1988ApJ. . .333. .2193 12 6 5 13 1613 6 (JVfcore ^0.6M)staraspointed outbyIben(1983).Thus,Tc is technetiumnotomnipresentinasampleofstarsenrichedto important aspectsofitsoccurrenceremaintobedefined.In by themixingofprotonsin the C-richHeshell:(p,y) might beexpectedtopresent instarsofvirtuallyallmasses very closetotheTP-AGBlifetimeof2x10yrfora1 M produced inexpectedquantitiesrelativetoitsfellows-process duration ofthethermalpulsing(TP)stage.Simple,LTE the s-processhasahalf-life(2x10yr)comparableto the tion ontheAGBbecauseisotope"Tcthatisproduced by thesis oftechnetium? same neutroncaptures-processthatisresponsibleforthe syn- similar levelsintheheavyelementsthatareproductsof the asymptoticgiantbranch(AGB)leadstoquestion,Why technetium isfoundinnomorethanhalfoftheseresidents technetium inasampleof40MSandSstars.Theresultthat are rarelyprovided.Inthispaper,wereportonasearchfor particular, quantitativeestimatesoftheelement’sabundance recent andperhapscurrentnucleosynthesisinredgiants, tion. Althoughtechnetiumistheoutstandingindicatorof the neutronsourceC(a, n)0 withtheCproduced during theTP-AGBphaseof evolution. 6-7 half-lives.Thistranslatesto~(1-1.5)x10yr,which is tent withtheobservations,itshouldbedetectableforabout nuclei andthenmixedintotheMSSstarsatlevelsconsis- using theresonanceTcilinesnear4250Â,showthatif is spectrum-synthesis calculationsbySmithandLambert(1986), observational inquiriesintostellarnucleosynthesisandevolu- of redgiantsisalandmarkinthenowextensiveliteratureon The AstrophysicalJournal,333:219-226,1988October1 0 0 © 1988.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. Technetium isanespeciallyimportanttracerofstellarevolu- In low-massstars,thes-process isprobablydrivenby Merrill’s (1952)discoveryoftechnetiumintheatmospheres 6 5 dredge-up and(2)Tc-poorstarswhichrepresentthecoolestmembersofbariumstarclass. (AGB) starshouldbevisibleinastar’satmospherefor~10yr.Thistimeiscomparabletothemaximum Subject headings:stars:abundances—Banbinarieslate-typeS-type two majorgroups:(1)Tc-containingstarswhicharecurrentlythermallypulsingAGBundergoingthird probably theresultofbinarymasstransferinpast.WesuggestthatMSandSstarsarecomposed relative tonormalGandKgiants,ofthebariumstars,whichalsolackTcwhoses-processenrichmentis Tc-poor MSandSstars,relativetoMgiants,iscomparable(withintheuncertainties)spacedensity, virtually alls-process-enrichedAGBstarsshouldshowTc.However,wearguethatthespacedensityofthese lifetime ofalow-mass,thermallypulsingAGBstar,andthusstandardstellarevolutionwouldsuggestthat surprising asTc(withahalf-lifeof2x10yr)produceddurings-processinginanasymptoticgiantbranch s-process-enriched starsdonotshowTc.ThislargefractionofTc-poor,issomewhat general s-processabundanceenhancements.Ofthetotalsample,wefindthat38%(22outof58)these provides Zri,Yn,LaandNdnlines.Theselinesallowustodeterminethatthesestarsdoindeedshow of TeI.Inaddition,previouslypublishedTcsurveysprovideanadditional18starsforatotalsample58 MS orSstars.Mostofthesestarshavealsobeenobservedinthenear-infrarednear27500,aregionwhich © American Astronomical Society • Provided by the NASA Astrophysics Data System High-resolution spectraat24260havebeenobtainedfor40starsoftypeMSorStosearchthepresence s-PROCESS-ENRICHED COOLSTARSWITHANDWITHOUTTECHNETIUM:CLUESTO I. INTRODUCTION ASYMPTOTIC GIANTBRANCHANDBINARYSTAREVOLUTION Department ofAstronomyandMcDonaldObservatory,UniversityTexas Verne V.SmithandDavidL.Lambert Received 1988January4;acceptedMarch23 ABSTRACT 8 22 13 8 2225 58 14ls222 14 13 13+13 We presentasurveyofsample ofstarsselectedprimarily been summarizedandnewdata presentedmostrecentlyby evidence forahots-processsite. ed atthehighertemperaturesbecauseneutronfluxes where "Tchasamoderatelylonghalf-life.Thisconclusion is spectrum (4238,4262,4297Â). Allobservationstodatehave strong resonancelinesofTc ilyinginthevioletpartof sence ofTcinstellaratmospheres, usingprimarilythethree absence ofTcinans-process-enrichedstarbetakenasstrong thermal pulses.Matthewsetal(1986)showthatTcisproduc- not supportedbydetailedstudiesofs-processingduring because thisisignitedatatemperature(T<1.5x1Ö K) and thatTc’spresenceinTP-AGBstarscouldbeunderstood thermometer fortheAGBs-processsitethanachronometer the Neneutronsource(CosnerandTruran1981;Schatz as anucleosyntheticclockwasrestatedrecently.Thehalf-lifeof Little, Little-Marenin,andHagen-Bauer (1987,hereafterLLB). offset thehigher/?