Lithium-Rich Field Giants in the Sloan Digital Sky Survey

5.6 MB·English

by  Sarah L. Martell

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
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Received25October2012/Accepted19December2012

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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
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6 SUMMARY Brown, J. A., Sneden, C., Lambert, D. L., & Dutchover, Jr., E.
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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.