197 9ApJ. . .230. .485A 79 The AstrophysicalJournal,230:485-496,1979June1 © 1979.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. clusters andlaterescapetobecomefieldstars,thereis Ap starsinclusters(YoungandMartin1973;Hartoog a problemregardingtheobservedlowfrequencyof inclusterscomparedwiththefieldstars,but dependence onageinthefrequencyofApstars, be seriouslydeficientinclusters.Arethefrequencies Hartoog foundstatisticallythesamefrequencies. Martin foundaseriousdeficiencyofcool(A0-A5)Ap same (10-10yr)asthetheoretical timescalesfor particularly sincetheclusteragesareroughly of Apstarsinclustersandthefieldreallydifferent, However, HartoogfoundthehotApstars(B5-B9)to in Astronomy,Inc.,undercontract No.AST74-04129with and ifso,why? the NationalScienceFoundation. 1976). Infact,theevidenceisinconflict:Youngand If essentiallyallPopulationIstarsforminopen A secondproblemconcernstheapparentlackofa * OperatedbytheAssociationof UniversitiesforResearch © American Astronomical Society • Provided by theNASA Astrophysics Data System 7 578 04 578 -15 5 025 1 types ofabnormalstars(Ap,Am,Be,shell,etc.).TheAp(Si),Ap(Hg,Mn),andAp(Sr,Cr) field stars,althoughthegreatestuncertaintyinthisresultisduetolackofsimilardatafor field stars.TherotationalvelocitiesofAp(Si)starsdecreasessteeplywithtimeataratepro- In eachcasetheaveragefrequencyinolderclusters(>10yr)issimilartoamong open clustersandassociations,wediscussthedependenceuponageoffrequenciesvarious show increasedfrequencieswithageafterinitialthreshholdsof10-,andyr,respectively. portional to(age)"-,probablyduemagneticbraking.TheBeandshellstarsshownostrong clusters. ThefrequencyofBestarsinclustersissimilartothatamongfieldstars.Stars,now dependence uponage,andsuchstarsoccuramongtheyoungest(lO-yr)oldest clusters, andtheirfrequencydecreasesas(age)*.TheAmstars(1)occurintheyoungest designated luminosityclassVb,withunusuallybroadhydrogenlines,occuronlyintheyoungest clusters, indicatingthatittakeslessthan10-yrtodevelopthem,(2)shownoobviousdepen- dence infrequencyuponage,(3)haveathatissimilartooffieldstars,and(4) rotational velocitiesofAmstarsdecreaseatarateproportionalto(age)"*,probablydue may showfluctuationsfromclustertocluster,buttheevidenceforthatisverymarginal.The tidal brakingincloselyspacedbinaries.TherotationalvelocitiesofAmstarstheyoungest clusters are~250kms“,whichistwicethemaximumforfieldAmstars. that certainphysicalparameterssuchasmagneticfieldsoroccurrenceincloselyspacedbinaries in ayoungercluster(OrionOBIassociation)showsthattheevolutionarysequenceisnotone slow rotatorsinthelateB’sdevelopabnormalspectra.Acomparisonwithrotationalvelocities rotators developabnormalspectra(AporAm)andthenslowdownrotationally.Thismeans occur first,causingspectralpeculiarities,andthattheslowrotationalvelocitiescomelater. of theslowrotatorsgraduallydevelopingabnormalspectra,butratherthatcertainmoderate Subject headings:clusters:open—stars:metallic-linepeculiarArotation — 1.3mag10*yr)clusters,buthefeltthattheresult found Amstarstooccuronlyinclustersolderthan there adiscrepancybetweenthoseclusteragesandthe in veryyoungclustersasSmith(1972è)found,is age inthefrequencyofAmstarsclusters,including ones muchyoungerthan10yr.IfAmstarscanoccur diffusion timescale(see Vauclair, and peculiar stars;e.g.,-NGC2516 hasfourAp(Hg,Mn) Michaud 1978)? stars (AbtandMorgan1969). Arethesestatistical 10 yrwhileHartoog(1976)foundnodependenceon A similarproblemmayexistfortheAmstars. Some individualclustersare richincertaintypesof 197 9ApJ. . .230. .485A 1 1 1 lines (Bestars)andshellspectra,thattheyhave accidents duetothesmallnumbersinvolved,orare frequencies ofBeandshellstarsvarywithage? tion ortheresultsofmassloss,particularlyduring circumstellar shells.Arethesetheremnantsofforma- there genuinedifferencesbetweenclusters? have abearingontheirorigins.Orspecifically,dothe stars thatareslowrotatorsdevelopabnormalspectra? short intervalsofcontraction?Evidenceforadecrease 486 or increaseinthefrequencyofshellswithagemay rence ofabnormalstarsinopenclusters. answered withamoreintensivestudyoftheoccur- two things:thatrelativelyhighdispersionsandwide spectra (describedbelow)cannotbedetectedatthe dispersions (90-130Âmm")usedinmostprevious of Ap(Hg,Mn)andallthemarginal“sn”shell