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Mon.Not.R.Astron.Soc.000,000–000(0000) Printed11December 2013 (MNLATEXstylefilev2.2) Lithium-rich field giants in the Sloan Digital Sky Survey S. L. Martell1⋆ and M. D. Shetrone2 1AustralianAstronomicalObservatory,NorthRydeNSW2122,Australia 3 2McDonaldObservatory,UniversityofTexasAustin,McDonaldObservatory,Texas79734,USA 1 0 2 Received25October2012/Accepted19December2012 n a J ABSTRACT 2 We present a search for post-main-sequence field stars in the Galaxy with atypically large lithium abundances. Using moderate-resolution spectra taken as part of the Sloan Digital ] R SkySurvey,alongwithhigh-resolutionfollowupspectroscopyfromtheHobby-EberlyTele- scope, we identify 23 post-turnoff stars with logǫ(Li) greater than 1.95, including 14 with S logǫ(Li) ≥ 2.3 and 8 with logǫ(Li) ≥ 3.0, well above the low level expected for evolved . h stars.ComparisonwiththeoreticalisochronesindicatesthatsomeofourLi-richstarsareaf- p filiatedwiththeupperredgiantbranch,theasymptoticgiantbranchandtheredclumprather - thantheRGBbump,whichisachallengetoexistingmodelsofLiproductioninevolvedstars. o r t Keywords: Stars:abundances s a [ 1 v 1 INTRODUCTION carbonandnitrogentoevolveconstantlyasstarsascendtheRGB 3 (see Martelletal. 2008 for a more thorough explanation of deep Theabundanceoflithiumisextremelysensitivetotheambientcon- 6 mixing).Inhigher-massstars,thedestructionoftheµ-barrieroc- ditionsinsidestars,sinceitscrosssectiontoprotoncaptureisquite 1 cursatseconddredge-up,lowontheasymptoticgiantbranch.The largeattemperaturesabove2.6×106K.Liisreadilydestroyedin 0 factthattheLi-richstarsidentifiedbyCharbonnel&Balachandran 1. tchoenvienctetirvioersenovfelloopwe-misaspsromgraeinss-isveeqluyendceeplsettaerds,aasnednvtheeloLpei imnatthee- (2000)werelocatednearthedestructionoftheµ-barrierledthose 0 authorstospeculatethattheonsetofdeepmixingproducesabrief rial is cycled past the innermost, hottest region of the convective 3 episode of Li production via the Cameron-Fowler mechanism zone.SurfaceLiabundanceisfurtherdepletedafterstarsleavethe 1 (Cameron&Fowler 1971). In this process, 7Be produced by the mainsequenceandundergofirstdredge-up(Iben1968),inwhich : proton-proton chain is transported to a cooler zone in the star, v theconvectiveenvelopedeepens,mixingfusion-processedmaterial whereitcapturesanelectronanddecaysinto7Li,whichisstable i from the stellar interior into the envelope and exposing envelope X againstprotoncaptureifitssurroundingsarecoolenough.Aphase materialtoevenhighertemperatures. r in which the deep-mixing mass transport speed was briefly high a Because of this, it was quite surprising when enoughtoallowthisprocesswouldcausesurfaceLiabundancesto Wallerstein&Sneden (1982) discovered a Li-rich K giant risesharplyandthendecay. serendipitously while investigating barium-rich field stars. More However, the existence of Li-rich stars in other regions of systematic searches for Li-rich stars (e.g., Brownetal. 1989; theHertzsprung-Russelldiagramchallengesthismodel:Kraftetal. Pilachowski 1986; Pilachowskietal. 1988; Pilachowskietal. (1999)foundonebrightredgiantwellabovetheRGBbumpinthe 2000) found that they are rare in the Solar neighborhood globularclusterM3,wheredeepmixingiswellknowntooperate and extremely unusual in globular clusters. The study of (Smith2002;Angelouetal.2011).Morerecently,sixindependent Charbonnel&Balachandran (2000) found that nearby Solar- studiesinthepastthreeyearshavereportedatotalof47newLi- metallicity Li-rich stars are either low-mass stars (M ≃ 1M⊙) richpost-main-sequencestars.ThesenewLi-richstarsfillabroad nearthe“bump”intheredgiantbranch(RGB)luminosityfunction or intermediate-mass stars (M ≃ 3M⊙) low on the asymptotic range of parameter space: low-mass (M ≤ 2M⊙) RGB stars in the thick disk with −1 ≤ [Fe/H] ≤ 0 (Monacoetal. 2011); an giant branch (AGB). In both cases, the hydrogen-burning shell intermediate-mass(M ≃ 4M⊙),Solar-metallicitystarinthethin in the stars in question has recently encountered, and destroyed, disk (Monacoetal. 2011); low-mass RGB stars with [Fe/H] ≃ the discontinuity in mean molecular weight (the “µ-barrier”) −0.9 in the Galactic bulge (Gonzalezetal. 2009; Lebzelteretal. left behind at the end of first dredge-up. In low-mass red giants, 2012); and low-mass RGB stars in Local Group dwarf galaxies, the destruction of the µ-barrier causes a brief loop in stellar twoofwhichhave[Fe/H]≃−2.7(Kirbyetal.2012). evolution, observed asthe RGB bump, and permits deep mixing, Twooftherecentstudies(Kumaretal.2011andRuchtietal. anonconvective processthatsteadilycyclesmaterialbetweenthe 2011) take the approach of selecting a well-defined subset from photosphereandtheradiativezone,causingsurfaceabundancesof anexistingcatalog(HipparcosandRAVE,respectively)forhigh- resolution spectroscopic followup and abundance analysis, a pro- ⋆ Email:[email protected](SLM);[email protected](MDS) cedurethatwealsoadopt.Inthispaper,wepresentasearchforLi- 2 S. L. Martell&M. D. Shetrone Figure1.DistributionofTeff andlog(g)forthe8535starsinourinitial Figure2.Metallicity histogramforourinitialdataset(heavysolidline), dataset. withhistograms forthethree data sources overplotted withadotted line (SEGUE),lightsolidline(Legacy)anddashedline(MARVELS). richfieldstarsinSloanDigitalSkySurvey(SDSS)spectroscopy. Bysurveying starsacrossawiderangeofmetallicityandsurface [Fe/H],andradialvelocity,alongwiththeequivalentwidthsand/or gravity,weareabletotakeabroadviewofthequestionofLi-rich lineindicesfor85atomicandmolecularabsorptionlines,bypro- post-main-sequence stars. This paper describes the 23 previously cessingthecalibratedspectrageneratedbythestandardSDSSspec- unknownLi-richstarsweidentifiedfromthisoriginaldataset,and troscopic reduction pipeline (Stoughtonetal. 2002). Additional exploreswhetherthevariousproposedmechanismsforLiproduc- validationsanddetailsofrecentupdatestoSSPParediscussedin tioninevolvedstarsaresufficienttoexplainthisgroupofLi-rich Smolinskietal.(2011). stars. 2.2 Targetselection WeselectedoursamplefromtheseventhSDSSdatarelease(DR7), 2 THEOBSERVATIONALDATASET using the online Catalog Archive Server (CAS)1 to identify stars In order to explore the physical conditions that produce large with a mean signal-to-noise ratio per pixel (SNR) of at least 40, amountsofphotosphericlithiuminsomeevolvedstars,weselected T ≤6000K,reasonableerrorson[Fe/H]metallicity(σ ≤ eff [Fe/H] asampleofpost-main-sequencestarsfromtheSDSSDataRelease 0.3) and surface gravity (σ ≤ 0.5), and surface gravity no log(g) 7.The8535starsinourinitialdatasetwererequiredtohavehigh- higherthan1.4dexgreaterthanthesurfacegravityatthe“bump” qualityspectraandrelativelylowerrorsonstellarparametersdeter- intheRGBluminosityfunction.Previousstudieshavefoundthat minedbytheautomatedSSPPpipeline(Leeetal.2008a),andwere stars with unexpectedly high lithium abundances tend to be near allowedtocoverabroadrangeofmetallicity,effectivetemperature the RGB bump, so we designed this gravity selection to include andsurfacegravity. stars at evolutionary phases from shortly before the RGB bump to the tip of the RGB. Empirically, the luminosity of the RGB bump is a function of metallicity: FusiPeccietal. (1990) report 2.1 SDSSspectroscopy [Fe/H]versusMbumpforaselectionofGalacticglobularclusters, V and we converted those M values to log(g) using metallicity- The imaging component of the original Sloan Digital Sky Sur- V matched 12 Gyr Yale-Yonsei isochrones (Demarqueetal. 2004), vey and its extensions (Fukugitaetal. 1996; Gunnetal. 1998; then fit a least-squares line to derive log(g) = 2.23235 + Yorketal.2000;Pieretal.2003;Gunnetal.2006;Stoughtonetal. bump (0.188018 ×[Fe/H]). This initial data set contained 8535 stars: 2002;Abazajianetal.2003,2004,2005;Adelman-McCarthyetal. 7667 fromSEGUE,471fromLegacyand 397fromMARVELS. 2006, 2007, 2008; Abazajianetal. 2009; Aiharaetal. 2011) in- Figure1showsT versuslog(g)forall8535stars,andFigure2 cludesugrizphotometryforseveralhundredmillionstars.SEGUE eff isahistogramofthemetallicities.InFigure2,theheavysolidhis- (Yannyetal.2009),oneofthreesub-surveysthattogetherformed togramshowsthemetallicitydistributionforthefulldataset,with SDSS-II,extended that imaging footprint by approximately 3500 deg2,andalsoobtainedR≃2000spectroscopyforapproximately thehistogramforjusttheSEGUEstarsshownasadottedline,the 240,000 stars over a wavelength range of 3800 − 9200A˚. Ad- histogramfortheLegacystarsdrawnwithalightsolidline,andthe histogramfortheMARVELSstarsshownwithadashedline. ditional spectroscopy with similar characteristics has been taken aspartoftheSDSSLegacyproject(Abazajianetal.2009)andin preparationfortheSDSS-IIIMulti-objectAPORadialVelocityEx- oplanet Large-area Survey (MARVELS) project (Geetal. 2008). 3 LOW-RESOLUTIONANALYSIS Tofacilitatethedevelopment andcalibrationoftheSEGUEStel- Inorder toidentify starswithpotentiallyhigh lithiumabundance lar Parameter Pipeline (SSPP; Leeetal. 2008a; Leeetal. 2008b; based on low-resolution spectra, we designed a spectral index, AllendePrietoetal. 