Chemical similarities between Galactic bulge and local thick disk red giants: O, Na, Mg, Al, Si, Ca and Ti

0.53 MB·English

by  A. Alves-Brito

Astronomy&Astrophysicsmanuscriptno.alvesbrito13jan10 (cid:13)c ESO2010
January14,2010
Chemical similarities between Galactic bulge and local thick disk
red giants: O, Na, Mg, Al, Si, Ca and Ti
A.Alves-Brito1,2,J.Mele´ndez3,M.Asplund4,I.Ram´ırez4,andD.Yong5
1 Universidade de Sa˜o Paulo, IAG, Rua do Mata˜o 1226, Cidade Universita´ria, Sa˜o Paulo 05508-900, Brazil; e-mail:
[email protected]
2 CentreforAstrophysicsandSupercomputing,SwinburneUniversityofTechnology,Hawthorn,Victoria3122,Australia
3 CentrodeAstrof´ısicadaUniversidadedoPorto,RuadasEstrelas,4150-762Porto,Portugal;e-mail:[email protected]
4 MaxPlanckInstitutfu¨rAstrophysik,Postfach1317,85741Garching,Germany
0
5 ResearchSchoolofAstronomyandAstrophysics,TheAustralianNationalUniversity,CotterRoad,Weston,ACT2611,Australia
1
0
Received:;accepted:
2

n ABSTRACT
a
J Context.The formationand evolution of theGalacticbulge and itsrelationship withtheother Galacticpopulations isstillpoorly
understood.
4
Aims.Toestablishthechemicaldifferencesandsimilaritiesbetweenthebulgeandotherstellarpopulations,weperformedanelemen-
1
talabundanceanalysisofα-(O,Mg,Si,Ca,andTi)andZ-odd(NaandAl)elementsofredgiantstarsinthebulgeaswellasoflocal

thindisk,thickdiskandhalogiants.
]
R Methods.Weusehigh-resolutionopticalspectraof25bulgegiantsinBaade’swindowand55comparisongiants(4halo,29thindisk
and22thickdiskgiants)inthesolarneighborhood. Allstarshavesimilarstellarparametersbutcoverabroadrangeinmetallicity
S
(−1.5 <[Fe/H]< +0.5).Astandard1DlocalthermodynamicequilibriumanalysisusingbothKuruczandMARCSmodelsyielded
.
h theabundancesofO,Na,Mg,Al,Si,Ca,TiandFe.Ourhomogeneous anddifferentialanalysisoftheGalacticstellarpopulations
p ensuredthatsystematicerrorswereminimized.
- Results.Weconfirmthewell-establisheddifferencesfor[α/Fe]atagivenmetallicitybetweenthelocalthinandthickdisks.Forall
o
theelementsinvestigated,wefindnochemicaldistinctionbetweenthebulgeandthelocalthickdisk,inagreementwithourprevious
r
studyofC,NandObutincontrasttoothergroupsrelyingonliteraturevaluesfornearbydiskdwarfstars.For−1.5<[Fe/H]<−0.3
t
s exactlythesametrendisfollowedbyboththebulgeandthickdiskstars,withastar-to-starscatterofonly0.03dex.Furthermore,
a bothpopulationssharethelocationofthekneeinthe[α/Fe]vs[Fe/H]diagram.Itstillremainstobeconfirmedthatthelocalthick
[
diskextendstosuper-solarmetallicitiesasisthecaseforthebulge.Thesearethemoststringentconstraintstodateonthechemical
1 similarityofthesestellarpopulations.
v Conclusions.Ourfindingssuggestthatthebulgeandlocalthickdiskstarsexperiencedsimilarformationtimescales,starformation
1 ratesandinitialmassfunctions,confirmingthusthemainoutcomesofourprevioushomogeneousanalysisof[O/Fe]frominfrared
2 spectrafornearlythesamesample.Theidenticalα-enhancementsofthickdiskandbulgestarsmayreflectarapidchemicalevolution
5 takingplacebeforethebulgeandthickdiskstructuresweseetodaywereformed,oritmayreflectGalacticorbitalmigrationofinner
2 disk/bulgestarsresultinginstarsinthesolarneighborhoodwiththick-diskkinematics.
.
1 Keywords.Stars:abundances–Galaxy:abundances–Galaxy:bulge–Galaxy:disk–Galaxy:evolution
0
0
1
1. Introduction Genzeletal.2008)formedthisway,andthatthebulgeandthick
:
v diskmayhaveformedatthesametime.Thus,thenatureofour
The Galactic bulge is the least understood stellar popula-
i Galacticbulgecanbeunveiledbydetailedchemicalcomposition
X tion in the Milky Way, as even its classification (classical or
analysisandbycarefulcomparisonswiththethickdisk.
r pseudo-bulge;Kormendy&Kennicutt2004)seemsunclear.The
a Galactic bulge has signatures of an old (Ortolani et al. 1995; Althoughall recentworks agreein enhancementsof the α-
Zoccalietal.2003)classicalbulgeformedrapidlyduringinten- elementsrelativetosolarabundancesinbulgefieldKgiants,the
sivestarformationasreflectedintheenhancementofα-elements levelofenhancementiscurrentlyunderdebate.Basedonacom-
(e.g.McWilliam&Rich1994;Cunha&Smith2006;Zoccaliet parisonofbulgegiantstarswiththickdiskdwarfstars,Zoccaliet
al. 2006;Lecureuret al. 2007;Fulbrightet al. 2007;Mele´ndez al.(2006),Lecureuretal.(2007)andFulbrightetal.(2007)sug-
et al. 2008; Ryde et al. 2009a,b). On the other hand its boxy gestedthatthebulgeandthethickdiskhavedifferentchemical
shapeisconsistentwithapseudo-bulgeindicativeofformation compositionpatterns,andthattheα-elementsareoverabundant
bysecularevolutionthroughdynamicalinstabilityofanalready inthebulgecomparedwiththethickdisk.Therefore,theyargued
establishedinnerdisk. forashorterformationtimescaleandhigherstarformationrate
Recently, Elmegreen et al. (2008) have shown that bulges fortheGalacticbulgethanthatforthethickdisk.Balleroetal.
formed by coalescence of giant clumps can have properties of (2007)alsoconcludedthattheinitialmassfunctionsmusthave
bothclassicalandpseudo-bulges,becausesecularevolutioncan been different between the two populations based on both the
take place in a very short timescale (< 1 Gyr). They suggest high[Mg/Fe]andmetallicitydistributionofthebulge(see also
that our Galactic bulge (and many z ∼ 2 early disk galaxies; Cescuttietal.2009).Nevertheless,thosecomparisonsshouldbe
2 Alves-Britoetal.:ChemicalsimilaritiesbetweentheGalacticbulgeandthickdisk
analysis of ∼180 clump giants by Mishenina et al. (2006), the
studyof∼300nearbygiantsbyLuck&Heiter(2007),thesurvey
of∼380giantsbyHekker&Mele´ndez(2007),andtheanalysis
of∼320giantsbyTakedaetal.(2008).Furthermore,theUVES
libraryofstellarspectra(Bagnuloetal.2003)wassearchedfor
suitablediskandhalogiants.
