198 6ApJ. . .309. .7 32J 3 The AstrophysicalJournal,309:732-736,1986October15 kf 1986.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. (AGB) tothewhitedwarfphaseisstillnotwellunderstood(see, for example,IbenandRenzini1983).OnediagnosticofAGB stars isthatintheirverylatestages,theyundergoextensive for example,Olofsson1985).WiththeIRASsurvey,itispos- that aremostprofitablystudiedintheradioandinfrared(see, mass losswhichresultsinsubstantialcircumstellarenvelopes fluxes toconstraintheirevolutionaryhistories. circumstellar material.IthaslongbeenthoughtthatRVTau have justevolvedoutoftheAGBphaseandstillretainsome therefore identifybothcandidateAGBstarsandthat sible toperformsystematicstudiesofcircumstellardustand proposed todescribetheirevolution(seeKwok1982).Very stars arehighlyevolvedobjects,andhereweusetheIRAS nebula stars(Zuckermanetal1976),andmodelshavebeen have previouslybeenidentifiedaspost-AGBpreplanetary material havealsobeenidentifiedsuchasNGC7027(seeJura AGB objectsbystudyingthepropertiesofRVTaustars. young planetarynebulaewithsomeresidualmolecular discuss theevolutionarystatusofthesestarsandin§V, we develop someinsightintothestars’histories.In§IV we we presentdetailedmodelsforthecircumstellarenvelopes to Gingold andEggen(1986)onindependentgrounds.In§ III, present ourconclusions. the RVTaustarsareprobablypost-AGBobjects,ashave G, andK.Fromtheiroptical classification,theyestimatethat posed that sistent withthisresult,Barnes andDupuy(1975)havepro- RV Taustars;theyareluminouspulsatorsofspectralclass F, 19846). Here,weaimtoincreaseourunderstandingofpost- M ^—3magwhichindicates aluminosityof~10L©.Con- where Pistheperiodindays. Withatypicalperiodof75days v The evolutionofstarsfromtheasymptoticgiantbranch Some stars,suchastheEggnebula(CRL2688)orCRL618 The structureofthispaperisasfollows.In§II,wearguethat Preston etal(1963)havedescribedthebasicproperties of -51 4-31 from about10Myr,dependinguponthemetallicityofstarsandgrainemissivityat60/mi.It with theviewthatwearewitnessingsubsetofstarsundergoingpost-AGBevolutionlowmass seems likelythatthesestarshavejustevolvedfromthephaseofrapidmassloss,characteristiclast that themass-lossratesfromRVTaustarshaveapparentlysignificantlydecreasedduringpast~500yr and atleastinsomecasesoflowmetallicity.MostRVTaustarswillprobablybecomeplanetarynebulae; roughly 6x10"kpcyr,aboutatenthofthebirthrateallplanetarynebulae,andthisisconsistent stages oftheasymptoticgiantbranch(AGB).ThebirthrateRVTaustarsinsolarneighborhoodisvery Subject headings:infrared:sources—stars:evolutionmasslossRVTauri others, however,mayevolvesufficientlyslowlythattheirenvelopeswilldissipatebeforebeingphotoionized. 