ThemoststrikingcaseinpointisFornax,whosestars account naturallyfortheprevalenceofredhorizontal son 1983)andSculptor(DaCosta1984),whichreachthe that havenowbeenpublishedforCarina(MouldandAaron- more directlybydeepcolor-magnitudediagrams(CMDs) (AGB) carbonstars(seeAaronsonandMould1985), through identificationofluminousasymptotic-giant-branch tablished formanyofthespheroidals.Thiscameaboutfirst found ingalacticglobularclustershasnowbeenfirmlyes- occurred. Zinn’s earlierreview,anumberofmajordevelopmentshave lent reviewsofthesesystemshavebeenwrittenbyHodge become intenselystudiedobjects,anddeservedlyso,forthey THE ASTRONOMICALJOURNALVOLUME90,NUMBER11NOVEMBER1985 (1971) andZinn(1980,1985;seealsoCarney1984).Since evolutionary processesintheMilkyWay’souterhalo.Excel- provide anoutstandinglaboratoryinwhichtoinvestigate Reasonably accuratemetalabundancesforallsevenhalo © American Astronomical Society • Provided by the NASA Astrophysics Data System To beginwith,thepresenceofstarsyoungerthanthose The dwarfspheroidalgalaxieshaveinthelastfewyears blue stragglers.UrsaMinoristhereforeanextreme-agegalaxy,unlikesuperficiallysimilarobjectssuch must havelostitsgaseouscontentverysoonafterformation. discovery ofdumpinessinthedistributionstars.Thisfindingmaygivemoreweighttoideathat to anomalousCepheidsinUrsaMinorisestimatedbe~100,anumberthatmayprovideanimpor- the horizontalbranchinourcolor-magnitudediagramispoorlypopulated.Theratioofbluestragglers turnoff iseasilyvisible.FitstoevolutionaryisochronesandtheglobularM92indicatethatUrsaMinor from charged-coupleddevice(CCD)observationswiththeKittPeak4mtelescope.Themain-sequence We haveconstructedacolor-magnitudediagramoftheUrsaMinordwarfspheroidalto=24.8mag dwarf spheroidalgalaxieswerepreviouslyirregulargalaxies,althoughclearly,ifso,UrsaMinor tant constraintonbinarymodelsfortheoriginofthesestars.Asurprisingresultourstudyis derived fromaslidingfittotheM92ridgelines.However,thismodulusisuncertainby~0.1mag,for lives uptotheclassicalidealsofaPopulationIIsystem.Adistancemodulus(m—Af)=19.0magis as theCarinadwarf.Indeed,UrsaMinormaybeonlyouter-halospheroidalwhosestellarcontent than about16billionyearsisseen,withthepossibleexceptionofapproximately20starsbelievedtobe has anageandmetalabundanceverysimilartothatofthelattercluster.Noevidenceforstarsyounger 0 Dominion AstrophysicalObservatory,HerzbergInstituteofAstrophysics,Victoria,BritishColumbiaV8X4M6,Canada THE URSAMINORDWARFGALAXY:STILLANOLDSTELLARSYSTEM I. INTRODUCTION Steward Observatory,UniversityofArizona,Tucson,Arizona85721 Steward Observatory,UniversityofArizona,Tucson,Arizona85721 Received 25March1985;revised5August1985 Edward W.Olszewski^ a) Marc AARONSON ABSTRACT and trasts mayilluminatemanyoftheremainingproblemsin several othergalacticglobulars. low-luminosity, blue-colorvariety alsofoundincoCenand Aaronson, Olszewski,andHodge 1983),buttheseareofthe These systemsarethetwolowest-luminosityhalodwarfs understanding thestellarcontentofdwarfgalaxies. contain carbonstars(Aaronson, Liebert,andStocke1982; city, andbothhavestarsthat show enhancedCNO(e.g., similar toM92,buthavesomestarsofverydifferentmetalli- cropped upinseveraloftheotherspheroidals(Baadeand to possessredgiantvariables,whichhaveoccasionally variables andanomalousCepheids,whileneitherareknown comparable togalacticglobulars.BothcontainRRLyrae co, respectively,afterZinn1985),andinthisregardaremost (M =—8.8magand8.5forUrsaMinorDra- Kinman eiû/.1981;Stetson1984; Suntzeffeia/.1984).Both Swope 1961;vanAgt1973).Bothhavemeanabundances suggested (seeAaronson1983;FaberandLin may containsubstantialamountsofdarkmatterhasbeen Faber 1983). dwarf ellipticals.Second,thepossibilitythatspheroidals dals arecloserinkinshiptodwarfirregularsratherthan oidals havealsobeenrecentlyraised.First,LinandFaber evidence asyetforanextendedperiodofstarformation. galaxies suchasDracoandUrsaMinor,wherethereislittle cient timetooccur,butitperhapsdoespresentdifficultiesin latter resultispossiblynotunexpectedinsystemssuchas to accumulate(Zinn1978;Demers,Kunkel,andHardy (1983) andKormendy(1985)havearguedthatthespheroi- Fornax, whereenrichmentprocessesshouldhavehadsuffi- v 1984; andBuonannoetal.1985;butseeBell1985).This 1979; Kinman,Kraft,andSuntzeff1981;etal. Ursa MinorandDracoformapairofgalaxieswhosecon- On theotherhand,UrsaMinor andDracodifferintwo Some ratherspeculativeideasconcerningthedwarfspher- 1985AJ 90.22210 nation, butforstellarphotometrythisismorethanade- eliminate large-scalegradientstobetterthan2%-3%, fields weredividedbythedomeflats.Weunableto the frameswerebiassubtractedandtrimmed,allstellar vations inBandVconsistedofexposurestheUrsaMinor June 1984,withtheRCAprime-focusCCDsystemon which weattributetothenonuniformityofdomeillumi- seeing wasmeasuredtobe^1arcsecforallexposures.All exposures oftheilluminateddome,andbiasframes.The field, twostandardfields(describedbelow),ablank-sky run throughacosmic-raycleaningprogram,whicheliminat- quate. Thefour500sVexposuresoftheblankfieldwere picture wassettozero,leavingbehindthefringepattern. ed thestars,andmodeofintensitiesinresultant each offsetbyafewstellardiameters.Theseexposureswere KPNO 4mtelescope.The‘Mould’filtersetwasused.Obser- logical effectsexistedinthatfilter.Aseriesofscaledfringe tailed. Theuninterestedreaderisinvitedtoskipdirectly one ofthefirstpaperstomakeextensiveusedaophot, ference inhorizontal-branchtype,itisverytemptingtospec- the UrsaMinorfieldatvarious timesduringthenight,and later cleared.Wethereforetook threeshort-framepairsof were takenduringamildlycloudy period,althoughthenight removal offringingwasaccompanied. frames werethensubtractedfromallVpicturesuntilthebest Similar exposuresweretakeninBtoensurethatnopatho- apparent stellarsubclusteringintheUrsaMinorfield. V. InSec.VIwepresentanddiscusstheimplicationsofa distance modulus.AsummaryofourfindingsisgiveninSec. ed, alongwithisochronefitsandthederivationofage discussion inSec.Illhasbeenmaderatherlengthyandde- reduction, performedentirelywithPeterStetson’sdao dard system. will usethemtoaccomplishthe transformationtothestan- startling andunexpectedresultpertainingtothepresenceof Sec. IV,wherethefinalcolor-mangitudediagramispresent- tude diagramforUrsaMinor.Twogroups(Carneyand test thishypothesisbyprovidingaverydeepcolor-magni- Minor. Theprimarypurposeofthepresentpaperistohelp ulate thatDracoisafewbillionyearsyoungerthanUrsa the famous“second-parameter”problem. phot program,isfullydescribedinSec.III.Becausethis scription oftheobservationsispresentedinSec.II.Thedata reached inbothsystems,sothatthecrucialtestcanbemade. detector technologyreadilyenablesthemainsequencetobe abundance exhibitsperhapsthemostextremecaseknownof zontal branchliketheotherspheroidals,andgivenitsmean a bluehorizontalbranch.Draco,incontrast,hasredhori- cant