198 6ApJ. . .303 . .226S 6 37_ 6 12 (Cameron andFowler1971). dance declinesasaresultofthemixing. 2 x10K,andmixedintoastellaratmosphere;theLiabun- mass transferacrossabinary.Thankstothepossibilitythat the the chainHe(a,'y)Be(^,v)Liinahotconvectivelayer by Boesgaard1976).NucleosynthesisofLiisattributedto stars suchasTSgrandCWZCas(seereview some residentsoftheasymptoticgiantbranch(AGB):rareS exposed tomoderatetemperatures,say6x10>T elements ofhigheratomicnumberunaffected.UnlikeBeandB, nous classicalbariumstars, as suggestedbyLuckandBond illustrate onetestprovidedby Li,considerthehypothesisthat constraints onthehypothesisthatCHsubgiantsaremass- in onegiantand10subgiantCHstarsexploretheimplied comparisons oftheabundanceindifferentclassespeculiar be destroyedanddilutedinthecourseofstellarevolution, test ofthemass-transferhypothesis.SinceatmosphericLi can companion. Inparticular,theclassicalandmildbariumstars variety ofpeculiarstars,thestarscreatedfromlessevolved companion inabinarysystemisadvancedastheoriginof provides Liwiththenearlyuniqueroleofatracermaterial boron, isdestroyedbyprotonsattemperaturesthatleavethe the subgiantCHstarsare progenitorsofthemorelumi- and s-processabundances(see reviewbyLambert1985).To share manyofthesameabundance anomalies,e.g.,highC transfer products.Inparticular,weexaminewhetherthese stars mayprovidecluestotheirkinship. abundance inthebariumandotherpeculiarstarsprovide a AGB starmayhavebeenLi-rich,measurementsofthe Li and thesubgiantgiantCHstarsmaybeproducts of stars. Theconjunctionofthesenuclearandatomicproperties Li isdetectablespectroscopicallywithrelativeeaseincool stars arerelatedtothemildandclassicalbariumstars,which (1982). Thesubgiantsareexpected tohaveashallowconvec- The AstrophysicalJournal,303:226-232,1986April1 © 1986.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. Another distinguishingmarkoflithiumisitsproductionin Today, masstransferfromanAGBstartoalessevolved Lithium, withitsneighboringlightelementsberylliumand In thispaper,wedetermineupperlimitstotheLiabundance 12 e(Li) =1.5[loge(H)12.0]weredeterminedforthissampleofstars.ThesubgiantCHstarsareon,ornear, resonance doubletdetected,andupperlimitstothelithiumabundanceinrangeloge(Li)<0.7 with low-luminositycompanions,theymayrepresentmasstransferfromaformerasymptoticgiantbranch dances, wouldgivethesubgiantCHstarswhentheybecomeredgiantsdistinctlylowerLiabundancesthan lithium byafactorof~30-50.Thisdilutionfactor,alongwithourupperlimitstothecurrentabun- the mainsequence,andastheyevolveupredgiantbranch,deepeningconvectiveenvelopeswilldilute Subject headings:starsabundances—Banbinariesevolutionlate-type star (nowawhitedwarf)ontomain-sequencecompanionsubgiantCHstar). classical bariumstars.AsthesubgiantCHstarsdoshowCands-processenhancementsareallbinaries are observedintheclassicalbariumstars.ThissuggeststhatsubgiantCHstarsnotprogenitorsof © American Astronomical Society • Provided by the NASA Astrophysics Data System We reportonlithiumabundancesin10subgiantCHstarsandonegiantstar.InnostarwastheLii I. INTRODUCTION LITHIUM INLATE-TYPEGIANTS.IV.THESUBGIANTCHSTARS Department ofAstronomy,UniversityTexas,andMcDonaldObservatory Verne V.SmithandDavidL.Lambert Received 1985July22;acceptedSeptember25 ABSTRACT 226 was madebycalculatingthe noiseinasmooth,continuum “eye” estimate.Amorereproducible, quantitativeestimate eye