Ouranalysisof thin disk,thick disk andhalo starsis based
mostly on high-resolution (R= 65,000) optical spectra taken
in April 2007 with the MIKE spectrograph (Bernstein et al.
2003)ontheClay6.5mMagellantelescope,andcomplemented
with observations using the 2dcoude´ spectrograph (Tull et al.
1995, R = 60,000) on the 2.7m Harlan J. Smith telescope
at McDonald Observatory, the upgraded HIRES spectrograph
(Vogt et al. 1994, R = 100,000)on the Keck I 10m telescope,
the UVES library1 (Bagnulo et al. 2003, R = 80,000), and the
ELODIEarchive2(Moultakaetal.2004,R=42,000).Themag-
nitudes, population membership and instrumentation used for
thedisk/halosampleareshowninTable1.
Thedatawerereducedwith IRAFemployingstandardpro-
cedures: correction for bias, flat field, cosmic rays and back-
ground light, then optimal extraction of the spectra (using a
Fig.1. H-R diagram showing our program stars. The symbols brightstartotracetheorders),wavelengthcalibration,barycen-
are described in the plot. The two most luminous stars (filled tric and Doppler correction, and continuum normalization. In
trianglesenclosedbycircles)arebulgegiantswhichshowabun- somecases,asdescribedbelow,avariationtothereductionpro-
danceanomalieslikeO-deficiencyandNa-enhancementsimilar cedurewasnecessary.ThetiltofthelinesintheMIKEdataisse-
tothoseobservedinsomeglobularclusterstars.Atypicalerror vereandvariesacrosstheCCD(e.g.Yongetal.2006),therefore
barinT andloggisshown. itmustbecarefullycorrectedtoavoiddegradationofthespectral
eff
resolution.ThetiltwascorrectedusingMTOOLS3,specifically
developedbyJ.BaldwintoaccountforthetiltedslitsinMIKE
taken with care as systematic errors may be present due to the spectra. On the other hand, our HIRES spectra were extracted
verydifferentstellarparameters,modelatmospheres,andNLTE using a new version of MAKEE4, an optimal extraction pack-
effectsofdwarfandgiantstars.Indeed,inourconsistentanalysis age developed by T. Barlow specifically for data reduction of
ofhighresolutioninfraredspectraofbothbulgeand thickdisk the improved HIRES spectrograph. MAKEE also performs an
giantswithsimilarstellarparameters(Mele´ndezetal.2008),we automaticwavelengthcalibrationcross-correlatingtheextracted
have shown that the bulge is in fact chemically very similar to ThAr spectra with a database of wavelength calibration solu-
thethickdiskin[C/Fe],[N/Fe]and[O/Fe].Here,weextendthis tions. Both the UVES and ELODIE archive data were already
worktootherα-elements(Mg,Si,Ti,Ca),andshowthatallthe extractedandwavelengthcalibrated.Theextractedspectrawere
α-elementsinbulgeandlocalthickdiskgiantshaveessentially shiftedtotherestframeandcontinuumnormalizedusingIRAF.
identicalchemicalabundancepatterns. Thesignal-to-noiseratio(S/N)perpixelofthereducedspectra
rangesfromS/N ∼45−100forthebulgegiants(Fulbrightetal.
2006),whereasforthediskandhalogiantstheS/Nistypically
2. Observations
∼200 per pixel, ranging from ∼150 (2dCoude/McDonald) to
The sample consists of 80 cool giant stars (Fig. 1) with ef- ∼200(MIKE/Magellan,ELODIE/OHP)to∼250(HIRES/Keck,
fective temperatures 3800 ≤ T ≤ 5000K, surface gravities UVES/VLT),asestimatedfromrelativelyline-freeregionsofthe
eff
0.5 ≤ logg ≤ 3.5, and metallicities −1.5 <[Fe/H]< +0.5. spectra.
Similarnumberofthindisk(29),thickdisk(22)andbulge(25)
giantswere selected, anda few (4) metal-richhalogiantswere
alsoincluded. 3. AbundanceAnalysis
All of our bulge giants are located in Baade’s window and
are taken from Fulbright et al. (2006), who have cleaned the We have homogeneously performed all the equivalent width
samplefromnonbulgegiants.Forthesebulgestars,wehaveal- (EW) measurements for the disk and halo sample. In order
readypublishedanabundanceanalysisofC,N,OandFebased to check that the EW measurements of the bulge giants by
onIR spectra (Mele´ndezetal. 2008).For the presentstudy we Fulbrightetal. (2006,2007)areconsistentwithoursystem for
make use of the equivalent widths measured in optical spectra the disk and halo giants, we have observed one bulge star (IV-
usingtheHIRESspectrograph(atR=45,000or67,000)onthe 203) with the MIKE spectrographand comparedthe EW mea-
Keck-I10mtelescopebyFulbrightetal.(2006,2007). suredbyuswiththoseobtainedbyFulbrightetal.(2006,2007).