isthefactthatUrsaMinoronlyspheroidaltohave currently supplyingthecompaniondataforDraco.Current Seitzer 1985;Stetson,VandenBerg,andMcClure1985)are more metal-richabundancespread.Perhapssignifi- ence mayberelatedtoDraco’shavingalargerandslightly important respects.First,thecharacteristicsofUrsaMi- either oftheOosterhoffclasses(e.g.,Zinn1980).Thisdiffer- nor RRLyraesareverysimilartothoseinOosterhofFType II clusters,whilethoseinDracocannotbereadilyplaced 2222 E.W.OLSZEWSKIANDM.AARONSON:URSAMINOR II. OBSERVATIONSANDPRELIMINARYDATAREDUCTIONS The datareportedhereweretakenononenight,22/23 The long(totalof3000sperfilter) UrsaMinorexposures The organizationofthepaperisasfollows:Abriefde- Although thereareotherpossiblewaystoaccountfordif- © American Astronomical Society • Provided by the NASA Astrophysics Data System was chosenasafirstapproximation,thecoefficientsand were forcedtozero,andcoefficientsrecalculated.Since their errorswerecalculated,insignificantterms(c3andc4) the finaltransformationequations the data.Alaterattempt,whichwasfinallyadopted,allowed the standardfieldobservationscoveredonlyarangeof1.02- ness offitthemodeland/or qualityofthenight.With standard-star magnitudes.This numberdefinesthegood- from theerrorsininputinstrumental magnitudesandthe a quantitythatistheamountof scatterabovethatexpected proaches wasonly—0.01mag. Stetson’s programcalculates ence inUrsaMinorphotometry betweenthesetwoap- the airmasscoefficienttobecalculatedaswell.Thediffer- airmass coefficientof0.28andayellow0.17to observed magnitudefromstandardmagnitude.Thisformu- tions weremadeusingdaophotsubsidiaryprograms,all ter inthedata. lation issuperiortotheinversebecauseobservationsoffilter CCDSTD wasthenusedtocalculatethetransformation scribed inStetson1985.)Aminorvariationoftheprogram also writtenbyStetson.Theetymologyofdaophotisde- tines inStetson’sdaophotprogram,andalltransforma- that allinstrumentalmagnitudeswerederivedusingrou- and errors,thestandard-starmagnitudewithitsasso- scale —0.6"perpixel).Thetotalinstrumentalmagnitudes from pixel6to7beingoforder0.005mag(withthe to thermserrorindeterminingthatquantity.Thiscorre- tudes werethendeterminedbygrowingtheradiusuntil1 coefficients atthemeanairmassbetterreflectactualscat- where nearthemeanairmassofstandards,errorsin the programobjectphotometryispresumablydonesome- lation, whileidenticaltothelatter,ismoreintuitive.Since ciated error,werethentypedintotwocomputerfiles.(Note sponded toa7pixelaperture,thelastmagnitudegrowth pixel changeinstellarmagnitudewasapproximatelyequal each coloratagivenairmass.Theinstrumentaltotalmagni- yielding uptofourobservationsofeachstandardstarfor of theobservations,ratherthanatzeroairmass.Thisformu- coefficients andtheirassociatederrorsforthemeanairmass filter 2fromnight2.Thevariationsimplygivesthevaluesof measured, 12attwodifferentairmasses. from —0.1to1.7.Twentyindividualstandardstarswere 1.37 airmasses,afirsttransformationwastriedforcingblue colors oftheobservedstandardscoverawidecolorbaseline observed attwodifferenttimesduringthenight.TheB—V for eachobservationofobjectinfilter.M92was In eachcase,twoshortandlongerexposuresweremade observations weresecuredduringphotometricconditions. available andusedfromSandage1958).Allstandard-star dage 1969)andtheNGC7790videocamerafield(Christian also Christianetal.1985;otherphotometryfoundinSan- M92 CCDstandardfield(Davis1984;Christian1980;see 1 fromnightcanthenbecombinedwithobservationsof 1980; Christianetal.1985;photoelectricphotometryalso v= F+cO+c1*Z+c2*(j5-V)c?>*X*(B— An equationoftheform Aperture photometrywasthenperformedforeachframe, Short exposures,rangingfrom5to30s,weretakenofthe + cA*(B-V)*(BV) III. DATAREDUCTIONSANDCALIBRATIONS a) TheStandard-StarFrames 2222 2223 E. W. OLSZEWSKI AND M. AARONSON: URSA MINOR 2223
v=r+ 0.1909( + 0.0048) + 0.0421( + 0.0060)*^ - F instrumental values given in Table I. The transformation equation is also shown. Note that the standard deviation of + 0.1348( + 0.0216)*(X- 1.2) the data from the best-fit line is 0.017 mag in F and 0.019 and mag in B. Most of the scatter comes from the errors in the b = B + 0.0255( + 0.0083) - 0.1065( ± 0.0107)*J9 - F standard-star magnitudes, and the resultant error in the slope and zero point of the relations, for the error in the + 0.3049(±0.383)*(X- 1.2), instrumental magnitudes of the standards is generally less than 0.008 mag, although a few individual magnitudes were the additional scatter was <0.001 mag in both B and F. uncertain by as much as 0.02 mag. The scatter for the NGC Thus the model transformation describes the actual data 7790 stars is about 1.5 times that of the M92 stars. within the errors in the standard-star instrumental magni- tudes and within the best guesses for the internal plus exter- b) The Short-Exposure Ursa Minor Frames nal errors in the published standard-star magnitudes. (The assumed errors in the standard star magnitudes are some- Since clouds appeared during the long exposures, which what arbitrary: + 0.02 in both B —V and F for Sandage’s will be described below, three pairs of short (50 s in F, 100 s in M92 magnitudes, and ± 0.03 in F and either 0.03 or 0.04 in B ) exposures of Ursa Minor were taken and reduced. Each B — V for the NGC 7790 magnitudes. For Davis’ M92 data, exposure is believed to have been taken in clear conditions. we assumed the errors to be the standard deviation of her The aperture-photometry program in daophot was used individual magnitudes, with an additional 0.015 uncertainty for these reductions. For all stars discovered by the star- for systematics. Because the transformation program stops finding routine, the magnitude correction to a large aperture iterating once the fit is as good as allowed by the errors in the was determined. The program ccdcal was then applied to standard and instrumental magnitudes, it is important to each frame to calculate F and B — V for each star. Sums assign errors that are as close to reality as possible.) The final V\ — F2, Fj — F3, Bx — B2i and Bx — B3 were determined equations can be inverted to solve for the magnitudes of pro- for all the uncrowded high-signal-to-noise stars, excluding gram stars. A plot of y — F — c*X versus B — V and a simi- the known RR Lyraes. Table II shows these framewide dif- lar plot for B are shown in Fig. 1, with the standard and ferences and their standard deviations. The blue magnitudes
Fig. 1. Standard-star transformations. The abscissa is (I? — F) of the standard, and the ordinate is the instrumental mag- nitude minus the standard magnitude cor- rected for the derived extinction coeffi- cient. The line is the best fit to the data as described in the text. The upper figure is for V, and the lower figure fori?. It is easily seen that color terms are needed to fit each magnitude to the standards.
