1958ApJ. . .128 . .185C of 64mag Evolutionary effectsonthisclassificationsystemarediscussed,andrelativeagesfortheScorpio-Cen- reddening, isusedwiththeHßintensitiestodefineatwo-dimensionalspectralclassificationsystem to astandardsystem.Thecolorindex(U—B)oftheU,B,Vsystem,whencorrectedforinterstellar of theMcDonaldObservatoryforBstarswithavailableMKspectraltypesandinseveral are estimated.MeanabsolutemagnitudescomputedfortheB8VandB9starsinScorpio- taurus cluster,theOrionassociation,fPerseiaand.fieldstars clusters andassociations.Thevariousfiltersystemsusedarediscussed,themeasurestransformed photographic determinationsofhydrogen-lineintensities,togetherwithsometype Centaurus cluster,andacomparisonwiththePerseiclusteryieldsminimumdistancemodulusforit luminosity. Theend-resultisthatthestarreceivesadefinitespectraltypeandlumi- used eithervisualtwo-dimensionalspectralclassification,suchastheMKsystem,or Atlas ofStellarSpectrabyMorgan,Keenan,andKellman(1943).Onlyafewremarks for theBstarsaswell. will benotedhere. determination aregiveninapublicationbyJohnsonandMorgan(1953)the structure andotherfieldsofastronomy.However,thesuccessnarrow-bandphotome- about onesubdivisionineitherspectraltypeorluminosityTheempiricalcriteriavary, nosity class,which,foranexperiencedclassifierandagoodspectrum,isaccurateto estimates forspectraltype.Thesemethodshavegivenfundamentalresultsingalactic try fortheFstarsbyB.Strömgrensuggeststhatsuchmethodswouldbequiteuseful nitude. Thesystemitselfisdefinedbyanetworkofstandardstarswithwell-deter- mined typesclassifiedbyMorgan. correlation possibleforvisualestimateswitheffectivetemperatureandabsolutemag- depending ontheapproximateclass,butanattempthasbeenmadetoachievebest interstellar reddening.Itisanexampleofthecolor-difference methodsuggestedby a parametercalculatedfromboth{B—B)and -V)andwhichisindependentof system ofJohnsonandMorgan(1953).Theyhave alsoderivedaquantityQ,whichis The mostwidelyusedcolorindexforthispurpose hasbeenthe(B—V)ofU,B,V and ismoreofacontinuousvariablethanthedefinitespectraltypesMKsystem remarkably good,andhencethisintrinsiccolorisoftenusedasameasureofspectral would seemtobeagoodindexofspectraltypefor useintheBstarsalone.Thishas Becker (1938,1948).ThisQisalsoanaccurateindicator ofspectraltype. type. Thishastheadvantageofgivingawiderrangeforspectral-typeparameter TWO-DIMENSIONAL SPECTRALCLASSIFICATIONBYNARROW-BAND © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Photoelectric observationsoftheintensityHßlinehavebeenmadewith36-inchreflector Most ofthecurrentinvestigationsonluminositydeterminationsforBstarshave The MKsystemiswellreviewedandreferencestoearlierworkonspectraltype The systemdependsonvisualestimatesofcertaincriteriaforbothspectraltypeand However, since(U—B)hasthewidestrangeof valuesthroughouttheBstars,it The correlationofthespectraltypethusobtainedwithstar’sintrinsiccoloris * ContributionsfromtheMcDonaldObservatory,Universityof Texas,No287 PHOTOMETRY FORBSTARSINCLUSTERSANDASSOCIATIONS* Yerkes andMcDonaldObservatories David L.Crawford Received May6,1958 I. INTRODUCTION ABSTRACT 185 1958ApJ. . .128 . .185C L systemwillbediscussedinapublicationbyStrömgren andCrawford,butdetails range ofthenewindex,called7,,isafactor2greater thaninthelsystem.Thestandard mation meanerrorforthe linearpartis±0.0067mag.in1.ThevaluesofL for thestand- had ahalf-widthof15A.Thisshouldcutoutmost ofthecontinuumcontributionin interference filterwastriedinplaceofthe35A filter ofthelsystem.Theused was usedbyStrömgren(1956a)inhisworkonthe Fstars.Forthemheachievedan relative totheBstarswill begivenhere. accuracy inspectraltypeof±0.03andabsolute magnitudeof±0.3mag. half-widths ofabout90A.Theirtransmissionpeakswerecentered4700Aand cluding thecomparisonfilters,wassameasfor thelsystem.Itturnedoutthat the neighborhoodofHßthatpassedthrough 35Afilter.Allotherequipment,in- index isverysmall.Thisindex,togetherwithameasure oftheBalmerdiscontinuity, comparison filtersisclosetothatofHß,theinfluence ofinterstellarreddeningonthe where 7(4700),7(5000),and7(4861)denotetheintensitymeasuredthroughtwo 5000 A.Theindexlisdefinedby 35 Aandamaximumtransmissionofabout75percent.Thetwocomparisonfiltershad comparison filtersandtheHßfilter,respectively.Asaveragewavelengthof veloped atYerkesandMcDonaldObservatories,inadditiontotheoriginallsystemof in apparentmagnitude,whichisofgreatvalueproblemsgalacticstructure. in Mv,±0.22—allforoneobservation.(Meanerrorsareusedthroughoutthispaper.) from theearlyBstarstolateFstars,measureoftotalintensityHßand accuracy inherentinphotoelectricphotometry.Itispossiblealsotocoverawiderange These werederivedfromthephotometricaccuracyaloneanddonotincludeeffects tional refrigeratedphotoelectricphotometer.TheHßfilteritselfhadahalf-widthof Strömgren (19566).Thislsystemusedthreeinterferencefiltersemployedinaconven- of cosmicscatter,ifany.Thesephotoelectricmethodstakeadvantagethehigh should beasfollows:inspectraltype,+0.01ofatype;(B—F),±0.005mag.;and type, colorindex,andabsolutemagnitude.HeestimatedthattheaccuracyforBstars plates, theamountandpositionofBalmerdiscontinuity.Theyshowthatrela- on thesameplatesasthoseusedbyChalongeandDivanusingitwithamount ured theintensityofrelativelystrongfeaturesinobjective-prismspectra.Thiswork the amountofBalmerdiscontinuitygiveahighaccuracyfordeterminingspectral these methodsbytheuseofphotoelectricphotometry.Hefoundthat,overrange is wellcorrelatedwithMKtype. of theBalmerdiscontinuitytodefineaspectraltypeandluminosityclass.Thisagain tion betweenthesetwoparametersandMKtypesisverygood. Divan (1952)andtheirassociates.Thisconsistsofmeasuring,fromphotographic dispersion photographicplatestoachieveatwo-dimensionalclassification.Hemeas- showed thevalueofstrengthhydrogenlinesasaluminositycriterion. been donetosomeextentbyJohnsonandMorganwasusedHarris 186 