0 © American Astronomical Society • Provided by the NASA Astrophysics Data System RV Taustarsarerare,luminouspulsatorsofspectraltypesF,G,andK.AnalysistheIRASdatashows II. RVTAURISTARSASPOST-AGBOBJECTS M ==-5.3+0.021P, (1) v I. INTRODUCTION RV TAURISTARSASPOST-ASYMPTOTIGGIANTBRANCHOBJECTS stars :stellarstatistics Department ofAstronomy,UniversityCalifornia,LosAngeles Received 1986February5;acceptedApril8 ABSTRACT M.Jura 732 3 1 3 q most ofthedustisnotbeingcurrentlyejected. effective temperaturesandluminositiesofthelessmassive luminosity of~10L.Thesestarsthereforeseemtohavethe stars quiteoftenhavecircumstellardustbecausetheydisplay berner 1983). post-AGB stars(Gingold1974;IbenandRenzini1983;Schon- s~ forRVTau(FixandClaussen1984)—isconsistentwitha velocity inferredfromobservationsofOH—ofabout10km molecular outflowvelocityandluminosityforOH/IRstars and asmallbolometriccorrection,then,fromequation(1), significant amountsofinfraredradiation.Here,wesuggestthat (Jones, Hyland,andGatley1983;Jura1984a),theoutflow again, L~10.Finally,fromthecorrelationbetween contributes tothe100gmbandwhilewarmerdust known propertiesofcircumstellardustshells(see,forexample, long andshortwavelengthemission,itispossibletoestimate 0 mass-loss rateisconstantorchangingwithtime. the densityofmaterialthatisnearbytostarcompared to theemissionat12jam.Bycomparingrelativeamountsof Sopka etal1985).IntheIRASbandpasses,cooldustmainly that whichismoredistant,andthusdeterminewhether the 0 detected byIRASat60jam,thewavelengththatisprobably et al(1969),wehaveidentified17RVTaustarswhichwere the existenceofcircumstellarmaterial. study thetimehistoryofmassloss—hecouldonlyestablish the groundoutto20jam,hehadinsufficientinformation to The detectedstarsandtheIRASfluxesarelistedinTable 1; most suitableforestimatingthedustlossrate(seeJura1986). are detectedinallfourIRAS bands,welistqforthespectral and welistthederivedvalues ofqinTable1.Forstarswhich a powerlawtotheobservedfluxessuchthatFvariesas v, this listincludesabout20%ofallknownRVTaustars.We fit region between12jimand100 /xm;otherwisewelistqforthe average valueofqis1.54,while thepower-lawfitstooxygen- range between12jamand60jim. v Gehrz (1972)andLloydEvans(1985)foundthatRVTau The basicargumentofthispapercomesfromusingthewell- Because Gehrz(1972)couldonlyperformobservationsfrom Jura (1986)foundthatfor mass-losing carbonstars,the From thecomprehensivevariablestarcatalogofKukarkin 198 6ApJ. . .309. .7 32J +1 n(35-2) No. 2,1986 maximum valueofqisonly1.41whiletheaverage0.98. rich starsare,ontheaverage,evensteeper(Hackingetal Therefore, theRVTaustarshavesignificantlyflatterinfrared spectra thandomostmass-losingredgiants. density distributionofdustgrainsandtheevmissivity infrared colorsofRVTaustarsmustresultfromanunusual drastically differentfromanyothercircumstellardustand grains asafunctionofwavelength.Unlessthearound unlike anysubstanceusuallyproposedforgrains,theunusual RV Taustarsarecomposedofanunknownmaterialwhichis 1985). Incontrast,fortheRVTaustarsshowninTable1, son, andGatley(1979),iftheemissivityvariesasv can naturallybeidentifiedastheasymptoticgiantbranch.This the mass-lossratewasgreaterinpastthanitiscurrently. q =0.98,weexpectthatthedensityvariesasr“^implying density variesasr~,thenFshouldvaryv-’".Since density distributionofdustgrainsandtheemissivity stars havejustleftsomephaseofmorerapidmasslosswhich