2008), SEGUE included spectra for a col- lection of Galactic globular and open clusters. As described in Leeetal. (2008a), SSPP produces estimates of Teff, log(g), 1 http://skyservice.pha.jhu.edu/CasJobs/login.aspx Lithium-richgiantsin SDSS 3 Figure 4. Typical spectra from our initial SDSS low-resolution data set. Figure3.LithiumindexS(Li)versusTeff fortheinitialdataset(upper Thelowertwospectraappeartohavestrongabsorptioninthe6708A˚ Li panel);S(Li)versus[Fe/H](lowerpanel).Inbothpanels,starsselected resonanceline,whilethetopspectrumdoesnot. forthesecondarydatasetareshownasopentriangles. nitude(VmagnitudefortheMARVELSstars)andS(Li)measure- S(Li), that measures the magnitude difference between the inte- ment for the 36 stars initially flagged for followup spectroscopy. gratedfluxinthe6708A˚ resonancelineandtheintegratedfluxina MARVELSstarsaretoobrighttobeincludedinSEGUEphotom- smallregionofnearbyspectrum.Itisdefinedas etry, so their apparent V magnitudes were taken from the Vizier R6713I dλ catalog-searchservice2.Theplate,MJDandfiberIDinformation S(Li)=−2.5×log 6706 λ . isgiven because it isasimpleway for finding individual starsin 6738 R I dλ 6730 λ theCAS. Typically spectral indices are defined for use in small regions of parameterspace,anduseoneormorenearbyregion(s)ofspectrum independentofthepropertythatdrivesvariationsinthefeatureof 4 HIGH-RESOLUTIONANALYSIS interest as a comparison band; however, our initial data set con- tainsstarsacrosssuchawiderangeofT ,log(g)and[Fe/H]that Followupspectrawereobtainedfor34ofthese36starsusingthe eff therearenonearbyregionsofspectrumthatarethesameforallof High-Resolution Spectrograph (HRS) on the Hobby-Eberly Tele- ourstars.ThisfactintroducessomescattertoourS(Li)measure- scope(HET)atMcDonaldObservatory(Tull1998).Allbutoneof ments,requiringextracareinselectinglikelyLi-enhancedstarsfor these34starswerefirstobservedasfillertime(P4)duringnormal followupobservations. queueobserving(Shetroneetal.2007)duringperiods2010T2and TheupperpanelofFigure3showsS(Li)versusT forthe 2011T1.Thisfillertimeallowedustotakeaninitialshortsnapshot eff initial data set, and the lower panel shows S(Li) versus [Fe/H] spectrumtoconfirmthestengthoftheLiline;7werefoundtonot for the same stars. In each plane, S(Li) has a broad distribution beLirichand27wereconfirmedLirich.For23oftheseLirich atafixedvalueoftheindependentparameter(T or[Fe/H])and starswewereabletofollow-upwithadditional spectratogetthe eff a slight dependence on that parameter. To identify outliers in the SNR sufficient for determination of stellar parameters and abun- S(Li)-T andS(Li)-[Fe/H]distributions,weselectallstarswith dances.Oneofthe36stars(SDSSJ0831+5402) hadalreadybeen eff S(Li) ≥ 0.295− Teff andS(Li) ≥ 0.18+ [Fe/H] asoursec- abservedaspartoftheChemicalAbundancesofStarsintheHalo ondarydataset.The4r×e1a0r4e162starsmeetingbothcr1i0t0eria,andthese (CASH)project(Frebeletal.2008). areplottedasopentrianglesinFigure3,whileallotherstarsfrom theinitialdatasetarerepresentedbysmallfilledcircles. 4.1 SpectroscopicObservationsandReduction Fromby-eyeexaminationeachofthespectrainthissecondary datasetof162stars,wefoundthatourselectioncriteriaweregen- HRSwasconfiguredtoHRS 15k central 316g5936 2as 0sky IS0 GC0 2x5 erous:fewerthanhalfofthestarsselectedbasedonS(Li)-Teff duringourHETobservationstoachieveR=18,000spectracovering - [Fe/H] position had 6708A˚ Li lines that appeared legitimately 4233A˚ to5900A˚ onthebluechipand6000A˚ to7850A˚ onthered strong in the SDSS spectra. Figure 4 shows spectra for two of chip. Exposure times per visit ranged from 300 seconds to 1800 the starswe selected for high-resolution followup (thelower two secondsbasedontheobservingconditions.Becauseofthenature spectra), as wellas one star withlittleapparent absorption inthe oftheHETandthegenerallypoorconditionsoffillertime,some 6708A˚ Li line (top spectrum). We generated low-resolution syn- targetshadmultiplevisitstakenoveranumberofdaystobuildup theticspectrabasedonSSPPstellarparameterstoroughlyestimate enoughS/NtocleanlydetecttheLifeature. lithiumabundanceforthosepotentiallylithium-enhancedstars,and The spectra were reduced with IRAF3 ECHELLE scripts. selected36ofthemforhigh-resolutionfollowupobservations,with thegoalof determiningmoreprecisestellarparametersandǫ(Li) abundances.Table1listsSDSSIDnumber,datasource(SEGUE, 2 http://vizier.cfa.harvard.edu/viz-bin/VizieR LegacyorMARVELS),plug-platenumber,modifiedJuliandateof 3 IRAF(ImageReductionandAnalysisFacility)isdistributedbytheNa- observation,fiberIDnumber,rightascension,declination,gmag- tionalOpticalAstronomyObservatories,whichareoperatedbytheAssoci- 4 S. L. Martell&M. D. Shetrone Table1.Staridentifiers,position,photometryandlithiumindexforthe36starsselectedforfollowupspectroscopy. ApparentVmagnitudesarelistedfor MARVELSstars. SDSSID Source Plate MJD FiberID α δ g0 S(Li) ConfirmedLi-richwithHET SDSSJ2206+4531 SEGUE 2556 54000 640 22:06:19.75 45:31:57.19 15.732 0.209 SDSSJ2353+5728 SEGUE 2377 53991 337 23:53:29.76 57:28:18.41 12.960 0.209 SDSSJ0808-0815 SEGUE 2806 54425 256 08:08:34.33 -08:15:47.30 14.598 0.198 SDSSJ0301+7159 MARVELS 2843 54453 634 03:01:33.77 71:59:11.16 10.570 0.197 SDSSJ2019+6012 SEGUE 2554 54263 600 20:19:32.44 60:12:46.32 15.270 0.212 SDSSJ0652+4052 MARVELS 2847 54452 557 06:52:37.80 40:52:01.44 11.170 0.212 SDSSJ0245+7102 MARVELS 2843 54453 168 02:45:20.83 71:02:02.63 11.760 0.184 SDSSJ0632+2604 SEGUE 2678 54173 188 06:32:30.90 26:04:37.46 15.152 0.234 SDSSJ2356+5633 SEGUE 2377 53991 200 23:56:52.22 56:33:38.83 13.080 0.210 SDSSJ0535+0514 MARVELS 2841 54451 299 05:35:43.75 05:14:22.62 12.310 0.191 SDSSJ2200+4559 SEGUE 2556 54000 540 22:00:51.22 45:59:40.33 15.453 0.229 SDSSJ0304+3823 SEGUE 2441 54065 453 03:04:37.40 38:23:46.12 15.357 0.181 SDSSJ1901+3808 SEGUE 2536 53883 306 19:01:49.00 38:08:51.00 14.29 0.214 SDSSJ0720+3036 SEGUE 2677 54180 255 07:20:42.79 30:36:51.49 16.667 0.209 SDSSJ1909+3837 SEGUE 2536 53883 110 19:09:29.25 38:37:46.23 15.764 0.214 SDSSJ1105+2850 SEGUE 2870 54534 599 11:05:36.91 28:50:08.09 17.006 0.185 SDSSJ0654+4200 MARVELS 2847 54552 614 06:54:58.00 42:00:27.00 12.51 0.220 SDSSJ1607+0447 SEGUE 2178 54629 378 16:07:09.23 04:47:12.73 16.139 0.173 SDSSJ1310-0012 SEGUE 2901 54652 191 13:10:37.22 -00:12:44.40 15.252 0.185 SDSSJ0936+2935 SEGUE 2889 54530 164 09:36:27.44 29:35:35.80 16.240 0.185 SDSSJ1522+0655 SEGUE 2902 54629 33 15:22:11.86 06:55:55.22 15.912 0.165 SDSSJ0831+5402 SEGUE 2316 53757 395 08:31:55.28 54:02:45.55 15.430 0.186 SDSSJ1432+0814 SEGUE 2933 54617 107 14:32:07.15 08:14:06.15 17.047 0.221 ConfirmedLi-richwithHETbutnotenoughSNRforfullanalysis SDSSJ0515+1558 SEGUE 2668 54084 214 05:15:23.17 15:58:55.38 14.555 0.216 SDSSJ2048+5603 SEGUE 2555 54265 58 20:48:46.82 56:03:53.65 13.423 0.200 SDSSJ2119-0734 SEGUE 2305 54414 230 21:19:47.65 -07:34:26.89 14.526 0.167 SDSSJ1214+5027 SEGUE 2919 54537 630 12:14:54.88 50:27:00.99 18.205 0.177 ConfirmednotLi-richwithHET SDSSJ1421+0136 Legacy 534 51997 162 14:21:28.26 01:36:28.35 17.077 0.200 SDSSJ1213-0147 Legacy 333 52313 331 12:13:53.32 -01:47:14.93 16.558 0.180 SDSSJ1227-0026 SEGUE 2558 54140 115 12:27:12.72 -00:26:58.81 16.990 0.177 SDSSJ0144-1005 SEGUE 2850 54461 12 01:44:49.17 -10:05:31.09 16.738 0.171 SDSSJ0352-0540 SEGUE 2051 53738 357 03:52:43.59 -05:40:03.32 16.487 0.168 SDSSJ1725+0729 SEGUE 2797 54616 304 17:25:04.06 07:29:45.03 14.807 0.168 SDSSJ1849+1931 SEGUE 2812 54633 393 18:49:52.58 19:31:04.20 14.980 0.168 NotobservedwithHET SDSSJ1824+7911 SEGUE 2802 54326 372 18:24:22.51 79:11:24.47 16.548 0.201 SDSSJ2021+1257 SEGUE 2248 53558 291 20:21:25.73 12:57:51.33 15.263 0.198 The standard IRAF scripts for overscan removal, bias subtrac- Johnson&Pilachowski (2010), and Koch&McWilliam (2008). tion, flat fielding and scattered light removal were employed. Lithium abundance was determined by spectral synthesis of the For the HRS flat field we masked out the Li I and Na D re- 6708A˚ and 6104A˚ resonance lines. We also determined carbon gions because the HET HRS flat field lamp suffered from inter- abundance and 12C/13C ratios, where possible, by synthesis of mittent faint emission lines which would otherwise compromise the 4297A˚ CH feature. The CH linelist for the carbon analysis our analysis. Equivalent widths (EW) were measured using the wastakenfromPlez(priv.comm.);allotherlinesweretakenfrom IRAFtasksplotandforcingtheGaussianfittohavefixedFWHM a version of the Kurucz&Bell (1995) linelist modified to fit the for those lines below 150 mA˚ while allowing the strong lines strongest lines to the Solar spectrum. Model atmospheres were to have a variable FWHM. We also fit multiple Gaussian pro- takenfromtheOsmarcs2005grid(Gustafssonetal.2008),inter- files to any nearby lines to take blends into account. Metallic- polating between the grid points. These spherical models cover ity and α-element abundances were determined through equiva- metallicity ranges from +1.0 to -3.0 with [α/Fe] = +0.4 for lentwidthanalysis;oscillatorstrengthsadoptedfortheEWanaly- [Fe/H] <= −1.5, [α/Fe] = +0.3 for [Fe/H] = −0.75, sisweretakenfromFulbrightetal.(2006),Fulbrightetal.