Extremely metal-poor stars in SDSS fields

0.16 MB·

by  Piercarlo Bonifacio

Astron. Nachr./AN999,No.88,789–795(2011)/DOIpleasesetDOI!
Extremely metal-poor stars in SDSS fields,⋆
P.Bonifacio1,⋆⋆,E.Caffau2,1,P.Franc¸ois1,L.Sbordone3,1,H.-G.Ludwig2,1,M.Spite1,P.Molaro4,F.
Spite1,R.Cayrel1,F.Hammer1,V.Hill5,M.Nonino4,S.Randich6,B.Stelzer7,andS.Zaggia8
1 GEPI,ObservatoiredeParis,CNRS,Universite´ParisDiderot,PlaceJulesJanssen,92190Meudon,France
2 Zentrumfu¨rAstronomiederUniversita¨tHeidelberg,Landessternwarte,Ko¨nigstuhl12,69117Heidelberg,Germany
3 Max-PlanckInstitutfu¨rAstrophysik,Karl-Schwarzschild-Str.1,85741Garching,Germany
1 4 IstitutoNazionalediAstrofisica,OsservatorioAstronomicodiTrieste,ViaTiepolo11,34143Trieste,Italy
1 5 Universite´deNiceSophiaAntipolis,CNRS,ObservatoiredelaCoˆted’Azur,LaboratoireCassiope´e,B.P.4229,06304
0
NiceCedex4,France
2
6 IstitutoNazionalediAstrofisica,OsservatorioAstrofisicodiArcetri,LargoE.Fermi5,50125Firenze,Italy

n 7 IstitutoNazionalediAstrofisica,OsservatorioAstronomicodiPalermo,PiazzadelParlamento1,90134Palermo,Italy
a 8 IstitutoNazionalediAstrofisica,OsservatorioAstronomicodiPadovaVicolodell’Osservatorio5,35122Padova,Italy
J

9
Received20Dec2010,accepted Jan2011
1
Publishedonlinelater

]
R
Keywords Galaxy:abundances-Galaxy:formation-Galaxy:halo-stars:PopulationII-stars:distances
S
. Someinsightonthefirstgenerationofstarscanbeobtainedfromthechemicalcompositionoftheirdirectdescendants,
h extremelymetal-poor stars(EMP),withmetallicitylessthanorequal to1/1000ofthesolarmetalllicity.Suchstarsare
p exceedinglyrare,themostsuccessfulsurveys,forthispurpose,havesofarprovidedonlyabout100starswith1/1000the
- solarmetallicityand4starswithabout1/10000ofthesolarmetallicity.TheSloanDigitalSkySurveyhasthepotential
o
toprovidealargenumberofcandidatesofextremelylowmetallicity.X-Shooterhastheuniquecapabilityofperforming
r
t thenecessaryfollow-upspectroscopyprovidingaccuratemetallicitiesandabundanceratiosforseveralelements(Mg,Al,
s
Ca,Ti,Cr,Sr,...)forEMPcandidates.Weherereportontheresultsforthefirsttwostarsobservedinthecourseofour
a
franco-italianX-ShooterGTO.Thetwostarsweretargetedtobeofmetallicityaround–3.0,theanalysisoftheX-Shooter
[
spectrashowedthemtobeofmetallicityaround–2.0,butwithalowαtoironratio,whichexplainstheunderestimateof

2 themetallicityfromtheSDSSspectra.TheefficiencyofX-ShooterallowsaninsitustudyoftheouterHalo,forthetwo
v starsstudiedhereweestimatedistancesof3.9and9.1Kpc,thesearelikelythemostdistantdwarfstarsstudiedindetail
9 todate.
3
1 (cid:13)c 2011WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
3
.