-decayratesof"Tc.Nolongercan the only iftheneutronsweresuppliedbyC(a,n)reaction K) tojust5yratTæ3x10K,theexpectedtemperatureof core mass,i.e.,m>0.75M.Forlowermasses, expected tobethedominantneutronsourceforstarsoflarge expected tobethereactionNe(a,n)MgwithNepro- flash. InmoremassiveAGBstars,theneutronsourceis "Tc decreasesfrom2x10yratlowtemperatures(T< and thenN(a,y)O(a,Ne.Atpresent,Ne(a,n)is and Heburning;theCNOcyclesconvertto(mostly)N duced naturallyfromtheinitialCNOnucleiinpriorHburning 1983). SmithandWallerstein(1983)arguedthatTcwasmore a C(a, n)ispresumedtobetheneutronsource. N(/?, v)C.SynthesisofoccurspriortoanHeshell core0 e A numberofobservershaveconductedsearchesforthepre- An earlyconcern(Cameron1959)abouttheusefulnessofTc 1988ApJ. . .333. .2193 5 6 12 exhibit measurables-processenhancements([s/Fe]<+0.2 dwarf companions(Böhm-Vitense 1980;Böhm-Vitense, probably allbariumstarsare binariesandmayhavewhite including Tcmaybeexpected. Theremarkablediscoveryby the TPstagewhenfreshadditions ofs-processedmaterial s-process enrichmentsyetcontainnoTc.Suchcoolerbarium examples oftheGandKgiantbariumstars,whichalsoexhibit (Smith andLambert1986)foundthatfouroutofninestars ago thattheTchasdecayed. These starshavenotyetentered stars couldhaveacquiredtheirs-processenhancementssolong relative tosolar).(2)Somestarsmaybecooler(orevolved) classified asMSfromYamashita’s(1967)catalogdid not which toclaimadefinitives-processenhancement.Indeed, we may notbes-processenhanced;low-resolutionclassification conclusions andsuggestedthreealternatives:(1)Somestars modified asonlythestarsofverylowcoremass(M<0.6 McClure, Fletcher,andNemec (1980)andMcClure(1983)that spectra maynotbesufficienttoanadequatebasisfrom M ;<1)existasTP-AGBstarsforlongerthan the suggestionsbyScaloandMiller(1981)wouldhavetobe the ScaloandMiller(1981)estimate.Withthislongerestimate, thetic spectraandmodernalimitingageofabouttwice or approximately7x10yr.Asmentionedearlier,Smithand the initialabundance;thiscorrespondstoabout3.3half-lives, how longtheTcilinesareconsideredtoremainobservable. never mix.ScaloandMiller’s(1981)explanationsdependon Lambert (1986)estimatedfromacomparisonsampleofsyn- Scalo andMilleradoptedalimitingTcabundanceof0.1times 1.5 x10yr. ily untilitisreplenishedbyaflashinwhichmixingoccurs.(2) without mixingintotheenvelope,surfaceTcdecaysstead- involve radicalchangesintheacceptedpictureofAGBevolu- not occurateveryshellflash.Ifmanyconsecutiveflashes tion: (1)Themixingofs-processedmaterialtothesurfacedoes Tc insome30%ofthesestarsledScaloandMiller(1981)to (Boesgaard 1970;SmithandLambert1985,1986;Utsumi core suggest twopossibleexplanations,bothofwhichwould earlier surveyandcompilationofTcobservationsbyLittle- pointed outthatof30starstypeMS,S,SC,andCinan is atoddswiththeobservations.ScaloandMiller(1981)first exceeded theinitialabundancesinstar’satmosphere. Some starsaboveacertaincoremass(sayM>0.8) spectral typesexhibit,ingeneral,s-processandCenrichment before theamountofdredged-up,stables-processelementshas probably representtheearliestphasesofTP-AGBdredge-up measurably. Suchstarsarenotincludedinoursurvey,but found noneoftheothers-processelementstobeenhanced q0 indications ofs-processdredge-up.Inanabundanceanalysis enhancement. ManyofthepureM-typeMirasobservedby indicates anabundanceenhancementofZr(aswellasother Marenin andLittle(1979),ninestarsdidnotcontainTc.These s-process elements)asdemonstratedbyBoesgaard(1970)and classified aspureMspectraltype,oursamplecontainsonly to isolateasampleofstarswhichshowevidences-process from thecatalogofSstarsbyStephenson(1984).Whereas Mira (oGet),DominyandWallerstein(1986)detectedTcbut LLB doshowTciintheirspectra,yetseemtoexhibitnoother Smith andLambert(1985,1986).Thepurposeofthisstudywas stars classifiedgenerallyonthestrengthofZrO,whichusually survey byLLBcontainsalargenumberofMira-typevariables 220 SMITHANDLAMBERTVol.333 1985; Dominy,Wallerstein,andSuntzeff1986).Theabsenceof core0 Iben andRenzini(1983)criticizedScaloMiller’s(1981) An expectationthatTcshouldbeseeninallTP-AGBstars © American Astronomical Society • Provided by the NASA Astrophysics Data System 1 ments andnovisibleTcline. Thewavelengthofthefeature indicates theexpectedposition ofthe4262Tciline.Nextis giant gUMawhichdoesnot showtheTciline;arrow interval arealsoshown.The firstspectrumisthenormalM HD 