© American Astronomical Society Provided by the NASA Astrophysics Data System 1985AJ 90.22210 M92-LD 7 M92-LD 5 M92-LD 4 M92-LD 2 M92-LD 1 M92-LD 10 M92-LD 8 M92-LD 6 NGC 7790-H NGC 7790-M NGC 7790-K NGC 7790-10 M92-X23 M92-LD 23 M92-LD 14 M92-LD 12 M92-LD 11 M92-LD 2 M92-LD 1 NGC 7790-Z NGC 7790-9 M92-X22 NGC 7790-K M92-X22 M92-LD 14 M92-LD 8 M92-LD 6 M92-LD 5 M92-LD 4 NGC 7790-H NGC 7790-M NGC 7790-9 NGC 7790-10 M92-X23 M92-LD 23 M92-LD 12 M92-LD 11 M92-LD 10 M92-LD 7 NGC 7790-Z given inparentheses.Sinceforeachfieldataairmass,twolongand shortexposuresweremade,amaximumoffourmeasurementsperairmassis Column 1:StaridentificationsfromDavis1984(M92-LDnumbers),Sandage 1969 (M92-Xnumbers),Sandage1958(NGC7790letters),andChristian1980 Columns 4and5:Standardmagnitudes andcolorsfromreferencesabove. possible. Noobservationsofoverexposedstarsorwithderivederrorslarger than0.02magwereincluded. Columns 2and3:Totalinstrumentalmagnitudescorrectedforextinction.The numberofindividualmagnitudedeterminationsaveragedtogetthisvalueis Columns 9and10:Observeddifference minuspredicteddifference. Column 8:Differencespredictedbythe equationsdiscussedinthetext. Columns 6and7:Differencesbetween instrumentalandstandardmagnitudes. (NGC 7790numbers). 2224 E.W.OLSZEWSKIANDM.AARONSON:URSAMINOR Star © American Astronomical Society • Provided by the NASA Astrophysics Data System (1) Star (1) 16.197(4) 14.981(4) 16.240(2) 14.431(2) 16.098(4) 15.988(4) 15.920(4) 14.990(4) 13.311(2) 13.341(2) 14.223(2) 16.285(2) 17.205(2) 15.713(4) 13.281(2) 14.807(4) 16.549(2) 15.080(4) 14.956(4) 14.527(2) 13.161(2) 13.365(2) 14.770(4) 13.302(2) 15.473(4) 16.031(4) 15.416(4) 15.192(4) 16.837(2) 14.366(2) 15.201(4) 15.960(4) 16.456(2) 16.378(4) 16.110(4) 14.680(2) 14.664(2) 16.027(4) 14.100(2) 13.224(2) (2) (2) ¿i ”1 16.194(4) 14.978(4) 14.967(4) 13.294(2) 16.238(2) 14.416(2) 16.074(4) 16.008(4) 15.927(4) 17.015(2) 14.520(2) 15.699(4) 14.686(2) 14.662(2) 13.160(2) 16.831(2) 14.368(2) 15.202(4) 15.960(4) 16.462(2) 16.368(4) 16.117(4) 16.022(4) 14.089(2) V (3) (3) ^2 2 16.332 16.601 15.401 13.705 14.73 16.624 14.838 16.502 16.227 15.400 13.64 13.69 14.893 16.164 13.070 15.23 17.092 16.704 15.44 15.33 17.430 13.21 14.611 14.579 13.15 13.32 14.75 15.380 15.940 15.38 15.16 16.790 14.306 15.960 15.142 14.023 15.931 16.444 16.337 16.059 (4) (4) B V Table I.Standard-starrelations. -0.112 -0.110 -0.112 -0.110 B-V B- V 0.822 0.542 0.789 0.635 (5) 0.49 0.815 0.571 0.37 0.48 0.764 0.17 0.640 0.587 0.664 0.06 0.789 0.822 0.635 0.49 0.37 0.48 0.764 0.571 0.542 (5) 0.06 0.17 0.640 0.587 0.664 0.815 Notes toTableI 1.52 1.022 1.712 1.52 1.022 1.712 V magnitudes B magnitudes - 0.420 -0.344 -0.404 - 0.394 - 0.359 - 0.349 - 0.423 -0.405 - 0.384 -0.451 -0.407 -0.404 - 0.307 - 0.410 - 0.507 - 0.543 -0.419 -0.360 - 0.374 - 0.366 b— B v-V x x 0.069 0.085 0.091 0.045 0.092 0.091 0.059 0.029 0.012 0.041 0.051 0.074 0.020 0.093 0.032 0.047 0.060 0.067 0.077 0.036 (6) (6) -0.407 - 0.434 -0.411 - 0.324 -0.300 - 0.422 - 0.422 - 0.428 - 0.386 - 0.465 -0.415 - 0.373 b —B v-V 2 2 0.075 0.029 0.018 0.031 0.058 0.083 0.090 0.041 0.062 0.062 0.060 0.066 (7) (7) Predicted Predicted - 0.398 - 0.428 - 0.329 - 0.329 - 0.425 -0.408 - 0.393 - 0.380 - 0.502 - 0.523 - 0.422 - 0.392 - 0.347 - 0.359 -0.409 -0.411 -0.449 - 0.427 -0.401 -0.403 b-B v-V 0.062 0.061 0.024 0.052 0.064 0.056 0.045 0.049 0.093 0.101 0.032 0.036 0.072 0.053 0.024 0.050 0.056 0.054 0.057 0.063 (8) (8) (O-C), (O—C) (O-C), 2 -0.031 -0.005 - 0.020 -0.013 -0.015 -0.002 -0.003 -0.015 -0.006 - 0.029 -0.001 -0.008 -0.004 -0.009 -0.013 - 0.024 - 0.012 -0.001 0.008 0.014 (9) (10) 0.020 0.022 0.015 0.034 0.031 0.003 0.004 0.037 0.027 0.030 0.004 0.007 0.021 0.035 (9) 0.000 0.006 0.017 0.024 0.010 0.014 mean =0.003 mean =0.004 s.d. =0.017 32 values s.d. =0.019 32 values (0-C) 2 -0.003 -0.009 -0.006 - 0.027 - 0.016 -0.006 -0.015 - 0.012 - 0.024 -0.006 (10) 0.005 0.003 0.029 0.025 0.005 0.030 0.034 0.008 0.007 0.013 0.019 0.005 0.003 0.006 2224 1985AJ 90.22210 hms ; brighter thanVand0.04mag.Thesediffer- agreed tobetterthan0.01mag,whileVwas0.017mag ences arenodoubtareflectionofthelessthanidealphoto- frames. Asystematicerroraslarge0.02magmaythusbe simply averagedthederivedmagnitudesfromthree evidence thatthebrightermagnitudesofVarecorrect,we metric qualityofthenight,butsincewehavenoapriori dure. present inVandB—Vsimplyduetothisaveragingproce- ml-m2 -0.040±0.015(12)0.0020.012 sures tothestandardsystem. stars willbeusedtocorrectthemagnitudesoflongexpo- on theshortframesandtheirstandarddeviations.These m2-ml -0.017+0.011(12)0.0030.008(12) Table II.UrsaMinorshortframes:Differencesincalculatedmagnitudes 2225 E.W.OLSZEWSKIANDM.AARONSON:URSAMINOR the factthatVframe2wasbrighterthan eitherframe1or3. Columns 4-7.Magnitudesandobserved deviationsfromthethreeshort-framepairs.Notethatatypicalstandarddeviation ofabout0.02isexpectedfrom Column 3.NumberfromSchommer,Olszewski, andCudworth(1981),tobepublishedinCudworth,Olszewski, Schommer(1985). lated foreachstar.Theaperture-photometryroutineisim- aperture