DAVIDL.CRAWFORD (1956) fortheirstudyofM29. © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem In anefforttosecurealargerrangeintheparameter measuringHß,anarrowerHß The transformationbetween landLisgiveninFigure1fortheBstars.The transfor- Another methodthathasachievedgoodresultsisthesystemusedbyChalongeand Hack (1953)hasamplifiedthismethodbyutilizingthecentraldepthofH<$measured Several systemsofphotoelectricmeasurementthestrengthHßhavebeende- Strömgren (1951,1952,1956a,b)hasinvestigatedthepossibilityofextension Lindblad (1922,1925,1926)usedmethodsbasedonspectrophotometryfromlow- / =2.5{[log7(4700)+log7(5000)]-7(4861)}constant, II. THEOBSERVATIONS 1958ApJ. . .128 . .185C , has beensubtractedoutbythefirstfilter.Theoutputsfromphotocellsareampli- ferent nights. and entirelyonsomenights. Thetotalobservingtime,includingmoving the telescope at thetelescopebulbwasrunfromaregulated voltagesupply. variation ofthisratiowasnegligibleoverwideranges inthebulb’sintensity.Inaddition, 5 minutes,12.0mag.wasobtainedwithanaccuracy of±0.010. of theMcDonaldObservatoryisaboutV=1.75 mag.Withobservingtimesofabout giving anadditionalrangeof2.5mag.Thelimitfor brightstarswiththe36-inchreflector have arangeof10mag.Theintegrationtimeis variable from10to100seconds,thus a 15AHßfilter.Thelightpassingthroughthisfilterisreceivedbythefirstphotocell; ly wasinitiatedbyStrömgren.Theoriginalphotometer,sketchedinFigure2,utilizes vation is±0.0048mag.,asdeterminedfromobservationsofthestandardstarsdif- ated lP21s.Thissecondfilteryieldsanapparentdouble-peakedtransmission-curve, second Hßfilter,about90Awide,andintothephotocell.Bothcellsarerefriger- and thecomparisoninnearbycontinuousspectrumcouldbeobservedsimultaneous- ard light-source.Standard starsfromtheLsystemwereobservedaboutevery fivestars ordinary smalllightbulbwasobservedaftereach startofurnishastandardratio.The type, designedandbuiltbyR.Weitbrecht(1957). Foragivenintegrationtimethey being controlledbyasuitableprogrammerunit.The integratorsusedaretheMarkVI fied andintegratedthenrecordedonaBrownrecorderchart,thewholeprocess the peaksbeinglocatedincontinuumoneithersideofHß,ascentralHßpart the lightreflectedfromitiscollectedbyasmallsphericalmirrorandpassedthrough star andoneormoreonthe sky.Thenoneormoredeflectionsweremade on thestand- ard starsusedinthisworkaregivenTable1.Theinternalmeanerrorofoneobser- © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem In hopesofimprovingobservingefficiency,amethodwherebythemeasureHßitself The observingprocedurewasasfollows:Fourormore deflectionsweremadeoneach To removeanyeffectduetodriftintherelativesensitivity ofthetwophotocells,an Fig 1.—TransformationbetweentheLandlsystemsforBstars SPECTRAL CLASSIFICATION187 1958ApJ. . .128 . .185C 2 4 £ Ori 27 Tau v Cep... r¡ Aur. 21 Tau 20 Tau 0 Per HR 1035 HR 1034 9 Cep 55 Cyg ß CMa 7 CMa. X Ori. v Ori. p Aur.. 7T Ori. 35 Ari 53 Cas 0 Cas Y Peg o Peg... Y Aqr. p Leo 103 Tau Tr And k And.. L And. P Peg. 22 Sco. Y UMa /3UMa ô Hya 19 Tau £ Cas Y Lyr 67 Oph Y Oph ß Lib 18 Tau 16 Tau 1 Per a Peg.. 12 Lac. r Peg.. 10 Lac. a Lac. L Aql. r Her ô Sco K Hya rj Hya 19 Cep. o’ Cyg Ô Cyg rAql . 1 Her a Del. k Agi 7T Seo 109 Vir Cas Standard Stars © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 1 408 1 455 1 519 1 356 1 462 1 518 1 431 Í 233 1 493 1 1 411 1 1 1 453 1 453 1 382 1 272 1 329 Í 428 1 260 1 711 1 555 1 291 1 339 1 289 1 595 1 690 1 471 1 363 1 478 1 640 1 679 1 324 454 656 506 361 335 484 302 423 331 429 478 342 628 502 651 682 538 600 674 714 714 2 650 2 690 2 606 2 696 2 722 2 688 2 670 2 584 2 667 2 688 2 628 836 856 742 743 900 653 860 881 908 535 689 610 588 551 592 550 579 525 808 812 560 874 585 908 846 875 887 729 663 668 594 733 774 612 704 714 760 710 709 Standard Stars TABLE 1 2 606 2 854 2 676 2 833 2 733 2 857 2 784 2 580 2 907 2 853 2 605 2 556 2 708 2 661 2 539 2 577 2 535 2 798 2 703 2 566 2 863 2 747: 2 691 605 691 678 700 662 790 664 625 755 753 628 716 597 604 639 578 709 (3 (Adopted) 650 857 691 605 900 881 857 606 722 699 834 610 908 853 869 585 908 655 689 672 678 731 584 551 597 545 530 803 812 563 594 846 875 545: 733 790 779 578 663 668 612 597 578 709 758 710 787 706 716 750 748 690 628 709 594 627 639 665 662 688 709 14 19 16 12 17 18 11 14 14 16 13 13 11 13 4 4 3 4 4 3 2 4 3 3 4 3 4 3 2 8 2 6 2 3 3 3 2 5 5 1 5 2 7 1 1 1 1 7 7 1 1 1 1 1 2 6 2 1 7 1 1 1 MK Type B3 V B5 V B2 la BOV B2 V B3 V B2 III B8 III B8 V B9 V B2 III B8 V B9 III A2 la B3 la B9 V B9 5III B5 III B0 5HI B5 V B3 V B8 V B8 II B8 V B7 III B6 V B8 V B7 IV B1 III 09 V B2 16 B9 III B5 16 A0 V B3 V B2 V B5IV BOV Bl V B8 V A0 V A0 V Al V B1 16 B3 V 09 516 09 lab B9 la B3 V B8 lb B2 V B2 V B2 IV B2 V B2 V 1958ApJ. . .128 . .185C filter andthesensitivityofcellsweresuchthatsecondintegratoroperatedwith and settinguponthestar,averagedabout4|minutesperstar.Thetransmissionof was takenasthemeasureofHß. length, nocorrectionsshouldbenecessaryforinterstellarreddening.Inaddition,asthe 2 mag.lessgainthanthefirstforagivenstar.Theresultingratio, measures ofHßandthecomparisonaremadesimultaneously,atmosphericextinction different airmassesandalsothatobservationsinnon-photometricconditionsshould should benegligible.Thismeansthatnocorrectionisnecessaryforobservationsthrough an extinctionofasmuch3mag. be possible.Thiswasfoundtothecase,asnovariationoccurredinobservedratio over arangeofaboutfourairmassesorinobservation throughcloudsamountingto within thephotometeritself.Itcouldbecorrected for,however,utilizingthemany angle anddeclinationinaratherirregularway.This wasthoughttobedueflexure fraction passesthroughthe 15Afilter,whilethe30percentfractionpasses througha Here theincominglight is splitintopartsbya30/70beamsplitter.The 70percent It preservestheadvantage ofsimultaneousobservations.Itissketched in Figure3. internal meanerror.Itis±0011mag.Thisequipment wasusedduringthewinterof standard starsobservedaschecks.Theresultant measureshave,asaresult,higher of 1957withthe36-inchreflectoratMcDonald Observatory. Strömgren wasbuiltofsomewhatdifferentstructure andmorerigidineveryrespect. 