49368,anSstarwithlarge s-processabundanceenhance- are indicated.Themajorspectral featuresinthisshortspectral ences of0.1timestheintensitylocal“continuumpoint ” intensity scalebutareshiftedbyarbitraryamounts.Differ- 4262 Âregion.Thesespectraareplottedtothesamerelative been usedtoprovidequantitativeTcabundances(Smith and titative abundanceanalysis.Indeed,someofourspectrahave panion resonancelines,the4262Âlineisamenabletoaquan- this muchiseasilydetectableinourspectra.Unlikeitscom- line by0.1Âtotheblue.Asweshallshow,awavelengthshift of composite featureismuchweakerthantheTciresonance line Lambert 1986;Smith1988). at itsmaximumstrengthandisalsoseparatedfromthe Tc i Gd iiandNbi,withaweakercontributionfromCri.This line. The4262Âisitselfblendedwithafeaturethatprimarily line andthe4297ÂisblendedquitestronglywithaCen contained inasingleexposure,soitisnotpossibletoobserve the 4238Âlinelieswithinwingsofstrong4226Cai taneously. Wechosetocenterthespectraon4262Âline; all threestrongTciresonancelines(4238,4262,4297Â)simul- between thetwosystems. detector combinationsshowednomeasureabledifferences spectra. Spectraofstarsobservedwithbothtelescopeand very nearlythesamedispersionandresolutionas2.1m diode slitwidth,gavearesolutionof0.18Â.Afewspectrawere provide someestimateofthefractions-process-enriched the coudéspectrographequippedwithaReticondetectorat taken withtheMcDonaldObservatory’s2.7mtelescopeand dispersion ontheCCDwas0.06Âpixel~which,withathree- coudé spectrographequippedwithanRCACCDdetector.The with theMcDonaldObservatory’s2.1mtelescopeand needed. radical changesinthegeneralpictureofAGBevolutionare cooler andevolvedmembersofthebariumstarsorwhether whether thes-process-enhancedstarswithoutTcmaybe stars thatdonotcontainTc.Thisfractionshouldshow s-process enhancement,andthusthesespectra,asawhole,can liam 1987).Thesespectraprovidesomeconfirmationoftrue lines ofZri,Yn,LaandNdn(Smith,Lambert,MeWil- in thenear-infraredat7500Âwherewedetectcleanspectral their spectra.Mostofthesestarshavealsobeenobservedbyus stars classifiedasMSorSandtestforthepresenceofTciin the Tciresonancelines.Here,wediscussobservationsof40 significant fractionofMSandSstars,wemadeanewsurvey explanation promotedbyIbenandRenzini(1983)invokedthe temperature sensitivityoftheTclifetime.Theworkby easily happenedlongenoughagotohaveallowedanyTcthat once existedinthesystemtohavedecayedaway.(3)Thethird barium star)(Lambert1988).Thismasstransfercouldhave TP-AGB star(now,thewhitedwarf)toadonor Nemec, andProffitt1984)hasledtothehypothesisthat Mathews etal.(1986)wouldseemtorefutethisexplanation. barium starsaretheresultofatransfermassfrom In Figure1,weillustratethreetypicalCCDspectraofthe With theCCDdetector,onlyabout30Âofspectraare The majorityofthespectrapresentedherewereobtained In ordertoclarifytheclaimsthatTcisnotdetectableina II. OBSERVATIONSANDANALYSIS 1988ApJ. . .333. .2193 lengths, werechosenasthewavelength standards.Instarsthat feature inthe40MSandSstars andina“controlgroup”of10 stellar features,whichcorrespond toknownatomicwave- present inthesespectra,we measured thewavelengthofthis intensity scalebaseduponthesamenormalizationpoint as the actuallinedepthsofspectralfeaturesinthisregion. that usedfortherealspectra.Thisprovidessomeindication of dance ofTcbaseduponobserveds-processenhancements in e(H) =12.0;thisisnearthemaximumexpectedforabun- Eight nearby(<5Âtoeither side),apparentlyunblended, M giants.Weadoptedaninternal stellarwavelengthscale. with andwithoutTc.TheabundanceofTcusedinthesynthe- Downs (1980).Forthesyntheticspectrumweshowrelative sis withTcwasloge(Tc)=1.0,ontheusualscalewhere log one withouttoillustratetheexpectedlineshiftbetweenstars two syntheticspectraofthisregion;onespectrumwithTc and the muchweakerGdn-Nbi-Criblend.Finally,weillustrate tral featureattheproperwavelengthforTci,whichswamps an MSstarwiths-processoverabundancesandastrongspec- wavelength oftheGdn-Nbi-Criblend.NextweshowYLyn, The wavelengthofthisfeaturecorrespondswiththeexpected line depthofthisfeatureisnotmuchdifferentfrom//UMa. closest totheTciwavelengthisshiftedbluebyovertwo MS andSstars(Boesgaard1970;SmithLambert1985, diodes (~0.12Â)fromtheexpectedpositionofTciand intensity. TheMgiant¡iUMaandthes-process-enrichedSstarHD49368donotshowTci,whileMSYLynclearly does containTci. One syntheticspectrumcontainsnoTc,whiletheotherapproximatelymaximumTcabundanceexpectedforobservedMS/Sstars. 