magnitude[—2.5log(counts)-f-c\wasthencalcu- (1967). Column 2.Numberfromunpublished work bySchommer,Olszewski,andKunkel(1978).StarOthreedigitnumbers largerthan154arefromvanAgt Column 1.Numberfromthispaper. pixels (seeStetson1985)andallstellarobjectsidentified.An l3 function (PSF)methods.First,thepictureswereconvolved mental magnitudewasaccomplishedbyatwo-stepproce- together, yieldingaVframewith66photonsperADU,and with aloweredtruncatedGaussianhavingFWHMof2.0 2 dure involvingbothaperturephotometryandpoint-spread- frame isshowninFig.2[Plate231].Thereductiontoinstru- and readoutnoiseof4.6ADUinB.AreproductiontheV readout noiseofabout3.3ADU,and33photonsperADU relatively freeofbrightobjects.Theframeswereaveraged foreground doublestar,andwaschosenbecausetheareais this nomenclature).ThepositionisR.A.15075(1950), and Dec.67°2356"(1950),~260arcsecwestofthecentral Q3,328 =starCOS313(seeTableIIIforanexplanationof 2 in Vandthree1000sframesBcenterednearstarSOK Table IIIgivesthemagnitudesofwell-observedstars The deepUrsaMinorexposuresconsistofsix500sframes © American Astronomical Society • Provided by the NASA Astrophysics Data System 1022 1479 1242 Star 290 745 612 648 517 300 863 160 113 c) TheLong-ExposureUrsaMinorFrames 66 5 V B for brightstars. Q2, 11 Q2, 41 Q2, 17 Q3, 324 Q3,40 Q3, 39 Q3, 22 Q3, 266 Q3, 296 Q3, 394 Q3, 327 SOK O Table III.Well-observedstarsonUrsaMinorshortframes. COS 211 345 332 338 342 339 319 351 321 311 180 189 Notes toTableIII 16.885 19.363 19.341 18.832 18.375 17.995 17.011 19.189 19.118 19.107 18.886 18.592 18.551 17.467 same telescope,andthefilters,daophotisefficientat taken inSeptember1983withthesamechip(RCA#1), this effectwasnoticedonframesoftheglobularclusterM56 remaking thePSFuntileffectsofneighborsdisappeared. brightest starofinteresthadaradiuslessthan6pixels,and were fit,theysubtractedfromthepicture(whereas the PSFinpeakroutine(similartorichfldsoftwareat these frames. this chip,theyareeithertransientortoosmalltonoticefor conclude thatifcharge-transferinefficienciesarepresentin made ofthecolor-magnitudediagramuncrowdedstarsin did notshowanysuchsystematicresiduals.Plotshavebeen charge-transfer inefficiencyorflatnessvariations.Indeed, of theycoordinateonchip,presumablybecause There issomeconcernthatthePSFitselfmaybeafunction brightness. Anystarwithacentralpixelofmorethan15000 routine); and(3)itistheonlyroutineinwhichvalueof Kitt Peak)usingafittingradiusof2pixels.Afterallstars systematic effectsatthelevelof0.02magcouldbeseen.We each ofthetop,middle,andbottomthirdschip.No pixel residuals.NowinthecaseofUrsaMinor,PSFstars measured tobelessthanorabout2%inthelargestsingle ordinates wereseentobemorespreadout.Thiseffectwas als studied.InthecaseofM56,starswithincreasingyco- each PSFstarissubtractedfromthepictureandresidu- discovering thisproblem,forinthewayPSFisderived, chosen forthePSF,subtractingthemoutofpicture,and an iterativeprocedureinvolvingfittingneighborstothestars ADU wasrejectedasapotentialPSFstar,wereanytoo beyond 7pixelscouldnotbemeasured.) having sufficientskypixelstorejectintrudingstars.Wealso of thepixelvaluesinanannulusradii8to15pixels.(The sky iscalculated.Thewascalculatedbytakingthemode eventual instrumentalmagnitudestothestandardsystem; portant forthreereasons:(1)itprovidesthelinkfrom crowded, ortooclosetoblemishes.ThePSFwascreatedin knew fromthestandard-starframesthatmagnitudegrowth we wantedthemostlocalskyvaluepossibleconsistentwith (2) itprovidesstartingguessesforthemagnitudeofstar (starting coordinateguessesarecarriedalongfromthefind All starsintheaperture-photometryfilewerethenfitto After theaperturephotometry,starsweresortedby sigma 0.019 0.020 0.075 0.008 0.016 0.018 0.026 0.011 0.022 0.017 0.016 0.020 0.040 0.019 B —V 0.740 0.728 0.800 0.734 0.856 0.808 0.616 0.891 0.895 0.436 0.891 1.144 1.226 1.506 sigma 0.089 0.010 0.019 0.025 0.026 0.022 0.043 0.028 0.023 0.020 0.009 0.023 0.023 0.044 2225 2226 E. W. OLSZEWSKI AND M. AARONSON: URSA MINOR 2226
RiCHFLD subtracts stars as it goes along). The find algo- mately the level at which the change in error per change in rithm was then run on the subtracted picture, picking up magnitude becomes very large. The starlist after these two stars hidden in wings of other stars, or stars which now can operations consists of 1486 stars. Table IV gives the pass sharpness or roundness tests. (Of course, a few bogus numbers, coordinates, magnitudes, and deprived errors for ‘stars’ caused by poor fits are also selected.) Aperture pho- these stars. Note that the uncertainties in Table IV reflect the tometry for these new stars was then performed on the origi- internal errors only. To these should be added a transforma- nal picture. The two groups of stars were then added togeth- tion error of ~0.005 mag. As discussed earlier, an unknown er and the routine group was run. group separates stars systematic error <0.02 mag may also be present. A finding into natural groupings, given a critical radius such that any chart with every tenth star explicitly identified is provided in star within one critical radius of another star is included in