1
. starsthatcanbefoundwilltelluswhenthestar-formation
0
1 switched from producing only massive stars to producing
1 stars of all masses, like observedtoday. Finally the chem-
1 Introduction
: ical composition of the most metal-poorstars, that are the
v
i direct descendants of the first generation, provides us in-
X Startingfromtheverysimple chemicalcompositionofthe
directinformationonthe first generationandthe chemical
r Universe emerged from the Big Bang nucleosythesis (hy- elementsthattheyproduced.
a
drogen,helium and a trace of lithium, which we shall call
The data of the Sloan Digital Sky Survey (SDSS,
the primordial chemical composition), the more complex
Adelman-McCarthyetal.2008;Yorketal.2000)provides
elementshavebuiltupasproductsofseveralgenerationsof
anexcellentopportunityforsearchingforprimordialstars.
stars. Our Galaxy holds the fossil record of this build-up:
Thankstoitshighefficiency,alongwithintermediatereso-
thechemicalcompositionoftheoldeststars.Theveryfirst
lution and broad spectral coverage,X-Shooter (D’Odorico
generationofstarsnecessarilyhadtheprimordialchemical
etal.2006)offerstheuniquepossibilityoffollow-upspec-
composition.Ifinthisfirstgeneration,starsoflowmasshad
troscopy of the EMP candidates in the magnitude range
been formed, they would still be shining on the Main Se-
18 ≤ g ≤ 20. The surface density of EMP stars in this
quencetoday.Thesearchforstarsofextremelylowmetal-
magnituderangeissolow(lessthanonepersquaredegree)
licitycouldallowustofindsuchstars,iftheyeverexisted.
thatitisnotworthtousemulti-objectspectrographs.
If not, we can then conclude that the first generation stars
wereallmassiveandarethusextinct.Thelowestmetallicity
2 Targetselection
⋆ basedonspectraobtainedwithX-Shooteratthe8.2mKueyenESO
telescope,GTOprogrammes085.D-0194and086.D.0094
⋆⋆ Correspondingauthor:e-mail:[email protected] Ourtargetswereselectedamongthestarswhichhavespec-
traintheSloanDigitalSkySurvey.Thetwostarsherede-
(cid:13)c 2011WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
790 P.Bonifacioetal.:Extremelymetal-poorstarsfromSDSS
aimed at obtaining any spatial information (the stars are
point sources), but to collect as many photons as possible
with resolution corresponding to that of a slit of 0′.′6, i.e.
R=7900 in the UVB arm and R=12600 in the VIS arm.
SDSS J135046.74+134651.1 was observed on April 7th
2010, with a single integration of 5400 s, in the IR arm
theintegrationtimewasdividedinto6DITsof900seach.
SDSS J135516.29+001319.1 was observed on May 17th
2010, with two integrations of 3150s each, in the IR arm
eachintegrationwassplitinto 4DITsof787s. Thediffer-
ent integrations were co-added. The spectra were reduced
usingtheX-Shooterpipeline(Goldonietal.2006).TheIR
armdidnotcontainanyusefulinformation,thepresentpa-
perisbasedontheanalysisoftheUVBandVISarms.The
UVBarmcoverstherange300-559.5nmandtheVISarm
559.5-1024nm.
Fig.1 The SDSS spectra of the program stars in the region The signal to noise ratios achieved were S/N=37 at
around the CaII K line compared to the X-Shooter spectra. The 415nm and38 at 660nm for SDSS J135046.74+134651.1
factthatthislineisweakerthantheneighbouringHlinesisadiag-
and S/N=18 at 415nm and 20 at 660nm for SDSS
nosticforextremelylowmetallicity.IntheX-Shooterspectrumof
J135516.29+001319.1.
SDSSJ135046.74+134651.1 welabelled“IS”theweakinterstel-
larCaIIKline.