To enableapropercomparisonwe havecompileda sample Theagreementissatisfactory,withameandifference(Thiswork
of thin disk, thick disk and halo stars for which we have ob- -Fulbrightetal.2006,2007)of−2.0mÅandaline-to-linescat-
tained our own optical spectra. The assignment of population
membershipwasbasedonUVWvelocities(Bensbyetal.2004; 1 http://www.sc.eso.org/santiago/uvespop/
Reddyetal.2006).Thesampleselectionwasbasedonevaluat- 2 http://atlas.obs-hp.fr/elodie/
ingpopulationmembershipin morethan 1500giantstars from 3 http://www.lco.cl/telescopes-information/magellan/instruments-
the literature, in particularan updatedversion of the Cayrel de 1/mike/IRAF tools/iraf-mtools-package/
Strobel (2001) catalog (see Ram´ırez & Mele´ndez 2005a), the 4 http://www2.keck.hawaii.edu/inst/hires/hires.html
Alves-Britoetal.:ChemicalsimilaritiesbetweentheGalacticbulgeandthickdisk 3
Table1.Programstarsdata Table 2.Sensitivitiesintheabundanceratiosbyemployingthe
Kurucz models (Castelli et al. 1997). The atmospheric param-
eters and α-enhancement were changed by ∆T = ±75K,
eff
Star V[mag] P∗[%] Instrument ∆logg = ±0.30dex, ∆v = ±0.20kms−1, and ∆[α/Fe] =
t
(1) (2) (3) (4) ±0.10dex. The total internal uncertainties are given in the last
column
Halo
HD041667 8.533 00:01:99 MIKE/Magellan
Abundance ∆T ∆logg ∆v ∆[α/Fe] (Px2)1/2
HD078050 7.676 00:00:100 ELODIE/OHP eff t
(1) (2) (3) (4) (5) (6)
HD114095 8.353 00:29:71 MIKE/Magellan
HD210295 9.566 00:00:100 HIRES/Keck
HD078050
ThickDisk [FeI/H] -0.09 0.01 0.07 0.00 0.11
[FeII/H] 0.02 -0.12 0.06 -0.02 0.14
HD023940 5.541 03:96:01 2dcoude/McDonald [O/Fe] -0.01 -0.13 0.00 -0.02 0.13
HD032440 5.459 08:91:01 MIKE/Magellan [Na/Fe] -0.06 0.01 0.01 0.00 0.06
HD037763 5.178 15:84:01 MIKE/Magellan [Mg/Fe] -0.06 0.05 0.04 0.00 0.09
HD040409 4.645 25:74:01 MIKE/Magellan [Al/Fe] -0.06 0.00 0.00 0.00 0.06
HD077236 7.499 00:58:42 MIKE/Magellan [Si/Fe] -0.03 -0.03 0.01 0.00 0.04
HD077729 7.630 25:74:01 MIKE/Magellan [Ca/Fe] -0.08 0.02 0.06 0.00 0.10
HD080811 8.35 00:97:03 MIKE/Magellan [Ti/Fe] -0.11 0.01 0.06 0.01 0.13
HD083212 8.335 00:94:06 2dcoude/McDonald
HD099978 8.653 00:99:01 MIKE/Magellan IV203
HD107328 4.967 43:57:01 2dcoude/McDonald
HD107773 6.355 20:78:02 MIKE/Magellan [FeI/H] 0.01 -0.07 0.06 -0.01 0.09
HD119971 5.454 14:85:01 MIKE/Magellan [FeII/H] 0.18 -0.18 0.04 -0.02 0.26
HD124897 -0.049 13:85:01 MIKE/Magellan [O/Fe] -0.01 -0.11 0.01 -0.02 0.11
HD127243 5.590 00:96:04 ELODIE/OHP [Na/Fe] -0.08 0.04 0.05 0.01 0.10
HD130952 4.943 01:98:01 MIKE/Magellan [Mg/Fe] 0.01 -0.04 0.03 -0.01 0.05
HD136014 6.195 24:75:01 MIKE/Magellan [Al/Fe] -0.06 0.02 0.03 0.01 0.07
HD145148 5.954 11:88:01 MIKE/Magellan [Si/Fe] 0.11 -0.09 0.03 -0.01 0.14
HD148451 6.564 00:64:36 UVES/VLT [Ca/Fe] -0.09 0.02 0.09 0.01 0.13
HD180928 6.088 00:74:26 MIKE/Magellan [Ti/Fe] -0.14 0.00 0.03 0.00 0.14
HD203344 5.570 00:97:02 ELODIE/OHP
HD219615 3.694 29:70:01 ELODIE/OHP HD083212
HD221345 5.220 14:85:01 ELODIE/OHP
[FeI/H] -0.09 -0.01 0.04 0.00 0.09
ThinDisk [FeII/H] 0.05 -0.12 0.05 -0.02 0.14
[O/Fe] 0.00 -0.13 0.01 -0.02 0.13
HD000787 5.255 98:02:00 2dcoude/McDonald [Na/Fe] -0.07 0.01 0.01 0.00 0.07
HD003546 4.361 78:22:00 ELODIE/OHP [Mg/Fe] -0.07 0.05 0.05 0.00 0.09
HD005268 6.163 86:14:00 HIRES/Keck [Si/Fe] -0.01 -0.04 0.00 -0.01 0.04
HD029139 0.868 98:02:00 ELODIE/OHP [Ca/Fe] -0.09 0.02 0.05 0.01 0.10
HD029503 3.861 96:04:00 2dcoude/McDonald [Ti/Fe] -0.16 0.00 0.07 0.02 0.17
HD030608 6.362 49:51:00 2dcoude/McDonald
HD045415 5.543 99:01:00 UVES/VLT HD045415
HD050778 4.065 87:13:00 2dcoude/McDonald
HD073017 5.673 86:14:00 ELODIE/OHP [FeI/H] -0.04 -0.02 0.09 -0.01 0.10
HD099648 4.952 99:01:00 MIKE/Magellan [FeII/H] 0.08 -0.14 0.08 -0.03 0.18
HD100920 4.301 99:01:00 MIKE/Magellan [O/Fe] 0.00 -0.14 0.00 -0.03 0.14
HD115478 5.333 99:01:00 MIKE/Magellan [Na/Fe] -0.06 0.08 0.05 0.00 0.11
HD116976 4.753 99:01:00 MIKE/Magellan [Mg/Fe] -0.02 0.02 0.03 -0.01 0.04
HD117220 9.010 95:05:00 MIKE/Magellan [Al/Fe] -0.05 0.02 0.04 0.01 0.07
HD117818 5.205 99:01:00 MIKE/Magellan [Si/Fe] 0.03 -0.05 0.03 -0.02 0.07
HD128188 10.003 98:02:00 MIKE/Magellan [Ca/Fe] -0.08 0.04 0.10 0.00 0.13
HD132345 5.838 97:03:00 MIKE/Magellan [Ti/Fe] -0.11 -0.01 0.01 0.00 0.11
HD142948 8.024 94:06:00 MIKE/Magellan
HD171496 8.501 98:02:00 2dcoude/McDonald
HD172223 6.485 91:09:00 MIKE/Magellan
HD174116 5.24 98:02:00 2dcoude/McDonald terofσ 5 =5.5mÅ(Fig.2a).Additionally,A.McWilliamhas
QD
HD175219 5.355 99:01:00 MIKE/Magellan kindly made available to us the HIRES/Keck spectrum of an-
HD186378 7.21 97:03:00 2dcoude/McDonald other bulge giant (I-322) for comparison purposes. Again, the
HD187195 6.022 99:01:00 MIKE/Magellan agreementisgoodwithadifference(Thiswork-Fulbrightetal.