Fig. 3 [Plate 232], and the accurate coordinates listed in Ta- that star’s group. The stars in a given group were then re- ble IV should enable the remaining stars to be easily located. duced simultaneously in a profile-fitting routine named NSTAR. Instrumental magnitudes, errors (in fit, plus readout IV. THE COLOR-MAGNITUDE DIAGRAM noise and photon statistics), and values of chi are produced. a) The CMD and Reddening The fitting error dominates the derived uncertainty for bright stars, which is why the quoted errors are significantly Two forms of the Ursa Minor color-magnitude diagram higher than simple photon statistics would imply, nstar are given here to allow readers to assess the data. The first, also has a star-rejection feature which largely eliminates the Fig. 4, presents all 1486 stars fit in both the B and F solu- bogus stars described above, and also removes most nonstel- tions. The second version shows only those 783 stars that lar local maxima. have no neighbor stars within 5 pixels of themselves; these In the special case of these data, the reduction to the stan- stars are flagged in Table IV. This CMD is made up of the dard system was performed by an iterative procedure. The cleanest and presumably best stars, and is analogous to a difference between the instrumental v and b — v was com- subset of stars that would have been hand-picked for other pared to the photoelectric system magnitudes from the short computer programs, although without unknown human bi- frames, ccdcal was run, yielding preliminary system mag- ases. Only this final diagram, shown in Fig. 5, will be used in nitudes, which were again compared to the short-frame the following discussion. magnitudes until no further improvements were noticed. The Ursa Minor CMD morphology at the level of the The long frames had to be adjusted by — 0.257 mag in Vand horizontal branch and above is similar to that published by — 0.384 mag in B to put them on the system of the short van Agt (1967), Schommer, Olszewski, and Kunkel (1983), frames. Of course, this amount includes the aperture correc- and Schommer, Olszewski, and Cudworth (1981). In parti- tion from the 2 pixel radius to a large radius, which is of cular, a blue horizontal branch is clearly present, along with order 0.2 mag, so the clouds indeed were thin. The fit to sparse but steeply sloping giant branch, two indications of a the short-exposure system was excellent; for 15 of the bright- metal-poor population. As noted by Schommer, Olszewski, est stars, Fshort — Fjong = 0.003 + 0.019(s.d.), and and Cudworth (1981), a small percentage of red horizontal- (B - ^(short -(B- F)long = - 0.006 ± 0.018. branch stars may also exist. Below the level of the horizontal There were approximately 3000 stars photometered on branch, a broad but well-defined subgiant branch is appar- each color frame. If the entire chip area is to be measured, it ent. Most important, at F—23.5 mag the main sequence is is imperative to make as good a model of the frame as possi- clearly reached. Finally, a small blue straggler population is ble, which means including all the star centers in the solution in evidence, which we discuss further below. for the coordinates and magnitudes. The approach of nstar Cudworth, Olszewski, and Schommer (1985) have per- differs here from that of richfld, where it is generally not formed a proper-motion survey of the Ursa Minor field to possible to fit more than one star at a time, nstar can fit up mv = 20, whose publication has been delayed because the to 60 stars at once, and the solution will converge more ra- color terms in the photographic photometry could not be pidly and accurately if all the stars in a group are identified adequately pinned down given the photoelectric standards and fit. Therefore, the nstar approach precludes using the available. The short-frame photometry here will be used as identical coordinate list for both ¿and V frames. Instead, the secondary standards in the Cudworth, Olszewski, and two starlists are cross correlated after the nstar reductions, Schommer survey. For convenience, the program stars in yielding in this case 1762 stars. Approximately 1000 stars our study, which are also in the proper motion and earlier were lost from the total on each frame because only stars CMD work, are identified in Table V using both SOK and whose coordinates agree to better than 1.3 pix- COS numbers, since a variety of other studies (e.g., Aaron- els = VtATf+lAFf are considered a match. For faint son and Mould 1985; Suntzeff et al. 1984) employ those stars, it is easy to have the coordinates disagree solely be- numbering schemes. Note that the SOK numbers are the cause of one deviant pixel. Of course, by relaxing the fit crite- same as those of van Agt (1967) with additional stars added. rion, more (and presumably noisier) stars can be added to the Reddening to Ursa Minor is known to be small. Cantema starlist. and Schommer (1978) used the value E(B — V) = 0.05 mag. The starlist of 1762 stars was manipulated in two further This seems