1956-1957 withthe40-inchrefractorofYerkes Observatoryandduringthespring © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem As bothfiltersandtheresultanttransmission-curveshavesameeffectivewave However, asystematiceffectwasobservedwhich appearedtobeafunctionofhour In anefforttoeliminatethetroublesomeflexure effect, asecondmodeldesignedby f ß —2.5[log7(90Afilter)log7(15filter)]+constant, Fig. 2.—SketchoftheoriginalsimultaneousHßphotometer Fig. 3—SketchofthesecondsimultaneousH/3photometer SPECTRAL CLASSIFICATION189 Beam Splitter 1958ApJ. . .128 . .185C photoelectric methodbut alsointheincreasedobservingefficiencyover photographic is IF=—19.1+38(5—2.000),withameanerror of±06.Hack(1953)measuredthe values of¡3.ThedeviationfromlinearityintheregionstrongHßintensitiesisproba- photographic determinationsoflineintensity,whichisabout5percentforanaverage been appliedtoputthemeasuresonsamesystem,hascalledg• above, individualtreatmentisoflittlevalue.Williams’ andHack’svalueshavetheleast against [3inFigure6.Thetransformationmean error is±0.011from(ß—2.000)= relation between5andthevaluesgivenforHßby Williams(1936).Thetransformation bly duetothelinewingsbecomingimportantin regionoutsidethe15Afilter.The for thisobservingsessionwas4.5.ThetotalrangeoftheparameterfromearlyBsuper- when usingthe15Afilter. narrow filter.Thisratioshouldbethesameforallstarsandwashenceusedtoeliminate strengths werealsomade,but,astheyall similar inappearancetothosegiven 2.451 +1.040Hô.Comparisonof5withother measuredvaluesofhydrogen-line siderable workonH7,inordertodeterminevaluesoftheabsolutemagnitudeearly- The finaladoptedvaluesofgaregiveninTable1forthestandardstars. of overtwoplatesperstar. giants tothestrongesthydrogenabsorptionobservedwasabout0.4mag.;hence the McDonald36-inchreflector.Theresultantindex,aftersmallnightcorrectionshad In thismethod,measuresaremadethroughthe15Aand150filtersinsuccession. “apparent centralintensity”forHôonlow-dispersion plates,andhervaluesareplotted transformation inthelinearregion(wheremostof thestarsare)isIF=—16.7+34 type stars.InFigure4hismeasuredvaluesforHyareplottedagainstthephotoelectric the Land[3'systemsweretransformedto5system.Thetransformationsused error ofoneobservationis1.5percent.Thistobecomparedwiththatachievedin was suchthatthesecondintegratoroperatedatabout2.5mag.lessthanfirstone on thestar’smagnitude.Inaddition,twoormoredeflectionswerethenmade,wherea These measuresareonthesamesystemasabove,withachangeinzeropoint,because the literature.Petrie(1953,1956a,b)andMaunsell(1949)havedonecon- the celldriftinsteadofanykindstandardlamp.Therelativesensitivitysystem the filtersaresame. remove thebeamsplitterandobserveduringphotometricweatherwithonecellonly. deflection onthesky.Theintegrationtimesvariedfrom10to40seconds,depending the samewayasthoseofpreviousmodel.Asanadditionalcheck,itispossibleto 190 DAVIDL.CRAWFORD transformation meanerrorofanytreated. to thecontinuumcontributionshouldbesmall.Theoutputsofcellsweretreatedin (ß —2.000),withatransformationmeanerrorin IF of±0.8.Figure5showsasimilar 150 Afilter,cutfromthesamepieceasotherone,wasmovedintoplaceof 150 AH/3filter.Thesecondfilter,ofcourse,passestheHßlinealso,butitseffectrelative ± 0.006mag.foroneobservation.Theaveragenumberofobservationsoneachstandard © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The valuesofgforthestandardstarsaregiveninTable1.Foruse,allmeasureson The advantageofthemethod liesnotonlyinthehigheraccuracypossiblethrough the There aremanyphotographicallydeterminedvaluesforhydrogen-linestrengthsin The internalmeanerror,asdeterminedfromthemeasuresofdifferentnights,was Observations weremadeon13nightsduringOctober,1957,withthisequipment In observing,aseriesoffourormoredeflectionsweremadeforeachstarbesides g =2.5[log7(150Afilter)—log7(15filter)]+constant. (ß -2.000)=+0.306+0.843(7,1.000). (5 -2.000)=-0.021+1.00, 1958ApJ. . .128 . .185C between ßandWilliams’Hßvalues(W);Fig.6,transformation betweenßandHack’sHôvalues © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Figs 4-6.—Fig.4,transformationbetweenßandPetrie’s Hyvalues(W)\Fig.5,transformation CO CÓCM (0 O (\j 1958ApJ. . .128 . .185C Hiltner (1956)fortheBstars,abestfitwasobtained fromthemain-sequencestars, A andFstars,cistobepreferred,asitonlyintherangeofBstarsthat in excellentagreementwiththerelationobtained byJohnsonandHiltnerfromthe line. ByuseofthedatagivenbyHiltnerandJohnson(1956)forOstars with atransformationmeanerrorof±0.036;and(C7—B)=1.32+1.17cforthe gone throughagainwith newcoefficientsintheequationforratio of thecolor step usingthelinearterm,E{u-b)/E(b-v)=0.70. Then(U—B)1.233{U supergiants, andOstars.Onlythosestarswith normal spectrawereused.Therela- order todeterminetheamountofinterstellarreddening.However,incase forty starsincommon,withatransformationmeanerrorof±0.030mag. cases. Therelationsare(U—B)o=—1.20+2.2529forthethirtystarsincommon, index, (U—B)o,against29,theindexusedbyChalongeandDivan.Figure8plotsit good representationoftheBalmerdiscontinuity.Figure7plotsthiscorrectedcolor measured colorsshouldbeuseful,ifitiswellcorrelatedwiththeBalmerdiscontinuity. hundredth ofaspectralclass. 