HD211075 8.190 99:01:00 HIRES/Keck
2006,2007)of+2.2mÅandσ =5.5mÅ(Fig.2b).Sincemost
HD212320 5.938 99:01:00 UVES/VLT QD
oftheemployedlinesarerelativelystrong,thetypicalimpacton
HD214376 5.036 99:01:00 HIRES/Keck
HD215030 5.92 98:02:00 ELODIE/OHP
HD221148 6.252 93:07:00 HIRES/Keck 5 weuseherearobuststandarddeviationbasedonthequartiledevi-
ationQD(=Q3-Q1),σ =QD/1.349;seeforexampleAbu-Shawiesh
Notes.— (*): The membership probabilities of the thin disk, QD
etal.(2009)
thickdiskandhalogiantsaregivenasthin:thick:halo.
4 Alves-Britoetal.:ChemicalsimilaritiesbetweentheGalacticbulgeandthickdisk
Fig.2.Comparisonofourequivalentwidthmeasurementswith Fig.4.Microturbulentvelocityasafunctionofeffectivetemper-
linesincommonwithFulbrightetal.(2006,2007)fortwobulge ature (upper panel) and log g (lower panel). Giant stars with
giant stars: (a) IV-203 and (b) I-322. The median (solid lines) [Fe/H]<–0.70and[Fe/H]≥–0.70are,respectively,represented
andarobustproxyofstandarddeviation(σ ;dashedlines)are byfilledandopencircles.Thesolidlineisalinearleastsquares
QD
alsodisplayed. fittothedata,whoseresultsarelabeledinthefigure.
pleofgiants.However,wedonotexpectanysignificantdiffer-
enceswithrespecttoRam´ırez&Mele´ndez(2005b)fortherele-
vantstellarparameters,exceptperhapsforasmall(∼1%)zero-
pointoffsetintheT scale,whichisirrelevantheresinceweare
eff
performingadifferentialstudy.
Reddeningforthebulgestarswasestimatedfromextinction
maps(Stanek1996),while forthecomparisonsamplebothex-
tinction maps (Mele´ndezetal. 2006b) and Nai D ISM absorp-
tion lines were used. The E(B-V) values based on the D lines
wereobtainedasfollows.Intheopticalthincasetherelationbe-
tweencolumndensityN (unitscm−2)andequivalentwidthEW
(unitsmÅ)is:
N =1.13×1017EW/(f λ2), (1)
(Spitzer1968).The f valuesare0.64and0.32,respectively,for
the5889.950and5895.924Ålines(NISTdatabase6).Notethat
theaboverelationbetween N(NaI)andequivalentwidthholds
onlyforlinesonthelinearpartofthe curveofgrowth,i.e., for
smallvaluesofE(B-V);forreddeninglargerthanafew0.01mag
the interstellar lines must be modeled in detail (e.g. Welty et
Fig.3. Comparison between evolutionary gravities from Y2 al. 1994) to avoid underestimation of the column densities. In
particular we use the profile fitting program FITS6P (Welty et
(filled circles) and Padova (open circles) isochrones, and
trigonometricgravitiesforgiantstarswithgood(σ(π)/π≤10%) al.1994).
The N(NaI)densitywastransformedto N(H)usingthere-
Hipparcosparallaxes.Thesolidanddashedlinesdepictperfect
agreementandvariationsof±0.1dex,respectively. lationfoundbyFerletetal.(1985):
logN(HI+H )=(logN(NaI)+9.09)/1.04, (2)
2
abundancesfrom these EW differencesis negligible. Thus, the wherebothN(NaI)andN(H)areincm−2.Finally,E(B-V)was
analysis of the faint bulge giants, the bright disk and the halo computedfromthetotalhydrogendensity(Bohlinetal.1978):
giants,isessentiallyinthesamesystem.
E(B−V)= N(HI+H )/5.8×1021, (3)
Photometric temperatures were obtained using optical and 2
infrared colors and the infrared flux method Teff-scale of whereN(H)isincm−2andE(B-V)inmagnitudes.Althoughthis
Ram´ırez&Mele´ndez (2005b). We note that the new improved
relationseemsnotwellestablishedforE(B-V)<0.1,Ramirezet
IRFM calibration of Casagrande et al. (2010) only applies to
dwarfandsubgiantstarsandthuscannotbeappliedtooursam- 6 http://physics.nist.gov/PhysRefData/ASD/lines form.html
Alves-Britoetal.:ChemicalsimilaritiesbetweentheGalacticbulgeandthickdisk 5
al.(2006)haveshownittobeveryaccurateforaE(B-V)=0.01 havestellarparametersinthesamesystem,i.e.intheRam´ırez&
star. Mele´ndez(2005b)temperaturescaleandloggintheHipparcos
Albeitnotusedin thepresentwork,we shouldmentionfor scale.
completeness that in addition to reddening maps and Na D in- Themicroturbulencewasobtainedbyflatteninganytrendin
terstellar lines, E(B-V) can also be estimated from other inter- the [FeI/H] versus reduced equivalent width diagram. The mi-
stellar features such as the diffuse interstellar band at 862 nm croturbulencefollow tight relations with temperature and logg
(Munarietal.2008),aswellasmulticolorphotometry(e.g.Sect. (Fig.4),withascatterofonly0.20kms−1:
4.2ofMele´ndezetal.2006b;Ram´ırezetal.2006)andpolariza-
tion(e.g.Fosalbaetal.2002). vt(Teff)=3.33−4.23×10−4Teff (MARCS)(8)
The stellar surface gravities were derived from improved
v(T )=3.40−4.41×10−4T (Kuruczovershooting)(9)
Hipparcos parallaxes (vanLeeuwen 2007) for the sample of t eff eff
nearbygiantstarsandassumingadistanceof8kpcforthebulge v(logg)=1.82−0.186logg (MARCS)(10)
t
giants.Inaddition,Yonsei-Yale(Y2;Demarqueetal.2004)and
Padova isochrones (daSilvaetal. 2006) were employed to de- v(logg)=1.84−0.202logg (Kuruczovershooting)(11)
t
termine evolutionarygravities, as well as the inputmasses that
The lines used for analysis (presented as online material)
were adopted for the trigonometric gravities. In order to esti-
mate the Y2 gravities, we generated a fine grid of isochrones, have been carefully selected to minimize the impactof blends.