(2007), [α/Fe] = +0.2 for [Fe/H] = −0.5, [α/Fe] = +0.1 for [Fe/H]=−0.25and[α/Fe]=+0.for[Fe/H]>=0.0). Theabundancecalculationswereperformedusingamodified ationofUniversitiesforResearchinAstronomy,Inc.,undercontractwith versionofthe2010versionoftheLTElineanalysisandspectrum theNationalScienceFoundation. synthesis code MOOG (Sneden 1973). This code has some sig- Lithium-richgiantsin SDSS 5 Figure5.The23starsforwhichwederivehigh-resolutionstellarparame- Figure6.Effective temperature andsurface gravityfromhigh-resolution tersandabundances,intheα–Feplane.Dashedlinesmarkthebordersof analysis, with measurement errors, for our 23 Li-rich stars. Lower- ourroughageestimations. metallicitystarsareintheleftpanel,andhigher-metallicitystarsareinthe rightpanel.Thedifferentsymbolsrepresentgroupsinevolutionaryphase, andrepresentativeisochronesareoverplotted.Horizontallinesmarkthepo- nificant differences from the earlier versions including using the sitionoftheRGBbumponeachisochrone,ascalculatedfromourempir- predictedabundancevs.reducedEWfordeterminingthemicrotur- icalrelationgiveninSection2.2.TrianglesareRGBstars,circlesarered bulence.Initialestimatesofthestellarparametersweretakenfrom clumpstars,andsquaresarestarsthathavenotyetreachedfirstdredge-up the SSPP pipeline. From this starting point the temperature was (whoseLiabundancesarenotrelevanttothequestionofLiproductionon adjusteduntilthetherewasnosignificant slopeintheFeIabun- theRGB).Thethreeplaincrossesatthetopoftheleftpanelarestarswhose danceswithrespecttolineexcitationpotential.Thestellargravity log(g)andTeff arenotcompatiblewithanyreasonableisochrone,andwe wasadjustedsuchthattheFeIandFeIIlinesproducedthesame believethemtobepost-AGBstars. abundance withinthe errors. Microturbulence wasdetermined by requiringthattherewasnosignificantslopeintheFeIabundance vs. predicted Fe I reduced equivalent widths based on the mean Figure6demonstratesthatourinitial[α/Fe]–[Fe/H]ageesti- abundanceofthelines. mationwasreasonable:bothpanelscompareourvaluesofT and eff Table2listsSDSSID,alongwithT ,log(g)and[Fe/H]and eff log(g) measured from high-resolution spectra against representa- theirassociatederrors,plusmicroturbulentvelocityandrotational tiveisochrones,witholderstarsintheleftpanelandyoungerstars velocity,wheremeasurable,derivedfromourhigh-resolutionspec- intherightpanel,andthemajorityofstarslienearthegiantbranch tra,andTable3listsSDSSIDandabundancesof[α/Fe],[C/Fe], correspondingtotheappropriateinitialvalues.Thehorizontallines 12C/13C, and logǫ(Li) determined from synthesis of the 6707A˚ ineachpanelmarkthelog(g)attheRGBbumpforeachisochrone and6104A˚ lines.Alpha-elementabundancesaretheerror-weighted shown,calculatedfromourempiricalcalibrationgiveninSection average of [Mg/Fe], [Ca/Fe] and [Ti/Fe], and the total error on 2.2.Thesquaresrepresentstarsthathavenotyetbeguntoascend [α/Fe] is the larger of the propagated errors or the dispersion in theRGB,andtheirhighLiabundancesarepresumablyprimordial. thethreeinputabundances. ThetrianglesrepresentRGBstars,bothyoungandold,andthecir- clesshowredclumpstars,whichallsittotheleftoftheisochrone attheirmetallicity,andareallatintermediateage.Thethreecrosses 4.2 Evolutionarystatesofthestars atthetopoftheleftpanelarethreelow-metallicitystarswithpa- PrevioussearchesforLi-richstarshavefocusedongroupsofstars rameters that are apparently inconsistent withthe isochrones; we withsimilarities:forexample,Lebzelteretal.(2012)selectedonly willaddressthesestarsinSec.5.ThelasttwocolumnsofTable2 RGB stars in the Galactic bulge, with metallicities in a narrow giveourfinalestimatesforageandevolutionaryphase.Theseval- range,andKumaretal.(2011)usetheHipparcoscatalog,inwhich uesarenotgivenforthethreestarswithparametersincompatible thedistancestoallstarsareknownquiteprecisely,astheirinitial withtheisochrones. data set. However, our data set is heterogeneous in terms of age, Weareabletodetermine12C/13Cforallsixofourredclump compositionandpositionintheGalaxy.Basedonthemetallicities stars,usingsynthesisofthe4297A˚ CHfeature.Inallofthesestars, andalpha-enhancementsderivedfromthehigh-resolutionspectra, theratioisfairlylow(12C/13C≤15),indicatingthatfirstdredge- wedivideourLi-richstarsintofourbroadgroupsandassignrep- uphasefficientlymixedthesurfacematerialwiththestellarinte- resentative ages. Figure 5 shows the four groups in the [α/Fe] rior. – [Fe/H] plane, which is commonly used to compare the star- The Li-rich stars are mainly found low on the RGB, which formation and chemical-enrichment histories of composite stel- might be expected from Li production associated with the onset larpopulations indwarfgalaxiestothecomponents oftheMilky ofdeepmixing.However,thepresenceofsomebrightLi-richgi- Way(e.g.,Vennetal.2003;Shetroneetal.2003;Kochetal.2008; antsandredclumpstarsindicatesthatitmustalsobepossiblefora Kirbyetal.2011).Briefly,moderate-andlow-metallicitystarsare startoproduceLiduringotherevolutionaryphases.Currentmodels assumedtobeold,whilehigher-metallicity,non-α-enhancedstars of thermohaline mixing on thered giant branch are fairlyuncon- are significantly younger. For the highest-metallicity stars in the strained in terms of whether matter is transported away from the sample([Fe/H]≥−0.2),weuseisochronestoestimatetheages. hydrogen-burning shell quickly enough for the Cameron-Fowler 6 S. L. Martell&M. D. Shetrone Figure 7. Lithium abundance versus surface gravity for the stars in our Figure8.Lithiumabundanceversusmetallicityforthestarsinoursample. sample.Thetypicalvalueforlogǫ(Li)decreasesinthemoreevolvedstars. Thehigher-metallicitystarsshowabroaderrangeinlogǫ(Li),andahigher SymbolsareasinFigure6,andthereisnocleardifferenceinthebehavior mean,thanthelower-metallicitystars.Therateofdeepmixingisasensitive ofRGBstarsandredclumpstarsinthisplane. functionofstellarmetallicity, sothismayindicatethathigher-metallicity starsarebetterabletoproducelithium,orlessabletodestroyit.Symbols areasinFigure6. machanism toproduce Li.Figures 7and 8show our resulting Li abundancesversuslog(g)and[Fe/H],respectively,usingthesame symbolsasinFigure6. quitedifferent,andwebelievethatthediscrepancyislikelydueto uncertaintiesintheKepleranaylsistechniques.SDSSJ1909+3837 isKIC3531579,whichdoesnothaveanypublicallyavailableKe- 5 DISCUSSION plerdatayet. Thethreestarsinourhigh-resolution datasetthathavestel- We have identified 23 new Li-rich post-main-sequence stars: 5 larparametersincompatiblewiththeisochronesarequiteunusual. bright redgiants, 6red clump stars, 6RGB-bumpstars, 3 lower- With their very low surface gravities, we identify them as post- RGBstarsand3post-AGBstars.Althoughschematicmodelsexist AGB stars. Themetallicitieswe derive for two of them are quite for the production of Li at onset of deep mixing, near the RGB low([Fe/H]∼ −2.5).Withtheseverylowmetallicities,thesetwo bump,thereisnotacleartheoreticalexplanationfortheprocesses starsaremarginallyconsistentwithbeingRGB-tipstars,buttheex- thatproduceanddestroyLiinpost-main-sequencestars. tremelylowgravitiesarealsoconsistentwithaknownpopulation There are no targets in common between our data set and of metal-deficient post-AGB stars (Parthasarathy 2000). We note the six recent studies mentioned in the Introduction. However, that it has been hypothesized that very low metallicities in post- ourinitialsearchoftheDR7CASincludedSEGUEobservations AGBstarsarearesult ofdust formationinextremely cool atmo- in Galactic globular and open clusters, allowing a possible over- spheresandarenotrepresentativeoftheinitialstellarcomposition. lap with the studies of Pilachowski (1986) and Pilachowskietal. (1988).OurstarSDSSJ2356+5633istheknownLi-richgiantNGC 7789 K301 (Pilachowski 1986). It is chosen as a likely followup 5.1 Internalandexternalsourcesoflithium candidateaccordingtoourlow-resolutionselectioncriteria,andour high-resolution analysis confirms itshigh Liabundance. Thefact The initial photometric association of Li-rich stars with the thatthisstarisinaclustercanbeusedtocheckouranalysis.The RGB bump and the red clump led naturally to a suggestion meanmetallicityofNGC7789is−0.04±0.05and0.04±0.07 (Charbonnel&Balachandran 2000) that the destruction of the µ- from Tautvaisˇiene˙etal. (2005) and Pancinoetal. (2010), respec- barriercreatedbyfirstdredge-upcausesabriefepisodeofLipro- tively,whichisconsistentwithourvalueof−0.05±0.07.Inaddi- duction.ThiscannotbetheexplanationforhighLiabundancesin tion,forthisstarPilachowski(1986)notethatitisaweakG-band brightgiants,farfromtheRGBbump,butanyprocessthatcauses star,andwefindthatthisstarhasoneofthelowestcarbonabun- rapidmassmotionbetweentheenvelopeandthehydrogen-burning danceratiosinoursample,[C/Fe]=−0.7.Further,thepositionof shellcouldinprincipleproduceLiatthesurfaceviatheCameron- K301ontheCMDofNGC7789suggeststhatitmaybeanAGB Fowlermechanism:3He(α,γ)7Be(e−,ν)7Li. star,oritslowcarbonabundancemaycausethestartomoveblue- One potential mechanism for such stirring is proposed by wardintheCMDviatheBond-NeffEffect(Bond&Neff1969). Denissenkov (2012), who note that, in their models of red giants WedonotrecovertheLi-richstarsdiscussedinRoedereretal. withrapidinteriorrotation,thetemperaturegradientintheregion (2008), Kumar&Reddy (2009), Carneyetal. (1998), Kraftetal. betweenthehydrogen-burningshellandtheenvelopecanbecome (1999),Smithetal.(1999)orCharbonnel&Balachandran(2000), quitelarge,permittingdeepmixingtooperatemuchmorequickly whichwerenotobservedbySEGUE.TwooftheLi-richstarswe thanusual.Oneconsequenceofthisaccelerateddeepmixingisthat selected for high-resolution followup are serendipitously located thephotometricloopexecutedduringtheRGBbumpismuchmore in the Kepler field. SDSS J1901+3808 is KIC 2968828, which extended,pushingRGB-bumpstarsintothesamecolor-magnitude has asteroseismology-based stellar parameters of log(g)= 2.393, region as red clump stars. This indicates that perhaps the Li-rich T =4270,[Fe/H]=−0.823.Whilethesurfacegravityisagood starsthathavebeenidentifiedasredclumpstars(includingthesix eff match to our determination, the temperature and metallicity are inthisstudy)areactuallyRGB-bumpstarswithrapidinternalro- Lithium-richgiantsin SDSS 7 Table2.IDandstellarparametersfromhigh-resolutionanalysis,forthe23starsforwhichwecoulddeterminehigh-resolutionstellarparameters SDSSID Teff σ log(g) σ [Fe/H] σ vturb(kms−1) σ vrot(kms−1) Age(Gyr) EP SDSSJ2206+4531 4700 159 3.5 0.51 +0.41 0.17 1.95 0.29 ... 