andsketchingalineprofile thatwasjustlargerthanthe equivalent width(W)wereestimatedintwoways.Initially, an Note theexpandedintensityscale. coudé echellespectrometer(Enard1979)andthe1.5m tele- coudé spectrograph,andeitheraDigiconintensifieddiode were madeusingtheMcDonaldObservatory’s2.7mtelescope, for theclassicalbariumstarsand,hence,hypothesismust first toprovideinformationonLiinanumberofsubgiantCH evolve tobariumstars,theseinferredstructuraldifferences convective envelopeofaredgiant.Then,ifthesubgiants spectra wereaveragedandthe standarddeviationcrcomputed. region. Approximately4Â(~35 Digicondiodes)ofsmooth noise fluctuations;werefer to thismethodasprovidingan upper limitwasestablishedbydrawingacontinuumlevel by trates theLiiregionintwoDigiconspectrafromMcDonald. scope oftheEuropeanSouthernObservatory.Figure1illus- HD 176021,wasobservedataresolutionof0.12Âwith the  ofspectrawererecordedataresolution0.2Â.One star, diode array(Vogt,Tull,andKelton1978).Approximately 100 array (Tull,Choisser,andSnow1975)oraReticonsilicon subgiant CHstarsandthegiantstarHD26.Observations classical bariumstars. combine theavailableinformationofLiwithotherevidence be rejected.Afterpresentingourabundanceanalysesin§II,we deep convectiveenvelopeandtheshallowouterzoneof at least50timesasgreattheabundancesfoundforbarium demand thattheLiabundanceofsubgiantCHstarsshouldbe tive envelope,butthebariumstars,beinggiants,havedeep CH starsandthemoreevolvedgiants,mildbarium, and explorepossibleevolutionarylinksbetweenthesubgiant stars, showthatLiisatorbelowtheconcentrationsreported subgiant inwhichLisurvives.Ourabundanceanalyses,the stars; thefactorof50isexpectedmassratioagiant’s Á In nostardidwedetecttheLiidoublet.Upperlimitstoits We obtainedspectracoveringthe6707ÂLiidoubletin10 II. OBSERVATIONSANDABUNDANCES 198 6ApJ. . .303 . .226S mined thatanyfeaturedeeperthanabout2crindepthwould width athalf-depth,andaGaussianapproximationtothe line consists oftwounresolvedcomponentsforeachisotope split could bemeasured(typically,fullwidthathalf-depth was be readilydetectable.Thewidthsofweaklinesinourspectra mated fromthecomputercr’s,andupperlimitsto the magnitudes, thesignal-to-noiseratioofspectraas esti- by ~0.15Â.Usingacentral-linedepthof2a,anestimated full about 0.28Â)andthiswidthappliedtotheLiidoublet,which By identifyingotherweakfeaturesinthespectra,itwasdeter- line-depth methodandbyeye. Notethatthe“eye”estimates equivalent widthsoftheLiifeatureobtainedfrom 2 a Table 1welistthestarsobserved,theirapparent V- previous observers(Snedenand Bond1976;Luckand generally agreefairlywellwith themorequantitativeestimates. shape, upperlimitstotheequivalentwidthwerecomputed. In 1985), andwehaveadopted theiratmosphericparameters 1982, hereafterLB;Sneden1983; KrishnaswamyandSneden HD 89948 HD 88446 HD 11377 HD 4395.. HD 26.... HD 216219 HD 207585 HD 204613 HD 182274 HD 176021 HD 125079 Most starsinthissamplehave beenstudiedindetailby Star © American Astronomical Society • Provided by the NASA Astrophysics Data System (mag) Ratio 9.8 7.7 8.2 7.5 8.2 7.8 7.6 8.5 7.7 7.9 8.5 V Signal-to-Noise Observational Information Fig. 1—SampleMcDonaldObservatoryDigiconspectraoftheLiidoubletregionintwosubgiantCHstars TABLE 1 106 110 108 128 40 79 81 86 55 35 85 2 (7LineDepthByEye Upper LimittoW{Lii) x LITHIUM INLATE-TYPEGIANTS.IV. 15 11 17 4 7 6 7 7 7 5 5 (mÂ) 20 10 4 4 6 7 8 5 5 1 -1 -1 halo objects).LBestimatethatthesevaluesofMareaccurate within afactorof2giventhatthesestarsareolddiskoreven lent widths.AlsoinTable2welistvaluesfortheabsolute will, ofcourse,haveanegligibleeffectonderivedLiabun- preferred, andforstarswhichweuseLB’svaluesT,« higher qualitythanLB’s,wepresumethathisvaluesaretobe to within±0.8mag. visual magnitude,M,asestimatedbyLBfromderivedvalues dances becauseoftheverysmallupperlimitstoLiiequiva- and logg,weassumethat£=1.5kms.Largervaluesof¿ information. Sneden(1983)pointsoutthatLB’smicro- eters, themetalabundance[Fe/H],andsourceofthis (effective temperatureT,surfacegravityg,andmicro- of T,logg,andanassumedmass1.0M(accurateto turbulent velocitiesareconsiderablylargerthanvalueshe turbulent velocity£).InTable2wepresenttheadoptedparam- opposed to3.0-4.0kms).SinceSneden’sspectraareof obtained forthesamestars(typically1.5-2.0kmsas v y e{( eff0 1983. HD 216219 HD 207585 HD 204613 HD 182274 HD 176021 HD 125079 HD 89948 HD 88446 HD 11377 HD 4395.. HD 26.... a b Two starsinourlist,HD125079and207585,havenot SB=SnedenandBond1976;LB =LuckandBond1982;SSneden Assumed(seetext). Star 5600 5400 5650 6000 6000 6000 6000 5300 5950 5450 5250 (K) Tff x Stellar Parameters log 0£_ (cgs) (kms) 4.50 4.00 4.10 4.50 4.10 3.25 2.50 3.50 3.75 3.50 3.30 TABLE 2 b b b b b 2.0 2.0 2.5 1.0 1.5 1.5 1.5 1.0 1.5 1.5 1.5 3 [Fe/H] MSource v -0.3 -0.5 -0.5 -0.4 -0.4 -0.3 -0.5 -0.4 -0.2 -0.4 -0.3 4.8 4.8 0.3 2.0 2.1 3.1 3.5 3.8 3.8 This study This study LB LB LB LB SB S S S S 227 198 6ApJ. . .303 . .226S -1 0.00 (Lii).Thesearenotsignificant toourresults,hencescaled log g»32-3.1.Forbothstarsweassignavalueof=3.5. been analyzedbefore,andwehaveestimatedloggfor solar atmospheresareappropriate forourpurposes. log e(X),were+0.11(Fe i), +0.07(A11),-0.03(Caand The metallinesofthestarappearquiteweak,soweadopt licity range-0.5<[Fe/H]0.0leadtoT=5400-5540K. perature calibrationfromBöhm-Vitense(1981)forthemetal- 207585 fromEggen(1975).Its(B—V)=+0.70andthetem- them. Broad-bandUBVRImagnitudesareavailableforHD BEGN). Thedifferences,in the formofloge(X)— other starsinthissamplewithT»5400-5600Khave (V. V.S.),andthestar’s(7^—T)colorof+0.38plus for thismetal-poorstar.Washingtonphotometricmagnitudes, lower temperatureofT=5400Kasthemoreappropriate the speciesFei,A1andCai.Linesfromthesewere log g,sowehaveestimatedavalueforgravitybynotingthat temperature calibrationfromHarris(1980)yieldT=5300 CMTi T,forHD125079havebeenmeasuredbyoneofus from theSun,soscaledsolaratmospheresweregenerated program fromSneden(1974).Elementabundanceswerevaried line verystrong. values ofIF,excitationpotentials,andLTEsolar¿//-values values of[Fe/H]extractedfromtheliteratureorestimatedby analyzed aschecksontheatmosphericparametersand K. Ourspectralcoverageisinadequatetoyieldaspectroscopic Andersen, Gustafsson,andLambert(1984). profile integratedforanequivalentwidth.Theg/-values the until amatchwiththemeasuredequivalentwidthswas using amodifiedversionoftheLTEspectrumsynthesis the blendingcanbeestimated,andinnocaseisoffending Ca ilineisslightlyblendedtothebluebyaweakFeline,but turbulent velocityof0.8kms.Wenotethatthe6717.69 derived usingourmeasuredsolarequivalentwidthsfromthe the HolwegerandMüller(1974)solaratmospherefor par- obtained. TheLiifeaturewassynthesizedandthepredicted atmosphere ofHolwegerandMüller(1974)withamicro- Liège atlas(Delbouille,Neven,andRoland1973)thesolar ourselves. InTable3welistthelinesusedalongwiththeir dances werealsodeterminedusingamodelatmospherefrom 26 isthemostdifferentfromSun,and,asatestof the ticular T-log¿/-[Fe/H]combination.ThegiantCHstar HD the gridofmetal-poorgiantsinBelletal.