as narrow0.3or0.5mm.AndtheworkofHartoog almost anydispersions.YoungandMartinpointed studies, althoughAmstarscanberecognizedwith spectra areneededtodetectcertainpeculiarities.Most clusters andclusterstarsmustbesurveyed.Also,if (1976) hasshownthat,inviewofthesmallpercentages are seeninAp(Hg,Mn)onlyoccasionallyspectra out (correctly)thatfaintlines,suchasthoseofHg, ages mustbesampled. obtained inmostcaseswiththeKittPeak2.1mand age dependencesareelusive,awidevarietyofcluster of Apstarslikelytobediscovered,alargenumber for normalstars,particularlyearly-typestarswith a dispersionof39Âmm",aswelltheconventional determination ofspectraltypesandluminosityclasses 92 cmCassegrainspectrographs,respectively.For a varietyoflinewidths.Thesetwosetsspectrawere one of128Âmm"thathasprovedtobeidealforthe These areexamplesofquestionsthatmightbe Some starsshow,throughthepresenceofemission Given enoughtime,wouldalllateB-typeorA-type The workofYoungandMartin(1973)emphasizes To meetthefirstoftheserequirementsweemployed © American Astronomical Society • Provided by theNASA Astrophysics Data System NGC 6633.. NGC 2516.... IC 4665 IC 2602 Coma. Ursa Major... NGC 6475.. Pleiades Lacerta OBI.. Orion OBI... a Persei Orion . M34 M39 II. THEMETHOD Cluster Log Age (yr) 7.1 9.042LevatoandAbt1976a 7.5 18AbtandLevato1975 7.1 5.8818AbtandMorgan1972 6.7 8.0-8.5152AbtandLavato1977c 5.7 8.3726LevatoandAbt19766 8.0 7.817AbtandLevato1976 7.7 5.5449AbtandLevato1978 7.4 6.1052Abt1978 8.8 4.49 35 AbtandLevato1977a 8.5 various154 LevatoandAbt1978 8.4 7.0 27 Abt1975 8.2 7.71 26 LevatoandAbt1977 8.1 7.97 27 AbtandMorgan1969 8.1 8.218AbtandLevato19776 The ClustersStudied Modulus Distance TABLE 1 (mag) ABT 578 the secondrequirementwewidenedbothsetsof tions whoseagesrangedfrom10-toyr. ments westudied661starsin14clustersandassocia- to reducegrain.Thespectralclassificationwasdone and theEastmanKodakIla-Oemulsionswere spectra to1.2mm.Forthethirdandfourthrequire- new spectralatlasbyMorgan,Abt,andTapscott standards byMorganandKeenan(1973)inthe overexposed andunderdeveloped(10minutesinD76) (1978). Theresultsforthe14individualclustershave on aBoiler&Chivensspectracomparatoragainst two independentclassifications.Heiscontinuingwith present paperwillbeconcernedprimarilywitha been publishedorareinpress(seereferencesTable summary ofresultsontheabnormalstars. of interestforthestudiesindividualclusters; been addressedineachindividualstudy.Thebasic respectively; theseareadequateforouranalyses. curacies areabout±0.2dex(s.e.)and0.3mag, to theclassifiers.Theclusteragesanddistancemoduli listed inorderofageTable1.Thelasttwocolumns of theclusters;publishedtypesareresult criterion ispropermotion,anditusuallytheonly come mostlyfromphotoelectricstudies.Theirac- similar studiesofsouthernclusters. position inacolor-magnitudediagramwithcurrent criterion formembership.Theconsistencyofastar’s give thenumbersofstarsclassifiedandreferences velocities aregenerallynotavailableinmostclusters, not usedasgroundsformembershipselection.Radial evolutionary ideasisariskyapproach,especiallyfor abnormal stars.Theindividualstudiesusuallynoted 1 below).Theclassificationsforthenormalstarsare such consistenciesandinconsistencies,buttheywere The twospectrographshaveexcellentresolution Dr. HugoLevatoworkedwiththeauthoronmost The questionofthemembershipineachclusterhas The 14openclustersandassociationsstudiedare Number of Classified Stars III. THEBASICDATA Classification Reference to Vol. 230 197 9ApJ. . .230. .485A -1 74 576 and onewouldhavetonumerousspectraper No. 2,1979 toallowforbinarymotions. that thestrengthsofpeculiarityincreasewithtime this apparentincreaseofspectroscopicpeculiarity in theyoungerclusters.Wehavevisualimpression of thesestarsaremarginallypeculiar,especiallythose with age;itshouldbedoneatcoudédispersions on theaverage.Adesirablelaterprojectistostudy relative tostandards. in Table2.Thefirstcolumngivestheclusterand last columngivestheindividualrotationalvelocities, magnitudes correctedforinterstellarabsorption.The Draper number;thethirdgivesabsolutevisual stellar designations;thesecondgivesHenry from our39Âmmspectra. compilation; theonesinparenthesesareestimates referenced inTable1orarefromUesugi’s(1978) (3) ofTable2areintherange—1.3mag