0.862 (B—F),and F)=+0.27{U—B).ThenE-v)iscomputed fromthis data ontheOstarsalone. tion obtainedfortheratioofcolorexcesseswas relatively easytofree(1/—B)oftheeffectsinterstellarreddening. against c,Strömgreffisindexforthediscontinuity.Thelinearfitisquitegoodinboth corrected fortheinfluenceofinterstellarreddening,aparameterderivedfrom achieves theseends.Heestimatesaresultantaccuracyinspectraltypeofaboutone- color difference(hisindexc),wellcorrelatedwiththeBalmerdiscontinuity,which from photoelectricmeasures.Strömgren(19566),infact,hasmeasuredanultraviolet hope, however,toachievesomewhatbetteraccuracy,andafainterlimitingmagnitude are ratherinsensitivetochangesinrelativechemicalcompositionofthestars. percentage accuracywillbeobtained.Itshouldindependentoftheeffectsinter- for theBstarsshouldbequitesensitivetochangesindiscontinuity,sothatahigh the Z7,B,VsystemofJohnsonandMorgansincethesecolorindicescanreadilybe sufficient accuracytoenablethemuniquelydeterminethespectraltype.Onewould the discontinuitydirectlyfromlow-dispersionphotographicplates.Theirindex,D,isof stellar reddening,ifpossible.Anadvantagealsoisthatcriteriabasedonhydrogenalone continuity isanexcellentindicatorofspectraltypealongthemainsequence(see,e.g., observations issimple,especiallysincenocorrectionsneedbemadeforatmospheric utes forstarsbrighterthantheninthmagnitude.Inaddition,reductionof work. Thetotaltimetoobtainameasurewithanaccuracyof+0.004isunder5min- Strömgren 1956Ôandreferencesgiventhere).Anyphotometricmeasureofthisquantity extinction. (B —F)o.ThenE(jj-b)/E{ -v) isredetermined,usingthisE(b-v),theprocess being (1956) forM29,utilizing,however,animprovedrelationtheinterstellarreddening 192 DAVIDL.CRAWFORD 0 0 0{B B © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem In thispaper(1/—B)hasbeendeterminedbythemethodofMorganandHarris However, sincemostBstarshavealreadybeenwellobservedbymanyobserverson The processofcalculating(U—B)proceedsby successive approximations,thefirst Using (U—B)oalsohastheadvantagethatittobecomputedinanycase, The colorindex(U—B),whencorrectedforinterstellarreddening,provestobea It hasbeenshownbyseveralauthorsthatsometypeofmeasuretheBalmerdis- Chalonge andhisassociatesintheirwork(ChalongeDivan1952)havemeasured 0 0 0 III. THEDETERMINATIONOESPECTRALTYPEFROM£/,B,VPHOTOMETRYANDg jCL(b-v) f+^= 0.70+0.06£-), (iJF 1958ApJ. . .128 . .185C Hence (U—B)o=—0.80forthefirststep.Therefore,{BF)o—0.22,andso E(b-v) =0.29.Then0.06Eb-f)0.02andE{u-b)/E{b-v)0.72.Therelationinthis values fortheintrinsiccolorandreddeningderivedonthissystemare(U—B) excesses. The(£/—.BVsfromthesecondandthirdapproximationsagreeexceptin This isthedesiredvalue.ThestarinthisexampleofclassB2V(HD23625). the casesofhighestreddening,sothattwostagesareallusuallyrequired. case is(£/-B)=1.242(£70.894(£F),whichgives{U-0.81. (j 0 0 B stars;Fig.8,transformation betweenStrömgren’sindexcand(U—B)qfortheBstars. © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem As anexample,taketypicalvaluesof(U—B)=—060and(BV)+0.07. Figs. 7-8.—Fig.7,transformation betweenChalongeandDivan’sindexD(U— B)oforthe O o o öÖÓÔ O —<\Jro'frm<0 SPECTRAL CLASSIFICATION193 1958ApJ. . .128 . .185C for aB2Vstar),E(b-v)=0.31.Usingthe“Qmethod”ofJohnsonandMorgan(1953) B2 Vstar,(B—V)=—0.24(notethat—0.22lieswithintherangeofpossiblevalues 194 DAVIDL.CRAWFORD on theMKsystem.Theagreementbetweenthesemethodsissatisfactory. that itisaB2star,andhenceitsintrinsiccolorreddeningwouldbethesameas and latetype,littleorheavyreddening,ofhighlowluminosity,suggeststhat — 0.81,{BF)o=—0.22,andE{b-v)0.29.TakingtheMKintrinsiccolorfora there islittleornosystematicerrorinvolvedinthedeterminationof(£7—B)o. should benoticedthattheagreementisquitegood.Theaccuracyofapointonthisplot dots arethediscordantstars.Thetrianglesgiants,andcrosses,supergiants.It solid dotsarestarswhosephotometrictypeagreeswiththeMKtype,andopen the dividinglinesbetweenspectralandluminosityclassesonMKsystem.The (see alsoJohnson1957),onegetsQ={U—B)0.72(BF)—0.65,whichshows in Table2.TheseagreewellwiththestandardMKvalues.Themeanerrorofpredicting is givenbythemeanerrorin[5of±0.006mag.andanestimated (£7 —B)oofabout±0.015mag. supergiants. stars, thelinesseparatingMKtypesforthesestarsThe opencirclesarethosestarswhose{U—B)^ ß typedisagreeswiththeMKtype.Trianglesindicateluminosity classIIIstars;crossesindicatethe B0 B1 B2 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The goodagreementwithChalonge’sworkandStrömgren’s,forallstarsofearly Figure 9plots(£7—B)oand5forthestandardstarsofTable1.Thesolidlinesare The meanvaluesof(£7—B)foragivenMKtypealongthemainsequencearelisted 0 Fig. 9—The(U—B)^ßdiagramforthestandardstarsof Table1Circlesindicatemain-sequence MK Type (U-B)o -1 06 -0 86 -0 96 Intrinsic ColorsofMain-SequenceStars B6 5 B5 . B3 MK Type TABLE 2 (U-B)o - 63 -0 74 -0 49 A0 B9 B8 MK Type (U-B)o - 14 -0 33 0 00 SPECTRAL CLASSIFICATION 195 a spectral class is about ±0.01 of a class. Definite values for the luminosity classes above the main sequence have not been computed because of the small number of stars in each class.

IV. EVOLUTIONARY EPEECTS ON THE (U — B)0, ? DIAGRAM The (U — £)o, 5 diagram in Figure 9 has been shown to comprise an accurate two- dimensional spectral classification system that agrees well with the MK system. The diagram agrees with those of similar type that have been determined from photographic results based on hydrogen criteria. The inherent accuracy of Figure 9 is, however, higher because of the photoelectric method. This method of two-dimensional classifica- tion proposed in Section II depends on objectively determined measures of high accura- cy for both the indices used: the (L — jB)0 index being closely correlated with spectral type, and the Hß index with luminosity. The method, therefore, offers a means of in- vestigating, with a precision not previously possible, the population characteristics of galactic clusters and of associations. For this purpose, member stars of four galactic clusters and associations have been observed. These groups are (1) the northern section of the Scorpio-Centaurus associa- tion; (2) the Orion association (excluding the group of stars in and around the Orion Nebula); (3) the f Persei association; and (4) the a Persei cluster. For comparison with these groups, a number of non-cluster stars were observed, as well as the ones utilized as standards on the 5 and L systems. In addition, a few members of the Pleiades cluster have been observed, in order to give an idea of the characteristics of this group. TABLE 3 The Scorpio-Centaurus Cluster Stars