(1976,hereafter appropriateness ofusingascaledsolaratmosphere,abun- Li ilinesweretakenfromtherecentstudyofisotopes by Fe, Al,andCa,anabsolute upperlimit,baseduponthe 228 BEGN eff scaled solar eff 2 eff eff 2 A eff All 6696.033.14-1.442838201949113021 Cai 6717.692.70-0.541191019266761105370816899 Fei 6703.572.76-3.0545322017195010132123 The subgiantCHstarsarenotverydifferentinTandlog g Additional linesareavailableinthisspectralregiondueto Synthetic equivalentwidthswerecomputedforeachline Our resultsarepresentedin Table 4asvaluesof[X/H]for eff Species A(Â)(eV)log(3/)264395113778844689948125079176021182274204613207585216219 © American Astronomical Society • Provided by the NASA Astrophysics Data System 6705.11 4.61-1.12474026242752152134 6698.67 3.14-1.7821231610326913... I HD Atomic LineDataandEquivalentWidths SMITH ANDLAMBERT TABLE 3 -1 cal giantCHstarHD26isindistinguishable bymetalabun- m Ali(6696Â)line,a30mÁFe(6705and5 Li include atruePopulationIIstar; note,too,thattheprototypi- mixture ofPopulationIand Population IIstars.Asthemetal i line,weobtainthedifferences,inAloge(X),shownTable 5. each ofthemodelparameters.Usinga60mÁCailine, a 20 km s,and[Fe/H]=—0.3)withthoseobtainedbyvarying from atypicalatmosphere(T=5800K,logg3.5,£ 1.5 eters areillustratedbycomparisonoftheabundancesderived improper analyses. These comparisonsdoindicatethatourupperlimitstotheLi lines wereoftensoweakastobenearthelimitsofdetectability. determination ofthemicroturbulenceisrequired,whileAli (Tomkin, Lambert,andBalachandran1985),thisagreement course, oursarenotcompletelyindependentbecausewe abundances (Tables2and4) show,ourselectiondoesnot abundances arenotstronglyinfluencedbysystematiceffects or line wasstrongenoughtobeaffectedbysaturation,soacareful cannot beclaimedtoverysignificant.IneachstartheCai with thoseobtainedformoderatelymetal-poordwarfs enhanced relativetoFe:themeanvaluesare[Al/Fe]=+0.16 scale loge(H)=12.0,forthelithiumabundance.Acomparison and tobepossibleprogenitors ofthegiantCHstars(Keenan Bond (1974),thesestarswereconsideredtobePopulation II and [Ca/Fe]=+0.12.Althoughtheseresultsareconsistent although thereisanoticeabletendencyforAlandCatobe adopted publishedatmosphericparameters.Thevaluesof of Table4and2showsthatourdeterminations [Fe/H] areingoodagreementwithotherinvestigations;of [Al/H] and[Ca/H]arealsoinfairagreementwith[Fe/H] 1942). TodaythesubgiantCH starsmaybeconsidereda e{{ The effectsofuncertaintiesinthemodelatmosphereparam- When theclassofsubgiantCHstarswasintroducedby HD204613 ....-0.54 HD 182274....-0.47 HD 125079....-0.30 HD 11377-0.30 HD216219 ....-0.39 HD207585 ....-0.74 HD 176021....