! 10- 150- O Rotational Velocities of Ap(Hg,Mn) Stars o / CO Field . Start T 100-

-C ^ w 50- oso I I \ L- \ I L o 7 8 o Log Age (yr.) Fig. 5.—The frequencies of cluster Ap(Sr,Cr) stars without 7 8 Si overabundances are shown, as well as for similar field stars Log Age (yr.) (arrow). The four groups (open circles) of clusters are the same ones as for Fig. 1. The sloped line, drawn dashed to indicate Fig. 4.—The rotational velocities of eight Ap(Hg,Mn) its uncertainty, is derived by dividing the oldest clusters into stars are shown as a function of age. The data are too few to two groups indicated by filled dots: two Ap(Sr,Cr) stars out establish an evolutionary slowing down. For field Ap(Hg,Mn) of 74 stars, or 2.770, among the four clusters with ages of stars, < Vsin /> = 29 km s-1 (Abt, Chaffee, and Suffolk 108 O-108-2 yr; and eight Ap(Sr,Cr) stars out of 99 stars, or 1972). 8.1%, among the three clusters with ages of 108*4-108-8 yr.

© American Astronomical Society • Provided by the NASA Astrophysics Data System 197 9ApJ. . .230. .485A 8 8 8 0 490 5781 threshhold of10yr,correspondingtotheageM39. the fieldisobtainedfromacomparisonofsuchstars 6.4 oratthe98.8%confidencelevel.Itimpliesa (without Sioverabundances)withV<6.0magin Osawa’s (1965)listwiththetotalstarsinCatalogue and apparentmagnitude;wederive5.4+2.3%.This of BrightStars'mthesamerangesintype,declination, frequencies forthefieldstarsandmembersofolder clusters (>10yr). indicates thatthereisapproximateagreementin vincing decreasewithagebecauseofthenarrowrange velocities. in ages(10-10-yr)includedandbecauseofafew stars (21Com,HD205116)withlargerotational listed inTable5.ThespectraltypesrangefromB1to based onthefourdatapoints1/11=9%,4/815%, stars inthatrangetheoldestclusters.Thefre- B7, andtheabsolutemagnitudesoccurinrange 7/45 =16%,and1/186%,.Theobservedmeanis quencies ofBestarsareshowninFigure6and frequency ofBestarsdepartsfrombeingconstantat 8.8 ±4.9%comparedwithanexpectedscatterof least. an unconvincing74%level ofconfidence.Bestars 5.8%. Thereforethescatterisonly0.8or,and occur inclusterswithages from 10-toyr,at of Bestarsinclustersisconsistent withtheearlier - 3.8mag