high given the H I column density (Burstein and ways to obtain a final starlist of stars with good photometry. Heiles 1982), which implies a reddening of between 0.00 and First, all star pairs having centers less than 1.7 pixels apart 0.03 mag. We will use Zinn’s (1981) value of were removed from the list. The cross-correlation program E(B — V) = 0.03, which is a compromise between the Can- can possibly identify two different V coordinates with one tema and Schommer value, the reddening estimates from blue coordinate. Many of the stars in this list (115 total) are in Zinn’s spectrophotometry, and the HI estimate. fact those with abnormally large chi values. Second, all stars As previously discussed, several dwarf spheroidals are with F> 24.8 mag were removed. This value was picked be- thought to have metallicity spreads, and it is of interest in cause in a plot of error versus magnitude, 24.8 is approxi- this regard to examine the width of features in the CMD
© American Astronomical Society • Provided by the NASA Astrophysics Data System 1985AJ 90.22210 2227 E.W.OLSZEWSKIANDM.AARONSON:URSAMINOR Star X © American Astronomical Society • Table IV.Coordinates,magnitudes,andderivederrorsforUrsaMinorstars. 0.176 0.111 0.104 0.149 0.076 0.043 0.111 0.042 0.016 0.085 0.074 0.113 0.021 0.017 1x Yra Flagl StarXYmB-Voa-VFlag_vavob-V vB Provided bythe NASA Astrophysics Data System 24.545 23.912 23.153 23.529 24.494 23.547 22 .878 24.198 21.783 23.501 23.357 24.097 23 .781 21.960 23.451 22.965 24.141 24.049 22.793 23.152 24.726 24.190 23.200 24.238 24.404 24.129 24.020 23.798 23.705 23.770 23.208 24.020 22.951 24.041 23.056 23.669 24.495 24.162 23.510 20.397 22.786 23.514 22.727 2227 1985AJ 90.22210 2228 E.W.OLSZEWSKIANDM.AARONSON:URSAMINOR © American Astronomical Society • Table IV.(continued). Provided bythe NASA Astrophysics Data System 1 Flag StarX 224.71 274.69 225.26 285.71 265.32 178.30 114.18 104.32 136.50 205.10 227.76 308.77 174.75 102.37 269.23 259 .51 283 .03 297.18 289 .99 98.48 197.61 43.24 55.52 38.14 145.56 80.47 16.95 43.76 8.12 24.722 -0.332 23.969 22.826 23.277 0.622 0.618 0.102 0.502 0.344 0.745 1 PB-V Flag 2228 1985AJ 90.22210 2229 E.W.OLSZEWSKIANDM.AARONSON:URSAMINOR © American Astronomical Society • 306.32 218.71 279.09 296.46 213.14 217.21 262.76 170.27 118.18 128.95 112.41 103.24 153.61 193.28 102.37 157.98 310.74 223.99 119.34 187.35 100.98 232.79 237.53 156.58 229.24 273.05 299.34 144.52 103.46 312.47 223.60 154.02 170.98 34.00 80.31 86.50 82.05 43.08 92.38 88.25 82.28 69.32 66.31 30.30 59.21 10.81 50.04 92.88 25.88 85.51 52.39 55.75 62.30 30.46 16.26 82.56 49.32 71.65 79.65 213.33 213.06 214.60 214.34 224.54 218.07 218.05 217.23 216.17 215.81 215.54 215.33 214.76 226.32 225.36 225.30 217.17 216.56 216.37 216.33 216.14 215.89 229.55 229.03 229.83 229.51 229.17 231.84 231.58 236.80 234.85 234.71 234.49 231.74 231.61 231.58 236.87 237.02 237.41 237.31 237.12 238.08 237.38 238.11 237.43 238.33 Table IV.(continued). Provided bythe NASA Astrophysics Data System 1008 1007 1010 1009 1014 1013 1016 1015 1022 1019 1018 1017 1032 1031 1030 1026 1025 1024 1023 1033 1029 1028 1027 2229 1985AJ 90.22210 2230 E.W.OLSZEWSKIANDM.AARONSON:URSAMINOR 1088 1074 1092 1090 1089 1086 1079 1078 1077 1076 1075 1073 105» 1099 1098 1097 1096 1093 1091 1087 1063 1062 1061 1058 1057 1138 1137 1136 1145 1144 1143 1140 1139 © American Astronomical Society • 1235 1234 1233 1232 1225 1224 1223 1222 1209 1207 1205 1204 1203 1208 1202 1240 1239 1237 1236 1247 1243 1242 1241 1238 1250 1249 1248 1246 1178 1164 1163 1162 1161 1180 1179 1262 1260 1259 1258 1264 1263 1261 Table IV.(continued). Provided bythe NASA Astrophysics Data System 1355 1354 1353 1352 1346 1345 1363 1360 1359 1358 1318 1317 1316 1275 1274 1273 1271 1268 1267 1266 1362 1361 1338 1323 1322 1320 1319 1290 1287 1286 1285 1284 1283 1282 1281 1280 1279 1278 1277 1276 1272 1369 1368 1367 1365 1364 1337 1336 1335 1334 1333 1332 1331 1330 1329 1328 1325 1324 1313 1312 1311 1309 1308 1307 1299 1298 1297 1289 1288 1373 1371 1370 1379 1378 1376 1374 1372 1380 1377 23.209 0.461 23.384 0.317 23.804 0.671 23.433 0.348 24.550 0.482 24.042 0.055 24.342 -0.027 24.180 0.424 22.718 0.481 23.876 0.340 21.804 0.185 24.398 0.398 24.182 0.654 23.934 0.407 23.808 0.277 23.871 0.569 24.641 0.442 24.591 0.444 21.069 0.616 20.016 0.277 22.167 0.598 22.943 0.503 23.947 0.340 23.501 0.472 24.439 0.304 22.557 0.478 24.284 0.307 23.987 0.223 24.118 0.773 24.187 0.587 24.189 0.584 22.984 0.333 23.505 0.582 24.391 0.504 22.958 0.734 22.212 0.445 23.372 0.267 24.354 0.087 24.66 21.863 22.503 0.498 22.960 0.281 21.829 1.418 23.910 0.551 23.421 0.411 24.571 0.354 24.351 0.295 24.598 0.335 21.398 0.594 23.212 0.402 23.790 0.528 23.221 0.462 22.872 0.493 23.752 0.627 21.718 0.107 22.397 0.532 24.436 -0.470 23.792 23.919 0.394 24.194 -0.044 23.156 0.466 20.019 -0.068 23.547 0.973 24.313 0.206 23.232 0.551 24.089 0.484 23.985 0.384 24.463 0.671 24.664 0.077 19.960 0.094 23.599 0.260 22.688 0.609 22.665 0.416 23.005 0.444 23.863 0.172 23.831 0.333 23.436 0.388 23.033 0.497 24.072 0.692 24.531 0.201 23.414 0.489 23.946 0.510 23.869 0.256 24.496 0.653 23.046 0.509 24.432 0.300 24.561 0.696 24.087 24.206 0.336 24.017 0.222 23.895 0.170 24.152 0.269 ¡3.623 0.469 0.780 0.535 2230 1985AJ 90.22210 1 bins, thesigmaofwidthsubgiantbranchormain compared totheexpectedwidth.TableVIgives,for0.2mag in eachbin. for whichthatsigmawascalculated,andthenumberofstars NSTAR, thesigmaofactualB—Vcolors,colorrange sequence thatisexpectedsolelyfromtheerrorscalculatedin the predictedones,whichmightbeinterpretedasevidence for allobservedUrsaMinorgiantsbutone(astarat[Fe/ entirely conclusive.WenotethatSuntzefFetal.