The Binary Nature of the Subgiant CH Stars

Repeated radial velocities have been measured for a sample of 10 subgiant CH (sgCH) stars over a period of about 15 years. Long-term velocity variations are exhibited by all but one star, and spectroscopic orbits have been calculated for six of them. The p

TheBinaryNatureoftheSubgiantCHStars

RobertD.McClure

DominionAstrophysicalObservatory,HerzbergInstituteofAstrophysics,

NationalResearchCouncil,Canada,5071W.SaanichRoad,Victoria,BC,V0S1N0

email:mcclure@dao.nrc.ca

ABSTRACT

Repeatedradialvelocitieshavebeenmeasuredforasampleof10subgiantCH(sgCH)stars

overaperiodof～15years.Long-termvelocityvariationsareexhibitedbyallbutonestar,andspectroscopicorbitshavebeencalculatedforsixofthem.Theperiodsarelong,rangingfrom876to4144days.ThedistributionofeccentricitiesandmassfunctionsforsgCHstarorbitsaresimilartothoseforgiantbariumandCHstars,exhibitingsigni cantorbitaldissipationrelativetonormallate-typebinaries.ItisconcludedthatallsgCHstarsarebinaries,andlikethebariumandCHstars,theyhavehadmasstransferredfromaformerasymptoticgiant-branchstar.

arXiv:astro-ph/9702034v1 3 Feb 19971.IntroductionFromexaminationofobjectiveprismspectra,Bond(1974)discoveredaclassofG-typestarswithenhancedfeaturesofCHandsprocesselementsintheirspectra,andwithabsolutemagnitudesplacingthemnearandabovethemainsequence.Withrelativelyhighvelocitiesandweakmetallines,theyappearedtobePopulationIIobjectsandtheywerereferredassubgiantCH(sgCH)stars,becausehesuggestedthattheywouldeventuallybecomeclassicalCHstarswhentheyevolvedawayfromthemainsequenceandupthegiantbranch.McClure(1984a,1985)hasreviewedstatusofthebarium,CHandrelatedstars,includingthesgCHstars.AbundanceanalyseshavebeendonebySnedenandBond(1976),LuckandBond(1982),Sneden(1983),KrishnaswamyandSneden(1985),SmithandLambert(1986),LuckandBond(1991),andSmithColemanandLambert(1993).TheseanalysesofalargersampleofstarsindicatedthatmostsgCHstarsaremoderatelymetalde cientwith[Fe/H]= 0.1to 0.8,andwithsprocessenhancementssimilartothebariumstars.TheclassicalCHstarshavehigheroverabundancesoftheheavysprocess

elements(BathroughSm),andC2bands(indicatingC/O>1)whicharelackinginthesgCHstars.ThevelocitiesofmostofthesgCHstarsarealsomoderate,beingmoreliketheolddiskthanthehalo.Theessentialconclusionofthesepapersisthatdespitethenamewhichwasoriginallyassignedtothem,thesgCHstarsareprobablytheprogenitorsofmoderatelymetal-de cientbariumstarsratherthanthehaloCHstarsthemselves.AlowLiabundanceinthesgCHstarswasthoughttobeaproblemwiththeirroleasprogenitorsofthebariumorCHstarsaspointedoutbySmithandLambert(1986),butLambert,SmithandHeath(1993)havedismissedthisasbeingduetocontaminationoftheLilinebyanunidenti edline.

TheterminologyforthesgCHstarshasbecomeconfusingasaresultofthe ndingsofonlymoderatemetal-de ciency.Theytendtobereferredtointheliteratureas“dwarfbariumstars”(e.g.JorissenandBo n1992,North,JorissenandMayor1996),andoverlapwithstarsreferredtoas“FStrλ4077”stars(NorthandDuquennoy1991).Hanetal.(1995)referto“pre-Ba/CH”starswhichundoubtedlyincludesthesgCHstars.

RegardlessoftheirroleasprogenitorsofthebariumstarsoroftheCHstars,thefrequencyofbinariesamongthesgCHstarsisofverysigni cantinterest.BoththebariumandCHstarshavebeenshownto

Repeated radial velocities have been measured for a sample of 10 subgiant CH (sgCH) stars over a period of about 15 years. Long-term velocity variations are exhibited by all but one star, and spectroscopic orbits have been calculated for six of them. The p

belongperiodbinaries,andtheirpeculiarabundancesarelikelytheresultofmasstransferfromaformerasymptoticgiant-branch(AGB)starwhoseatmospherehasbeencontaminatedfromproductsofheliumshell- ashing(McClure,FletcherandNemec1980;McClure1983;McClure1984b;JorissenandMayor1988;Webbink1986;McClureandWoodsworth1990).SmithandDemarque(1980) rstsuggestedthatthesgCHstarsmaybemass-transferbinariessincetheyfounditimpossibletoexplainthembyinternalmixingofheliumcore- ashmaterial.Aprogramwasbegunbythepresentauthor,therefore,intheearly1980’s,tomonitorradialvelocitiesofsgCHstars.Someverypreliminaryresultsfromthisprogramhavebeenmentionedintheliterature(McClure1985,1989)indicatingahighbinaryfrequency,andLuckandBond(1991)alsoobservedseveralsgCHstarswhichappearedtoexhibitvariablevelocity.Asaresult,mostauthorshaveassumedoverthelastfewyearsthatallsgCHstarsarebinaries.Theevidencefromthesepreliminarydata,however,issomewhatmeager.Thepresentpaperpresentsmorecompletevelocitydataforthissampleofstars,andsetsthebinaryfrequencyforthesgCHstarsona rmbasis.ResultsofsimilarvelocitymonitoringofsgCHstarshasrecentlybeenmentionedinaconferencepaperpresentedbyNorth,Jorissen,andMayor(1996).

2.RadialVelocityObservations

Theradial-velocityobservationsweremadeattheDominionAstrophysicalObservatoryinVictoria,usingtheradial-velocityspectrometeratthecoud´efocusofthe1.2metertelescope.SeeFletcheretal.(1982)andMcClureetal.(1985)foradescriptionoftheinstrument.Itiscapableofaprecisionofabout±0.3kms 1,althoughwiththelimitedintegrationtimeforeachobservation,theobservationalerrorforthesgCHstarsisclosertodoublethatvalue.ThestellarsamplewastakenfromBond(1974),supplementedbyafewstarsfromBond(privatecommunication)thatwereanalyzedlaterbyLuckandBond(1991).Oneofthese,HD122202,isrepresentativeofanearlierclassofmidF-typestarswithstrongSr,discussedbyBidelman(1981),andmorerecentlybyNorthandDuquennoy(1991).TheywerereferredtobytheseauthorsasFstrλ4077stars,buttheyarealmostcertainlyjustearlierexamplesofG-typesgCHstars.

FormostofthestarsinthissampleLuckandBond(1991)listradialvelocitiesmeasuredfromthesamecoud´eplatesfromwhichtheyanalyzedabundances.Thesevelocitiesareofcomparableprecisionandcanbecombinedwiththepresentdataafterasmallsystematicvelocityshiftof 0.8kms 1(seebelow).Theradial-velocitiesandJuliandates( 2400000days)foreachsgCHstararelistedinTable1.ThosefewentrieswithJuliandatesearlierthan2444000andmarkedwithanasteriskarefromLuckandBond(1991).

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