9 SGB SDSSJ2353+5728 5025 76 3.0 0.21 +0.23 0.08 1.67 0.11 ... 4 RC SDSSJ0808-0815 4900 103 3.0 0.41 +0.14 0.10 1.72 0.14 ... 2 RGB SDSSJ0301+7159 5075 63 3.1 0.18 +0.14 0.06 1.79 0.08 ... 1 RC SDSSJ2019+6012 4800 100 3.0 0.41 +0.06 0.10 1.70 0.13 ... 3 RGB SDSSJ0652+4052 4900 62 2.9 0.22 +0.04 0.07 1.73 0.08 ... 2 RC SDSSJ0245+7102 4850 70 2.4 0.25 +0.01 0.07 1.77 0.07 ... 4 RC SDSSJ0632+2604 5050 84 2.8 0.35 +0.00 0.09 1.84 0.11 ... 4 RC SDSSJ2356+5633 4750 73 2.5 0.26 −0.05 0.07 2.00 0.07 ... 1 RGB SDSSJ0535+0514 5200 64 2.8 0.22 −0.08 0.06 1.66 0.08 ... 4 RC SDSSJ2200+4559 4675 100 2.4 0.37 −0.12 0.11 1.99 0.39 6 2 RGB SDSSJ0304+3823 5125 64 2.6 0.24 −0.20 0.07 1.33 0.09 ... 4 RC SDSSJ1901+3808 4450 86 2.3 0.45 −0.29 0.08 1.78 0.09 ... 4 RGB SDSSJ0720+3036 5250 142 3.4 0.52 −0.29 0.15 1.40 0.25 10 4 SGB SDSSJ1909+3837 4450 123 2.2 0.47 −0.30 0.13 2.13 0.13 ... 4 RGB SDSSJ1105+2850 5625 156 3.9 0.58 −0.46 0.18 1.05 0.31 ... 4 SGB SDSSJ0654+4200 5200 50 2.6 0.18 −0.47 0.05 1.88 0.06 12.5 4 RC SDSSJ1607+0447 5175 100 2.0 0.34 −1.37 0.13 1.49 0.15 ... 10 RGB SDSSJ1310-0012 4550 66 1.0 0.28 −1.54 0.06 1.74 0.07 ... 12 RGB SDSSJ0936+2935 5125 120 0.0 0.60 −1.62 0.15 2.59 0.23 ... ... ... SDSSJ1522+0655 4975 140 1.9 0.58 −2.34 0.15 1.89 0.18 ... 12 RGB SDSSJ0831+5402 4725 106 0.2 0.66 −2.46 0.12 2.32 0.23 ... ... ... SDSSJ1432+0814 4625 38 0.3 0.3 −2.65 0.06 1.90 0.13 9.1 ... ... Table3.IDandabundancesfromhigh-resolutionanalysis,forthe23starsforwhichwecoulddeterminehigh-resolutionstellarparameters SDSSID [α/Fe] σ [C/Fe] σ 12C/13C logǫ(Li) σ logǫ(Li) σ 6708 6104 SDSSJ2206+4531 −0.29 0.07 −0.5 0.3 ... 2.80 0.30 ≤2.9 ... SDSSJ2353+5728 −0.08 0.06 −0.5 0.2 8 3.10 0.15 ≤3.2 ... SDSSJ0808-0815 −0.34 0.18 −0.6 0.2 ... 2.45 0.15 ≤2.7 ... SDSSJ0301+7159 −0.15 0.08 −0.6 0.1 15 2.70 0.10 ≤2.9 ... SDSSJ2019+6012 −0.08 0.06 −0.7 0.2 ... 2.77 0.20 ≤2.8 ... SDSSJ0652+4052 −0.05 0.06 −0.4 0.2 15 3.30 0.20 3.0 0.3 SDSSJ0245+7102 −0.07 0.04 −0.4 0.1 15 2.58 0.10 2.4 0.4 SDSSJ0632+2604 −0.08 0.05 −0.4 0.1 ... 4.20 0.2 4.1 0.2 SDSSJ2356+5633 +0.06 0.05 −0.7 0.1 ... 2.88 0.15 2.9 0.2 SDSSJ0535+0514 +0.06 0.04 −0.5 0.2 15 2.90 0.10 2.7 0.3 SDSSJ2200+4559 +0.05 0.08 −0.8 0.2 ... 3.05 0.20 3.0 0.2 SDSSJ0304+3823 −0.01 0.04 −0.3 0.2 ... 2.40 0.10 ≤2.6 ... SDSSJ1901+3808 +0.17 0.07 −0.7 0.1 ... 2.52 0.15 2.4 0.3 SDSSJ0720+3036 −0.23 0.13 −0.4 0.2 ... 4.55 0.20 4.2 0.2 SDSSJ1909+3837 −0.20 0.07 ≤−0.4 ... ... 1.95 0.15 ≤2.2 ... SDSSJ1105+2850 +0.09 0.48 −0.4 0.4 ... 3.24 0.30 ≤2.9 ... SDSSJ0654+4200 +0.02 0.06 −0.4 0.1 4 3.30 0.20 3.1 0.3 SDSSJ1607+0447 +0.33 0.06 −0.6 0.2 ... 2.55 0.15 2.7 0.3 SDSSJ1310-0012 +0.32 0.09 −0.7 0.2 ... 2.15 0.15 2.2 0.3 SDSSJ0936+2935 +0.38 0.11 0.3 0.4 ... 2.70 0.30 ≤3.0 ... SDSSJ1522+0655 +0.50 0.20 −0.7 0.3 ... 2.25 0.20 2.6 0.4 SDSSJ0831+5402 +0.33 0.11 0.1 0.3 ... 2.55 0.20 ≤2.7 ... SDSSJ1432+0814 +0.12 0.10 −0.2 0.2 ... 4.02 0.30 3.6 0.3 tation.Asteroseismicdataonthesestarscouldclarifytheirevolu- deepmixing.Thisisaprocessthatcouldinprinciplehappenatany tionary phase. This process is an interesting possibility, but does pointinstellarevolution,thoughthedramaticradialexpansionthat notexplainLi-richbrightRGBstars. occursasastarascendstheRGBwouldmakeitfarmorelikelyto happenduringthatphase. The engulfement of a giant planet has also been suggested (Carlbergetal.2012)asawayto“replenish”thesurfaceLiabun- danceofaredgiant.Thisprocessshouldbedistinguishablefrom intrinsic Li production by deep mixing, since the 12C/13C ratio shouldrisefollowingplanetaccretion,butshouldfallasaresultof 8 S. L. Martell&M. D. Shetrone 6 SUMMARY Brown, J. A., Sneden, C., Lambert, D. L., & Dutchover, Jr., E. 1989,ApJS,71,293 Theworkpresentedheredemonstratesthepotentialoflargespec- Caffau,E.,Bonifacio,P.,Franc¸ois,P.,etal.2011,Nature,477,67 troscopic surveys such as SEGUE as a source of unusual targets Cameron,A.G.W.&Fowler,W.A.1971,ApJ,164,111 forfollowupobservations.Theabilitytoselectourinitialdataset Carlberg,J.K.,Cunha,K.,Smith,V.V.,&Majewski,S.R.2012, fromaverybroadparameterspaceallowsustosearchforLi-rich ApJ,757,109 stars in a relatively unbiased way. Other followup studies based Carney,B.W.,Fry,A.M.,&Gonzalez,G.1998,AJ,116,2984 on SEGUE data (e.g., Caffauetal. 2011; Bonifacioetal. 2011; Charbonnel,C.&Balachandran,S.C.2000,A&A,359,563 Aokietal.2012) followthesameapproach: aninitialsearch ina de LaReza, R., Drake, N. A., & da Silva, L. 1996, ApJL, 456, large, multidimensional parameter space, which introduces some L115 heterogeneityinthedataset,butcanavoidbiasesinherentinmore Demarque, P.,Woo, J.-H.,Kim, Y.-C.,&Yi,S. K.2004, ApJS, focusedstudies. 155,667 AlthoughweagreewithKirbyetal.(2012)thatLi-richfield Denissenkov,P.A.2012,ApJL,753,L3 giantsareperfectlynormalstarsinmostrespects,wemustdisagree Frebel, A., Roederer, I. U., Shetrone, M., et al. 2008, in Astro- with their conclusion, originally expressed by deLaRezaetal. nomicalSocietyofthePacificConferenceSeries,Vol.393,New (1996), that the onset of deep mixing produces a brief phase of HorizonsinAstronomy,ed.A.Frebel,J.R.Maund,J.Shen,& Li enrichment in all stars. Such a universal process would imply M.H.Siegel,207 thatLi-richstarsshouldonlybefoundneartheRGBbump,which Fukugita,M.,Ichikawa,T.,Gunn,J.E.,etal.1996,AJ,111,1748 theyarenot.Weproposeinsteadthatdeepmixingiscapableofpro- Fulbright, J. P.,McWilliam, A.,& Rich, R.M. 2006, ApJ, 636, ducingLienrichmentatanypointabovetheRGBbump,basedon 821 thermohalinemixingmodels.Thisdoesnotexplainwhattriggersa Fulbright, J. 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S.D’Odorico,387–398 Venn,K.A.,Kaufer,A.,Tolstoy,E.,etal.2003,inIAUSympo- sium,Vol.212,AMassiveStarOdyssey:FromMainSequence toSupernova,ed.K.vanderHucht,A.Herrero,&C.Esteban, 30 Wallerstein,G.&Sneden,C.1982,ApJ,255,577 Yanny,B.,Rockosi,C.,Newberg,H.J.,etal.2009,AJ,137,4377 York,D.G.,Adelman,J.,Anderson,Jr.,J.E.,etal.2000,AJ,120, 1579 ACKNOWLEDGEMENTS We appreciate thoughtful comments from an anonymous referee, whichhavemadethispaperclearer. Duringthiswork,SLMwassupportedbySonderforschungs- bereich SFB 881 “The Milky Way System” (subprojects A2 and A5)oftheGermanResearchFoundation(DFG). The Hobby-Eberly Telescope (HET)isajoint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximillians-Universita¨t Mu¨nchen, and Georg-August- Universita¨tGo¨ttingen.TheHETisnamedinhonorofitsprincipal benefactors,WilliamP.HobbyandRobertE.Eberly. Funding for SDSS-III has been provided by the Alfred P. SloanFoundation, theParticipatingInstitutions,theNationalSci- enceFoundation,andtheU.S.DepartmentofEnergy.TheSDSS-III websiteishttp://www.sdss3.org/. SDSS-IIIismanagedbytheAstrophysicalResearchConsor- tium for the Participating Institutions of the SDSS-IIICollabora- tion including the University of Arizona, the Brazilian Participa- tionGroup,BrookhavenNationalLaboratory,UniversityofCam- bridge, Universityof Florida,theFrenchParticipationGroup, the German Participation Group, the Instituto de Astrofisica de Ca- narias,theMichiganState/NotreDame/JINAParticipationGroup, Johns Hopkins University, Lawrence Berkeley National Labora- tory, Max Planck Institute for Astrophysics, New Mexico State 10 S. L. Martell&M. D. Shetrone APPENDIXA: COMPARISONOFHIGH-RESOLUTION ANDSSPPSTELLARPARAMETERS During the course of this project, revised SSPP stellar parame- ters(includingeffectivetemperature,surfacegravityandmetallic- ity) were published for our stars as part of SDSS Data Release 9(SDSS-IIICollaborationetal. 2012).Sincewehave derived at- mosphericparametersfromourhigh-resolutionspectra,wepresent hereabriefstudy of thecomparison betweentheDR9SSPPand high-resolutionparameters,forthe20starswithreasonablehigh- resolutionparameters.High-resolutionstellarparametersarelisted inTable2;DR9SSPPparametersfor thesamestarsaregivenin TableA1. FiguresA1,A2andA3comparethehigh-resolutionvaluesfor T , log(g) and [Fe/H], respectively, to the DR9 SSPP values, eff forthe20starswithreasonablehigh-resolutionparameters.There isscatter, but not asignificant trend, between the high-resolution FigureA1.Differencebetweenhigh-resolutionandDR9SSPPdetermina- andSSPPdeterminations ofT and[Fe/H],butwefindasig- nificanttrendwithlittlescatterebfeftweenhigh-resolutionandSSPP tionsofTeff,asafunctionofhigh-resolutionTeff.Thebest-fitdashed line is consistent with nooffset between the two temperature scales, but determinations of log(g). We alsofind a significant trend, witha significantscatter.SymbolsareasinFigure6. slopeof102±39K/dex,betweenTHRS−TSSPPand[Fe/H]SSPP, eff eff indicating that the SSPP algorithms are somewhat susceptible to temperature-metallicitydegeneracy.Thefactthatweusedifferent iron lines to determine [Fe/H] for the metal-poor and metal-rich starsinoursamplecouldalsocontributetothistrend,butwewould notexpectthattocauseaneffectaslargeaswhatisobserved. FigureA2.Differencebetweenhigh-resolutionandDR9SSPPdetermina- tionsoflog(g),asafunctionofhigh-resolutionlog(g).Thebest-fitdashed lineshowesasignificanttrend.TheSSPPisknowntohavedifficultydeter- mininglog(g)inlow-gravitystars,sincetheindicatorsituseslosesensi- tivity;however,thetrendtohighgravityisunexpected.Symbolsareasin Figure6.