view isconsistentwiththeirluminosities,temperatures,and, as consider ACHer,whichisoneofthebrightestRVTau stars described below,theirstatisticalpropertiesintheMilkyWay. (1985), itseemsthat and iscarbon-rich(Baird1981;Cardelli1985)sothatwe can Gehrz (1972),Prestonetal(1963),andBairdCardelli stars. apply theanalysisofJura(1986)formasslossfromcarbon In equation(2),Disthedistance tothestarinkpc.Since amount ofinterstellarreddening isuncertain,weconsiderboth v kpc The fluxdistributionintheinfrareddependsupon A reasonableinterpretationofallthesedataisthatRVTau As anexampleofobjectatime-varyingmassloss,we First, fromtime-averagingtheobservedfluxesgivenby (1969) listitasanMstarwellRVTauvariable.Weassume is anunusualMira. 60 /imbecauseitsperiodislongerthan200daysandKukarkinetal. TW Cam. GK Car.. IW Car... UY CMa. SX Cen... RU Cen.. U Mon... AC Her.. SU Gem. AI Seo... AR Pup.. DYOri .. CT Ori... V Vul.... RV Tau.. R Set R Sge III. DETAILEDMODELSFORCIRCUMSTELLARENVELOPES Note.—We excludeBICepeventhoughitwasdetectedbyIRASat Star © American Astronomical Society • Provided by the NASA Astrophysics Data System RV TauriStarsDetectedat60/¿mbyIRÁS 32 32 L =2.1x10D,A =l. (2b) L =1.1xÍ0Z),A =0(2a) kpc0v kpc0v 12 fim2560/¿m100/mi 101 124 131 41 21 22.5 12 18 10.6 12 2.9 8.3 6.0 3.5 7.9 5.4 6.2 a) ACHer TABLE 1 96 94 65 88 11 15 11 18 2.5 2.5 5.6 9.3 5.7 3.6 7.6 5.6 5.7 26 21 34 26 0.78 0.57 4.1 2.2 2.1 5.6 2.9 6.4 8.1 1.8 1.1 1.2 1.3 <139 <12 <12 <11 <46 <1.0 <1.7 <1.5 <1.7 <1.5 <7.0 13 12 9.2 2.0 7.8 2.5 RV TAURISTARS 0.47 0.97 0.82 0.95 0.78 0.79 0.67 0.59 1.13 1.05 1.02 1.23 1.04 1.13 1.01 1.13 1.38 1 s2-1 + 1 15 can computethetemperatureofcarbongrainsat,say,1"from below). values forthisquantity(seeBairdandCardelli1985 A =0andAy—1maginordertobrackettheproposed the startogive In equation(4),vistheoutflowvelocityinunitsof10kms~ carbon-rich redgiants(KnappandMorris1985;Zuckerman and Xóoitheopacityofdust(cmg)at60¿un.We for RVTauwhereOHhasbeendetected(FixandClaussen and Dyck1986;Zuckerman,Dyck,Claussen1986), assume thati^o=1asseemsappropriateformass-losing for simplicity,amodelwithconstantmass-lossrate would derivefromequation(3)ofJura(1986)andtheobserved IRAS fluxat60fim material, then loss stops.Ifwefurtherassumeasimplefreeexpansionofthe time, until,thestarabruptlyevolvesoffAGBandmass of aconstantmass-lossratewithtimeisnotcorrect.Assume, emission atfrequencyvisgivenby loss stopped. We definer=vtwheretisthetimeelapsedsincemass v o512x 1984). 10 where Tisthegraintemperatureatr=.Aftersomemanip- ulation, wefindthat v ,then Since weassumethattheemissivityofgrains,Xvvariesas In equation(8),/i,c,andkaretheusualconstantsx= hv/ 9 kT. Wedefinex=atr. 0 is afiniteradiustotheinnershellofdustdistribution, then 17 1 we findadifferentspectralvariation ofLsincexisafunction tion (8),Lvariesasvdescribedabove.If,however,there malized tothefluxat25pm. Though noneofthemodelsgives inner radiusofthecircumstellar envelope.Allresultsarenor- of v. the JR^S-measuredcolorsof ACHerfordifferentvaluesofthe 0 0 v0 v L =20nv-xMJ(hcv)rkTÍ'(e1)~ dx . +1 M =4.0x10z;D(150/o)gs“,^11.