HD No Star Fo (U-B)o 5 MK Type 142096 X Lib 49 -0 72 710 B3 V 142165 HR 5906 95 -0 51 743 B6 V 142185 HR 5907 86 -0 74 668 B2 Vnn 142301 3 Sco 43 -0 71 681 142315 52 -0 23 803 142378 47 Lib 45 -0 65 706 142669 p Sco 80 -0 91 646 B2 V 142990 HR 5942 06 -0 73 678 143018 7T SCO 75 -0 98 612 Bl V 143275 ô Sco 80 -1 04 594 BOV 143567 69 -0 23 846 144334 HR 5988 60 -0 65 708 144844 HR 6003 43 -0 50 788 145482 13 Sco 43 -0 80 664 B2 5 V 145554 97 -0 25 837 145631 91 -0 21 856 145792 HR 6042 74 -0 62 718 146001 HR 6054 54 -0 49 747 146029 97 -0 16 873 146284 91 -0 34 753 146285 97 -0 32 818 146416 HR 6066 36 -0 23 832 147009 09 -0 08 920 147010 59 -0 50 755 147165 a Sco Var. 594 B1 III 147889 59 -0 95 650 B2 V 148199 44 -0 33 778 148579 29 -0 28 881 148605 22 Sco 54 -0 78 668 B2 V 149438 r Sco 77 -1 08 2 588 BOV 157056 d Oph 22 -0 91 2 632 B2IV

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1958ApJ. . .128 . .185C McDonald Observatory.ThismovingclusterhasbeendiscussedbyBlaauw(1946, 196 DAVIDL.CRAWFORD located onthemainsequence,however,andhencelittlecorrectiontoameanrelation sequence starsinthisgroupareBOV. for main-sequencestarswouldbeexpected. type, ifany,becauseofthesmallnumberstarsinanyoneclass.Mostare the upperpartofcluster.Nocorrectionhasbeenmadeforvariationduetospectral the Scorpio-Centaumscluster.ThedistancemodulusisbasedonBlaauw’sresultsfor Scorpio-Centaums movingcluster.Thelistcontainsonlytheupperpartofgroup while theHßchangeincreases. However,whentheHßdifferenceislarge, theweight is thesmallest.Asseparation inluminosityincreases,theMvdeviation decreases, points areofhighweight.Thescatterinthisdiagramissmall.Insuchathe 1957), andthelistofmembersgivenbyhimarestarstreatedhere.Theearliestmain- (north of—30°declination),whichcanbeconvenientlyobservedfromthelatitude perpendicular distanceoftheobservedpointfrom themeanlinewillremainsmall.If part oftheScorpio-Centaumsassociation.Eachplottedvaluesforstarscon- either (17—B)or^fromthevaluesforstar alone.Infact,onecanseethatfor sists ofmultipleobservations,bothforU,BjVandthereforetheobserved two starsofnearlyequal luminosity theMvdeviationislargest,while Hß deviation the twostarsareofsamespectraltypeandluminosity, therewillbenochangein these twoindiceswilltendtomovetheplottedpoints fordoublestarsoffthemeanrela- index willbelargerthanfortheprimarycomponent alone.Thecombinedeffecton Hß line(exceptforstarsclosetothemaximumof hydrogenabsorption).Henceitsg in general,andhencethe(U—B)oindexfor combinedlightwillbemorepositive tion forsinglestars,but,asthedirectionofchange isthesameforbothindices, than fortheprimarycomponentalone.Likewise, later-typestarwillhaveastronger effect ofunresolveddoublestars,suchasspectroscopic binaries,issuchastochangeboth (U —B)oand§inthesamesense.Thesecondary starwillhavealaterspectraltype, 0 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Table 3givesthevaluesofVo,(U—B),and(5thathavebeendeterminedfor The relationbetweenabsolutemagnitudeandHßstrengthisshowninFigure10for Fig. 10—^AbsolutemagnitudeandH/3relationforthenorthernpartofScorpio-Centaumscluster Figure 11plotsthe(U—B),grelationformain-sequencestarsinthisnorthern 0 0 -5 0 -3 0 -2 0 -I 0 •4 0 0 I 0 1958ApJ. . .128 . .185C association {opencircles),thePleiades{triangles),andfieldstarsofTable1{crosses). association. Thedatafor thememberstarsarecontainedinTable4.This clusteris Harris (1956)whichare membersoftheassociationIIPersei.Thisis the fPersei values forthisstarare—0.95and2.650,respectively. ItshighrelativeHßstrengthalso are plottedinFigure11also.Itisofconsiderableinteresttoseethat,withtheexception tions fromameanlinemustbeduetoothercauses. is expectedinanHß,Mvdiagram,suchasFigure10,forthesamereason.Therefore, This isnottrue,ofcourse,inanabsolute-magnitude-spectral-typediagram(HRdia- indicates thatitisyoungerthantheaveragemember. TheotherstarisXLibrae,with The firstisHD147889,ahighlyreddenedmember ofthisnorthernparttheassocia- to havestrongerHßstrengthsthananyoftheother Scorpio-Centaurusclustermembers. line absorptionatHßisweakerfortheolderstars thanfortheyoungerones. Scorpio-Centaurus clusterstarsofthesame(U—B)q.Thisisanimportantfact,forit of thestarkAndromedae,allfieldstarshaveindicessmallerthanthosefor gram). Here,fortwostarsofequalluminosity,theMyparametershiftsthree-quarters of thefainterstartocombinedH/3issmall.AnydeviationsintotalHßlinedue its relativelygreaterHßstrength. expected tobeamongtheyoungestinScorpio-Centaurus cluster.The(U—B),(5 association aretobefoundinsuchlocations(as inOrion),thenthisstarwouldbe the index5formain-sequencestarsshowsanevolutionaryeffectsuchthathydrogen- appears toindicatethatanageeffectisdirectlyobservableinthe(U—B)o,§diagram: the effectofduplicityon(U—ü)o,5diagramissmall,andanyratherlargedevia- of amagnitude,whilethespectral-typeparameterremainsunchanged.Alargerscatter to duplicityare,therefore,expectedbesmall.Thesameistruefor(£/—B)qaswell. tion. Itislocatedinthedensedarkcloudsnearp Ophiuchi. Iftheyoungeststarsinan (U —B)oand§valuesof—0.722.710,respectively. Itappearsnormalexceptfor 0 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Fig. 11.