-0.61 HD 89948-0.31 HD 88446-0.38 HD 4395-0.38 HD 26-0.45 Equivalent Width(mÁ) III. LITHIUMANDTHEMASS-TRANSFERHYPOTHESIS Star [Fe/H][Al/H][Ca/H]loge(Li) Derived Abundances TABLE 4 -0.53 -0.63<1.2 -0.04 -0.27<1.0 -0.29 -0.20<1.3 -0.27 -0.46<1.2 -0.10 <0.7 -0.33 -0.27<0.8 -0.24 -0.48<0.9 -0.29 -0.40<1.2 -0.07 0.00<1.5 -0.39 -0.06<0.7 -0.51 -0.71<1.5 Vol. 303 198 6ApJ. . .303 . .226S 12 No. 1,1986 classical bariumstars—e.g.,themildBastaroVirhas[Fe/ dance fromtheselectedsubgiantCHstarsandmanymild ments ofoursubgiantCHstarsspantherangefoundformild H] =—0.4(TomkinandLambert1985)theclassicalBa star HR774has[Fe/H]=—0.3(TomkinandLambert1983; and classicalBastars(seeLambert’s1985reviewfordiscussion binary systems(McClure,Fletcher,andNemec1980;McClure and references).ThediscoverythattheBastarsoccuronlyin Smith 1984).Furthermore,thecarbonands-processenrich- process enrichmentsincarbon-richandoxygen-richAGBstars ion. SincesubgiantCHstarsarebinariestoo(McClure1985), mass transferacrossabinaryfromanAGBstartoitscompan- and thoseseeninBastars(Lambert1985;SmithLambert with acompositionsimilartothatoftheBastars,thissuggests that subgiantCHstarsarealsoproductsofmasstransfer.In H-R diagram.RepresentativeclassicalandmildBastarsfrom do notshowthetwosubgiantCHstarsforwhichwe Böhm-Vitense, Nemec,andProffitt(1984)arealsoshown.We Figure 2weshowthelocationofsubgiantCHstarsinan 1983) andtheclosecorrespondencebetweenCs- 1985) encouragethehypothesisthatBastarsareformedby NGC 2420{cross),whichhasawell-determined distance,fromMcClure,Forrester,andGibson(1974).Theoretical modelsfromIben(1967a,b)areshownwith signs) andmildbariumstars{opentriangles) fromBöhm-Vitense,Nemec,andProffitt(1984)areshownalong withtheclassicalbariumstarinoldopencluster predicted Lidepletions,relativetothe main-sequenceabundance,indicatedalongthemodeltracks. Fig. 2.—Luminosity-surfacetemperature diagramforthesubgiantCHstars{filledcircles)andgiant star HD26{opencircle).Classicalbariumstars{plus © American Astronomical Society • Provided by the NASA Astrophysics Data System All -5.52+0.04-0.010.00 Ca i...... -5.05+0.06-0.04-0.080.00 Fei -5.35+0.05-0.01-0.040.00 Li i-6.13+0.09-0.010.00 1 Species log{W/X)AT=+100KA#+0.5A£kms“A[Fe/H]+0.1 eff Dependence ofDerivedAbundanceonModelAtmosphereParameters LITHIUM INLATE-TYPEGIANTS.IV. TABLE 5 -8 _1 evolving tobecomeacataclysmicvariable(Paczyñski1976; naries asthecentralobjectsofplanetarynebulae(Bond,Liller, and Mannery1978)supportthisscenario. Meyer andMeyer-Hofmeister1979).Discoveriesofclosebi- were chosenbecauseLidepletionfactors,relativetoitsmain- from Iben(1967a,b)arealsoshown;theseparticularmodels have spectroscopicallyderivedgravities.Theoreticalmodels expected resultforratesM>/t,whereisthemass- develops anextendedenveloperesemblingagiant.Thisisthe collapse oftheorbit(a“spin-down”)isfollowedbyejection common envelopeiscreatedaroundthetwostars,andrapid companion (seeIbenandTutukov1985).Iftransferoccursvia tracks for1.0,1.25,and1.5Mstars. sequence value,weregivenandareshownalongthemodel its Kelvin-Helmholtztime;thisconditionisM>3x10 transfer rate,Misthemain-sequencestar’smass,andt the envelopetocreateaplanetarynebulawithbinary Roche lobeoverflowatahighrate,itappearsmostlikelythat M yrfora1star.Suchtransferratesmightoccuras trmskh 0 tr mskh q0 At alowermass-transferrate,themain-sequencestar We beginbydiscussingmasstransferontoamain-sequence 229 198 6ApJ. . .303 . .226S -4 2 4 9 9 10 9 -1 -1 fer, thesmallerenvelopecollapsesandsecondary mixing betweenthetransferredmassandunderlyinglayers, peculiar composition.Thiscomposition,evenaftersome reassumes amain-sequenceappearance,albeitnowwith collapse backtothemainsequenceoccursquickly(ona or anejectionoftheenvelope.Oncessationmasstrans- the staraccretesmasslostfromAGBthroughawind 230 seems likelytoresemblethatofthesubgiantCHstars.The (1985) radialvelocitysurveysuggestsstronglythatthestars may accountforthesubgiantCHstars. (MacConnell, Frye,andUpgren1972).However,thisscenario giant Bastars.Withthecollapsespanningjustafraction(10) may mixtheouterlayersandsoslightlyreduceabundance giants, whereastheobservedfrequencyisabout1among10 quency ofBastarsis,atmost,1amongevery10normal of thenuclearlifetimeanormalgiant,predictedfre- remark, thisscenariocannotaccountforthemajorityof Kelvin-Helmholtz timescale).AsIbenandTutukov(1985) sequence companionissimplycoatedwithmassfromtheAGB ignores interstellarextinction,which,foracolorexcessofas dwarf companionswithauniformdistributionofagesover dwarf cancooltothistemperatureinabout10yr(Sweeney have asurfacetemperaturecoolerthan12,000K.Awhite subgiants. Heconcludesthatthewhitedwarf,ifpresent,must ultraviolet excessattributabletoawhitedwarfinsome21CH the InternationalUltravioletExplorer(IUE)satelliteforan companion. Bond(1984)describesanunsuccessfulsearchwith belong tolong-periodbinarysystemswithalow-luminosity products ofmasstransferacrossabinary.Certainly,McClure’s anomalies. Again,thestarwouldbeclassifiedasasubgiantCH star. Thethinconvectiveenvelopeofthemain-sequencestar mechanism, thentheBa/CHstarphenomenashouldbe seen little asafewhundredthsofmagnitudein(B—F),couldhave CH subgiants,asagroup,areolderthan5x10yrandcloser white dwarfcompanions.However,onecouldarguethatthe 20% ofthesample,oraboutfourstars,shouldhavedetectable star. discernible atF2V.Bond’s(1974)discoverypaperofthe CH range ofspectraltypesseenforthesubgiantCHstars.Ifmass a significant(>50%)effectontheultravioletfluxnear1500Â. statistically significantevent.Inaddition,Bond’sargument only twodetections.Thelackofdetectionswouldnotbea to 10yr,inwhichcasetheaboveargumentwouldpredict CH-like stars.InAbt,Morgan,andTapscott(1968),itisnoted dispersion classificationwouldnecessarilydetectthehotter all alongthemainsequence.Itisnotclear,however,that low- transfer fromanAGBstaronto“innocentbystander”is the sis, aspointedoutbyBond(1984),concernstherathernarrow 5 x10yr,andifawhitedwarfisdetectableforsome and earliertypesofmain-sequence examplesoftheBa/CH and someofthesubgiantCH starsinthisstudyfallquiteclose sequence spectraltypeofF2 corresponds toroughly1.5M, subgiants wasbaseduponspectra of108Àmm.Amain- that, attheirdispersionof125Âmm,theGbandis first stars probablyrequirescareful, high-resolutionstudiesofa to 1.5Mevolutionarytracks (seeFig.2).ThedetectionofF ence, orlackthereof,ofdegeneratecompanions.Arigorous IUE doesnothavethesensitivitytotestthoroughlypres- analysis mustawaittheSpaceTelescope. 1976). BondpointsoutthatiftheCHsubgiantshavewhite 0 0 At lowmass-transferrates,Mmass transfer by Roche lobe overflow seems likely to lead to a common envelope and finally to a close binary containing a pair of white dwarfs. Lower rates of mass transfer will lead to pol- lution of the giant’s convective envelope. Large amounts of mass (Mtr) must be transferred to create the extreme Ba stars with carbon and s-process enhancements close to those esti- mated for the carbon AGB stars, say Mtr ~ MCE ~ 0.5 M*. We speculate that the giant with its large cross section captures substantially more material than a main-sequence companion would. The latter does lack a deep convective envelope, and so little mass is required to produce marked abundance anom- alies ; on the other