(4b) M =5.9xlO^v.oD^lSO/xeo)gs“,A0(4a) vduo dust1()kpcZ6F dustv Following themethoddescribedbySopkaetal(1985),we If weassumeaconstantmass-lossratefromthisstar, Because oftherelativelylowfluxat12¿mi,simplemodel Following Sopkaetal(1985),wemaywritethattheinfrared Note thatifx=0andvariesasv,thenfromequa- In Figure1weshowtheresults ofcalculationstoreproduce 0v 2 L =(47t)i(xMJ4nrv)B(T)4nrdr.(6) vdu 2 p =0r.(5b) 0 àu0 T(l") =123K,A0(3a) T(l") =144K,Al.(3b) v v Jro 0A T =(r/r),(7) 0 Jxo 733 (8) 198 6ApJ. . .309. .7 32J 17_ 1 15 21 21- 734 the dustshellof300K. x(12 fim);allpointsarenormalizedat25pm.Althoughnotperfect,thebestfitisforpm)=4,whichcorrespondstoatemperature inner boundaryof perfect agreementwiththeobservations,modelx(12 making themorerealisticassumptionofasuddencessation lar shellof300K.Fromequation(3)andanassumeddistance is notverysensitivetoA. losing phaseofthisstar7x10gs(D=1);result mass loss[eq.(5)].Weinferadust-lossrateduringthemass- with theassumptionofaconstantmass-lossrate[eq.(3)]is cm fromthestar(seebelow),andmass-lossratederived km s"ofabout150years. shell, r,is5x10cm.Thisrequiresanexpansiontimeat to agraintemperaureattheinnerboundaryofcircumstel- model usedtoreproducetheIRAScolors.Wewrite lower byonlyafactorofabout1.3fromthemass-lossrate of 1kpc,wefindthattheradiusboundaryinner from theresultsinJura(1986),thenwithparameters /xm) =4matchesthedatareasonablywell.Thiscorresponds from opticalstudies,which,however,iscontaminatedbyinter- derived above,wefindthatt=0.23whichcorresponds to If wetake%=18,000cmg“atFasasimpleextrapolation stellar extinctiontowardACHer.Inthegeneraldirection of lower boundsincetheremay bemolecularandionizedgasin However ourvaluesofthe interstellar reddeningisonlya that theinterstellarcontribution toE(B—V)is0.15±0.1. seems thatE(B—V)>0.25mag (Bohlin,Savage,andDrake 21 cmdatais1.5x10(Heiles1975),andtherefore it this star,thecolumndensityofatomichydrogenderivedfrom A =025. the lineofsightaswell observedatomichydrogen.In 0 0 v kpc 0 1978), consistentwiththeestimate byBairdandCardelli(1985) v 16 Fig. 1.—PlotofIRASobservationsvs.simplemodelsforthecircumstellaremissionfromACHer.Thedifferentsetspointscorrespondto valuesof The circumstellarextinction,t,canbeestimatedfromthe Most oftheemissionat60/unisproducedbydust~10 We cannowcomparethisresultwiththereddeningderived © American Astronomical Society • Provided by the NASA Astrophysics Data System ? =xM/(47rrp).(9) dust JURA -51 4 0.44 ±0.05(BairdandCardelli1985),itseemsthatthecontri- any case,sincethetotalcolorexcesstowardthisstaris bution tothereddeningbycircumstellarmatteris the formulaforCOfluxgivenbyKnappandMorris(1985), J =1-0rotationallinegivenbyZuckermanetal.