Fig. 6.—The frequency of Be stars shows no obvious variation with age, considering the standard error bars in the Fig. 7.—This shows the frequencies of stars with pro- frequencies. The mean frequency is similar to that (arrow) nounced shell spectra plus those with “sn” spectra that have for field stars. both sharp and broad lines probably due to thin shells. The frequency statistically does not vary with age, despite the appearance of a decrease after IQ8 yr. B1-B7 III-V), there are 10 Be stars for a frequency of 5.6%. His dispersion, emulsion, and widening were similar to ours, and we both obtained single isolated In addition, there are stars here called “sn” be- plates (the emission in many Be stars tends to appear cause they have both sharp and broad lines in their and disappear with time); his frequency for field Be spectra. I suspect that these will prove to be due to stars and ours should be comparable. Slettebak’s thin shells. An illustration is given by Abt (1978). A frequency for field stars is shown in Figure 6 and nearly complete listing is given in Table 7; we re- implies that, within the standard errors, field and surveyed all the clusters except IC 2602 and NGC 2516 cluster stars do not differ in their frequencies of Be to bring this list nearly up to date. It includes 28 stars. cluster members (excluding HD 37232) in 12 clusters. Note that the sn characteristic can occur simul- VIII. THE FREQUENCY OF SHELL STARS taneously with a variety of peculiarties. All the shell and “sn” stars occur in —2.8 mag < The 16 stars with pronounced shell spectra are M7<+1.3mag. Figure 7 shows the combined listed in Table 6. They include well-known shell stars frequencies for the four groups of clusters: the data like if* Per (HD 22192), Pleione (HD 23862), and 14 points are 3/20 = 15%, 15/93 = 16.1%, 7/100 + Com (HD 108283). One star (HD 21551) does not 8/80 = 16.1%, and 5/145 + 5/120 = 8.4%. The currently have a shell, but one was reported for it apparent decrease in frequency of shell stars in the earlier (Kraft 1967). oldest clusters is not statistically significant (69% level

TABLE 6 Summary of the Pronounced Shell Spectra

Cluster and Star HD or BD Mv Spectral Type Orion Nebula: B655 — 5°1318 -0*?1 B2: V + shell B786 + 1.4 A0: V + shell B907 37115 -1.4 B7 Ve + weak shell Orion OBI 34959 -1.6 B7 Hin + shell + late B (213801 -0.9 B9 V + shell Lacerta OBI.... *< 214783 -0.5 AO Vp +shell (218674 -2.8 B4 Ve + shell a Persei : HL 904 21551 -0.5 B8 Vn HL 1164 22192 -2.3 B5 Ve + shell IC 4665: K32... 161261 +0.4 B8 V + shell Pleiades: Hz 878 23862 -0.6 B8 Vn + shell M34: B62 16655 +0.1 A0 V + weak shell NGC 2516: C41. — 60°968 +0.8 B9 V + shell NGC 6633: H42 + 6°3765 +1.3 A2 V + shell H81 170095 +1.3 A2.5 V + shell Coma: TI25 108283 +0.5 A9 V + shell