(1984)find for anabundancespread,butwedonotconsiderthetest Notes: Flag=0if 2231 E.W.OLSZEWSKIANDM.AARONSON:URSAMINOR 1384 1390 1388 1387 1386 1383 1382 1381 1389 1399 1398 1397 1396 1400 Flag =1if The calculatedsigmastendtobesomewhatlargerthan 18 © American Astronomical Society • Provided by the NASA Astrophysics Data System 1r- only appearsinFigure3; appears inbothFigures3and4. 1 Flag StarXYmB-Vaaß-VoUB-V v Ursa Minor 1438 1440 1439 1430 1429 1428 1435 1434 1433 B-V Table IV.(continued). (1985a) andVandenBergBell(1985)releasedanewpre- unambiguously established. might beentirelyduetoobservationalscatter.Furtherwork Ursa MinorsamplethescatterwaslessthanforDraco,and seems requiredbeforeanabundancespreadinUrsaMinoris than 0.1dex.Also,Stetson(1984)foundthatforhisobserved H] 3.5)thatthescatteraboutM92abundanceisless At thetimeourCMDwasbeingcompleted,Vandenberg b) FitstoNewEvolutionaryIsochronesandM92 1454 1453 1452 1473 1472 1455 1475 1474 1480 1479 1478 1477 1476 brighter thanm=24.8magareplotted All starsfoundinboththeVandBframes, here. (Thescalesizeis—0.6"perpixel.) with noneighborswithin1.7pixels,and stars measuredintheUrsaMinorfield. Fig. 4.Color-magnitudediagramof1486 v 2231 1985AJ 90.22210 a 2232 E.W.OLSZEWSKIANDM.AARONSON:URSAMINOR SeenotestoTableIIIforanexplanation ofSOKandCOSnumbers. Table V.ConversionfromnumbersinIVtoSOKandCOS © American Astronomical Society • Provided by the NASA Astrophysics Data System Star 1022 1412 1297 1479 1242 1066 1226 1481 1318 1271 1169 1152 1149 1237 1018 1244 1401 745 612 209 980 290 465 260 710 663 648 802 508 810 826 300 517 928 889 863 353 895 113 160 63 85 numbers. a Q3, 327 Q2, 11 Q3, 323 Q3, 37 Q2, 12 Q3, 376 Q2, 396 Q2, 19 Q2, 41 Q2, 17 Q3, 265 Q3, 39 Q3, 22 Q2, 40 Q3, 266 Q3, 296 Q3, 394 Q2, 14 Q3, 10 Q3, 23 Q2, 20 Q3, 38 Q3, v6 Q3, 20 Q3, 9 Q2, 22 Q3, 11 Q3, 324 Q3, 40 Q2, 24 Q3, 43 Q2, 26 Q2, 21 Q2, 16 Q3, 7 Q2, 15 Q2, 13 Q2, 18 Q3, 8 Q2, 30 Q3, v5' SOK O Ursa Minor a COS 209 345 210 211 351 321 311 352 316 320 309 332 338 342 339 319 341 343 310 333 344 340 318 346 334 317 314 189 187 185 180 177 183 179 188 184 174 175 173 178 181 182 B —0.1)=0.88.TheresultsareshowninFig.6,where B —Vby0.03maganddecreeingthatthemidlevelof fit isuncertainbyabout0.1mag.WenotethattheSandage the smallnumberofhorizontal-branchstarsmeasured,this dage (1983),whofounditnecessarytoarbitrarilyshiftthe it canbeseenthatthebestfitistooldestisochrone. and Walker(1966)normalpointsforM92giveM(H.B.at RR LyraegapbeatM=0.63mag(Sandage1983).Given laid ontheUrsaMinordiagramafterreddeningtracksin zeff etal.(1984)havemeasuredthemeanabundanceofUrsa gies wereknowntobesimilarthoseofM92.Second,Sunt- reasons: First,thegiantandhorizontal-branchmorpholo- model wasdeemedtobethebestfitUrsaMinorfortwo open clusterisochrones.TheV=0.2,Z0.0001,a1.6 cally tooblue.AsimilarproblemwasencounteredbySan- However, itisapparentthattheisochronesareallsystemati- of thesemodelstoM92itself. Sandage (1983)hadpartialsuccessfittinganearlierversion Minor tobeveryclosethatofM92.Wealsonote v v of thevalueinColumn1. “Calculated fromstarsinTableIV,which areflagged1,andwithin0.1mag Table VI.Calculatedandobservedwidthsofsubgiantbranchmain The isochronesfor12,14,and18billionyearswereover- mag 24.5 24.0 23.4 23.0 22.5 22.0 21.0 21.0 sigma from a Calculated NSTAR 0.142 0.100 0.074 0.055 0.057 0.042 0.044 0.036 There are783starsplotted. are assumedtobethebestsetavailable. no neighborswithin5pixels.Thesestars subset ofstarsplottedinFig.3thathave Fig. 5.Color-magnitudediagramofthe sequence. Observed sigma® 0.20 0.15 0.13 0.08 0.05 0.07 0.04 0.06 0.05-0.9 0.1 -0.8 0.15-0.6 0.25-0.6 0.45-0.8 0.4 -0.85 0.5 -0.8 0.5 -0.7 Color range 2232 63 76 60 28 N 10 9 2 6 1985AJ 90.22210 fit toUrsaMinor.ThisisshowninFig.7,wherewehavenow population havingayoungerage. the bluestragglers,thereislittleevidenceforanysubstantial translated theisochronesby0.05mag.Abest-fitageof new tracks,anevenlargershiftseemsrequiredtogetthebest older VandenBergtracksby0.03magtofitM92.Withthe (Sandage 1983;SandageandWalker1966).Thetwocolor- 2233 E.W.OLSZEWSKIANDM.AARONSON:URSAMINOR 16 +2Gyrissuggestedbythisfigure.Exceptpossiblyfor 18 In Fig.8weshowafittothemeanridgelinesofM92 © American Astronomical Society • Provided by the NASA Astrophysics Data System 0 0.5 —i—i—i—«—i—i—i—«—r- Ursa Minor Ursa Minor » % B-V * 1.0 ies. Hence,itdoesnotseemlikelythatsomecombinationof magnitude diagramsareseentoberemarkablysimilar,in good