(1976) and loss ratefromACHeris2xl0Myr.Thismass-loss is then4.5x10",anorderofmagnitudesmallerthanthe as inbipolarnebulae(seeMorris1981). Given alltheuncertainties,thereisnocompellingreasonto extinction of0.25derivedfromanalysistheIRASdata. reasonable agreementwithourvalueofthecircumstellar circumstellar extinctionislessthanorequalto0.57±0.15,in E(B— F)<0.19±0.05whichimplies,forR=3,thatthetotal scaled toanappropriatelylowerabundanceofCO. rate isconsistentwiththeupperlimitofCOemissionin the typical valueforcarbonstars(Jura1986),thenthetotalmass- think thatthemasslossisdrasticallynonsphericallysymmetric 1981 ;Cardelli1985)andifweassumethatthedust-to-gasratio Jura (1986).Weusethissimplemodel,because,asshown assume thatthefluxfromthese starsissimplygivenbythe able, andbecausethebolometric correctionsaresmall,we mass-loss process. emitting materialareinsensitivetothedetailedhistoryof the model forcircumstellaremissionbySopkaetal.(1985) and Tau stars,andinTable2,welistourderivedratesbasedupon stars forwhichthedataareavailable, weassumethevaluesof time-averaged valueofvFat F,theusualvisualfilter.For above forACHer,ourderivedmass-lossratesthe60 pm o the IRASobservationsof60pmfluxandsimplified v We arealsointerestedinthemass-lossratesfromotherRV If weassumeametallicityof0.1thesolarvalue(Baird Because detailedphotometry atallwavelengthsisnotavail- b) SimplifiedMass-LossEstimates Vol. 309 198 6ApJ. . .309. .7 32J _1 are not + 1,5 2_ 1 1 51 17- 1 16 No. 2,1986 we assumeA=1mag.Wetaketheabsoluteluminosityand luminosities. insensitive tothismodelwheretheemissivityofgrains is straightforwardtoshowthatthederivedmass-lossratesare analysis byJura(1986)formasslossfromcarbon-richstars.It although someofthesestarsareoxygenrich,weusethe assume anoutflowvelocityofthegas10kms.Finally, E(B—V) givenbyDawson(1979)andthatA=3E(B—V). Mdust especiallysensitivetothedistancesandabsolute rates scaleonlylinearlywiththedistancesothatourvaluesof b =2?7thatis10kpcfromtheSun.Thederivedmass-loss therefore derivedistancesfromequation(1),andinallcases,we amount ofextinctionseemstobemuchtoosmallforastarat derived distanceforGKCarisprobablytoolargebecausethe rates scaleinverselyas%,theemissivityat60fim.Herewe varies asv,insteadof,forexample,.Allthemass-loss For thetwostarswhereDawsondoesnotestimateE(B—V), metallicity whichisoftenuncertainforthesestars. and inagreementwiththosegivenbyDawson.However,our which 0.5ofallthe60/anradiationisemitted(seeSopkaet al. Her, thenthetypicaltotalmass-lossratefromthesestars is the metallicityofallthesestarsisrelativelylow(Wahlgren assume thatXeo=150cmg(seeJura1986). is 10kms“,thegrainsthat emitmostoftheradiationat60 correlated withPasexpected fromeq.(1).]Iftheoutflowspeed luminosity increasesforthesestars,thevalueofrisanti- emitting regionsandbecausetheperioddecreasesas the about 10“Myr“.Thisratedependsinverselyupon the scatter butaretypicallynear3x10gs.Ifweassume that the star.Inthiscase,current massloss,whichpresumably envelope variesasr“,where risthedistancemeasuredfrom /an wereejectedfromthestar about 500yrago. [Note thatstarswithlargerluminositieshave60 /an v 1985) andthatthedust-to-gasratioissameasof AC v 1985). Acharacteristicvalueofthisquantityis1.5x10 cm. 60 1/2 0 The distancesthatwederiveinTable2aremostlyplausible The