assuming[α/Fe]=0and+0.3for[Fe/H]> 0and[Fe/H]< −1, Completelyavoidingblendsisalmostanimpossibletaskincool,
relatively metal-rich giants as in our sample, since their spec-
respectively,andlinearlyinterpolatedinbetween.Allsolutions
tra are heavily blended with many atomic and molecular lines
allowed by the error bars were searched for, adopting as final
(e.g. Coelho et al. 2005), in particular due to CN. We have
result the median values. The Padova gravities were obtained
usingtheBayesiantoolPARAM7.AsshowninFig.3,both(Y2, tried to avoid blendingby performingspectralsynthesisof CN
(using the line list of Mele´ndez & Barbuy 1999) and discard-
Padova) evolutionary gravities are in excellent agreement with
ing the atomic lines whose equivalentwidths are contaminated
thetrigonometricgravitiesofournearbygiantswithreliable(un-
certainties≤10%)Hipparcosparallaxes.Theevolutionarylogg by more than 10% by CN. The cool giant Arcturus (Hinkle et
values required small zero-pointcorrectionsof −0.04 (Y2) and al. 2000) was also carefully inspected to discard lines that are
+0.07dex (Padova),to be on the same scale as the Hipparcos- severely contaminated with other features. In some cases even
lineswhichareblendedbymorethan10%havetobeincluded,
basedresultsforoursamplegiantstars(Fig.3).Bolometriccor-
especiallyforelementsotherthanironbecauseonlyafewuse-
rectionsfromAlonsoetal.(1999)wereadopted.
fullineswereavailable.Forheavilyblendedlineswehaveper-
We use iron lines to check our T and log g, but we do
eff
notassumeapriorithatouradoptedeffectivetemperatures,sur- formedthe measurementsby fitting onlythe unblendedpartof
theprofile,ordeblendingthefeatureusingtwoormorecompo-
face gravities, 1D model atmospheres, gf-values, selection of
nents.Apreliminaryversionofourlinelist(Hekker&Mele´ndez
lines, equivalent width measurements and LTE line formation,
would result in absolute excitation (zero slope of Fei abun- 2007)hasbeentestedin∼380field(Hekker&Mele´ndez2007)
dances vs. excitation potential) and ionization (A = A ) and39opencluster(Santosetal.2009)giants,andthefinallist
FeI FeII
equilibria.We use the nearbydisk/halo giants to determinethe hasbeenalreadyusedinfieldbulge(Rydeetal.2009b)andglob-
slopes (d(A )/d(χ )) and differences between Fei and Feii, ularcluster(Mele´ndez&Cohen2009)giants.
FeI exc
The stellar chemical abundances were obtained from an
followedbymoststars.Ourtestsoftheionizationandexcitation
balancesofFeiandFeiilinesrevealedthatmostofthesample equivalent width analysis using the 2002 version of MOOG
(Sneden 1973). The same transition probabilities were applied
giants(58stars)satisfyourrelativespectroscopicequilibriumof
to both the bulge and comparison samples. In the present
iron lines within the uncertainties, therefore the overall agree-
work, we employedboth Kuruczmodelswith convectiveover-
ment is encouraging. Nevertheless, the photometric stellar pa-
shooting(Castellietal.1997)andspeciallycalculatedMARCS
rametersof22stars(8thindisk,5thickdisk,1halo,and8bulge
(Gustafssonetal.2008)1Dhydrostaticmodelatmospheres.For
stars) required some adjustmentsto be on our relative spectro-
theMARCSmodels,bothα-enhanced([α/Fe]=+0.2and+0.4)
scopicequilibriumscale.Thecorrectionsbasedonthetrendfol-
lowedbythebrightdisk/halogiants,forwhichthephotometric and scaled-solar abundances models were constructed; for the
Kurucz models, adjustments of [Fe/H] were applied to simu-
stellar parameters (and stellar spectra) were more reliable than
late the effects of α-enhancement on the model atmospheres
forthebulgesample,is:
(Salarisetal.1993):
d(A )/d(χ )=0.008dexeV−1 (Kuruczovershooting), (4)
FeI exc ∆[Fe/H]=log(0.64×10[α/Fe]+0.36) (12)
d(A )/d(χ )=0.003dexeV−1 (MARCS), (5)
FeI exc
Theeffectsoffailingtoaccountforthevariationsin[α/Fe]
stars within 2-σ (σ = 0.011dex eV−1) were consideredto ful- arerelativelysmallforadifferenceof[α/Fe]=+0.1dex(Table
fill our relative excitation equilibrium. Ionization balance was 2), but could be important(∼ 0.1 dex) for the typicalenhance-
achievedif ment of [α/Fe] ∼ +0.3-0.4 seen in thick disk, halo and bulge
stars.
A(FeII)−A(FeI)=0.08dex(Kuruczovershooting), (6) We estimate that our stellar parameters have typical un-
certainties of ∆T ≈ ±75K, ∆logg ≈ ±0.3dex and ∆v ≈
A(FeII)−A(FeI)=0.00dex(MARCS), (7) eff t
0.2kms−1. The impactof these uncertaintieson the abundance
and stars within ±0.07dex were considered to fulfill our rel- ratios,aswellasthetotalabundanceerrorsduetouncertainties
in T , logg, v and [α/Fe] added in quadrature, are shown in
ative ionization equilibrium. After these corrections were per- eff t
Table 2, butnote thatsome uncertaintiesare likelyto becorre-
formedthedeviatingthindisk,thickdisk,haloandbulgestars,
latedtosomedegree(see,e.g.,Fulbrightetal.2007).Theuncer-
7 http://stev.oapd.inaf.it/cgi-bin/param taintiesgiveninTable2areprobablyconservativeinsomecases,
6 Alves-Britoetal.:ChemicalsimilaritiesbetweentheGalacticbulgeandthickdisk
Table 3. Internal zero-point abundances adopted for our giant
starsusingMARCSandKuruczmodels.