1986) andtheoreticals-processpredictionsfromCowley and No. 1,19885-PROCESS-ENRICHEDCOOLSTARS221 In ordertodecidewhetherTcicanbeconsidered Fig. 1.—SamplespectraofthreestarsandtwosynthesesneartheTciline.Thestellarareshownnormalizedtosamescalebutshifted inrelative © American Astronomical Society • Provided by the NASA Astrophysics Data System (1982) andthemeasuredwavelength,À,ofblendthat may and nameofthestaralongwithspectraltypefromHofïleit feature inthe10controlMgiants.ListedareHRnumber mine thewavelengthoffeaturethateithercorrespondsto wavelengths of±0.02-0.03Â.Thesefitsarethenusedtodeter- the Tcilineornearbyblend. features providermsdifferencesbetweenactualandfitted sition. Second-orderpolynomialfitstothecentroidsofthese the chosenfeaturesaredominatedbyasingleatomictran- are thiscool,nospectralfeatureistrulyunblended;however, x In Table1wepresentthewavelengthsdeterminedfor 1 HR 8775,ßPeg.. HR 7405,aVul.. HR 6406,aHer. HR 6146,30Her. HR 4483,a)Vir.. HR 4069,pUMa HR 2286,pGem. HR 921,pPer.... HR 867,RZAri. HR 259 Star The ControlSampleofMGiants TABLE 1 M2.5 II-III M0 III M5 Ib-II M6 III M4 III M0 III M3 Illab M4 II M6 III M4 Illab Spectral Type Wavelength 62.06 62.07 62.08 62.03 62.11 62.10 62.04 62.10 62.06 62.10 - 4200 (À) 1988ApJ. . .333. .2193 222 SMITHANDLAMBERTVol.333 wavelength forfeatureXisthengivenfollowedbyourdecision expected Gdn-Nbi-Criblend,andthestandarddeviationof 4262.11 Â,withameanof4262.08Âandstandarddeviation note whetherthestarislistedinLLB’scompilationandtheir discuss belowthecriteriongoverningthisdecision.Finally,we the HR,HD,orBDnumbersaregivenalongwithvariable dispersion solutions. the meanisclosetotypicalrmsdeviationsoffitted seen, thewavelengthsoffeatureXrangefrom4262.03to two surveys:starsconsideredbyustohaveTcarelabeledas verdict onTcinthestar.Thereisgoodagreementbetween as towhetherTciispresent(Y=yes)ornot(Nno);we star designationorname,ifany.Thespectraltypeisfrom of 0.03Â.Thismeanwavelengthiswithin0.01Âthe Stephenson’s (1984)compilation,unlessnotedotherwise.The include Tciandwhichwenowdub“featureX.”Ascanbe In Table2welistthedatafor40MSandSstars.Either © American Astronomical Society • Provided by the NASA Astrophysics Data System 0 type fromHoflleit(1982).(4)Foundby usinasurveyofMGiantsfromHoflleit(1982). Smith andLambert(1986)thatarenot s-processenhancedbutclassifiedMSbyYamashita(1967).(3)Spectral HD 54587 HR 2508 HD 49368 HD 37536 HD 35155 HR 1556 HR 1105 HD 14028 HR 363 HR 8714 HR 8062 HD 199799.... HD 191589.... HD 191226.... HD 189581.... HR 7442 HD 177175.... HD 170970.... BD +16°3426 HR 6702 BD —18°4320 HD 142143.... HD 110813.... HR 4647 HD 96360 HR 4088 HR 3639 BD +06°2063 HD 64332 HD 63733 HR 2967 HD 58521 BD +33°4573 BD —11°5880 BD —185539 BD —13°4495 BD —02°3726 BD —10°3156 BD +23°654.. BD +18°145.. a PresenceofTcasdescribedbyLLB: dbfl=doubtfulandprobprobable. Notes--(1) Near-infraredspectrasuggest thatthisstarmaynotbes-processenhanced.(2)Starsstudiedby HR, HD, Number or BD Star 1 NQ Pup V915 Aql DE Leo NZ Gem Y Lyn AA Cam V613 Mon o Ori W And HR Peg OP Her ST Her S UMa RS Cnc BD Cam Name The SampleofMSandSStars TABLE 2 S5/2 S3/1 S3/6e S5/2 S4/3 S3/2 S S4/2 S7/le S3/2 S4/1 S (wk) S M3S S4*2 S2*3 M5S M4S (wk) M5 IlbS S (wk) M2S M6.5S MIS (wk) M4S M3S (wk) M3S M3 HlabS M6S M3S M3 II-III M6S M5S Ml Ib-IIa M2S S3/2 M3S M25 M4S M4S MS Spectral Type 4262.08 Â.Thissampleincludesfourstars(thecross-hatched all, theMSandSstarscontainTc. The wavelengthbinswere0.02Âwide.InspectionofTable2 grams (Fig.2)forthreeclassesofstars:theM,MS,andSstars. Â line. blended withaCenline.Oursisbasedexclusivelyonthe4262 decisions regardingthepresenceofTc.LLB’sdecisionabout and especiallyFigure2showsthatsomeof,butbynomeans the presenceofTcisbasedlargelyon4297Âlinethat that differentTciresonancelinesmostheavilyinfluencethe Table 2[SmithandLambert1986,1987].)Itmightbenoted no Tcarelabeledas“dbfl”or“no”byLLB.