—The(U—B)o,ßdiagramfortheScorpio-Centauruscluster{filledcircles),fPersei Figure 11alsocontainsthe plotteddataforthesixmain-sequencestarsobserved by Two starsareworthcheckinginmoredetail.These arethetwostarswhichappear The dataforthenon-clustermain-sequencestars,orfieldtakenfromTable1, SPECTRAL CLASSIFICATION197 o d o 1958ApJ. . .128 . .185C expected tobeyoungerthantheScorpio-Centaumsassociation.Itsearliest-typestar is £Persei,an07starontheMKsystem.TheseparationinHßindexbetweenthese points agreemorecloselyinpositionwiththefieldstarsthanScorpio-Centaums sequence memberstarstendtoliebelowtheScorpio-Centaumsmembersforsame cluster. The(U—jB),5valuesforthesestarsarealsoplottedinFigure11. stars andtheScorpio-Centaumsclusteriswellmarked;ingeneral,allmain- 198 DAVIDL.CRAWFORD observed groups,itwouldappearthattheageofPleiadesclusteriscloserto cluster members.FromthephenomenaofageeffectonHßevidencedinother is thatofSharpless(1952,1954).TheveryyoungstarsimbeddedintheOrionNebula earliest main-sequencestarinthePleiadesisB6V,alsoindicatingaconsiderablyolder spectral typeisBOV. age forthePleiadesthanScorpio-Centaumsassociationmembers,whoseearliest the fieldstars,takenasagroup,thantoageofScorpio-Centaumscluster.The main-sequence starsareoftypeB0V.Theage this outergroupoftheOrionassocia- 0 The solidlineindicatesthemeanrelationfor Scorpio-Centaumsclustermembers. the associationstarsnotincentralarea(thatis,oraroundnebula),earliest discussion here.NostarsassociatedwithanynebulosityareincludedinTable5.Among This linefitstheOriondatasatisfactorily,confirming thattheageofthesetwogroups tion isthereforeprobablyneartothatoftheScorpio-Centaums association. cluster stars. is approximatelyequal.ThelargerscatterintheOrion dataperhapsindicatesthatthe spectral typeis06.Owingtotheeffectsofnebula,whichinemissionatHß,on range ofagestheouterOriongroupmembersis largerthanintheScorpio-Centaums cluster. Figure12includes theplottedvaluesforthesememberstars.The positionof Scorpio-Centaums association.However,itwould stillbeyoungerthanthePleiades cluster appearstobeolderthaneithertheOrionassociation orthenorthernpartof of Harris(1956).Inthisgrouptheearliestmain-sequence typeisB3V.Hencethis the observedlinestrengthsforthesestars,however,theyhavebeenomittedfromfurther itself havebeentreatedbyJohnson(1957a,b)andStrand(1957).Theirearliest the starsin(U—B) , ?diagramwouldindicatethattheclusteris ofsomeage 0 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Table 1forthestandardstarscontainsthreemain-sequenceofPleiades Table 5givestheobserveddataforassociationIOrionis.TheU,B,Vphotometry Figure 12showsthe(U—B),5diagramforthese outerOrionassociationstars. Table 6givesthedataforaPerseicluster, £/,B,Vphotometrybeingthat 0 21843 23180 21856 24398 23478 23060 24912 24640 24190 24131 23625 HD No HR 1074 HR 1215 HR 1191 £ Per o Per T Per Star The ?PerseiAssociation Vo 42 91 25 25 08 65 97 84 87 71 60 TABLE 4 (U-B)o -0 75 -0 96 -0 80 -1 01 -0 77 -1 05 -1 01 -0 81 -1 15 -0 90 -0 81 2 634 2 618 597 682 679 632 664 563 691 650 581 MK Type B3 HI Bl V B1 III B2 Vp B3 IV B2 V B2 V Bl V B1 lb B2 V 07 1958ApJ. . .128 . .185C 31237 34511 33647 30836 35079 35039 35007 34989 35762 35715 35640 35588 35575 35502 35411 35407 35299 35792 35777 35718 35912 35910 35899 36234 36120 36013 36166 36151 36429 36285 36392 36512 36487 36430 36560 36541 36513 36627 36607 36591 36824 36741 36697 36655 36954 36779 36695 36959 36958 36867 36960 37321 37209 37001 37000 37481 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem HD No 8 21 8 55 6 15 9 23 5 07 8 28 8 57 8 43 5 04 7 47 8 89 49 47 48 44 61 24 56 46 43 32 99 58 51 54 68 90 73 15 29 44 41 12 (Var) 67 65 76 11 11 05 78 35 52 60 75 06 82 54 34 58 65 27 90 62 87 65 15 Vo Orion Association TABLE 5 (U-B)o -0 95 -0 21 -0 60 -0 85 -0 -0 -0 -0 38 -0 85 -0 76 -0 73 -0 77 -0 74 -0 62 -0 87 -0 57 -0 65 -0 65 -0 97 -0 64 -0 92 -0 76 -0 60 -0 39 -0 67 -0 40 -0 87 -0 69 -0 90 -0 66 -0 80 -1 08 -0 57 -0 13 -0 40 -0 49 -0 98 -0 55 -0 14 -1 00 -0 74 -0 82 -0 03 -0 27 -0 16 -0 79 -0 94 -0 67 -0 72 -0 -1 -0 25 -0 59 -0 94 -0 93 0 00 89 70 74 07 70 2 621 624 906 822 628 689 592 605 601 675 675 601 687 630 692 663 732 779 720 637 712 676 714 701 681 629 777 647 723 796 904 682 711 586 715 910 756 902 852 777 912 626 691 664 661 768 634 718 654: 598 707 835 700 715 635 751 MK Type B9 V: B8 V B2 III B2 III Bl V B2 V B3 V B5 V B0 5V B5 V Bl V B3 V B2 IV B3 V Bl V B5 V B6 V B2 V B2 V B5 V B3 V B2 V B2 V B5 V B3 V B2 V B2 V B5 V B2 V B0 V Bl V B2 V Bl V B6 V B2 V B2 V B3 V B3 V BOV Bl V B5 V Bl V B3 V Bl V 200 DAVID L. CRAWFORD TABLE 5—Continued

HD No Fo (U-B)o 5 MK Type 37526 55 -0 56 737 B5: V: 37606 90 -0 33 757 B8 V 37700 90 -0 49 735 B5 V 37744 11 -0 92 628 Bl V 37756 88 -0 85 639 B3 III 37776 70 -0 92 640 B2 V 37807 7 65 -0 70 705 37887 67 -0 10 870 37888 06 -0 08 884 37889 678 B2 V 37903 87 -Ó 84 639 B2 V 38051 88 -0 69 673 38755 62 -0 52 733 B5 V 38771 66 -1 12 549 BO 5 la 39291 18 -0 90 629 39377 45 -0 83 658 B2 V

Fig. 12 —The (U — B)0, ß diagram for the outer part of the Orion association (filled circles), the a Persei cluster (open circles), and the field stars (crosses). The solid line indicates the mean relation for the Scorpio-Centaurus cluster intermediate between the field stars and the Scorpio-Centaurus cluster members, but definitely younger than the Pleiades group. It is interesting to consider the difference in this evolutionary effect between earlier and later B-type stars. In the earlier-type stars the “turnup” effect in the HR diagrams is most evident. Hence an age criterion, such as the Hß strength, would be expected to separate clusters of various ages better for the earlier-type B stars than for the late- type stars. This is actually seen in Figures 11 and 12. While the range of the parameter 5 is largest in the late-type B stars, the separation between the groups is not so apparent as it is in the earlier types. It is especially noticeable for the case of the a Persei cluster, where for the earlier types the plotted points tend definitely to lie above the Orion members, but in the later types the points are more intermingled.