3.1 Analysis
scribedwereextractedfromtheanalysisofSDSSDataRe- Our abundanceanalysis is based on 1D ATLASmodel at-
lease 6 (SDSS-DR6 Adelman-McCarthy et al. 2008), and mospheres.Webeganwithatmospherescomputedwithver-
have colours compatible with the Halo Turn-Off. Temper- sion 9 of the ATLAS code (Kurucz 1993, 2005) on Linux
atures are derived from g −z colour by using the colour- (Sbordone et al. 2004; Sbordone 2005). In this version of
temperature calibration presented in Ludwig et al. (2008). ATLAS the line opacity is treated through Opacity Distri-
For the X-Shooter sample we concentrated on the stars bution Functions (ODFs) and we used the ones computed
fainter than g = 18 with colours −0.3 ≤ g − z ≤ 0.7 byCastelli&Kurucz (2003),foramicroturbulentvelocity
andu−g > 0.7.Themetallicitieswereestimatedusinga of 1kms−1. For SDSS J135516.29+001319.1, given that
version of the automatic abundancedeterminationcode of itsabundancepatternissignificantlydifferentfromthatas-
Bonifacio & Caffau (2003), tailored to the resolution and sumedin thecomputationofthe ODFs, we also computed
coverageoftheSDSSspectra.Allthe starswithestimated aspecificmodelatmospherewithversion12oftheATLAS
metallicitiesbelow–2.0wherethenvisuallyinspected. code,whichusesopacitysampling(Castelli 2005a;Kurucz
ThetwostarsselectedareSDSSJ135046.74+134651.1 2005),andourfinalanalysisisbasedonthismodel.Convec-
and SDSS J135516.29+001319.1, whose basic data are tionwastreatedinthemixing-lengthapproximationwitha
summarized in Table 1. From the analysis of the SDSS mixing-lengthparameterα =1.25.
MLT
spectra SDSS J135046.74+134651.1was expectedto have
We also used a 3D hydrodynamicalmodelatmosphere
a metallicity [M/H]∼ −31 . As a first try we did not se- (3Dmodelhereafter)computedwiththeCO5BOLDcode,
lectparticularlyextremeobjects,the goalbeingto demon-
(for details see Freytag et al. 2011) to compute 3D
strate the possibility of using X-Shooter spectra for abun- corrections. This CO5BOLD model has parameters T /
eff
dance analysis. For SDSS J135516.29+001319.1there are
logg/[M/H]=6320K/4.5/–2.0,and the series of 20 selected
two SDSS spectra, one provided a metallicity around –3,
snapshotscovers2.5h. Asa reference1D modelswe used
the other around –2. In Fig. 1 the SDSS spectra in the re-
two 1D model atmospheres, h3Di and 1D : h3Di is
LHD
gionaroundtheCaIIKlineareshownandcomparedtothe obtained from the 3D model by averaging all snapshots
X-Shooter spectra, to emphasize the increase in resolution
horizontally at surfaces of equal Rosseland optical depth;
andS/Nratio.
1D is computedwith the code LHD, that employsthe
LHD
same microphysics and radiative transfer scheme as the
3 Observations anddata reduction CO5BOLD code. For both these models, in the spectral
synthesisweusedamicroturbulenceof1.5kms−1,andthe
mixing-lengthparameteroftheLHDmodelwasfixedat0.5.