agreementwiththeaforementionedabundancestud- nor astheM92ridgeline.VandenBerg(1985b)suggeststhat regions simplydonotprovideasgoodamatchtoUrsaMi- fully fortheisochronecolordiscrepancy.Furthermore, reddening uncertaintiesandphotometricerrorscanaccount missing opacityand/oraslightlyerroneouscolor-effective shape oftheisochronesthroughturnoffandsubgiant 1.5 bluer thanthedata. billion yearisochronesareplotted.Thefit E(B —V)=0.03mag.The12,14,and18 a =1.6,andanUrsaMinorreddeningof Z =0.0001,amixinglengthparameter gap isatanabsolutemagnitudeofM was madeassumingthattheRRLyrae are foracompositionof7=0.2, en fromVandenBerg(1985a).Thetracks overlaid withevolutionaryisochronestak- Fig. 6.Color-magnitudediagramof5 good afittotheUrsaMinordataasM92 Even so,noneofthetracksprovideas trarily reddenedbyanadditional0.05 city probablymakesthetrackstooblue. mag. Asdiscussedinthetext,missingopa- Fig. 7.Sametracksplottedin6arbi- v (see Fig.8). = 0.6.Notethatthetracksarerather 2233 1985AJ 90.22210 2234 E.W.OLSZEWSKIANDM.AARONSON:URSAMINOR fected. Inanyevent,itwillclearlybeofinteresttocompare the isochrones,yetleaveabsolutemagnitudeslittleaf- marque, andKing1984)whenthesebecomeavailable. Ursa MinorwiththenewsetofYaleisochrones(Green,De- temperature relationcouldexplainthecolorproblemswith by slidingtheDracolinestosmaller distancemoduli.They derive relativemoduliof~0.4 mag. the distancemodulustoDraco is19.4(Stetson1979),they Ursa Minorissubstantiallynearer thanDraco.Giventhat uncertainty ofatleast+0.1mag tothisresult. main sequenceandsubgiantsyieldsamodulusof19.0.In gives atruedistancemodulusof18.9,whilethebestfitto M92 (Fig.8).Aslidingfittomatchthehorizontalbranch distance moduluscomesouttobe19.0. can onlyfittheridgelinesofDraco tothoseofUrsaMinor earlier assumedvalues,butitseemsappropriatetoattachan summary, thebestmodulusis19.0,somewhatsmallerthan brighter by—0.1magbeforetheblueedgeofinstability 0.09). Bytakingthemeanofthesemagnitudes,andbynoting the techniquehasalargerthannormalerrorassociatedwith which themeanmagnitudeofblueedgeRRLyrae that theM92bluehorizontalbranchgetsbothredderand strip, at(F,2?—V)=(19.76,0.14),(19.85,0.11),and(19.96, it. Therearethreestarsneartheblueendofinstability our CMDhasveryfewhorizontal-branchstarsmeasured, gap isassumedtobeatM=0.6.Unfortunately,because distance. First,theapproachofHarris(1976)isusedin strip isreached,wederiveanapparentdistancemodulusof 19.1. Correctingfor0.1magofvisualabsorption,thetrue v The bestdistanceestimatescomefromcomparisonto Stetson, VandenBerg,andMcClure (1985)confirmthat We canemployseveralwaystomeasuretheUrsaMinor © American Astronomical Society • Provided by the NASA Astrophysics Data System c) TheDistanceModulus Ursa Minor 2 be aboutten(BahcallandSoneira1980).Giventhatthe al. 1983).Finally,—10galaxiesareexpected(Shankset Ursa Minor,whichseemsunlikely.Thenumberofquasars stars atthisapparentmagnitudewouldbemoredistantthan the numberofwhitedwarfspersquaredegreeisexpectedto stragglers inthisframeistherefore20. should bedescribed. as bluestragglers,butfirstafewtestsandcomparisons off andbelowthehorizontalbranch.Weinterpretthesestars in thisfieldissimilarlycalculatedtobeonly0.4(Marshalet stragglers claimedforUrsaMinor.Thetotalnumberofblue CMD samples0.005deg,weexpect<1whitedwarf.A-type servative willnotincludetheminthenumberofblue fied eightstarsasverycrowded,orpoorlyfit,andtobecon- tracted, theBframe,andsubtractedframe.Weidenti- clude thatall20bluestragglercandidatesareprobable ined objectswereseentobenonstellar.Wethereforecon- frame, theframefromwhichall3000-oddstarsweresub- Each ofthesestarswasexaminedonfourframes:theV explained assmallerrorsinphotometryofturnoffstars. 28 starsoccupyingthebluestragglerregimewhichcannotbe members. Minor is92(vanAgt1973).There are38inthecentralre- culated asfollows:Thetotalnumber ofRRLyraesinUrsa from thesedata?Ifbluestragglers aremain-sequencestars, scales upto—300totalbluestragglers inUrsaMinor. Hence, (92/38)X7—15,sothat 20bluestragglersinourfield gion, whichisafactorof7larger thantheCCDframe. 1984), butfewoftheseshouldbesoblue.Nonetheexam- A fewstarsareseenbluewardofthemain-sequenceturn- Now intheapparent-magnituderange20
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© American Astronomical Society • Provided by the NASA Astrophysics Data System 2238 E. W. OLSZEWSKI AND M. AARONSON: URSA MINOR 2238
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© American Astronomical Society • Provided by the NASA Astrophysics Data System 1985AJ 90.22210 © American Astronomical Society• Provided bytheNASA Astrophysics Data System 24i6 PLATE 231 arcsec, asjudgedbytheFWHMof a stellarprofile. E. W.OlszewskiandM.Aaronson (see page2225) Fig. 2.VframeoftheUrsaMinorfield describedinthetext.Itisaverageofsix500sframes.Theseeingwas ~0.9 1985AJ 90.22210 identified byreferringtoTableIV. Fig. 3.CCDframeof2withmeasured starsidentifiedandeverytenthstarnumberedtotheupperright ofthestar.Otherstarscanbe E. W.Olszewskiand M.Aaronson(seepage2226) © American Astronomical Society •Provided bythe NASAAstrophysics Data System PLATE 232 2417