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Preview Lithium-Rich Field Giants in the Sloan Digital Sky Survey

Mon.Not.R.Astron.Soc.000,000–000(0000) Printed11December 2013 (MNLATEXstylefilev2.2) Lithium-rich field giants in the Sloan Digital Sky Survey S. L. Martell1⋆ and M. D. Shetrone2 1AustralianAstronomicalObservatory,NorthRydeNSW2122,Australia 3 2McDonaldObservatory,UniversityofTexasAustin,McDonaldObservatory,Texas79734,USA 1 0 2 Received25October2012/Accepted19December2012 n a J ABSTRACT 2 We present a search for post-main-sequence field stars in the Galaxy with atypically large lithium abundances. Using moderate-resolution spectra taken as part of the Sloan Digital ] R SkySurvey,alongwithhigh-resolutionfollowupspectroscopyfromtheHobby-EberlyTele- scope, we identify 23 post-turnoff stars with logǫ(Li) greater than 1.95, including 14 with S logǫ(Li) ≥ 2.3 and 8 with logǫ(Li) ≥ 3.0, well above the low level expected for evolved . h stars.ComparisonwiththeoreticalisochronesindicatesthatsomeofourLi-richstarsareaf- p filiatedwiththeupperredgiantbranch,theasymptoticgiantbranchandtheredclumprather - thantheRGBbump,whichisachallengetoexistingmodelsofLiproductioninevolvedstars. o r t Keywords: Stars:abundances s a [ 1 v 1 INTRODUCTION carbonandnitrogentoevolveconstantlyasstarsascendtheRGB 3 (see Martelletal. 2008 for a more thorough explanation of deep Theabundanceoflithiumisextremelysensitivetotheambientcon- 6 mixing).Inhigher-massstars,thedestructionoftheµ-barrieroc- ditionsinsidestars,sinceitscrosssectiontoprotoncaptureisquite 1 cursatseconddredge-up,lowontheasymptoticgiantbranch.The largeattemperaturesabove2.6×106K.Liisreadilydestroyedin 0 factthattheLi-richstarsidentifiedbyCharbonnel&Balachandran 1. tchoenvienctetirvioersenovfelloopwe-misaspsromgraeinss-isveeqluyendceeplsettaerds,aasnednvtheeloLpei imnatthee- (2000)werelocatednearthedestructionoftheµ-barrierledthose 0 authorstospeculatethattheonsetofdeepmixingproducesabrief rial is cycled past the innermost, hottest region of the convective 3 episode of Li production via the Cameron-Fowler mechanism zone.SurfaceLiabundanceisfurtherdepletedafterstarsleavethe 1 (Cameron&Fowler 1971). In this process, 7Be produced by the mainsequenceandundergofirstdredge-up(Iben1968),inwhich : proton-proton chain is transported to a cooler zone in the star, v theconvectiveenvelopedeepens,mixingfusion-processedmaterial whereitcapturesanelectronanddecaysinto7Li,whichisstable i from the stellar interior into the envelope and exposing envelope X againstprotoncaptureifitssurroundingsarecoolenough.Aphase materialtoevenhighertemperatures. r in which the deep-mixing mass transport speed was briefly high a Because of this, it was quite surprising when enoughtoallowthisprocesswouldcausesurfaceLiabundancesto Wallerstein&Sneden (1982) discovered a Li-rich K giant risesharplyandthendecay. serendipitously while investigating barium-rich field stars. More However, the existence of Li-rich stars in other regions of systematic searches for Li-rich stars (e.g., Brownetal. 1989; theHertzsprung-Russelldiagramchallengesthismodel:Kraftetal. Pilachowski 1986; Pilachowskietal. 1988; Pilachowskietal. (1999)foundonebrightredgiantwellabovetheRGBbumpinthe 2000) found that they are rare in the Solar neighborhood globularclusterM3,wheredeepmixingiswellknowntooperate and extremely unusual in globular clusters. The study of (Smith2002;Angelouetal.2011).Morerecently,sixindependent Charbonnel&Balachandran (2000) found that nearby Solar- studiesinthepastthreeyearshavereportedatotalof47newLi- metallicity Li-rich stars are either low-mass stars (M ≃ 1M⊙) richpost-main-sequencestars.ThesenewLi-richstarsfillabroad nearthe“bump”intheredgiantbranch(RGB)luminosityfunction or intermediate-mass stars (M ≃ 3M⊙) low on the asymptotic range of parameter space: low-mass (M ≤ 2M⊙) RGB stars in the thick disk with −1 ≤ [Fe/H] ≤ 0 (Monacoetal. 2011); an giant branch (AGB). In both cases, the hydrogen-burning shell intermediate-mass(M ≃ 4M⊙),Solar-metallicitystarinthethin in the stars in question has recently encountered, and destroyed, disk (Monacoetal. 2011); low-mass RGB stars with [Fe/H] ≃ the discontinuity in mean molecular weight (the “µ-barrier”) −0.9 in the Galactic bulge (Gonzalezetal. 2009; Lebzelteretal. left behind at the end of first dredge-up. In low-mass red giants, 2012); and low-mass RGB stars in Local Group dwarf galaxies, the destruction of the µ-barrier causes a brief loop in stellar twoofwhichhave[Fe/H]≃−2.7(Kirbyetal.2012). evolution, observed asthe RGB bump, and permits deep mixing, Twooftherecentstudies(Kumaretal.2011andRuchtietal. anonconvective processthatsteadilycyclesmaterialbetweenthe 2011) take the approach of selecting a well-defined subset from photosphereandtheradiativezone,causingsurfaceabundancesof anexistingcatalog(HipparcosandRAVE,respectively)forhigh- resolution spectroscopic followup and abundance analysis, a pro- ⋆ Email:[email protected](SLM);[email protected](MDS) cedurethatwealsoadopt.Inthispaper,wepresentasearchforLi- 2 S. L. Martell&M. D. Shetrone Figure1.DistributionofTeff andlog(g)forthe8535starsinourinitial Figure2.Metallicity histogramforourinitialdataset(heavysolidline), dataset. withhistograms forthethree data sources overplotted withadotted line (SEGUE),lightsolidline(Legacy)anddashedline(MARVELS). richfieldstarsinSloanDigitalSkySurvey(SDSS)spectroscopy. Bysurveying starsacrossawiderangeofmetallicityandsurface [Fe/H],andradialvelocity,alongwiththeequivalentwidthsand/or gravity,weareabletotakeabroadviewofthequestionofLi-rich lineindicesfor85atomicandmolecularabsorptionlines,bypro- post-main-sequence stars. This paper describes the 23 previously cessingthecalibratedspectrageneratedbythestandardSDSSspec- unknownLi-richstarsweidentifiedfromthisoriginaldataset,and troscopic reduction pipeline (Stoughtonetal. 2002). Additional exploreswhetherthevariousproposedmechanismsforLiproduc- validationsanddetailsofrecentupdatestoSSPParediscussedin tioninevolvedstarsaresufficienttoexplainthisgroupofLi-rich Smolinskietal.(2011). stars. 2.2 Targetselection WeselectedoursamplefromtheseventhSDSSdatarelease(DR7), 2 THEOBSERVATIONALDATASET using the online Catalog Archive Server (CAS)1 to identify stars In order to explore the physical conditions that produce large with a mean signal-to-noise ratio per pixel (SNR) of at least 40, amountsofphotosphericlithiuminsomeevolvedstars,weselected T ≤6000K,reasonableerrorson[Fe/H]metallicity(σ ≤ eff [Fe/H] asampleofpost-main-sequencestarsfromtheSDSSDataRelease 0.3) and surface gravity (σ ≤ 0.5), and surface gravity no log(g) 7.The8535starsinourinitialdatasetwererequiredtohavehigh- higherthan1.4dexgreaterthanthesurfacegravityatthe“bump” qualityspectraandrelativelylowerrorsonstellarparametersdeter- intheRGBluminosityfunction.Previousstudieshavefoundthat minedbytheautomatedSSPPpipeline(Leeetal.2008a),andwere stars with unexpectedly high lithium abundances tend to be near allowedtocoverabroadrangeofmetallicity,effectivetemperature the RGB bump, so we designed this gravity selection to include andsurfacegravity. stars at evolutionary phases from shortly before the RGB bump to the tip of the RGB. Empirically, the luminosity of the RGB bump is a function of metallicity: FusiPeccietal. (1990) report 2.1 SDSSspectroscopy [Fe/H]versusMbumpforaselectionofGalacticglobularclusters, V and we converted those M values to log(g) using metallicity- The imaging component of the original Sloan Digital Sky Sur- V matched 12 Gyr Yale-Yonsei isochrones (Demarqueetal. 2004), vey and its extensions (Fukugitaetal. 1996; Gunnetal. 1998; then fit a least-squares line to derive log(g) = 2.23235 + Yorketal.2000;Pieretal.2003;Gunnetal.2006;Stoughtonetal. bump (0.188018 ×[Fe/H]). This initial data set contained 8535 stars: 2002;Abazajianetal.2003,2004,2005;Adelman-McCarthyetal. 7667 fromSEGUE,471fromLegacyand 397fromMARVELS. 2006, 2007, 2008; Abazajianetal. 2009; Aiharaetal. 2011) in- Figure1showsT versuslog(g)forall8535stars,andFigure2 cludesugrizphotometryforseveralhundredmillionstars.SEGUE eff isahistogramofthemetallicities.InFigure2,theheavysolidhis- (Yannyetal.2009),oneofthreesub-surveysthattogetherformed togramshowsthemetallicitydistributionforthefulldataset,with SDSS-II,extended that imaging footprint by approximately 3500 deg2,andalsoobtainedR≃2000spectroscopyforapproximately thehistogramforjusttheSEGUEstarsshownasadottedline,the 240,000 stars over a wavelength range of 3800 − 9200A˚. Ad- histogramfortheLegacystarsdrawnwithalightsolidline,andthe histogramfortheMARVELSstarsshownwithadashedline. ditional spectroscopy with similar characteristics has been taken aspartoftheSDSSLegacyproject(Abazajianetal.2009)andin preparationfortheSDSS-IIIMulti-objectAPORadialVelocityEx- oplanet Large-area Survey (MARVELS) project (Geetal. 2008). 3 LOW-RESOLUTIONANALYSIS Tofacilitatethedevelopment andcalibrationoftheSEGUEStel- Inorder toidentify starswithpotentiallyhigh lithiumabundance lar Parameter Pipeline (SSPP; Leeetal. 2008a; Leeetal. 2008b; based on low-resolution spectra, we designed a spectral index, AllendePrietoetal. 