(Inpart,the “yes” or“prob”byLLB,andstarsconsideredustohave our earlierdiscussionofTcinseveralstarsnowlistedagain agreement isduetothefactthatLLB’scompilationsdrawson As thefirststepinanalysis,wecompiledfrequencyhisto- For theMstars,histogramisapproximatelypeakedat Wavelength 62.23 62.09 62.08 62.07 62.25 62.14 62.12 62.19 62.10 62.14 62.05 62.20 62.17 62.07 62.04 62.10 62.08 62.23 62.14 62.11 62.19 62.18 62.06 62.05 62.14 62.04 62.20 62.03 62.05 62.15 62.09 62.01 62.10 62.24 62.17 62.16 62.07 62.20 62.06 62.07 - 4200 (A) Paper This No No No No Yes Yes No No No No No Yes Yes No Yes No Yes No Yes Yes No No Yes No Yes No Yes No No No No No No Yes Yes Yes Yes Yes Yes Yes Tc a LLB Notes No dbfl dbfl dbfl Yes Yes Yes Yes No dbfl No Prob Prob Prob Prob Prob Prob Yes 4 2, 3 2, 3 2, 3 2, 3 3 1 1 1988ApJ. . .333. .2193 with aproperabundance analysisstillneeded(theMSpanel). s-process overabundancesbasedupon equivalent-widthmeasurements,but based uponanabundanceanalysis (the Mpanel),orprobablydonothave classified asMS,buteitherdefinitely donothaves-processenhancements feature, respectively.Thecross-hatched boxesindicatestarsthathavebeen indicate theextentofhfsinTci line andtheapproximatecentroidof the M,MS,andSstars.Thehorizontal barandverticalarrowintheMSpanel length offeatureXwillbeshiftedtothered.Inmiddle ive abundanceanalysis,weretaintheassumptionthatstars for theMSandSstars.Forformersample,cross- wavelength matchestheofblendinoursyn- 4088, andHR7442.Thishistogramis,weassume,representa- are MSstars. to normalMgiants.Wesuspectthatthisquartetmayinclude near-infrared spectraofthisquartet(BD+18°145,—13°4495, richment isapparentlyverysmallorevennonexistentonour hatched areasidentifyfourstarsforwhichthes-processen- thetic spectrawhentheTcilineisexcludedfromlist. tive ofmeasurementsacrossasampleTc-freestars;the show measurableenhancementsofthes-processelementson areas ofFig.2)classifiedpreviouslyasMS,butthatdonot La ii,andNdnsuggestlittleornostrengtheningwithrespect our spectra.ThisquartetcomprisesHR2508,2967, M ratherthanMSstars,butpendingtheresultsofaquantitat- No. 1,1988 +16°3426, and—H°5880);theequivalentwidthsofZrn,Y Fig. 2.—Frequencyhistogramsofthe wavelengthofspectralfeatureXfor When theTcilinecontributestospectrum,wave- In thelowertwopanelsofFigure2,weshowhistograms © American Astronomical Society • Provided by the NASA Astrophysics Data System WAVELENGTH OFFEATUREX(A) 4262.10 4262.20 s-PROCESS-ENRICHED COOLSTARS 6 extended with stars inLLB’slist.Thiscompilation basedon included, theTciislackinginspectraof50%MS/S stellar evolutionandnucleosynthesis. and compilationasweattempt aninterpretationintermsof stars. Thesestrikingresultsare combinedwithLLB’ssurvey evidence oftheTciline.IffourquestionableMSstars are we findthat14of32or44%theMS/Sstarsshow no contains adegreeofarbitrariness.Ifthesamplesarecombined, pleted, thedistinctionbetweenMSandSspectraltypes larly lacktheTciline.Untilquantitativeanalysesarecom- ments suchasZr.IfthefourpossibleMgiants(seecross- hatched areasinFig.2)areexcluded,fiveofthe15MSstars Tc. Thesestarsareenrichedincarbonandthes-process ele- many MSandSstarsdonotcontainadetectableamount of show noevidenceoftheTciline.Nine17Sstarssimi- Table 2)andTc-containing(“yes”instars. limiting abundanceinourdiscussionoftheTc-free(“no” log [e(Tc)/e(Zr)]»—4.4for[Zr/Fe]=+1.0.Weadoptthis e(Tc) =1.0.Atthefairlysharptransitionfrom24262.18A its initialabundanceisclosetothemaximumorlog to À=4262.08Á,theTcabundanceislog€(Tc)«—0.8,or The syntheticspectrasuggestthatTc,asmonitoredbythe observed wavelength2,isdetectablefor(1.0-1.5)x10yrif lations weredonewithamodelatmospherecorrespondingto but, however,line-blendingisalsostronger.Theabovecalcu- parameters; atcoolertemperaturestheTcilineisstronger, mates aresomewhatdependentuponthemodelatmosphere 4262.14 ÂfeatureXisdominatedbytheGdn-Nbi-Criblend. nated byTciasdemonstratedoursyntheticspectra.Below value of2,and,forvalueslargerthanthis,featureXisdomi- criterion fordecidingwhetherTciispresentornot,wechoose 4262.08 Â,thewavelengthofGdn-Nbi-Criblend.Asa by 7half-livesisessentiallyundetectable.Ofcourse,theseesti- maximum, theTci4262Âlineisquiteseverelysaturated. the wavelengthÀshiftstoblueasTcabundance the wavelengthof4262.14Â.NoMstarsarefoundwiththis N(Tc) =+1.0],butbeginstoshiftbluewardat6half-livesand e(Tc) =1.0ontheusualscalewherelog6(H)12.0.Ofcourse, spectra remainsconstantforseveral(here5)half-lives Consequently, thewavelengthoffeatureXfromsynthetic have decayed.Withanabundanceclosetotheestimated but someoftheTcfromearliestepisodesdredge-upwill as thedredge-upphaseprogresses,Zrisenrichedprogressively, stars is[Zr/Fe]«+1.0,thustheTcabundancelog range ofneutronexposures.TheZrenhancementformanyS Downs (1980):log[Ar(Tc)/AT(Zr)]«—2.2to—2.5forawide using thes-processpredictionspreparedbyCowleyand