Sun Giants
Specie Literaturea Thisworkb MARCSc Kuruczd
(1) (2) (3) (4) (5)
Fe 7.50,7.45,7.56,7.50 7.49±0.04 7.53 7.54
O 8.83,8.73.8.71,8.69 8.74±0.04 8.83 8.84
[OI]
Na 6.33,6.27,6.27,6.24 6.24±0.04 6.24 6.24
Mg 7.58,7.54,7.58,7.60 7.56±0.04 7.65 7.66
Al 6.47,6.28,6.47,6.45 6.39±0.04 6.56 6.56
Si 7.55,7.62,7.54,7.51 7.54±0.03 7.60 7.63
Ca 6.36,6.33,6.36,6.34 6.34±0.02 6.32 6.30
Ti 5.02,4.90,4.92,4.95 4.94±0.05 4.83 4.81
Notes.—(a):SolarphotosphericabundancesfromGrevesse&
Sauval(1998),Reddyetal.(2003),Bensbyetal.(2003,2004) Halo (4)
Bulge (24)
andAsplundetal.(2009);(b):solarabundancesbasedonour Thick disk (22)
Thin disk (29)
previous work (Mele´ndez et al. 2006a; Mele´ndez & Ram´ırez
2007; Mele´ndez et al. 2009) using different (e.g. McDonald,
Keck, Magellan) solar spectra;(c,d):Ourinternal zero-points
forgiantsrepresent thethin-diskabundances at[Fe/H]=0.0. Fig.5.[O/Fe]vs.[Fe/H]forthesamplestarsemployingMARCS
Thesezero-pointsarenotabsoluteabundancesandshouldonly
(top)and Kurucz (bottom)modelatmospheres.Symbolsare as
beusedwhenbothoursamegf-valuesandanalysistechniques
explainedin the figure. Note, however,that hereafter the bulge
areadopted
starsI-264andIV-203areomittedfromallfigures(refertothe
textfordetail).
asshownbytherelativelylowscatter(asafunctionofmetallic-
veresothatat[Fe/H]=−1our1D-basedabundancescouldbein
ity)ofourabundanceratios;the uncertaintiesintheabundance
ratios[X/Fe]areprobably≤0.10dex. errorby<∼ 0.2dex.However,giventhesimilarityinparameters
The uncertainty of ±75K in T is the based on the up- betweenthe bulgeanddisk giants,the relative abundanceratio
eff
differences – which we are primarily interested in here – will
per and lower envelopes(excludingoutliers) of the differences
besignificantlysmallerandthusinconsequentialforourconclu-
between the slopes of iron abundance vs. excitation potential
sions.
(d(A )/d(χ ))oftheinitialphotometrictemperaturesandthe
FeI exc
adopted zero-points (relations 4 and 5). Note that these differ- Of particularimportanceto our work are the adoptedzero-
encesareduenotonlytoerrorsinthetemperaturecalibrations, pointsofourabundancescale.MostworksusetheSuntodefine
photometric errors and the quality of the spectra, but also due the zero-point of the thin disk at [Fe/H] = 0.0, but due to the
to errors in E(B-V), which although for the nearby giants are differencesbetweendwarfsandgiants,thisapproachmayintro-
low,forthebulgegiantsmaybehigher.Nevertheless,sincewe ducesystematicerrors.Instead,inthepresentworkweuseseven
correctalloutliersfromouradoptedzero-points(whichinsome thindiskgiantswith−0.1 <[Fe/H]<+0.1dex(HD29503,HD
cases may be due to incorrect reddenings), we are inmune to 45415, HD 99648, HD 100920, HD 115478, HD 186378, HD
large errors in E(B-V). Our error of 0.3 dex in log g is based 214376)to define our zero points, which are shown in Table 3
onthedifferencesbetweenFeIIandFeIfromtheinitialtrigono- for both the Kurucz and MARCS models. In Table 3 we also
metricloggandtheadoptedzeropointinFeII-FeI(relations6 show for comparison different abundance analysis of the Sun.
and7).Notethatsincewearebasingouruncertaintiesontheup- Ascanbeseen,ourzero-pointsforFe,NaandCaareroughlyin
perandlowerdiscrepanciesoftheinitialinputstellarparameters agreementwiththesolarabundances,butforO,Mg,andSithe
and the adoptedzero-points,our uncertaintiesin T and log g giantsshowahigherzero-pointby∼+0.1dex,andforAldiffer-
eff
areconservative.Forthebrightdisk/halostarsinternalerrorsof encesashighas+0.15arefound.Ontheotherhand,Tiislower
50KinT and0.2dexinloggmaybemoreadequate.Rydeet by∼0.1dex.Thesezero-pointoffsetsof−0.1to+0.15dexshow
eff
al.(2009b)suggeststhattheuncertaintiesadoptedinthestellar thatitisnotstraightforwardtocompareabundancesobtainedin
parametersofourmethod(whichwasusedtodeterminetheat- giantswiththosefoundindwarfs.
mosphericparametersintheirsample)aresoundfortheirbulge These zero points we have found for giants are internal
giants. In particular,uncertaintiesin T higher than ∼75K are for ourparticularset of gf-valuesandanalysis techniques.For
eff
excludedbasedontherelativelylowstar-to-starscatteroftheir comparisonwithchemicalevolutionmodels,theabsolutezero-
[O/Fe]ratios. pointsshouldbe adoptedfromanalysis of the Sun (Asplundet
No predictions of the effects of 3D hydrodynamical mod- al. 2009),which representswell the localthindisk at[Fe/H] =
elsinsteadofclassical1Dmodelsusedhereareavailableasyet 0.0,exceptforsmallpeculiaritiesof a few 0.01dex(Mele´ndez
for the exact stellar parameters of our targets (Asplund 2005). etal.2009;Ram´ırezetal.2009).
Colletetal.(2007)haveperformedsuchcalculationsforslightly ThefinalstellarparametersaregiveninTable4andTable5
less evolved red giants (T ≈ 4700 and logg ≈ 2) and found fortheMARCSandtheKuruczmodels,respectively,whilethe
eff
that the the 3D abundance corrections for the species consid- abundanceratiosaregiveninTable6andTable7.Theequivalent
ered herein are expected to be modest: |∆logǫ| <∼ 0.1dex at widthmeasurementsaregiveninTables8-15,whichisavailable
[Fe/H]∼0.Atlowermetallicitythe3Deffectsbecomemorese- onlyintheelectronicversionofthearticle.