2008), SEGUE included spectra for a col- lection of Galactic globular and open clusters. As described in Leeetal. (2008a), SSPP produces estimates of Teff, log(g), 1 http://skyservice.pha.jhu.edu/CasJobs/login.aspx Lithium-richgiantsin SDSS 3 Figure 4. Typical spectra from our initial SDSS low-resolution data set. Figure3.LithiumindexS(Li)versusTeff fortheinitialdataset(upper Thelowertwospectraappeartohavestrongabsorptioninthe6708A˚ Li panel);S(Li)versus[Fe/H](lowerpanel).Inbothpanels,starsselected resonanceline,whilethetopspectrumdoesnot. forthesecondarydatasetareshownasopentriangles. nitude(VmagnitudefortheMARVELSstars)andS(Li)measure- S(Li), that measures the magnitude difference between the inte- ment for the 36 stars initially flagged for followup spectroscopy. gratedfluxinthe6708A˚ resonancelineandtheintegratedfluxina MARVELSstarsaretoobrighttobeincludedinSEGUEphotom- smallregionofnearbyspectrum.Itisdefinedas etry, so their apparent V magnitudes were taken from the Vizier R6713I dλ catalog-searchservice2.Theplate,MJDandfiberIDinformation S(Li)=−2.5×log 6706 λ . isgiven because it isasimpleway for finding individual starsin 6738 R I dλ 6730 λ theCAS. Typically spectral indices are defined for use in small regions of parameterspace,anduseoneormorenearbyregion(s)ofspectrum independentofthepropertythatdrivesvariationsinthefeatureof 4 HIGH-RESOLUTIONANALYSIS interest as a comparison band; however, our initial data set con- tainsstarsacrosssuchawiderangeofT ,log(g)and[Fe/H]that Followupspectrawereobtainedfor34ofthese36starsusingthe eff therearenonearbyregionsofspectrumthatarethesameforallof High-Resolution Spectrograph (HRS) on the Hobby-Eberly Tele- ourstars.ThisfactintroducessomescattertoourS(Li)measure- scope(HET)atMcDonaldObservatory(Tull1998).Allbutoneof ments,requiringextracareinselectinglikelyLi-enhancedstarsfor these34starswerefirstobservedasfillertime(P4)duringnormal followupobservations. queueobserving(Shetroneetal.2007)duringperiods2010T2and TheupperpanelofFigure3showsS(Li)versusT forthe 2011T1.Thisfillertimeallowedustotakeaninitialshortsnapshot eff initial data set, and the lower panel shows S(Li) versus [Fe/H] spectrumtoconfirmthestengthoftheLiline;7werefoundtonot for the same stars. In each plane, S(Li) has a broad distribution beLirichand27wereconfirmedLirich.For23oftheseLirich atafixedvalueoftheindependentparameter(T or[Fe/H])and starswewereabletofollow-upwithadditional spectratogetthe eff a slight dependence on that parameter. To identify outliers in the SNR sufficient for determination of stellar parameters and abun- S(Li)-T andS(Li)-[Fe/H]distributions,weselectallstarswith dances.Oneofthe36stars(SDSSJ0831+5402) hadalreadybeen eff S(Li) ≥ 0.295− Teff andS(Li) ≥ 0.18+ [Fe/H] asoursec- abservedaspartoftheChemicalAbundancesofStarsintheHalo ondarydataset.The4r×e1a0r4e162starsmeetingbothcr1i0t0eria,andthese (CASH)project(Frebeletal.2008). areplottedasopentrianglesinFigure3,whileallotherstarsfrom theinitialdatasetarerepresentedbysmallfilledcircles. 4.1 SpectroscopicObservationsandReduction Fromby-eyeexaminationeachofthespectrainthissecondary datasetof162stars,wefoundthatourselectioncriteriaweregen- HRSwasconfiguredtoHRS 15k central 316g5936 2as 0sky IS0 GC0 2x5 erous:fewerthanhalfofthestarsselectedbasedonS(Li)-Teff duringourHETobservationstoachieveR=18,000spectracovering - [Fe/H] position had 6708A˚ Li lines that appeared legitimately 4233A˚ to5900A˚ onthebluechipand6000A˚ to7850A˚ onthered strong in the SDSS spectra. Figure 4 shows spectra for two of chip. Exposure times per visit ranged from 300 seconds to 1800 the starswe selected for high-resolution followup (thelower two secondsbasedontheobservingconditions.Becauseofthenature spectra), as wellas one star withlittleapparent absorption inthe oftheHETandthegenerallypoorconditionsoffillertime,some 6708A˚ Li line (top spectrum). We generated low-resolution syn- targetshadmultiplevisitstakenoveranumberofdaystobuildup theticspectrabasedonSSPPstellarparameterstoroughlyestimate enoughS/NtocleanlydetecttheLifeature. lithiumabundanceforthosepotentiallylithium-enhancedstars,and The spectra were reduced with IRAF3 ECHELLE scripts. selected36ofthemforhigh-resolutionfollowupobservations,with thegoalof determiningmoreprecisestellarparametersandǫ(Li) abundances.Table1listsSDSSIDnumber,datasource(SEGUE, 2 http://vizier.cfa.harvard.edu/viz-bin/VizieR LegacyorMARVELS),plug-platenumber,modifiedJuliandateof 3 IRAF(ImageReductionandAnalysisFacility)isdistributedbytheNa- observation,fiberIDnumber,rightascension,declination,gmag- tionalOpticalAstronomyObservatories,whichareoperatedbytheAssoci- 4 S. L. Martell&M. D. Shetrone Table1.Staridentifiers,position,photometryandlithiumindexforthe36starsselectedforfollowupspectroscopy. ApparentVmagnitudesarelistedfor MARVELSstars. SDSSID Source Plate MJD FiberID α δ g0 S(Li) ConfirmedLi-richwithHET SDSSJ2206+4531 SEGUE 2556 54000 640 22:06:19.75 45:31:57.19 15.732 0.209 SDSSJ2353+5728 SEGUE 2377 53991 337 23:53:29.76 57:28:18.41 12.960 0.209 SDSSJ0808-0815 SEGUE 2806 54425 256 08:08:34.33 -08:15:47.30 14.598 0.198 SDSSJ0301+7159 MARVELS 2843 54453 634 03:01:33.77 71:59:11.16 10.570 0.197 SDSSJ2019+6012 SEGUE 2554 54263 600 20:19:32.44 60:12:46.32 15.270 0.212 SDSSJ0652+4052 MARVELS 2847 54452 557 06:52:37.80 40:52:01.44 11.170 0.212 SDSSJ0245+7102 MARVELS 2843 54453 168 02:45:20.83 71:02:02.63 11.760 0.184 SDSSJ0632+2604 SEGUE 2678 54173 188 06:32:30.90 26:04:37.46 15.152 0.234 SDSSJ2356+5633 SEGUE 2377 53991 200 23:56:52.22 56:33:38.83 13.080 0.210 SDSSJ0535+0514 MARVELS 2841 54451 299 05:35:43.75 05:14:22.62 12.310 0.191 SDSSJ2200+4559 SEGUE 2556 54000 540 22:00:51.22 45:59:40.33 15.453 0.229 SDSSJ0304+3823 SEGUE 2441 54065 453 03:04:37.40 38:23:46.12 15.357 0.181 SDSSJ1901+3808 SEGUE 2536 53883 306 19:01:49.00 38:08:51.00 14.29 0.214 SDSSJ0720+3036 SEGUE 2677 54180 255 07:20:42.79 30:36:51.49 16.667 0.209 SDSSJ1909+3837 SEGUE 2536 53883 110 19:09:29.25 38:37:46.23 15.764 0.214 SDSSJ1105+2850 SEGUE 2870 54534 599 11:05:36.91 28:50:08.09 17.006 0.185 SDSSJ0654+4200 MARVELS 2847 54552 614 06:54:58.00 42:00:27.00 12.51 0.220 SDSSJ1607+0447 SEGUE 2178 54629 378 16:07:09.23 04:47:12.73 16.139 0.173 SDSSJ1310-0012 SEGUE 2901 54652 191 13:10:37.22 -00:12:44.40 15.252 0.185 SDSSJ0936+2935 SEGUE 2889 54530 164 09:36:27.44 29:35:35.80 16.240 0.185 SDSSJ1522+0655 SEGUE 2902 54629 33 15:22:11.86 06:55:55.22 15.912 0.165 SDSSJ0831+5402 SEGUE 2316 53757 395 08:31:55.28 54:02:45.55 15.430 0.186 SDSSJ1432+0814 SEGUE 2933 54617 107 14:32:07.15 08:14:06.15 17.047 0.221 ConfirmedLi-richwithHETbutnotenoughSNRforfullanalysis SDSSJ0515+1558 SEGUE 2668 54084 214 05:15:23.17 15:58:55.38 14.555 0.216 SDSSJ2048+5603 SEGUE 2555 54265 58 20:48:46.82 56:03:53.65 13.423 0.200 SDSSJ2119-0734 SEGUE 2305 54414 230 21:19:47.65 -07:34:26.89 14.526 0.167 SDSSJ1214+5027 SEGUE 2919 54537 630 12:14:54.88 50:27:00.99 18.205 0.177 ConfirmednotLi-richwithHET SDSSJ1421+0136 Legacy 534 51997 162 14:21:28.26 01:36:28.35 17.077 0.200 SDSSJ1213-0147 Legacy 333 52313 331 12:13:53.32 -01:47:14.93 16.558 0.180 SDSSJ1227-0026 SEGUE 2558 54140 115 12:27:12.72 -00:26:58.81 16.990 0.177 SDSSJ0144-1005 SEGUE 2850 54461 12 01:44:49.17 -10:05:31.09 16.738 0.171 SDSSJ0352-0540 SEGUE 2051 53738 357 03:52:43.59 -05:40:03.32 16.487 0.168 SDSSJ1725+0729 SEGUE 2797 54616 304 17:25:04.06 07:29:45.03 14.807 0.168 SDSSJ1849+1931 SEGUE 2812 54633 393 18:49:52.58 19:31:04.20 14.980 0.168 NotobservedwithHET SDSSJ1824+7911 SEGUE 2802 54326 372 18:24:22.51 79:11:24.47 16.548 0.201 SDSSJ2021+1257 SEGUE 2248 53558 291 20:21:25.73 12:57:51.33 15.263 0.198 The standard IRAF scripts for overscan removal, bias subtrac- Johnson&Pilachowski (2010), and Koch&McWilliam (2008). tion, flat fielding and scattered light removal were employed. Lithium abundance was determined by spectral synthesis of the For the HRS flat field we masked out the Li I and Na D re- 6708A˚ and 6104A˚ resonance lines. We also determined carbon gions because the HET HRS flat field lamp suffered from inter- abundance and 12C/13C ratios, where possible, by synthesis of mittent faint emission lines which would otherwise compromise the 4297A˚ CH feature. The CH linelist for the carbon analysis our analysis. Equivalent widths (EW) were measured using the wastakenfromPlez(priv.comm.);allotherlinesweretakenfrom IRAFtasksplotandforcingtheGaussianfittohavefixedFWHM a version of the Kurucz&Bell (1995) linelist modified to fit the for those lines below 150 mA˚ while allowing the strong lines strongest lines to the Solar spectrum. Model atmospheres were to have a variable FWHM. We also fit multiple Gaussian pro- takenfromtheOsmarcs2005grid(Gustafssonetal.2008),inter- files to any nearby lines to take blends into account. Metallic- polating between the grid points. These spherical models cover ity and α-element abundances were determined through equiva- metallicity ranges from +1.0 to -3.0 with [α/Fe] = +0.4 for lentwidthanalysis;oscillatorstrengthsadoptedfortheEWanaly- [Fe/H] <= −1.5, [α/Fe] = +0.3 for [Fe/H] = −0.75, sisweretakenfromFulbrightetal.(2006),Fulbrightetal.