hand, the hot wind off the main-sequence star may inhibit accretion. The observed correlation between the carbon and s-process abundances of mild and classical Ba stars (Lambert 1985) follows if the stars capture increasing amounts of material from the AGB star. Moreover, the composition of the extreme Ba stars is very similar to that of carbon AGB stars, although the Li abundances in Ba stars appear to be below those (uncertain) values reported for carbon stars. Another concern is the fact that the lifetimes of giants (~ 108-109 yr) are shorter than the cooling time for a white dwarf. Hence, white dwarf companions ought to be seen in all cases. Dominy and Lambert (1983) reported on Ba stars for which the white dwarf, if present, was cooler than about 7000 K. This limit corresponds to about -1.0 0 1.0 2 x 109 yr and would seem to require that the Ba star had Log €(U) formed as a main-sequence star. Perhaps the addition of a substantial amount of material lengthens the giant’s lifetime. If Fig. 3.—Histograms of the derived Li abundances in subgiant CH, “evolved” subgiant CH, classical barium, and mild barium stars. Cross- the mass transfer leaves material in and around the binary, hatching indicates upper limits. “Evolved” subgiant CH stars are the subgiant local obscuration may result in an overestimate of the observa- abundances diluted by a factor of 45 (see text). ble white dwarfs luminosity. Significantly, Ba stars do possess an infrared excess (Catchpole and Feast 1977), which may well the key criterion, the CH stars could be the progenitors of be the signature of circum-binary dust. some of the mild Ba stars—again the Li abundances are pro- One clear conclusion is provided by this paper : the subgiant vided by Pinsonneault, Sneden, and Smith (1984). Such an CH stars are not the progenitors of the mild and classical Ba evolutionary connection is broken by the fact that some mild stars. This follows from the comparison of the Li abundances Ba stars are accompanied by hot white dwarfs (Böhm-Vitense, and the presence of hot, observable white dwarf companions Nemec, and Proffitt 1984; Böhm-Vitense and Johnson 1985). with some of the giants. The true descendants of the subgiant Böhm-Vitense et al. note that all mild Ba stars may have a hot CH stars may be giants with very small s-process enhance- white dwarf companion. If true, mild Ba stars join the classical ments, i.e., a subset of the mild Ba stars or giants with even Ba stars in having no evolutionary tie to the CH stars. smaller abundance anomalies. If the mass transferred to the subgiant CH star is confined to a thin layer, abundance anomalies will be reduced greatly by We thank Dr. David Dearborn for helpful conversation and the giant’s convective envelope which encompasses about half correspondence, Dr. J. Tomkin for assistance at the McDonald the stellar mass. Highly precise abundance analyses of a Observatory, and Dr. A. C. Danks for assistance in obtaining sample of giants might reveal such stars. and reducing the spectrum of HD 176021. This research has To complete discussion of mass transfer, we comment on the been supported in part by the National Science Foundation speculation that the classical barium stars may result from (grant AST 83-16635) and the Robert A. Welch Foundation.

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