declines. Theinitial(i.e.,maximum)abundancewasestimated [assuming simpledecayfromtheinitialabundanceoflog ing totheTciline,butasizablefractionalsoclusteraround extending from4262.11to4262.27Â.Clearlyalargefractionof Tff =3400Kandlogg1.0,typicalofaspectraltypeM4. the MSandSstarsgrouparoundwavelengthcorrespond- of thisbroadenedlineis4262.18Âwiththetotalhfssplitting as measuredfromoursyntheticspectra;thetypicalwavelength and Luc1977),and,withthearrow,centroidofthisfeature hyperfine splittingofthe4262ÂTciline(Wendlandt,Bauche, panel ofFigure2,whereweshowthehistogramforMS X stars, weindicatewiththehorizontalbarextentof x X X x C Our statisticsonMSandSstars withandwithoutTccanbe The strikingobservationhighlightedbyFigure2isthat Our syntheticspectrawerecomputedinordertoshowhow in. DISCUSSION 223 1988ApJ. . .333. .2193 6 (Pedley andMarshall1983).However,technetiumismost decayed tolowlevelsintheTc-poor stars.Therequireddelay duction rate(Mathewsa/.1986). Tc-poor andTc-richMSSstarsinasimilarareaof the peratures. Sincethecharacteristicsofsitesurelyvary with low levelsbecause"Tcdecaysveryquicklyathightem- must beentertained: monoxide TcO.TherearenoexperimentaldataonD(TcO). D(ZrO) =8.00±0.14eV,but(TiO)6.92eV with ZrOandothermonoxidesratherthanTiO: S starsofZrO,LaO,andothermonoxidesattheexpense present mightariseinoneof severalways.Perhapsthemost of about10yrbetweenthe lastepisodeofmixingandthe HR-diagram. However,asnotedin§I,thereducedhalf-life of stellar (core)massandluminosity,thisproposalmightprovide stars istoberetained,one(orboth)ofthefollowingscenarios absence ofTcObandsinSstarsasobservedbySanner(1978). another molecule.Thisconclusionisinagreementwith the blamed ontheassociationofTcatomswithTcO or to TcO.EveninthecaseofZrO,Zrilinesareprominent With PedleyandMarshall’s(1983)estimateofZ>(TcO)=5.6 atom orion.Themostabundantmoleculeislikelytobethe unlikely toresideinamoleculeratherthanremainasfree not associatedintoCOdecreases,oxygenatomsare factor behindtheriseofZrOandothermonoxidesisthat present inTc-poorstarsasTcO?Thecompetitionforfree "Tc athightemperaturesmay beoffsetbyahigherpro- spectra ofMSandSstars.Inshort,anabsenceTccannot be the C/OratioapproachesunityandnumberofOatoms their dissociationenergies(D)exceedthatofTiOsothat,as and otherheavyelementsisacontributingfactor,thekey stars. Althoughtheenhancementvias-processofZr,La, the TiOmoleculeswhosebandsdominatespectraofM largely responsiblefortheappearanceinspectraofMSand tion ofanelementmaybeassociatedintomolecules:IsTc account fortheTc-poorMSandSstars.Hereweattempta As the§Iillustrates,avarietyofscenariosmaybeenvisagedto more extensivesurveyconfirmsScaloandMiller’s(1981)con- tention thatTc-poors-process-richAGBstarsarecommon. oxygen atomsamongvariousmonoxidesofmetalis stars. one thatappearstoaccountforthemajorityofTc-poor systematic summaryofthesescenariosanddiscussindetailthe whether Tcilinesaredetectableornot.Thecombinedsample 4088, HR7442,and57Peg(SmithLambert1986).Forthe classified orverymildMSstarsareHR2508,2967, all) havesignificantenrichmentsofthes-processelements.Our of 58stars)donothaveTcilines,butalargefraction(perhaps are asfollows:11(or46%)ofthe24MSand32%) of 58starsconsists24MSand34Sstars.ThestatisticsonTc s-process enhancementsonourspectra—thesefivemis- ± 0.9eV,wecanexpectnoappreciablelossoffreeatomicTc S starsdonotshowtheTcilines.Oftotalsample,38%(22 stars thefiveclassifiedasMS,butshowingnodetectable of 58MSandSstars;weremovefromthetotalsample63 Hence, thecombinationoftwosurveysprovidesasample their ownspectraandpublishedsearchesforTccontains41 224 SMITHANDLAMBERTVol.333 MS andSstars,but18oftheseareincludedinoursurvey. 0 0 18 starsincommon,weandLLBareperfectagreementasto 0 0 2. Tcwassynthesizedandmixed tothesurface,buthas If thehypothesisthatTc-poorstarsareactiveTP-AGB In thecoolatmosphereofanAGBstar,appreciablefrac- 1. TheyieldofTcfromthes-processsiteisreducedtovery © American Astronomical Society • Provided by the NASA Astrophysics Data System 1 8 6 4 5 1316 13 luminous bariumgiants.)Thequestionmaybeanswered by physical association,and,with anassumedMfortheGOV projected separationof2". The radialvelocitiessuggesta Gru (anSstarwithTciLLB) ofspectraltypeGOVata catalog iscompletetothislimit.Theabsolutemagnitude is by Feast(1953)ofacommon radialvelocitycompanionton based on17Sstars.Keenan (1954) pointsoutanobservation needed inordertoconvertthestarcountaspacedensity. comparing thespacedensitiesofbariumandMS/Sstars. (MacConnell, Frye,andUpgren1972).YettheTc-poor MS Keenan (1954)quotesMä —1.0 fromastatisticalparallax such starsbrighterthanVä9.9mag.Weassumethat the to estimatethespacedensityofMSandSstars.Helists 178 estimated toaccountforonlyabout1%ofallGandKgiants evolved bariumstars?Afterall,theGandKgiants are the MSandSstarsmayhavebeencreateddirectlycool as and Sgiantscompose38%oftheobservedsample.