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1958ApJ. . .128 . .185C may betakenasameasureoftheclusteragerelativetoothergroupsorfield scatter duetothisshouldbemorenoticeableinanassociationthanacluster,whichis for itisnotlikelythatallstarswithinanassociationareformedatthesametime(the accidental errorsofthephotometry;(2)varyingagesstarswithingroup, stars ingeneral.Thescatteraboutthismeanlinemaybeduetoseveralcauses:(1)the spectral typeinthegroup.TheTable7henceyieldsagegiven determined agesforanumberofgroupsandgivesrelationbetweenagetheearliest in thegroup.ThelastcolumnlistsagesasdeterminedbyvonHoerner(1957).He of theirdecreasingH/3strengthmaybetakenasanindicationrelativeages of sometheobservedstars;and(4)effectactualfieldstarsbeingincludedin the lastcolumn. The relativeagesarealsoindicatedbytheearliestmain-sequencespectraltypeobserved more likelytoconsistofstarsaboutthesameages);(3)scatterdueduplicity © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Over theentirerangeof(£/—B)qwithinagroupmemberstarsdefinelinewhich Table 7summarizestheobserveddataforfivegroupsdiscussedabove.Theorder 20418 21641 21181 20961 20701 20391 20365 21672 21551 21479 21362 21278 21091 21071 21699 21481 21428 21398 21375 20809 HD No Pleiades Field stars £ Persei. Orion (lessnebulacluster) Scorpio-Centaurus . a Persei Association orCluster HR 1063 HR 1051 34 Per HR 1037 HR 1034 HR 1029 HR 1011 31 Per 29 Per Star Summary ofEvolutionaryEffects SPECTRAL CLASSIFICATION201 a PerseiCluster 4 96 4 67 4 77 8 06 7 64 Vo 27 39 32 98 56 33 26 62 60 29 85 71 13 17 16 TABLE 7 TABLE 6 Decreasing Strength Order in of H0 {U-B)o -0 63 - 38 - 07 - 22 - 29 - 12 - 33 - 38 - 66 - 54 - 63 - 56 - 65 - 11 - 64 -0 68 Spectral Type Main-Sequence of Earliest Star 07 B6 B3 B0 B0 871 893 902 925 873 849 896 923 711 694 695 787 696 671 781 770 766 708 734 688 Approximate 6 6 80X10 6 6 10X10 6 (Years) 4X10 4X10 1X10 Age MK Type A0 V A2 V Al V A0 V B6 V A0 V Al V B9 V B3 V B9 V B5 V B5 V B8 V B9 V B8 IV B3 IV B6 V Al V B3 V B8 III 1958ApJ. . .128 . .185C Therefore, howmuchfainter thefPerseimainsequenceisthanthatofScorpio-Centau- are B3VandB6V,respectively.Theyalsohave increasinglyweakerHßintensities. 202 DAVIDL.CRAWFORD rus isnotaccuratelyknown. Thedataofthepresentpaperdonotpermita newcalibra- few exceptions,suchaskAndromedae. Persei clusterandthePleiadesstarsappearolder, fortheirearliestmain-sequencestars systematically weakerintensitiesoftheHßline, asmeasuredbytheindex§.Thea to themeanlinesindicateorderindecreasingH/3strength,asgivenTable7. pendent ofanyothercriteriaage,suchastheturnupobservedinclustersand of theScorpio-Centaurus clusterareaccurateindividualabsolutemagnitudes available. The oldest“group”ofall—thefieldstars—havethe weakestHßintensityofall,witha well correlated.Thestrongesthydrogen-lineabsorptionforBstarsofagivenspectral be lessthanforthememberstars. the listsofmembersagroup,,forHßvaluesthesefieldstarswould,ingeneral, the samespectraltype—or(U—B)o—innorthern Scorpio-Centaurusgrouphave the highestrelativestrengthofHßlineinits stars. Thosemain-sequencestarswith owing totheextensionofmainsequenceastar oftype07.Thisgroupalsoshows groups discussedinSectionIV,thefPerseigroup wouldbejudgedtotheyoungest, type arepresentinthosestarswhichmembers oftheyoungestgroups.Offive so thattheintensityofHßabsorptionastarandstageitsevolutionare associations. can estimatetherelativeagesofdifferentclustersandassociations.Thiseffectisinde- evolutionary effecttobeshowninsuchdiagramsasFigure11.Byuseofthiseffect,one more groups. been takenintoaccount.Thequestionofitsrealitywillhavetoawaitobservation for themain-sequencestarsofgroupsinTable7,andresultantcurveswere smoothed toalinearrelation.The“hump”observedinFigure11,therefore,hasnot the fivegroupsdiscussedabove.Meanvaluesfor§weretakenasafunctionof(U—B)o © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Hence thehighaccuracywithwhich(JJ—B)qandgcanbedeterminedenablesan A fullexplanationofthiseffectisdifficultatthe presenttime,foronlyinthecase In the(U—B)o,gdiagram,awell-definedrelationthereforeappearstobepresent, Fig. 13—Themean(U—B),ßrelationforthegroupsdiscussedinthispaperThenumbersnext Figure 13illustratesthisphenomenonofvaryingHßstrengthasafunctionagefoi 0 ? 9I V. DISCUSSIONOFTHEAGEEFFECT 1958ApJ. . .128 . .185C nificant trigonometricparallaxes.FortheAstarsevolutionaryeffectsproceedata lution. Inaddition,therelationwouldfurnishan accuratemethodofdeterminingthe would bedeterminedforallthestarsindifferentclusters.Ifsuchafitwerepossible stars mightnotbesoapparentasinthecaseofBstars.Ifso,anactualcombination above. FieldstarsofspectraltypeAshouldalsobeobserved,especiallythosewithsig- also forabetterunderstandingoftheevolutionaryphenomenaaswholewouldbeto approach. for use.Thiswouldnotbeasimpleproblembutisprobablythemoststraightforward in associationswheremain-sequencestarsofdifferentagesmightexist.Theaimsuch be ofprimeimportanceinthefieldstellarevolution forpopulationIstars,especially attempted oftheevolutionaryphenomenadiscussed inthispaper.Theinformation parallaxes wouldbemostusefulfordeterminingthe(U—B),?andMvrelation. slower rate,andhencetheobservedagedifferencesin(U—B),gdiagramforthese observe theindices(U—B)and5forA-typemembersofgroupsdiscussed relative MVs,whichwouldthenbetiedintotheScorpio-Centaurusabsolutemagnitudes and 5indicestheabsolutemagnitudes.Theresultantrelationwouldgiveaccurate a programwouldbetoestablishanaccuraterelationbetweentheobserved(U—B) in surfacegravity.Thesechanges,however,seemtobethemostlikelyexplanationof in termsofabsolutemagnitude. groups illustratedinFigure13.Wecanconclude,therefore,thatthemain-sequence in theAstars,thengroupswithknowndistancescouldbeusedtogivezero-point only ontheageandchemicalcompositionofstars. observed phenomena.Therefore,anypointonthe(U—F>),5planeshoulddepend do nothaveenoughavailableinformationtoallowacalculationoftheeffectschanges seriously withanyquantitativeexplanationoftheobservedevolutionaryeffect,forwe galactic distancescale,bymeansoftheindividual absolutemagnitudesobtainablefor greatest importanceforseveralreasons.Itwouldenableaphysicalexplanationtobe moving cluster.Inaddition,ofcourse,A-andF-typestarswithsignificanttrigonometric useful todeterminethe mean absolutemagnitudesforthelate-typeB stars inthe then derivedconcerninglinestrength,absolutemagnitude, andsurfacegravitywould out thespectralrangefromlateBstarsthroughAandevenintoF correction necessarytoapplytherelativeIfV’s,inorderstrengthendetermina- those intheScorpio-Centauruscluster.Butexactdifferencescannotbeestimated stars ofthefPerseiclusterarefainterthanthoseinScorpio-Centaurusand tion tobemadeoftheabsolutemagnitudesformain-sequenceBstars,even Strömgren andassociatesmaymakethispossible. the