mass-lossratesinTable2showsomerelativelysmall In Table2,wealsolistr(60/un),thecomputedradius in Above, wesuggestedthatthe densityinthecircumstellar 1/2 © American Astronomical Society • Provided by the NASA Astrophysics Data System extrapolated byassumingthatB—V=1mag. V Vul.... AI Seo... RV Tau.. R Sge AR Pup.. DYOri .. R Set..... CT Ori... U Mon... SU Gem. GK Car.. TW Cam. SX Cen... RU Cen.. IW Car... UY CMa. a b EstimatedasthemeanofphotographicmagnitudesgiveninKukarkinetal.(1969)and Note.—Magnitudes andextinctionsarefromDawson(1979)exceptforthefootnotedentries. Assumedvalue. Star a a a (mag) a a a 10.1 11.9 10.0 11.1 10.0 12.2 10.7 Sun. BecauseACHerisalsonearby,welloverhalfofall the photometric propertiesthatthecatalogofKukarkinetal. are sufficientlyluminousandhavedistinctive stars byKukarkinetal(1969)suchasAOCep(Antonelloand there aresomeobjectsinappropriatelyclassifiedasRVTau Mantegazza 1984)andZSex(Dawson1977),theRVTaustars d 250 630 With anumberdensityof~10 RVTaustarswithin2kpcof The durationoftheRVTauphaseevolutionisuncertain. While thedistancestothesestarsareuncertain,andwhile 7(1") 200 113 167 111 136 IV. EVOLUTIONARYSTATUSOFRVTAURISTARS K 98 94 96 92 89 77 77 87 73 94 59 18 17 17 17 17 17 17 17 17 16 18 18 17 18 Mdust gS“ 4.0 x10 6.6 x10 3.7 x10 4.1 x10 4.5 x10 2.2 x10 9.7 x10 2.4 x10 2.8 x10 1.7 x10 1.8 x10 1.3 x10 1.2 x10 1.5 x10 16 (10 cm) 0.9 0.9 2.3 2.6 1.7 1.6 1.8 1.8 1.7 1.6 1.5 1.9 1.8 1.9 1.2 1.6 735 198 6ApJ. . .309. .7 32J -31 4 .1986,Ap.J.,303,327. 736 .19846,Ap.J.,286,630. Jura, M.1984a,Ap.J.,282,200. Jones, T.J.,Hyland,A.R.,andGatley,1.1983,Ap.273,660. -.1979,Ap.J.Suppl.,41,97. (Cahn andWyatt1976).ThehypothesisthattheRVTaustars which planetarynebulaeforminthesolarneighborhood can becomparedwiththerateof4-6x10kpcyrat Knapp, G.R.,andMorris,M.1985,Ap.J.,292,640. Gehrz, R.D.1972,Ap.J.,178,715. (Preston etal1963;Stothers1964),thatatleastafewofthem Iben, I.,andRenzini,A.1983,Ann.Rev.Astr.Ap.,21,271. Heiles, C.1975,Astr.Ap.Suppl.,20,37. Gingold, R.A.1974,Ap.J.,193,177. Dawson, D.W.1917,Pub.A.S.P.,89,919. very wellknown)inviewoftheirluminosities(Barnesand amount ofmassthathasbeenlostduringtheirevolutionisnot are asubsetofallthenewlyformedplanetarynebulaeiscon- Harvey, P.M.,Thronson,H.A.,andGatley,1.1979,Ap.J.,231,115. Hacking, P.,etal.1985,Pub.A.S.P.,97,616. Fix, J.D.,andClaussen,M.1984,Ap.{Letters),287,L35. Eggen, O.J.1986,A.J.,91,890. Cardelli, J.A.1985,A.J.,90,1494. Cahn, J.H.,andWyatt,S.P.1976,Ap.J.,210,508. mass starstakemuchlongertoevolveacrosstheH-Rdiagram mass post-AGBstarsareparticularlyobservationallysalient have lowmetallicities(Baird1981;LuckCardelli1985), DuPuy 1975),theirmembershipinanintermediatepopulation sistent withtheavailablestatistics. Bohlin, R.C,Savage,B.D.,andDrake,J.F.1978,Ap.J.,224,132. Antonello, E.,andMantegazza,L.1984,Astr.Ap.,133,52. evolve totheblueregionofH-Rdiagraminorder Their rateofmasslosshasbeensuchthatduringthepast10 mass sensitive(seeIbenandRenzini1983).Becausethelower ently havetakentoevolvefromtheAGBbeingGstars(see