(Afew of ments originatedinaTP-AGBstar,i.e.,starliketheTc-rich created. Earlier,wenotedthatmasstransferacrossabinaryis an attractivehypothesis.Ifthisisthecase,s-processenrich- probably independentofthemethodbywhichabariumstaris long beforethestarevolvesupAGB.Evolutionofabarium and Sstarsmustbeevolvedbariumstars.Thisassertionis star uptheAGBisnotinquestion;someofTc-poorMS barium GandKgiants.Ifthelatterevolveintoformer,all AGB. WeprefertolinktheTc-poorMSandMstars evolve fromthecoreflashthroughHeburningto for thes-processenrichment,Tcwouldnotbeexpectedin example, theHecoreflashinalow-massstarwereresponsible MS andSstarsinoursample. of theTcpresentatformationbariumstarwilldecay ment ofs-processelementsincludingTctoaneventnot directly linkedtotheirpresentevolutionarystage.If,for derived byKoester,Schulz,andWeidemann(1979),whichhas would notagreewiththewhitedwarfmassdistribution M giantdescendantbecauseabout2x10yrisrequiredto that thesestarsareTP-AGBandassignstheirenrich- appears toaccountfortheobservedfrequencyofTc-poor surgery oncurrentmodelsofTP-AGBstarsandwhich a rathersharppeaknear0.6M. MS andSstars.Thepreferredscenariorejectsthesupposition Such alowcoremassforsubstantialfractionofAGBstars v v require anunrealisticallylowcoremassofM<0.35. mass of0.8M.Usingthecore-mass-AirelationfromIben and Renzini(1983),aninterpulsetimeof>10yrwould is solongthatTcdecaystolowlevelspriorapulse.Thisis, and Renzini1983)decreasestoamere10yrforcore about 10yrfora1Mstarwithcoremassof0.6(Iben however, unlikelyasthetimebetweenpulses,Ai,isonly interval betweenthermalpulseswhichreplenishtheTcsupply pulses. Anextremelysimplescenariowouldsuggestthatthe is difficulttoattaininmodelsoflow-massTP-AGBstars.If suppose thatmixingmaybeinterruptedformanythermal this difficultyisreflectedinnature’sTP-AGBstars,onemight 0 advertised thatmixingbetweentheHeshellandenvelope be inhibitedinlow-massTP-AGBstars.Ithasbeenwidely production and,hence,theneutronsourceC(a,n)0may products. HollowellandIben(1988)speculateonhowC carbon addedtotheenvelopehasnotbeenmixedwiths- recent pulseshavenotignitedtheneutronsourcesothat c0 0ip 0 ip Stephenson’s (1984)catalogofGalacticSstarsmaybeused Is oursampleofMSandSstarsseriouslycontaminatedby Finally, wediscussthescenariowhichrequiresnoradical 1988ApJ. . .333. .2193 3-1 3612 63- 1 2 _263-1 -1 6 3_ 1 263-1 m 263-1 3-1 No. 1,1988s-PROCESS-ENRICHEDCOOLSTARS225 consist oftwocomponents:theTP-AGBTc-richstars(62%) comparable luminosity(M<—1),MihalasandBinney(1981) pc) forM=—3.Wesupposethatthesespacedensities and iV(MS/S)=6.7xIO“(10pc)“1.6IO" extremes ofMSandSstarscaleheights. Wing (1979)estimatez-directionscaleheightsof465pcand lute visualmagnitudes,bothStephenson(1984)andYorka poor MSandSstarsrangesfrom72to1375—afactorof20in quote aspacedensityofAT(M)~(3-3.5)(10pc)^Theratio and (evolvedbarium?)Tc-poorstars(38%).ForMgiantsof 200 pc,respectively.Thesetwodeterminationsrepresentthe ly, Jura(1988)hasestimatedthatthesurfacedensityofSstars in therangeof130-920:again,theseratiosarenotgreatly densities oftheMSandSstars,wediscussotherestimates. G andKgiantstobariumstarsof~100. and Sstarsarenotwellknown;however,thisrangeofratios the extremeestimates!ClearlyspacedensitiesofMS of spacedensitiestheluminousMgiantstotechnetium- MS andSstarabsolutemagnitudes.Usingtheirderivedabso- (1979) concludesthatthemeanvalueofMatmaximumlight companion, Feast(1953)estimatesM«0to—1fornGru. (1988) Sstardensitymustbeloweredbysomefactorof 2 or is >10kpc"withazscale-heightof200pc.Thisnumberdoes different fromtheratio^(GKViVíbariumGK)~100.Recent- ratios ofMgianttoTc-poorMSandSstars(38%thetotal) cate thatMy~—1to—3wouldincludethegreatmajorityof is (My)=—1.5to—2.0.Thesevariousdeterminationsindi- derive M=—0.4.LloydEvans(1983)providesphotometry reversal widthoftheCanKlineforSstarHR1105and more, whichagainplacestherangeofMgianttoTc-poor MS “numerous”; hence,theratiosof300-370derivedfromJura’s clude