Both stars were observed in visitor mode with the IFU
in stare mode2. In our case the use of the IFU was not The3DcorrectionsaredefinedasinCaffauetal.(2010),
thedifferenceoftheabundancederivedfromthe3Dmodel
1 [X/H]=log(X/H)-log(X/H)⊙
2 For a full description of X-Shooter observing modes and //www.eso.org/sci/facilities/paranal/instruments/
related acronyms see the instrument documentation at http: xshooter/index.html
(cid:13)c 2011WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2011) 791
Table1 CoordinatesandSDSSphotometryofthetargetstars
Name α(2000) δ(2000) g (g−z)0 (i−z)0 E(B−V) Vr
kms−1
SDSSJ135046.74+134651.1 13:50:46.740 13:46:51.00 18.29 0.320 0.048 0.027 −331±1
SDSSJ135516.29+001319.1 13:55:16.290 00:13:19.00 18.97 0.131 −0.004 0.040 −9±1
andtheonederivedfromthe1Dreferencemodels:∆ = low and ignoring it would result in an effective tempera-
gran
A(X) −A(X) toisolatethegranulationeffects,due turelowerby65K.Asacheck,wealsoderivedT byfit-
3D h3Di eff
tohorizontalfluctuations;∆ =A(X) −A(X) tingtheHαwings.Thetheoreticalprofileswerecomputed
LHD 3D 1DLHD
to measure the effects that both the horizontal fluctuation using the self broadening theory of Barklem et al. (2000)
andthetemperaturestructurehaveontheabundancedeter- andtheStarkbroadeningofStehle´ &Hutcheon(1999).As
mination. doneinSbordoneetal.(2010),weusedtheCIFISTgridof
In the spectrum synthesis we used the atomic parame- CO5BOLDmodelsLudwigetal.(2009),agridof1DLHD
tersusedinthe“FirstStars”survey(Bonifacioetal.2009; modelswithamixinglengthparameterof0.5,agravityof
Cayreletal.2004;Franc¸oisetal.2007).Thereferenceso- 4.5,andmetallicity-2.0and–3.0,andananalogousgridof
lar iron abundance 7.52 is from Caffau et al. (2010). For ATLAS9models.FromtheCO5BOLDmodelsweobtaina
theotherelementsthesolarreferenceisfromLoddersetal. temperatureof6420K(seeFig.A1),fromthe1DLHDmod-
(2009). els 6240K, andfromthe ATLASmodels6400K. Thedif-
We measured the equivalent widths (EW) of the lines ferencebetween1DLHD andATLASmodelsisnotsurpris-
ing, given the differenttemperaturestructures of the mod-
intheobservedspectrausingtheIRAFtasksplotandde-
els. The photometric and Hα-based temperatures are con-
rivedtheabundancewiththecodeWIDTH(Castelli 2005b;
Kurucz2005).We usedtheline positionsofthe linesused sistent within ∼ 100K, and each of them has an associ-
ated error which is again of about 100K. Moreover, the
fortheabundanceanalysisintheUVBspectrumtoderivea
accuracy of the present analysis is limited mainly by the
radialvelocityofthestars.TheresultsaregiveninTable1
limited S/N ratio and resolutionof the spectra, rather than
along with the error on the mean of the velocities. We did
by the temperature uncertainty. We thus decided to adopt
not observe any radial velocity standards, thus we do not
thephotometrictemperature.Thegravityhasbeenfixedat
have a good estimate of the external error on these radial
velocities. For star SDSS J135046.74+134651.1the SDSS logg=4.5,basedontheFeIandFeIIequilibrium.Theres-
spectrumprovidesaradialvelocityof(−299±24)kms−1, olutionof X-Shooter,evenwith the IFU, and the achieved
S/N ratio, do not allow to measure weak lines, on the lin-
compatiblewithourmeasurementatlessthan2σ.Forstar
ear part of the curve of growth. Thereforethe microturbu-
SDSS J135516.29+001319.1,there are two SDSS spectra,
whichprovide(−43±34)kms−1and(−33±30)kms−1, lencecannotbederivedfromthe observation.Thisparam-
eter is not a really independent parameter in 1D analysis,
although both measurements are compatible with ours, it
andit canbe calibratedasa functionof effectivetempera-
is possible that the star is a radial velocity variable.These
ture and, mainly, surface gravity. We decided to apply the
comparisonssuggestthattheexternalerrorisnotlargerthan
afewkms−1. relation of Edvardssonet al. (1993)which yields a micro-
turbulenceof1.5kms−1.Itmustbenotedthatinthisway
The large negative radial velocity of SDSS
microtrubulenceandsurface gravityare strictly correlated.
J135046.74+134651.1 allows to clearly resolve the in-
Anychangeinsurfacegravityimpliesachangeinmicrotur-
terstellar CaII K line, as shown in Fig. 1. The line has an
bulence,whichimpliesachangeintheabundancesofboth
EWof1.6pm,suchaweaklineisexpected,giventhelow
FeI and FeII, and thus a change in gravity. Therefore the
reddeningofthisstar.ForstarSDSSJ135516.29+001319.1
finallyadoptedvalueshadtobedeterminediteratively.
the low radial velocity implies that the photospheric line
is contaminated by the interstellar absorption, however,
since this star has an even lower reddening,we expectthe
4.2 Abundances
interstellar line to be even weaker, thus the contamination
shouldbelessthan1%.