early-typestarsandforgalacticclusters. Theworknowbeingcarriedonby those ofspectraltypeB,wherethephenomenon is mostmarkedduetotherapidevo- stars, ifpossible.Someofthegroupswhichwouldbeparticularlyuseful tion fromtheScorpio-Centaurusmovingclusteralone. of thediagramswouldbepossible,andthereforeaccuraterelativeabsolutemagnitudes that themain-sequencestarsinaPerseiandPleiadesclustersarebrighterthan Scorpio-Centaurus cluster. Theseabsolutemagnitudesshouldbe,inasense, standard (1) theaPerseicluster,(2)Pleiades,(3)Hyadesand(4)UrsaMajor 0 0 0 0 0 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Another approachwhichwouldbeusedforthetie-insbetweenvariousgroupsand A promisingattackontheproblemshouldbeinvestigationoflineintensities In thisconnectionusefulclusterswouldbethosewheremembersareknownthrough- The uncertaintyinabsolute-magnitudedifferencebetweenthegroupsinterferes In spiteoftheevolutionary effectsdiscussedintheprecedingsection, it maybe The resulting(U—B)o,5,andMvthree-dimensionalrelationwouldbeofthe SPECTRAL CLASSIFICATION203 VI. SOMEAPPLICATIONS 1958ApJ. . .128 . .185C , nitudes willbederivedfromtheB8VandB9starsinScorpio-Centauruscluster dependence iswellmarked. and thenappliedtotheaPerseiclusterobtainlimitingvalueforitsdistance dependence on(£/—B)o isslight.Forhigherluminosities,ofcourse,the (U—B)q inspection ofthe(U—B)o,5diagramitisprobable that,fortheB8VandB9stars, where theturnoffoccursatlatermain-sequence typesthanB0V.Infact,froman B8 VandB9starsofclustersassociations inwhichthemainsequenceextends photometry. Itshouldnotbeinerrorbymorethan ±0.1mag. mag. ThisvalueisderivedfromBlaauw’sabsolute magnitudesandavailableU,B,V faintest B9starintheuppergroupofScorpio-Centaurus cluster. some conclusionsaboutlater-typeclusters.Inthissectionthesestandardabsolutemag- absolute magnitudesforagroupofstarswhosemainsequenceextendstoBOV.The the Mv,5relationsmay be unique.Thatis,forstarsfainterthanMv= 0.0 mag.the to B0V.Theywillthereforebelower-limitvalues forB8VandB9starsofgroups the Fo’sforB8andB9stars.TheseMy’sshould beaccurateforthemain-sequence fainter apparentmagnitudesthantheseventh, the approximatemagnitudeof from theavailabletypesinHenryDraperCatalogue,whichiscertainlycompleteto modulus. absolute magnitudescouldthenbeused,alongwiththe(U—B),§diagram,todraw would belessthaninthecaseofB8’s.Alsooriginalworkstarswereselected for B8,whilethemeanvalueofFoB8starsis0.34mag.brighterthan for completenessofthedata,rangeinvaluesFoB9isaboutsameas have beentakenfortheB8andB9starsseparately.Nocorrectionsmade for theB8VandB9starsinupperScorpio-Centaurusgroup.Thecomputedvalue main-sequence spectraltypeshavinganumberofstarsavailable.Table8givesthedata 204 DAVIDL.CRAWFORD of (U—B)oyieldsaspectraltypeforthesetenstars,andmeanvaluesVo5 magnitude data,itisinterestingtolookintotheabsolute-magnitudecalibrationfor northern partofthewholemovingclusterhasavailable(U—B),5,andabsolute- B9 stars.Iftherewasaselectioneffectinapparentmagnitude,theranges 0 0 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Using thisvalue,themeanabsolutemagnitudegiven inTable8wasderivedfrom The distancemodulusofthisparttheclusterhas beentakenasFo—My=6.3 While thedataforScorpio-Centaurusassociationarenotextensive,asonly B8 V B9 V Mean B9V Mean B8V Spectral Type B8 VandB9StarsintheScorpio-CentaurusCluster 2 841 2 782 5 873 856 837 832 846 805 TABLE 8 (U-B)o -0 22 - 28 - 16 - 23 - 32 - 33 - 32 -0 34 - 21 - 25 - 23 - .23 6 29 6 44 5 91 6 74 6 97 6 42 Vo 97 36 91 97 52 52 -fO 12 +0 44 1958ApJ. . .128 . .185C and [3>whichmeasuresphotoelectricallytheintensity oftheHßlineabsorption,isa He obtainedI/,B,Vphotometryforthelate-typemembersofthisclusterdiscovered tion systemshouldbecapable offurnishingaccurateabsolutemagnitudes andhence applicable alsotofieldstarsnotassignedacluster orassociation. cluster. Henceasimplebutaccuratemethodisfurnished forarranginggalacticclusters system. clusters andassociations,aswellanumber of fieldstars,wereobservedonthis luminosity parameter.Bothparametersareinherently ofhighinternalaccuracy.Several ultraviolet colorindexontheU,B,Vsystem,is primarilyaspectral-typeparameter, distances ofhighprecision forclustersandassociations. in orderofevolutionarydevelopment.Thismethod ofagedeterminationis,course, which appearstobeofanevolutionarynature. For main-sequencestarsofthesame magnitudes foritslate-B-typemembers,comparedtootherclustersofthesameage. crepancy thatthiscluster,inwhichtheturnupisatB3V,shouldhavesuchlowabsolute give areliablevalueforthedistancemodulus.Hencelowofabsolutemagni- utilizing thetwoparameters(U—B)oand[3.TheparameterB)o,intrinsic given inTable9.Theresultantminimumdistancemodulusforeachtypeisthe being greatlyaffectedbytheevolutionaryturnupoccurringforB-starmembers,should closer agreementwiththedistancemodulusdeterminedbyMitchell(1957)of6.1mag. 5.52 mag.,basedontheassumptionthatclusterwasatsamedistanceas for thisgroup,usingitsB8andB9main-sequencestars.Thedatathesestarsare the equivalentpartsofotherclustersknowndistance.Theselate-typemembers,not occurs atB3V.Wecanthereforecomputeaminimumvalueforthedistancemodulus magnitudes. Forthisgrouptheabsolutemagnitudesarelowerlimits,asturnoff tudes determinedbyBlaauwforthisclusterarenotconfirmed.Thisremovesthedis- at HamburgandfittedtheF-G-starpartofmainsequenceclusterwith Cassiopeia-Taurus stream.ThelargervaluederivedfromtheHßconsiderationsisin table. (U —B)o,orspectraltype,theHßlinedecreases in intensitywithincreasingageofthe © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem From the(U—B)o,5diagramsforthesegroups, a newphenomenonhasbeenfound When apropercalibration canbemade,itappearsthatthe(U—B)o,5 classifica- An accuratetwo-dimensionalclassificationsystemforBstarshasbeenestablished, Blaauw (1956)inhisworkonthenorthern0-B5starsderivedavalueofF—Mv It isinterestingtolookatthecaseofaPerseicluster,usingthesemeanabsolute 0 B8 V B9 V B8 Vmean B9 Vmean Spectral Type B8 VandB9StarsintheaPerseiCluster SPECTRAL CLASSIFICATION205 2 849 2 849 2 788 2 805 2 781 2 787 2 770 5 VII. SUMMARY TABLE 9 (U-B) 0 • 21 ■ 22 • 35 ■ 38 0 22 0 29 33 39 6 54 Vo 39 08 04 59 60 56 15 Distance Modulus 6 42 6 64 1958ApJ. . .128 . .185C Lindblad, B.1922,Ap/.,35,85 .19576,ibid,p.134. 1925,NovaActsReg.SocScient.Upsala,SerIV,6,1. Johnson, HL.1957a,ApJ,126,121. 1957,unpublished. Morgan, WW,Keenan,PC,andKellman,E1943,AnAtlasofStellarSpectra,withanOutline Morgan, WW.,andHarris,DL1956,VistasinAstronomy,ed.Beer(London:PergamonPress), .1926,MeddAstrObsUpsala,11,3 Johnson, H.L.,andMorgan,W.1953,Ap/,117,313. Harris, D.L1956,ApJ,123,371 Hack, M.1953,Ann.Pap,16,417. 1956,ApJ,123,498 1948,Ap./.,107,278 Becker, W.1938,Zs.fAp,15,225 Mitchell, RI1957,PubAS.P,69,392. 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