that,relativetotheSstars,MSstarsshould be ratios ofN(M)/AT(Tc-poorS)~300-370,whichrepresents Af(MS/S). UsingJura’s(1988)density,however,wederive not includetheMSstarsandthusrepresentsalowerlimitto stars ofN(MS/S)=(1-6)x10(10pc),whichleadsto M giantstoTc-poorMSandSstarsencompassestheratioof following crudespacedensitiesfortheMSandSstarsfrom volume distributionofasampleSstarsbyYorkaandWing of SstarshaveMintherange—1.5to—2.5.Astudy of MS,S,andCstarsintheMagellanicClouds,hissample In addition,DominyandWallerstein(1986)havemeasuredthe carbon stars,deriveaspacedensityofN(C)ä1x10 (10 and SstardensitieswithinthevalueforiV(GK)/Â(barium ratios thataretoolargeduetonotincludingtheMSstars. Scalo andMiller(1979)derivespacedensitiesoftheMSS pc) forthetwoscaleheightsof465and200pc,respectively, are N(MS/S)=5.0x10“(10pc)and1.110" factor of~2ormore,closeto therangeofAr(GK)/AT(barium GK). Claussenetal(1987),usingaflux-limitedsample of Both WingandYorka(1977)ScaloMiller(1979)con- or —3,anda05correctionforabsorption,wederivethe Stephenson’s (1984)catalog:forM=—1thespacedensities GK). stars. IncludingtheMSstars wouldlowertheseratiosbya N(S) «5x10“(10pc)thusderivedwouldleadtoratios of thespacedensitiesN(C)/N(S)æ2.Thevalue of pc), whileWingandYorka(1977)estimatethattheratio of N(M)/N(Tc-poorS)~160-185 withnoinclusionoftheMS v v v v v v v Due tothelargeuncertaintyinourestimateofspace Using therangeofestimatedscaleheights,Meither—1 All oftheabovestudiesand estimatesdemonstratethat, v © American Astronomical Society • Provided by the NASA Astrophysics Data System is uncertain,thelimitsonthisratioencompassvalueof further evolvedalongtheAGBthanlatter(Wood,Bessell, evolved bariumgiants.Thisidentificationhasbeenproposed expectation thatthebariumGKgiantsshouldretaintheir although theratioofMgianttoMSandSstarspacedensities which ledtoanimprovedpaper. cussions andthehelpfulcomments ofananonymousreferee, we beforcedtoentertainradicalchangesthecurrentpicture enriched TP-AGBsinglestars.Onlyifalargefractionof the increases withincreasingperiod(Whitelock,Pottasch,and before (SmithandLambert1985,1987;LLB),butthecompari- abundance peculiaritiesastheyevolveuptheAGBencourages Ar(GK)/AT(barium GK).Thisclosecorrespondenceandour Welch Foundation.Wethank JohnScaloforfruitfuldis- Tc-poor MSandSstarsarediscoverednottobebinariesshall ments andphotometryshouldpermitthetwoeffectsto be variations, butathoroughprogramofradialvelocitymeasure- will complicatethesearchforsmallamplitudeorbital and Fox1985).Sincetheamplitudeoflightvariationsalso from theshortestperiodstars.Theformerareconsideredtobe barium starsissupportedbythecorrelationbetweenpre- that Tc-poor,butactive,TP-AGBstarsmustbequiterare. son ofthespacedensitiesprovidesnovelevidencethatsuggests us toidentifythemajorityofTc-poorMSandSstarsas of TP-AGBevolution. nate betweenevolvedbariumstarsandtheintrinsically the searchforspectroscopicbinariesmaybeusedtodiscrimi- have yettobeexaminedforTc.JorissenandMayornote that that atleastfivestarsarebinaries.TheTc-poorSstar HD stars withtheradialvelocityspectrometerCORAVEL,found separated. JorissenandMayor(1987),fromasurveyofnine S tions arisingfromsemiregularpulsationsoftheMSandSstars those shownbyallbariumstars.Ofcourse,thevelocityvaria- searching fortheradialvelocityvariationscharacteristicof stars asevolvedBariumisopentothesimpletestof variables willbeanapproximateseparationbyabsolutelumin- period: Tcispresentinthelongestperiodstarsandabsent period variablestars.Little-MareninandLittle(1979) sence ofTcandtheperiodlightvariationslong- Science Foundation(grantAST 86-14423)andtheRobertA. those withvariablestardesignationsandwithout,a osity. Whenwedividethecombinedsampleof58starsinto Feast 1987),adivisionofoursampleintovariablesandnon- 35155 (Table2)isoneofthesefive.Theremainingfourstars striking differenceemerges:onlyfiveoutof35starswithvari- LLB haveshownthatthepresenceofTciscorrelatedwith enrichments viamasstransfercouldappearanywherealong be undetectedvariables;however,themostobviousvariables tedly, somestarsthatarenotknowntobevariablemay,infact, stars withoutvariablestardesignationsdonot(74%).Admit- able starnamesdonotshowTc(14%),whereas17outof23 hence, highlyevolved.Starsthathaveacquireds-process to befoundwouldtheoneswithlargeamplitudesand, phase ofactiveTPsand,hence,synthesizeTc. tion. 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