(2007), [α/Fe] = +0.2 for [Fe/H] = −0.5, [α/Fe] = +0.1 for [Fe/H]=−0.25and[α/Fe]=+0.for[Fe/H]>=0.0). Theabundancecalculationswereperformedusingamodified ationofUniversitiesforResearchinAstronomy,Inc.,undercontractwith versionofthe2010versionoftheLTElineanalysisandspectrum theNationalScienceFoundation. synthesis code MOOG (Sneden 1973). This code has some sig- Lithium-richgiantsin SDSS 5 Figure5.The23starsforwhichwederivehigh-resolutionstellarparame- Figure6.Effective temperature andsurface gravityfromhigh-resolution tersandabundances,intheα–Feplane.Dashedlinesmarkthebordersof analysis, with measurement errors, for our 23 Li-rich stars. Lower- ourroughageestimations. metallicitystarsareintheleftpanel,andhigher-metallicitystarsareinthe rightpanel.Thedifferentsymbolsrepresentgroupsinevolutionaryphase, andrepresentativeisochronesareoverplotted.Horizontallinesmarkthepo- nificant differences from the earlier versions including using the sitionoftheRGBbumponeachisochrone,ascalculatedfromourempir- predictedabundancevs.reducedEWfordeterminingthemicrotur- icalrelationgiveninSection2.2.TrianglesareRGBstars,circlesarered bulence.Initialestimatesofthestellarparametersweretakenfrom clumpstars,andsquaresarestarsthathavenotyetreachedfirstdredge-up the SSPP pipeline. From this starting point the temperature was (whoseLiabundancesarenotrelevanttothequestionofLiproductionon adjusteduntilthetherewasnosignificant slopeintheFeIabun- theRGB).Thethreeplaincrossesatthetopoftheleftpanelarestarswhose danceswithrespecttolineexcitationpotential.Thestellargravity log(g)andTeff arenotcompatiblewithanyreasonableisochrone,andwe wasadjustedsuchthattheFeIandFeIIlinesproducedthesame believethemtobepost-AGBstars. abundance withinthe errors. Microturbulence wasdetermined by requiringthattherewasnosignificantslopeintheFeIabundance vs. predicted Fe I reduced equivalent widths based on the mean Figure6demonstratesthatourinitial[α/Fe]–[Fe/H]ageesti- abundanceofthelines. mationwasreasonable:bothpanelscompareourvaluesofT and eff Table2listsSDSSID,alongwithT ,log(g)and[Fe/H]and eff log(g) measured from high-resolution spectra against representa- theirassociatederrors,plusmicroturbulentvelocityandrotational tiveisochrones,witholderstarsintheleftpanelandyoungerstars velocity,wheremeasurable,derivedfromourhigh-resolutionspec- intherightpanel,andthemajorityofstarslienearthegiantbranch tra,andTable3listsSDSSIDandabundancesof[α/Fe],[C/Fe], correspondingtotheappropriateinitialvalues.Thehorizontallines 12C/13C, and logǫ(Li) determined from synthesis of the 6707A˚ ineachpanelmarkthelog(g)attheRGBbumpforeachisochrone and6104A˚ lines.Alpha-elementabundancesaretheerror-weighted shown,calculatedfromourempiricalcalibrationgiveninSection average of [Mg/Fe], [Ca/Fe] and [Ti/Fe], and the total error on 2.2.Thesquaresrepresentstarsthathavenotyetbeguntoascend [α/Fe] is the larger of the propagated errors or the dispersion in theRGB,andtheirhighLiabundancesarepresumablyprimordial. thethreeinputabundances. ThetrianglesrepresentRGBstars,bothyoungandold,andthecir- clesshowredclumpstars,whichallsittotheleftoftheisochrone attheirmetallicity,andareallatintermediateage.Thethreecrosses 4.2 Evolutionarystatesofthestars atthetopoftheleftpanelarethreelow-metallicitystarswithpa- PrevioussearchesforLi-richstarshavefocusedongroupsofstars rameters that are apparently inconsistent withthe isochrones; we withsimilarities:forexample,Lebzelteretal.(2012)selectedonly willaddressthesestarsinSec.5.ThelasttwocolumnsofTable2 RGB stars in the Galactic bulge, with metallicities in a narrow giveourfinalestimatesforageandevolutionaryphase.Theseval- range,andKumaretal.(2011)usetheHipparcoscatalog,inwhich uesarenotgivenforthethreestarswithparametersincompatible thedistancestoallstarsareknownquiteprecisely,astheirinitial withtheisochrones. data set. However, our data set is heterogeneous in terms of age, Weareabletodetermine12C/13Cforallsixofourredclump compositionandpositionintheGalaxy.Basedonthemetallicities stars,usingsynthesisofthe4297A˚ CHfeature.Inallofthesestars, andalpha-enhancementsderivedfromthehigh-resolutionspectra, theratioisfairlylow(12C/13C≤15),indicatingthatfirstdredge- wedivideourLi-richstarsintofourbroadgroupsandassignrep- uphasefficientlymixedthesurfacematerialwiththestellarinte- resentative ages. Figure 5 shows the four groups in the [α/Fe] rior. – [Fe/H] plane, which is commonly used to compare the star- The Li-rich stars are mainly found low on the RGB, which formation and chemical-enrichment histories of composite stel- might be expected from Li production associated with the onset larpopulations indwarfgalaxiestothecomponents oftheMilky ofdeepmixing.However,thepresenceofsomebrightLi-richgi- Way(e.g.,Vennetal.2003;Shetroneetal.2003;Kochetal.2008; antsandredclumpstarsindicatesthatitmustalsobepossiblefora Kirbyetal.2011).Briefly,moderate-andlow-metallicitystarsare startoproduceLiduringotherevolutionaryphases.Currentmodels assumedtobeold,whilehigher-metallicity,non-α-enhancedstars of thermohaline mixing on thered giant branch are fairlyuncon- are significantly younger. For the highest-metallicity stars in the strained in terms of whether matter is transported away from the sample([Fe/H]≥−0.2),weuseisochronestoestimatetheages. hydrogen-burning shell quickly enough for the Cameron-Fowler 6 S. L. Martell&M. D. Shetrone Figure 7. Lithium abundance versus surface gravity for the stars in our Figure8.Lithiumabundanceversusmetallicityforthestarsinoursample. sample.Thetypicalvalueforlogǫ(Li)decreasesinthemoreevolvedstars. Thehigher-metallicitystarsshowabroaderrangeinlogǫ(Li),andahigher SymbolsareasinFigure6,andthereisnocleardifferenceinthebehavior mean,thanthelower-metallicitystars.Therateofdeepmixingisasensitive ofRGBstarsandredclumpstarsinthisplane. functionofstellarmetallicity, sothismayindicatethathigher-metallicity starsarebetterabletoproducelithium,orlessabletodestroyit.Symbols areasinFigure6. machanism toproduce Li.Figures 7and 8show our resulting Li abundancesversuslog(g)and[Fe/H],respectively,usingthesame symbolsasinFigure6. quitedifferent,andwebelievethatthediscrepancyislikelydueto uncertaintiesintheKepleranaylsistechniques.SDSSJ1909+3837 isKIC3531579,whichdoesnothaveanypublicallyavailableKe- 5 DISCUSSION plerdatayet. Thethreestarsinourhigh-resolution datasetthathavestel- We have identified 23 new Li-rich post-main-sequence stars: 5 larparametersincompatiblewiththeisochronesarequiteunusual. bright redgiants, 6red clump stars, 6RGB-bumpstars, 3 lower- With their very low surface gravities, we identify them as post- RGBstarsand3post-AGBstars.Althoughschematicmodelsexist AGB stars. Themetallicitieswe derive for two of them are quite for the production of Li at onset of deep mixing, near the RGB low([Fe/H]∼ −2.5).Withtheseverylowmetallicities,thesetwo bump,thereisnotacleartheoreticalexplanationfortheprocesses starsaremarginallyconsistentwithbeingRGB-tipstars,buttheex- thatproduceanddestroyLiinpost-main-sequencestars. tremelylowgravitiesarealsoconsistentwithaknownpopulation There are no targets in common between our data set and of metal-deficient post-AGB stars (Parthasarathy 2000). We note the six recent studies mentioned in the Introduction. However, that it has been hypothesized that very low metallicities in post- ourinitialsearchoftheDR7CASincludedSEGUEobservations AGBstarsarearesult ofdust formationinextremely cool atmo- in Galactic globular and open clusters, allowing a possible over- spheresandarenotrepresentativeoftheinitialstellarcomposition. lap with the studies of Pilachowski (1986) and Pilachowskietal. (1988).OurstarSDSSJ2356+5633istheknownLi-richgiantNGC 7789 K301 (Pilachowski 1986). It is chosen as a likely followup 5.1 Internalandexternalsourcesoflithium candidateaccordingtoourlow-resolutionselectioncriteria,andour high-resolution analysis confirms itshigh Liabundance. Thefact The initial photometric association of Li-rich stars with the thatthisstarisinaclustercanbeusedtocheckouranalysis.The RGB bump and the red clump led naturally to a suggestion meanmetallicityofNGC7789is−0.04±0.05and0.04±0.07 (Charbonnel&Balachandran 2000) that the destruction of the µ- from Tautvaisˇiene˙etal. (2005) and Pancinoetal. (2010), respec- barriercreatedbyfirstdredge-upcausesabriefepisodeofLipro- tively,whichisconsistentwithourvalueof−0.05±0.07.Inaddi- duction.ThiscannotbetheexplanationforhighLiabundancesin tion,forthisstarPilachowski(1986)notethatitisaweakG-band brightgiants,farfromtheRGBbump,butanyprocessthatcauses star,andwefindthatthisstarhasoneofthelowestcarbonabun- rapidmassmotionbetweentheenvelopeandthehydrogen-burning danceratiosinoursample,[C/Fe]=−0.7.Further,thepositionof shellcouldinprincipleproduceLiatthesurfaceviatheCameron- K301ontheCMDofNGC7789suggeststhatitmaybeanAGB Fowlermechanism:3He(α,γ)7Be(e−,ν)7Li. star,oritslowcarbonabundancemaycausethestartomoveblue- One potential mechanism for such stirring is proposed by wardintheCMDviatheBond-NeffEffect(Bond&Neff1969). Denissenkov (2012), who note that, in their models of red giants WedonotrecovertheLi-richstarsdiscussedinRoedereretal. withrapidinteriorrotation,thetemperaturegradientintheregion (2008), Kumar&Reddy (2009), Carneyetal. (1998), Kraftetal. betweenthehydrogen-burningshellandtheenvelopecanbecome (1999),Smithetal.(1999)orCharbonnel&Balachandran(2000), quitelarge,permittingdeepmixingtooperatemuchmorequickly whichwerenotobservedbySEGUE.TwooftheLi-richstarswe thanusual.Oneconsequenceofthisaccelerateddeepmixingisthat selected for high-resolution followup are serendipitously located thephotometricloopexecutedduringtheRGBbumpismuchmore in the Kepler field. SDSS J1901+3808 is KIC 2968828, which extended,pushingRGB-bumpstarsintothesamecolor-magnitude has asteroseismology-based stellar parameters of log(g)= 2.393, region as red clump stars. This indicates that perhaps the Li-rich T =4270,[Fe/H]=−0.823.Whilethesurfacegravityisagood starsthathavebeenidentifiedasredclumpstars(includingthesix eff match to our determination, the temperature and metallicity are inthisstudy)areactuallyRGB-bumpstarswithrapidinternalro- Lithium-richgiantsin SDSS 7 Table2.IDandstellarparametersfromhigh-resolutionanalysis,forthe23starsforwhichwecoulddeterminehigh-resolutionstellarparameters SDSSID Teff σ log(g) σ [Fe/H] σ vturb(kms−1) σ vrot(kms−1) Age(Gyr) EP SDSSJ2206+4531 4700 159 3.5 0.51 +0.41 0.17 1.95 0.29 ... 