The derived abundancesare given in Table2, while in Ta-
bleA1 all the lines considered are listed with the atomic
parameters, the equivalentwidth (EW), the abundancede-
4 SDSS J135046.74+134651.1
rived from the photometric temperature and from the Hα
3D bestfit, the 3D corrections,and the abundancederived
4.1 Stellarparameters
afterapplyingthe3Dcorrections.
From the (g −z) colour we derived an effective temper- The star is metal-poor, [Fe/H]= −2.3, but not around
0
ature of 6284K. The colour excess providedby the SDSS −3.0, as expected from the analysis of the SDSS spec-
catalogue for this star is given in Table 1 and is is based trum.Thestrikingthingisthattheironcontentissimilarto
on the reddeningmaps of Schlegelet al. (1998).It is very the abundance of α-elements. This is unusual because the
www.an-journal.org (cid:13)c 2011WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
792 P.Bonifacioetal.:Extremelymetal-poorstarsfromSDSS
Table2 Chemicalabundancesfromthe1DATLASabundance LAS+SYNTHE synthetic profiles we fitted the observed
analysisofSDSSJ135046.74+134651.1. profile (see Fig.2). We determined the EW of the best fit
andusedthisEW toderivethe3D-NLTEabundanceofLi
El. [X/H] [X/H] N† A‡
⊙ byusingthefittingfunctionofSbordoneetal.(2010),which
T =6284K T =6420K
eff eff
MgI −2.30±0.06 −2.15±0.06 2 7.54 yields A(Li)=2.38.This value is compatiblewith the level
SiI −2.54 −2.43 1 7.52 of the Spite plateau (Sbordone et al. 2010; Spite & Spite
CaI −2.15 −1.99 1 6.33 1982).
CaII −2.21±0.21 −2.12±0.19 4 6.33
FeI −2.33±0.20 −2.17±0.20 12 7.52
5 SDSS J135516.29+001319.1
FeII −2.28 −2.25 1 7.52
†Numberoflines
5.1 Stellarparameters
‡A(X)=log(X/H)+12
The effective temperature from the (g −z) colour is ex-
0
tremely high, T =6765K. Also for this star the redden-
eff
ing (see Table 1) is low, ignoringthe reddeningcorrection
would provide a temperature which is only 106K lower,
thereforestillextremelyhigh.Accordingtopublishedstel-
lar isochrones, such a hot star should be very young, un-
less it is a Blue Straggler star, or lies on the Horizontal
Branch. We note however that the evolutionary tracks of
Piauetal.(2006),computedwithCNOenhancedchemical
composition, predict such a hot Turn-Off for a population
Fig.2 ThefitoftheLidoublet at670.7nmwithagridofAT-
of 12Gyrs. The coloursof thisstar couldin fact suggesta
LAS+SYNTHEprofiles.Asevidentfromthefeatureontheblue
sideoftheLi,theLilineiscomparablewiththenoiseofthespec- CNO-enhancedmetal-poorstar.
trum. The3DfitoftheH-αprofilegivesaconsiderablylower
effectivetemperatureof6300K.Amongthe othercolours,
(g − r) and (r − i) support the high T indicated by
0 0 eff
metal-poorHalostarsdisplaygenerallyanenhancedabun-
(g−z) ;the(i−z) colourisconsiderablysensitivetothe
0 0
dance of α-elements. This expected overabundance of α-
surface gravity,it supportsthe hightemperaturefor logg=
elements is in fact folded into our method of analysis of
4.5,butforlogg=4.0itimpliesT =6300K.
eff
SDSS spectra where metallicity is derived also from lines
We thereforedecidedtoadoptaneffectivetemperature
of α-elements. In factat low metallicity the dominantfea-
of6300K,whichisconsistentwiththeHaloTurn-Off.The
ture is the CaII K line. It is thus not surprising that such
iron ionisation equilibrium implies logg= 3.9 and the mi-
a non-α enhanced star is estimated to have a too low iron
croturbulencederivedfromtheEdvardssonetal.(1993)cal-
abundancefromtheSDSSspectra.
ibration is 2.3 kms−1. We note that the inconsistency be-
The elements detectable in the observed spectrum are
tweencolourandHα,aswellasthepossibleinconsistency
Fe (both FeI and FeII), Mg, Si, and Ca (both neutral and of(g−z) and(i−z) ,isahintofapeculiarspectralenergy
0 0
singlyionised).
distribution.