Alves-Britoetal.:ChemicalsimilaritiesbetweentheGalacticbulgeandthickdisk 7
Halo (4) Halo (4)
Bulge (23) Bulge (23)
Thick disk (22) Thick disk (22)
Thin disk (29) Thin disk (29)
Fig.6. [Mg/Fe] as a function of [Fe/H] for MARCS model at- Fig.8. [Ca/Fe] vs. [Fe/H] for MARCS model atmospheres.
mospheres.Symbolsasexplainedinthefigure. Symbolsasexplainedinthefigure.
Halo (4) Halo (4)
Bulge (23) Bulge (23)
Thick disk (22) Thick disk (22)
Thin disk (29) Thin disk (29)
Fig.7. [Si/Fe] vs. [Fe/H] for MARCS model atmospheres. Fig.9. Plot of [Ti/Fe] against [Fe/H] for the sample stars em-
Symbolsasexplainedinthefigure. ployingMARCSmodelatmospheres.Symbolsareasexplained
inthefigure.
4. Results
In Fig. 5 we show the [O/Fe] ratios obtained in this work for in excellentagreement.The mean differenceis only −0.04dex
bothMARCSandKuruczovershootingmodelatmospheres.As (OH - [Oi]) with a scatter of 0.10 dex. Since this is identical
canbeseen,thereisagoodoverallagreementbetweenMARCS totheestimatederrorin[O/Fe]fromOHfoundinMele´ndezet
and Kurucz models. In particular, the difference in iron abun- al. (2008),the uncertaintiesin our stellar parametersare likely
dance (MARCS - Kurucz) is only −0.02 dex (σ = 0.03 dex). somewhatoverestimated,asalreadydiscussedabove.Although
Thus, in the following figures, we present results based on the theoxygenabundancesobtainedfrom[Oi]andOH linesagree
MARCS models only. Even though both set of models give well, the results obtainedfromOH lines have less scatter, pos-
similar chemical abundance ratios (see Tables 6,7), for com- sibly becauseseveralOH lineswere usedinstead of relyingon
parison with chemical evolution models we suggest to adopt onlyoneor two forbiddenlines. Therefore,forcomparisonsof
theMARCSresults,sincetheywerecomputedwiththecorrect oxygenabundanceswithdetailedchemicalevolutionmodelsof
[α/Fe] ratio. Our disk/halo comparison sample shows that the thethindisk,thickdiskandbulge,webelievethattheOHlines
oxygenabundancesobtainedherefromthe[Oi]630and636nm arepreferable(Mele´ndezetal.2008;Rydeetal.2009a,b).The
lines and in Mele´ndez et al. (2008)from infraredOH lines are [Oi]-basedoxygenabundancesconfirmsthesimilaritybetween
8 Alves-Britoetal.:ChemicalsimilaritiesbetweentheGalacticbulgeandthickdisk
Halo (4)
Bulge (22) Halo (4)
Thick disk (22) Bulge (22)
Thin disk (29) Thick disk (22)
Thin disk (29)
Fig.10. Mean α-elements abundance ratio Fig.12. [Na/Fe] vs. [Fe/H] for MARCS model atmospheres.
([(O,Mg,Si,Ca,Ti)/Fe]) as a function of [Fe/H] for MARCS Symbolsasexplainedinthefigure.
modelatmospheres.Symbolsasexplainedinthefigure.
Halo (4)
Bulge (20)
Thick disk (22)
Thin disk (29)
Fig.13. [Al/Fe] vs. [Fe/H] for MARCS model atmospheres.
Fig.11.Fitof[α/Fe]vs.[Fe/H]formetal-poor([Fe/H]< −0.5) Symbolsasexplainedinthefigure.
and metal-rich ([Fe/H] ≥ −0.5) bulge (top),thick disk (center)
andbothbulgeandthickdisk(bottom)stars.Bothstellarpopu-
lationscanbefittedbysimilarrelations,withascatteraslowas al.2005;Carrettaetal.2009).Fulbrightetal.(2007)foundthat
σ=0.03dex. these two giants have high Na and Al abundances reminiscent
oftheabundanceanomaliesseeninglobularclusters,whichwe
confirm. Thus, the oxygen abundances of these two stars most
thebulgeandthelocalthickdisk,whichwepreviouslydemon- likelydonotreflectthetypicalbulgecomposition.
strated basedonOH lines. Thisis in contrastto someprevious Theresultsfortheotherα-elements(Mg,Si,Ca,Ti)studied
works on the topic (Zoccalietal. 2006; Fulbrightetal. 2007; hereareshowninFigs.6-9.Ascanbeclearlyseen,thechemi-
Lecureuretal. 2007), which argue that [O/Fe] in the bulge is calpatternsofthebulgeandthickdiskareindistinguishablealso
higherthaninthethickdiskbasedonacomparisontodiskdwarf for those elements, reinforcing our previous findings based on
stars. oxygenabundances.TheaverageofourMgabundancesforthe
As pointed out by Fulbright et al. (2007), two O-deficient 7starswith[Fe/H]≥0is[Mg/Fe]=0.1±0.1,ingoodagreement
stars(I-264andIV-203)at[Fe/H] ≈ −1.25havepeculiarabun- withtheresultsfrommicrolensedbulgedwarfs,whichtypically
dancessimilartotheO-Naanti-correlationseeninglobularclus- have [Mg/Fe]≈ +0.1 (Cohen et al. 2008, 2009; Johnson et al.
ters(e.g.Grattonetal.2004;Cohen&Mele´ndez2005;Yonget 2008;Bensbyetal.2009a).Thelatestpreliminaryresultsbased
Alves-Britoetal.:ChemicalsimilaritiesbetweentheGalacticbulgeandthickdisk 9
This work
Fulbright et al.
Fig.14. [C/O] vs. [O/H] for the bulge (triangles), thick disk Fig.16.[O/Mg]asafunctionof[Fe/H]forbulgegiantsaccord-
(filledcircles)andthindisk(opencircles)withdatatakenfrom ingtoFulbrightetal.(2007)(opentriangles)andourwork(filled
Mele´ndez et al. (2008) and Ryde et al. (2009b). The original triangles). Both studies show a shallow trend up to aboutsolar
(undepleted)C abundanceswere estimated fromC+N (referto metallicity and a step decrease in [O/Mg] for higher metallici-
the text). Overplotted, we show model predictions as given in ties. Recent predictions by Cescutti et al. (2009) are shown as
Cescuttietal.(2009;c.f.theirFigure5). solid(WW95model),shortdashed(WW95+M92),longdashed
(WW95+MM02)anddot-shortdashed(MM02)lines.Allmod-
elshavebeenshiftedby−0.2dexin[O/Mg](seeCescuttietal.