9 SGB SDSSJ2353+5728 5025 76 3.0 0.21 +0.23 0.08 1.67 0.11 ... 4 RC SDSSJ0808-0815 4900 103 3.0 0.41 +0.14 0.10 1.72 0.14 ... 2 RGB SDSSJ0301+7159 5075 63 3.1 0.18 +0.14 0.06 1.79 0.08 ... 1 RC SDSSJ2019+6012 4800 100 3.0 0.41 +0.06 0.10 1.70 0.13 ... 3 RGB SDSSJ0652+4052 4900 62 2.9 0.22 +0.04 0.07 1.73 0.08 ... 2 RC SDSSJ0245+7102 4850 70 2.4 0.25 +0.01 0.07 1.77 0.07 ... 4 RC SDSSJ0632+2604 5050 84 2.8 0.35 +0.00 0.09 1.84 0.11 ... 4 RC SDSSJ2356+5633 4750 73 2.5 0.26 −0.05 0.07 2.00 0.07 ... 1 RGB SDSSJ0535+0514 5200 64 2.8 0.22 −0.08 0.06 1.66 0.08 ... 4 RC SDSSJ2200+4559 4675 100 2.4 0.37 −0.12 0.11 1.99 0.39 6 2 RGB SDSSJ0304+3823 5125 64 2.6 0.24 −0.20 0.07 1.33 0.09 ... 4 RC SDSSJ1901+3808 4450 86 2.3 0.45 −0.29 0.08 1.78 0.09 ... 4 RGB SDSSJ0720+3036 5250 142 3.4 0.52 −0.29 0.15 1.40 0.25 10 4 SGB SDSSJ1909+3837 4450 123 2.2 0.47 −0.30 0.13 2.13 0.13 ... 4 RGB SDSSJ1105+2850 5625 156 3.9 0.58 −0.46 0.18 1.05 0.31 ... 4 SGB SDSSJ0654+4200 5200 50 2.6 0.18 −0.47 0.05 1.88 0.06 12.5 4 RC SDSSJ1607+0447 5175 100 2.0 0.34 −1.37 0.13 1.49 0.15 ... 10 RGB SDSSJ1310-0012 4550 66 1.0 0.28 −1.54 0.06 1.74 0.07 ... 12 RGB SDSSJ0936+2935 5125 120 0.0 0.60 −1.62 0.15 2.59 0.23 ... ... ... SDSSJ1522+0655 4975 140 1.9 0.58 −2.34 0.15 1.89 0.18 ... 12 RGB SDSSJ0831+5402 4725 106 0.2 0.66 −2.46 0.12 2.32 0.23 ... ... ... SDSSJ1432+0814 4625 38 0.3 0.3 −2.65 0.06 1.90 0.13 9.1 ... ... Table3.IDandabundancesfromhigh-resolutionanalysis,forthe23starsforwhichwecoulddeterminehigh-resolutionstellarparameters SDSSID [α/Fe] σ [C/Fe] σ 12C/13C logǫ(Li) σ logǫ(Li) σ 6708 6104 SDSSJ2206+4531 −0.29 0.07 −0.5 0.3 ... 2.80 0.30 ≤2.9 ... SDSSJ2353+5728 −0.08 0.06 −0.5 0.2 8 3.10 0.15 ≤3.2 ... SDSSJ0808-0815 −0.34 0.18 −0.6 0.2 ... 2.45 0.15 ≤2.7 ... SDSSJ0301+7159 −0.15 0.08 −0.6 0.1 15 2.70 0.10 ≤2.9 ... SDSSJ2019+6012 −0.08 0.06 −0.7 0.2 ... 2.77 0.20 ≤2.8 ... SDSSJ0652+4052 −0.05 0.06 −0.4 0.2 15 3.30 0.20 3.0 0.3 SDSSJ0245+7102 −0.07 0.04 −0.4 0.1 15 2.58 0.10 2.4 0.4 SDSSJ0632+2604 −0.08 0.05 −0.4 0.1 ... 4.20 0.2 4.1 0.2 SDSSJ2356+5633 +0.06 0.05 −0.7 0.1 ... 2.88 0.15 2.9 0.2 SDSSJ0535+0514 +0.06 0.04 −0.5 0.2 15 2.90 0.10 2.7 0.3 SDSSJ2200+4559 +0.05 0.08 −0.8 0.2 ... 3.05 0.20 3.0 0.2 SDSSJ0304+3823 −0.01 0.04 −0.3 0.2 ... 2.40 0.10 ≤2.6 ... SDSSJ1901+3808 +0.17 0.07 −0.7 0.1 ... 2.52 0.15 2.4 0.3 SDSSJ0720+3036 −0.23 0.13 −0.4 0.2 ... 4.55 0.20 4.2 0.2 SDSSJ1909+3837 −0.20 0.07 ≤−0.4 ... ... 1.95 0.15 ≤2.2 ... SDSSJ1105+2850 +0.09 0.48 −0.4 0.4 ... 3.24 0.30 ≤2.9 ... SDSSJ0654+4200 +0.02 0.06 −0.4 0.1 4 3.30 0.20 3.1 0.3 SDSSJ1607+0447 +0.33 0.06 −0.6 0.2 ... 2.55 0.15 2.7 0.3 SDSSJ1310-0012 +0.32 0.09 −0.7 0.2 ... 2.15 0.15 2.2 0.3 SDSSJ0936+2935 +0.38 0.11 0.3 0.4 ... 2.70 0.30 ≤3.0 ... SDSSJ1522+0655 +0.50 0.20 −0.7 0.3 ... 2.25 0.20 2.6 0.4 SDSSJ0831+5402 +0.33 0.11 0.1 0.3 ... 2.55 0.20 ≤2.7 ... SDSSJ1432+0814 +0.12 0.10 −0.2 0.2 ... 4.02 0.30 3.6 0.3 tation.Asteroseismicdataonthesestarscouldclarifytheirevolu- deepmixing.Thisisaprocessthatcouldinprinciplehappenatany tionary phase. This process is an interesting possibility, but does pointinstellarevolution,thoughthedramaticradialexpansionthat notexplainLi-richbrightRGBstars. occursasastarascendstheRGBwouldmakeitfarmorelikelyto happenduringthatphase. The engulfement of a giant planet has also been suggested (Carlbergetal.2012)asawayto“replenish”thesurfaceLiabun- danceofaredgiant.Thisprocessshouldbedistinguishablefrom intrinsic Li production by deep mixing, since the 12C/13C ratio shouldrisefollowingplanetaccretion,butshouldfallasaresultof 8 S. L. Martell&M. D. Shetrone 6 SUMMARY Brown, J. A., Sneden, C., Lambert, D. L., & Dutchover, Jr., E. 1989,ApJS,71,293 Theworkpresentedheredemonstratesthepotentialoflargespec- Caffau,E.,Bonifacio,P.,Franc¸ois,P.,etal.2011,Nature,477,67 troscopic surveys such as SEGUE as a source of unusual targets Cameron,A.G.W.&Fowler,W.A.1971,ApJ,164,111 forfollowupobservations.Theabilitytoselectourinitialdataset Carlberg,J.K.,Cunha,K.,Smith,V.V.,&Majewski,S.R.2012, fromaverybroadparameterspaceallowsustosearchforLi-rich ApJ,757,109 stars in a relatively unbiased way. Other followup studies based Carney,B.W.,Fry,A.M.,&Gonzalez,G.1998,AJ,116,2984 on SEGUE data (e.g., Caffauetal. 2011; Bonifacioetal. 2011; Charbonnel,C.&Balachandran,S.C.2000,A&A,359,563 Aokietal.2012) followthesameapproach: aninitialsearch ina de LaReza, R., Drake, N. A., & da Silva, L. 1996, ApJL, 456, large, multidimensional parameter space, which introduces some L115 heterogeneityinthedataset,butcanavoidbiasesinherentinmore Demarque, P.,Woo, J.-H.,Kim, Y.-C.,&Yi,S. K.2004, ApJS, focusedstudies. 155,667 AlthoughweagreewithKirbyetal.(2012)thatLi-richfield Denissenkov,P.A.2012,ApJL,753,L3 giantsareperfectlynormalstarsinmostrespects,wemustdisagree Frebel, A., Roederer, I. U., Shetrone, M., et al. 2008, in Astro- with their conclusion, originally expressed by deLaRezaetal. nomicalSocietyofthePacificConferenceSeries,Vol.393,New (1996), that the onset of deep mixing produces a brief phase of HorizonsinAstronomy,ed.A.Frebel,J.R.Maund,J.Shen,& Li enrichment in all stars. Such a universal process would imply M.H.Siegel,207 thatLi-richstarsshouldonlybefoundneartheRGBbump,which Fukugita,M.,Ichikawa,T.,Gunn,J.E.,etal.1996,AJ,111,1748 theyarenot.Weproposeinsteadthatdeepmixingiscapableofpro- Fulbright, J. P.,McWilliam, A.,& Rich, R.M. 2006, ApJ, 636, ducingLienrichmentatanypointabovetheRGBbump,basedon 821 thermohalinemixingmodels.Thisdoesnotexplainwhattriggersa Fulbright, J. 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S.D’Odorico,387–398 Venn,K.A.,Kaufer,A.,Tolstoy,E.,etal.2003,inIAUSympo- sium,Vol.212,AMassiveStarOdyssey:FromMainSequence toSupernova,ed.K.vanderHucht,A.Herrero,&C.Esteban, 30 Wallerstein,G.&Sneden,C.1982,ApJ,255,577 Yanny,B.,Rockosi,C.,Newberg,H.J.,etal.2009,AJ,137,4377 York,D.G.,Adelman,J.,Anderson,Jr.,J.E.,etal.2000,AJ,120, 1579 ACKNOWLEDGEMENTS We appreciate thoughtful comments from an anonymous referee, whichhavemadethispaperclearer. Duringthiswork,SLMwassupportedbySonderforschungs- bereich SFB 881 “The Milky Way System” (subprojects A2 and A5)oftheGermanResearchFoundation(DFG). The Hobby-Eberly Telescope (HET)isajoint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximillians-Universita¨t Mu¨nchen, and Georg-August- Universita¨tGo¨ttingen.TheHETisnamedinhonorofitsprincipal benefactors,WilliamP.HobbyandRobertE.Eberly. Funding for SDSS-III has been provided by the Alfred P. SloanFoundation, theParticipatingInstitutions,theNationalSci- enceFoundation,andtheU.S.DepartmentofEnergy.TheSDSS-III websiteishttp://www.sdss3.org/. SDSS-IIIismanagedbytheAstrophysicalResearchConsor- tium for the Participating Institutions of the SDSS-IIICollabora- tion including the University of Arizona, the Brazilian Participa- tionGroup,BrookhavenNationalLaboratory,UniversityofCam- bridge, Universityof Florida,theFrenchParticipationGroup, the German Participation Group, the Instituto de Astrofisica de Ca- narias,theMichiganState/NotreDame/JINAParticipationGroup, Johns Hopkins University, Lawrence Berkeley National Labora- tory, Max Planck Institute for Astrophysics, New Mexico State 10 S. L. Martell&M. D. Shetrone APPENDIXA: COMPARISONOFHIGH-RESOLUTION ANDSSPPSTELLARPARAMETERS During the course of this project, revised SSPP stellar parame- ters(includingeffectivetemperature,surfacegravityandmetallic- ity) were published for our stars as part of SDSS Data Release 9(SDSS-IIICollaborationetal. 2012).Sincewehave derived at- mosphericparametersfromourhigh-resolutionspectra,wepresent hereabriefstudy of thecomparison betweentheDR9SSPPand high-resolutionparameters,forthe20starswithreasonablehigh- resolutionparameters.High-resolutionstellarparametersarelisted inTable2;DR9SSPPparametersfor thesamestarsaregivenin TableA1. FiguresA1,A2andA3comparethehigh-resolutionvaluesfor T , log(g) and [Fe/H], respectively, to the DR9 SSPP values, eff forthe20starswithreasonablehigh-resolutionparameters.There isscatter, but not asignificant trend, between the high-resolution FigureA1.Differencebetweenhigh-resolutionandDR9SSPPdetermina- andSSPPdeterminations ofT and[Fe/H],butwefindasig- nificanttrendwithlittlescatterebfeftweenhigh-resolutionandSSPP tionsofTeff,asafunctionofhigh-resolutionTeff.Thebest-fitdashed line is consistent with nooffset between the two temperature scales, but determinations of log(g). We alsofind a significant trend, witha significantscatter.SymbolsareasinFigure6. slopeof102±39K/dex,betweenTHRS−TSSPPand[Fe/H]SSPP, eff eff indicating that the SSPP algorithms are somewhat susceptible to temperature-metallicitydegeneracy.Thefactthatweusedifferent iron lines to determine [Fe/H] for the metal-poor and metal-rich starsinoursamplecouldalsocontributetothistrend,butwewould notexpectthattocauseaneffectaslargeaswhatisobserved. FigureA2.Differencebetweenhigh-resolutionandDR9SSPPdetermina- tionsoflog(g),asafunctionofhigh-resolutionlog(g).Thebest-fitdashed lineshowesasignificanttrend.TheSSPPisknowntohavedifficultydeter- mininglog(g)inlow-gravitystars,sincetheindicatorsituseslosesensi- tivity;however,thetrendtohighgravityisunexpected.Symbolsareasin Figure6.

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