We apply the 3D corrections, ∆ , to the 1D analy-
LHD
sis based on the temperaturederivedfromthe photometry.
5.2 Abundances
∆ correctionsarenegative,meaningthattheabundance
LHD
derivedfrom3Dmodelatmosphereissmallerthantheone The abundances derived assuming both effective tempera-
derived from 1D reference model. This is due to the fact tures(photometricandHα)areprovidedinTable3.Among
thathydrodynamicalmodelsareusuallycoolerthanhydro- the α elements Si and Mg appear underabundantwith re-
staticalmodelsintheouterpartoftheatmosphere.Thelines spect to iron, but it should be noted that the measurement
formedintheoutermostlayersarethusmoreaffectedbythe is based on a single line for each of the two elements.
cooling effect. Usually lines of neutral elements with low Giventhisunusualchemicalcomposition,differentfromthe
excitationenergy,showthelargesteffects.∆graniscloseto onesavailableintheODFscomputedbyCastelli&Kurucz
zero,meaningthatfortheobservedlinesthehorizontalfluc- (2003), and the importance of Mg as electron donor, we
tuationsdonotplayadeterminantrole,thestrongesteffect computedmodelswithversion12oftheATLAScode,us-
isofalmost0.2dexforthelineofFeII. ingthedesiredchemicalcomposition.Theabundanceswere
thenrecomputedwiththisnewmodel.Thedifferenceswith
respecttothoseobtainedfromanATLASmodelcomputed
4.3 Liabundance
with version 9 and non-α-enhancedODFs are very small.
The Li feature at 670.7nm is not detected, its EW being At these low metallicities and high gravities, even consis-
at the level of the noise, so that we consider the abun- tentvariationsofMg abundancehaveminoreffectson the
dance derived as an upper limit. With a 1D grid of AT- modelstructure.
(cid:13)c 2011WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2011) 793
Table3 Chemicalabundancesfromthe1DATLASabundance flat part of the curveof growth are little sensitive to abun-
analysisforSDSSJ135516.29+001319.1. dance, so that a large error is always associated to abun-
dances derived from such lines. The situation is better for
El. [X/H] [X/H] N A⊙
lineswhichareevenstrongerandlieinthedampingpartof
T =6765K T =6300K
eff eff
MgI −1.85 −2.08 1 7.54 thecurveofgrowth,althoughinthiscase thedampingpa-
AlI −1.34 −1.66 1 6.47 rametersshouldbeaccuratelyknown.Formetal-poordwarf
SiI −1.95 −2.19 1 7.52 stars, however, even the CaII K line has weak wings and
CaII −1.31±0.25 −1.59±0.03 2 6.33 is usually on the flatpartof the curveof growth.A partial
TiII −0.84±0.53 −1.33±0.49 7 4.90 compensationis providedbytheverylargespectralcover-
CrI −0.82 −1.39 1 5.64 ageprovidedbyX-Shooter,whichallowstomeasuremany
FeI −1.29±0.31 −1.74±0.39 12 7.52 features,improvingtheabundanceaccuracy.This,however,
FeII −1.40±0.27 −1.78±0.30 3 7.52 for metal poor stars, applies only to iron, since the other
SrII −1.11±0.20 −1.65 1 2.92
speciesarerepresentedbyonlyoneorfewlines.