© American Astronomical Society • Provided by the NASA Astrophysics Data System 197 9ApJ. . .230. .485A 492 Orion OBI...... 35502 IC 4665:K49161480 Pleiades: a Persei: the “sn”spectracanbedetectedonlyamongBstars, NGC 6475: the Astarsthatpredominateinolderclusters; M39: E26205116 and shellspectraaremuchlesseasilydetectedamong of confidence)andmaybeduetoaselectioneffect: frequency of13%withthatfieldstars. sharp strongultravioletTinlines(AbtandMoyd shells areusuallydiscoveredinAstarsthroughtheir spectra, sowecannotcomparethemeancluster shells isconstantduringmostorallofastar’smain- usually broadhydrogenlines.Theoverallwidthsare measure ofsurfacegravityandthesestarsgiveno brighter starsintheOrionNebulaclusterhaveun- and Lodén1966)totheobservationthatmanyof sequence lifetime. explained. However,ithasbecomeapparentthatthe about twicenormal.Sincethelinewidthisnormallya (Morgan, Abt,andTapscott 1978)hasseparatedthe nated withtheletter“w,” butrecentlyMorgan youngest clusters.Theeffect wastemporarilydesig- the zero-agemainsequence and theyoccuronlyinthe stars withunusuallybroad hydrogenlinesareon other main-sequencestars,theeffecthasneverbeen other evidenceforhighsurfacegravitiesrelativeto 1973). Wetentativelyconcludethatthefrequencyof Cluster andStarHDM(mag)SpectralType v HL 67521071 HL 98521699 HL 62520961 HL 1153...... 22136 HR 118523950 Hz 242.23408 K108 162804 K42. 162515 K56 162586 Hz 91023873 K77 162679 There hasbeennosurveyoffieldstarsfor“sn” Morgan hasoftencalledattention(e.g., 162678 IX. THEFREQUENCIESOFSTARSWITHUNUSUALLY © American Astronomical Society • Provided by theNASA Astrophysics Data System Summary of“sn”Spectra BROAD HYDROGENLINES 36351 35792 37151 36485 36430 36429 36392 37000 36954 37129 37058 37232 37889 37321 TABLE 7 -2.0 B2.5Vsn -2.4 B2.5Vsn -0.9 B3Vsn -1.1 B5IVsnp(Hewk) -0.6 B3Vsn -1.2 B4Vsn -1.3 B2Vsn -0.7 B4IVsnp(Hewk) -1.0 B5Vsn -0.1 B8Vsnp(Si) -1.0 B2.5Vsn -1.3 B2Vsn -0.2 B7Vsn -0.8 B5Vp(Hewk)sn -0.8 B5Vsn -0.4 B6Vp(CII)sn +0.5 B8IV:sn -0.8 B2.5Vsn -1.8 B7Hlsn + 1.0B9.5Vsn -0.1 B9Vsn -0.7 B9Vsn -1.1 B8Vsn -0.9 B9.5V(Srst)sn + 0.5B9IVp(Hg,Mn)sn -0.2 B9Vsn -0.9 B9Vsn + 1.0B9.5IVsn B2.5 Vsn ABT 15 51 67 57 Orion Nebula: luminosity classVstarsintoVb(thosewithunusually broad hydrogenlines)andVa(theremainder). Orion OBI remainder ofOrionOBI.Morgandiscoveredone are confinedtoonlytheOrionNebulaclusterand frequency thatvariesasT"-.Thisisaverysteep in alltheotherclusters.Thefrequencyisnotconstant cluster is9/18=50%,;intheremainderofOrion The frequenciesofVbstarsintheOrionNebula NGC 2516(C29).Thatsuchstarsareveryrarein at the99.9%confidencelevel.Thepower-lawcurvein OBI itis2/120=1.7%.Thefrequenciesarenegligible such starinIC2391(notthisprogram)andone dependence onage. occur intherangeof—3.1mag

494 ABT Vol. 230 'o ft

7 8 Log Age (yr.) Fig. 10.—The rotational velocities of individual Am stars Fig. 9.—The frequency of Am stars in each of the 14 show a gradual decrease with age. The rotational velocities clusters is shown because there might be real differences from in the Orion Nebula cluster and Orion OBI far exceed the cluster to cluster. However, the scatter is only 1.9 for richest clusters show similar frequencies. The mean frequency the oldest cluster stars is similar to that (32 km s_1) for field for the cluster stars is similar to that for field stars (arrow). stars.