and therelativelylongtime(~500yr)thatthesestarsappar- Barnes, S.R.,andDuPuy,D.L.1975,Ap.J.,200,364. Baird, S.R.,andCardelli,J.A.1985,Ap.J.,290,689. could besomestarsareso“lazy”thatthecircumstellarmatter produce adetectableplanetarynebula.However,major yr, theymayhaveejected0.1Mwhichwouldbesufficientto selection ofthelowermassobjects. after havingbeenontheAGB,thereisastrongobservational since thetimescaleforevolutionofpost-AGBstarsisvery Baird, S.R.1981,Ap.J.,245,208. become hotenoughtophotoionizethesurroundinggas.It uncertainty isthelengthoftimeittakesforthesestarsto Schonberner 1983).Itisnotsurprisingthattherelativelylow- M. Jura:AstronomyDepartment,UCLA,LosAngeles,CA 90024 0 It seemslikelythatRVTaustarswereinitiallyof1M(the The futureevolutionofRVTaustarsissomewhatuncertain. 0 © American Astronomical Society Provided bythe NASA Astrophysics Data System JURA -51 4-31 evolve backtothered-giantregionofH-Rdiagram. cannot excludethepossibilitythatsomeofthesestarswill and significantamountsofcircumstellarmaterial. dissipates beforeitisphotoionized(Renzini1981).Wealso Zuckerman, B.,Gilra,D.P.,Turner,B.E.,Morris,M.,andPalmer,P. 1976, by afactorof100,from~10Myr,duringthepast Zuckerman, B.,Dyck,H.M.,andClaussen,M.J.1986,Ap.J.,304,401. Zuckerman, B.,andDyck,H.M.1986,Ap.J.,304,394. than theycurrentlyare;probablyarejustpasttheasymp- stars appearstohavebeenejectedwhenthewerecooler Wahlgren, G.M.1985,Bull.AAS,17,875. totic giant-branchphaseofevolution. Stothers, R.1964,Pub.A.S.P.,76,98. Sopka, R.J.,Hildebrand,R.,Jaffe,D.T.,Gatley,I.,Roellig,Werner,M., Schonberner, D.1983,Ap.J.,272,708. in somecasesthelowmetallicityportionofthosestars roughly of6x10“kpcyrinthesolarneighborhood, Renzini, A.1981,inPhysicalProcessesRedGiants,ed.I.IbenandRenzini Olofsson, H.1985,inWorkshoponSubmillimeterAstronomy,ed.P.A.Shaver probably becomeplanetarynebulae.However,someofthese general. about 0.1oftherateformationplanetarynebulaein Preston, G.W.,Krzeminski,Smak,J.,andWilliams,J.A.1963,Ap.137, Morris, M.1981,Ap.J.,249,572. Luck, R.E.1981,Pub.A.S.P.,93,211. Kwok, S.1982,Ap.J.,258,280. Kukarkin, B.V.,etal.1969,GeneralCatalogueofVariableStars(Moscow: transition fromtheasymptoticgiantbranchtowhitedwarfs. McAlary, C.W.,andWelch,O.L.1986,A.J.,91,1209. Lloyd Evans,T.M.N.R.A.S.,217,493. that theyneverphotoionizethesurroundinggasand objects mayevolvetotheblueinH-Rdiagramsoslowly NSF ;IthankMarkMorrisforhiscomments. appear asplanetaries. Because oftheirpreviouslyhighmass-lossrates,manywill ~ 500yr.Mostofthecircumstellarmatterthatsurroundsthese 0 3 Ap. J.(Letters),205,LI5. Jura, M.,andZuckerman,B.1985,Ap.J.,294,242. (Dordrecht :Reidel),p.431. 401. and K.Kjar(Garching:EuropeanSouthernObservatory),p.535. Academy ofSciences). 2. Themass-lossrateshavedecreasedsignificantly,perhaps 3. ItseemsthatRVTaustarsareformedataratevery 4. TheRVTaustarsareprobablythelowmass,andatleast 1. RVTaustarshaveluminositiesofapproximately10L This workhasbeenpartlysupportedbyNASAandthe 0 V. DISCUSSIONANDCONCLUSION Vol. 309