BaII −1.52 −2.10 1 2.17
The two stars studied in these first GTO runs did not
turnouttobeasmetal-poorasexpectedfromtheanalysisof
TheCaIIabundanceinTable3isbasedontwolinesof theSDSSspectra,themainreasonbeing,inbothcases,the
the IR triplet. It must be remarked that the CaII K line is weaknessoftheCaIIKlinewithrespecttotheironlines.In
not compatible with this abundance and is consistent with factouranalysisofSDSS spectraestimatesthe metallicity
a Ca abundancewhich is lower by 0.7dex.Since the CaII byusingallavailablefeaturesassumingthatαelementsare
K line is the main abundance indicator at low metallicity enhancedby0.4 dexoveriron.
fortheSDSSspectra,thisexplainswhywehadestimateda
InthecaseofSDSSJ135046.74+134651.1,forthefew
muchlowermetallicitythanwhatresultsfromtheanalysis
measurable elements, the abundances appear to be solar-
of the X-Shooter spectrum. Also the CaI line, implies an
scaled. The existence of metal-poorstars with low, almost
abundancewhichis0.5dexhigherthantheCaIIIRtriplet.
solar α to iron ratios was pointed outby Nissen & Schus-
ItshouldbenotedthataccordingtotheNLTEcomputations
ter(1997),however,themostmetal-poorstarsofthatsam-
ofMashonkinaetal.(2007)thecorrectionsforCaIItriplet
plehadametallicityabout–1.0.Remarkableknownmetal-
linesarealwaysnegative,thoseforCaI4226˙ nmarealways poor α-poor stars are BD+ 80◦ 245, with a metallicity of
positive. Thus the cause of this discrepancy should not be
–1.8 and a logarithmic α to iron ratio of –0.3 (Carney
due to NLTE effects, which would rather tend to increase
et al. 1997) and CS 22873-139 (Spite et al. 2000) with
thediscrepancy.
a metallicity of –3.4, [Mg/Fe]=–0.04, [Ca/Fe]=+0.16 and
We could not convincingly detect the LiI 670.7 nm [Ti/Fe]=+0.55.Thelatterstarisadoublespectrumspectro-
feature. The EW upper limit is 5.4pm corresponding to
scopicbinary.Atlowermetallicitiesstarswithbizarrecom-
A(Li)<2.98atatemperatureof6765Kand2.63at6300K.
positions are known, such as HE 1424-0241(Cohen et al.
Weinspectedcarefullythespectrumforanysignofcar-
2007),withametallicityaround–4.0andSiunderabundant
bonenhancement,lookingattheG-bandandalltheCIlines by 1dex, but magnesium “normally” enhanced, or SDSS
which are observed in carbon-enhanced metal-poor stars
J234723.64+010833.4(Lai et al. 2009) which has a small
(seee.g.Beharaetal.2010),butcouldnotdetectany.
underabundanceof Mg ([Mg/Fe]=–0.1) but a strong over-
abundanceofCa([Ca/Fe]=+1.1),atametallicityof–3.2.
6 Discussion SDSS J135046.74+134651.1appears to have an abun-
dance pattern similar to BD+ 80◦ 245, and CS 22873-
These first observations of metal-poor SDSS candidates 139. The most straight forward interpretation of these α–
withX-Shooterallowsustoestimatethestrengthsandlim- poormetal–poorstarsisthattheywereformedinlow-mass
itations of this instrument for this application. The great galaxies,satellitesoftheMilkyWay,characterizedbyalow
strength is the high efficiency of X-Shooter, which has al- or bursting star-formation rate. The original galaxies have
lowed us to obtain a workable spectrum of a star of 19th beendisruptedduetotidalinteraction,andtheirdebrispop-
magnitude in less than two hours. The use of the IFU ulates the Halo. It is interesting to point out that the sur-
(Guinouard et al. 2006) as image slicer has been success- viving satellite galaxies have indeed low α to iron ratios,
ful. The transmission of the IFU in the UVB arm is lower althoughonlyCarinadisplaysessentiallysolarαtoironra-
than at longer wavelengths. The opportunity of using the tiosata metallicityaround–2.0(see the reviewofTolstoy
IFU ratherthana slitof0′.′6 dependsofcourseonthesee- etal.2009).However,atevenlowermetallicity([Fe/H]=∼3
ing.Inthecaseoftheobservationsdescribedheretheseeing andbelow),solar-scaledαelementabundancesaredetected
wasalwaysbetween1′.′0and1′.′2,theadvantageofusingthe in stars of Sculptor and Sextans dSph (Tafelmeyer et al.