uncertain when small numbers are involved. A least- It does not appear likely that the data in Figure 9 squares solution shows that represent a simple dependence upon age because of the 71 28 1 occurrence of Am stars in the youngest cluster. = 77(10~ r)-°- km s" The hydrogen types, which correlate well with or approximately oc r"1/4, where T is the effective temperatures, for the Am stars in Table 8 cluster age. range from AO to FO. Among the AO-FO stars in This decrease with age is probably due to tidal Cowley et al. (1969), approximately 10.4% are Am or interactions in closely spaced binaries because Am Am: stars. This provides the frequency of Am stars stars are rich in binaries (Abt 1961). Note that this is a among field stars that is shown in Figure 9. slower decrease than the magnetic braking shown in Figure 9 shows the following results: Figure 2. 1. Am stars occur in the youngest cluster, so it 5 7 takes less than 10 * yr to develop the anomaly unless XI. ROTATION AND ABNORMAL SPECTRA the mechanism commences before the stars arrive on the main sequence. This was first pointed out by Abt and Moyd (1973) showed that among A5-A9 Smith (1972b). main-sequence field stars, statistically all the slow 2. The mean frequency (12.5%) of Am stars in rotators (F^lOOkms"1) have abnormal (Am) open clusters is not significantly different from that spectra, implying that a low rotational velocity is a of field stars (10.47o). sufficient condition for the formation of Am spectra. 3. The frequency of Am stars does not seem to Actually, this does not say whether the low rotation depend on age in a pronounced way. allows the Am characteristic to develop or whether 4. There is only marginal evidence for statistically the physical parameters associated with Am stars— significant differences in the frequency of Am stars especially the high duplicity—produced the low from cluster to cluster. rotational velocities; however, Am stars and low The projected rotational velocities of the Am stars rotational velocities generally go together. are shown in Figure 10, which shows very large In the case of Ap stars, Abt, Chaffee, and Suffolk velocities (up to 260 km s-1) for the youngest Am (1972) found that among the B7-A0 main-sequence stars. Such large rotational velocities are not known field stars, not all slow rotators have Ap spectra. Ap among field stars (Abt and Moyd 1973), for which stars have V < 135 km s-1, but not all such slow = 32 km s_1, and the maximum is 95 km rotators have Ap spectra. s“1 or 120 km s“1 (Smith 1972a). But one would In view of the contents of Figures 1, 3, and 5, we expect that escapees from very young clusters are wonder whether, if cluster stars had enough time, all rare in the general field. Nevertheless, we wonder slow rotators would develop abnormal spectra through about how pronounced or marginal Am stars can a diffusion process or whatever mechanism produces occur at V sin i ä 200 km s“1. them. In Figure 11 (top) we show the distribution of Figure 10 also shows a decrease in mean rotational projected rotational velocities for all the main- velocity with age. The distribution in Figure 10 is, of sequence or giant stars with —1.3 mag < Mv + 1.3 course, an envelope because the inclination effects mag in six old clusters (M39, M34, NGC 2516, NGC have not been removed; to remove them is rather 6633, NGC 6475, and Coma). This shows that after

© American Astronomical Society • Provided by the NASA Astrophysics Data System 197 9ApJ. . .230. .485A 83 8,3 1 -;L 1 -1 -1 No. 2,1979 nitudes intheOrionOBIassociation;Am,Ap(Si),andAp(Heweak)starsareindicatedwithhatching. is showninthetoppanel.TheAm,Ap(Hg,Mn),andAp(Si)starsareindicatedwithhatchingwhileblankareasrepresentnormal stars. Thebottompanelshowsthedistributionofprojectedrotationalvelocities78starsinsamerangeabsolutemag- an averageof10-yr,20/51=39%thestarswith creased throughmagnetic ortidalbraking.This unless ourdiscoveryrateisaslow69%,,wemust not Ap(SiorSr,Cr),Ap(Hg,Mn),Am.Ifacon- tors areabnormal,butrather thataselectionofstars conclude thatafter10yrnotallslowrotatorshave volution toVismadeassumingrandomorientation abnormal andhavetheir rotational velocitiesde- developing abnormalspectra untilalltheslowrota- in speed(inaccordwithFigs.2and10). (excluding theOrionNebulacluster)forstarswith shows adifferenttypeofevolutionforabnormal abnormal spectra. of rotationalaxes,wefindthat317thestarswith of variousspeeds—mostly moderate ones—become one oftheslowrotatorsinayoungclustergradually stars andthenthoseabnormalaresloweddown 225 kms",thatarebeingconvertedintoabnormal Ap(He weak)]areshown.Therefewerabnormal statistically identicalasfortheolderclustersinFigure absolute magnitudeintherange—1.3mag=159+82kms“for V