IFUisunquestionable. 2010). Thus, in a naive interpretation,the majority of sur-
ThemainlimitationofX-Shooterforabundanceanaly- vivingsatelliteshadahigherstarformationratethanCarina,
sisistheresolution,whichistoolowtoallowustomeasure Sculptor,Sextansorthedisruptedsatellites,suchastheone
linesonthelinearpartofthecurveofgrowth.Linesonthe in which SDSS J135046.74+134651.1originated. A more
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TableA1 Abundancefromeachline,fromthe1DATLAS+WIDTH,and3DcorrectionsforSDSSJ135046.74+134651.1.
Element λ ξ log gf EW† ∆gran ∆LHD [X/H] [X/H] [X/H]
eV [pm] TeffK TeffK 1D+∆gran
=6284 =6420
FeI 381.5840 1.485 0.237 16.1 –0.03 –0.21 –1.96 –1.78 –2.17
FeI 382.0425 0.860 0.119 15.0 –0.09 –0.33 –2.47 –2.28 –2.80
FeI 385.6371 0.052 –1.286 10.6 –0.14 –0.56 –2.30 –2.11 –2.86
FeI 386.5523 1.011 –0.982 8.6 0.00 –0.34 –2.17 –2.01 –2.51
FeI 404.5812 1.485 0.280 11.8 0.03 –0.26 –2.48 –2.31 –2.75
FeI 406.3594 1.557 0.062 9.9 0.04 –0.28 –2.51 –2.35 –2.79
FeI 413.2058 1.608 –0.675 7.0 –0.05 –0.32 –2.40 –2.26 –2.72
FeI 414.3868 1.557 –0.511 8.2 –0.01 –0.33 –2.36 –2.22 –2.69
FeI 426.0474 2.399 0.109 6.8 0.00 –0.18 –2.40 –2.28 –2.59
FeI 438.3545 1.485 0.200 10.6 0.03 –0.33 –2.65 –2.49 –2.98
FeI 440.4750 1.557 –0.142 10.5 0.03 –0.34 –2.26 –2.10 –2.61
FeI 495.7596 2.808 0.233 8.5 0.04 –0.21 –2.03 –1.90 –2.22
FeII 516.9033 2.891 –0.870 7.7 0.18 0.16 –2.28 –2.25 –2.12
MgI 517.2684 2.712 –0.380 15.0 –0.02 –0.21 –2.36 –2.21 –2.57
MgI 518.3604 2.712 –0.158 20.0 0.00 –0.21 –2.24 –2.08 –2.46
SiI 390.5523 1.909 –1.090 8.9 0.06 –0.16 –2.54 –2.43 –2.70
CaI 422.6728 0.000 0.240 14.1 –0.10 –0.42 –2.15 –1.98 –2.57
CaII 393.3663 0.000 0.134 159.5 –0.05 –0.19 –2.46 –2.31 –2.65
CaII 849.8023 1.692 –1.312 22.1 0.09 –0.25 –1.92 –1.85 –2.17
CaII 854.2091 1.700 –0.362 36.2 0.01 –0.20 –2.35 –2.27 –2.55
CaII 866.2141 1.692 –0.623 35.3 0.02 –0.21 –2.11 –2.05 –2.32
†TherelativeerrorsonEWsareoftheorderof25%.
A Appendix A
Fig.A1 The X-Shooter spectrum of SDSS
J135046.74+134651.1 (solid black) with the best fitting
CO5BOLD profile (T = 6420K, solid green) in the top
eff
panel and the same for SDSS J135516.29+001319.1 (T =
eff
6288K˙,solidgreen)inthebottompanel.
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