2009for a descriptionof the modelsand an explanationof the
empiricaloffset).
and high [α/Fe] to [Fe/H]>0.0 and low [α/Fe], but Reddy et
al. (2006) and Ram´ırez et al. (2007) did not find any evidence
of sucha knee.A re-examinationofthe latter results including
newobservationsofkinematicallyselectedthick-diskmetal-rich
objects is underway (Reddy et al., in preparation)and will ad-
dressthisdiscrepancy.Theproblemisthatathigh[Fe/H]there
is a significant numberof thick-disk candidatesthat follow the
thin disk abundance pattern. That suggests that hot kinematics
alone cannot be used to separate the thin from the thick disk,
especially athigh[Fe/H]. Atsuper-solarmetallicities the prob-
lemmaybeunsolvablebecausetheabundancepatternsofboth
Halo (4)
Bulge (22) disksmerge.Interestingly,thechemicalsimilaritiesbetweenthe
Thick disk (22) Galacticbulgeandthelocalthickdiskgiantstarswefindinthis
Thin disk (29)
workinfactextendtosuper-solarmetallicities.Yet,asexplained
above,itremainstobedemonstratedthatthefewselectedthick
diskstarsarebonafidethickdiskmembersratherthankinemat-
icallyheatedthindiskstars.
Fig.15.[O/Mg]asafunctionof[Fe/H](toppanel)and[Mg/H]
FromTable 1 we see that three giants, which are kinemati-
(bottompanel).Symbolsasexplainedinthefigure.
cally classified as thick disk (HD 77236,HD 107328)andthin
disk(HD30608)members,presentambiguouskinematicalpop-
ulation.ThestarHD77236couldbeeitherathickdisk/halostar,
onmicrolensedbulgedwarfstars(Bensbyetal.2009b)alsoin- whilethestarsHD107328andHD30608bothhavesimilarlike-
dicate similarities between the bulge and thick disk for Ti and lihoodofbelongingtothethinorthickdiskpopulations.These
Mgatallprobedmetallicities(−0.8<[Fe/H]<+0.5). starshave[Fe/H]=(−0.67,−0.43,−0.28)and[α/Fe]=(+0.33,
Evenmoreclearresultsarefoundwhenwecombinethere- +0.30, +0.08), respectively, which means that both HD 77236
sults for all α-elements, as shown in Fig. 10. It is clear that andHD107328couldbeindeedthickdiskstars,whilethestar
the chemical patterns of the bulge and thick disk are indistin- HD30608hasanabundancepatternconsistentwithathindisk
guishableintheirabundancepatternsuptothemetallicityrange starat[Fe/H]∼−0.3.
where the thick disk is unambiguously identified, i.e., up to Linear fits of both the bulge and the thick disk [α/Fe] vs.
[Fe/H] ≈ −0.3. Bensby et al. (2003, 2004) reported the exis- [Fe/H]relationsupto[Fe/H]=−0.3,showthatbothpopulations
tence of a knee connectingthick-disk stars from[Fe/H]≈ −0.3 followidenticalpatterns,with a star-to-starscatter of onlyσ =
10 Alves-Britoetal.:ChemicalsimilaritiesbetweentheGalacticbulgeandthickdisk
0.03 dex. We thus set the most stringent constraints to date on the[Na/Fe]and[Al/Fe]trendsagreewellforthebulgeandthe
the chemicalsimilarity of bulge and local thick disk stars. The local thick disk, although this is not too surprising given that
metallicity of the bulge extends to significantly higher [Fe/H] there is no obvious offset between the thick and thin disk for
than that, which remains to be convincingly demonstrated for thosetwoelements.
thethickdisk,aspreviouslydiscussed. Previousworks(Fulbrightetal.2007;Lecureuretal.2007)
Inordertoquantifyhowsimilararethebulgeandthethick have found high [Mg/Fe] ratios in bulge stars, as well as high
disk at all metallicities, we have divided the stars in a metal- [X/Fe]ratiosinotherelementswithrespecttothethickdisk.It
poor and a metal-rich sample with the division set somewhat may seem surprising that using the same equivalent widths as
arbitrarily at [Fe/H] = −0.5, and we have performedlinear fits Fulbright et al. (2007) we find significantly lower [Mg/Fe] in
of [α/Fe] vs. [Fe/H]. We find that the metal-poor part of both bulge giants at all metallicities. However, as mentioned in the
stellarpopulationscanbefittedbyessentiallyidenticalrelations section 3, the zero-points we use in our analysis are based on
(Fig.11):[α/Fe]=0.22-0.10×[Fe/H]forthebulge(σ=0.03 seventhindisksolarmetallicitygiants,which,asshowninTable
dex)and [α/Fe] = 0.27- 0.07× [Fe/H] for the thick disk (σ = 3,arenotthesameasthezero-pointsbasedontheSun,whichis
0.03 dex). These relations are identical to within ±0.01 dex at ∼1400Khotterandhasasurfacegravity∼300timeshigherthan
[Fe/H]=−1.5andtowithin±0.02dexat[Fe/H]=−0.5,hence our giants. The differences shown in Table 3 between the Sun
bothdatasetscanbefittedbyasinglerelationfollowedbyboth and our giants could be due to the different impact of 3D and
stellarpopulations: non-LTEeffectson giantand dwarfs,as well as problemswith
lineblendingingiants.Furthermore,sincewecomparebulgegi-
[α/Fe]=0.268−0.065×[Fe/H] ([Fe/H]<−0.5), (13) antstothickdiskgiants,ourconclusionsonthesimilarityofthe
bulge and thick disk is independentof the adopted zero-point,
withaverylowstar-to-starscatterofonly0.03dex(Fig.11). unlikethecomparisonsofZoccalietal.(2006),Fulbrightetal.
A similar exercise for the most metal-rich bulge and thick (2007)andLecureuretal.(2007),thatcomparedbulgegiantsto
disk stars with [Fe/H] ≥ −0.5, results in a single relation fol- diskdwarfs.AlthoughbothFulbrightetal.(2007)andLecureur
lowedbybothstellarpopulations(Fig.11): et al. (2007) used the giants Arcturus ([Fe/H] ∼ −0.5) and µ
Leo ([Fe/H] ∼+0.3)as referencestars, their zero-pointsare ul-
[α/Fe]=0.104−0.381×[Fe/H] ([Fe/H]≥−0.5), (14) timately based on the Sun8, which as shown in Table 3, may
be inadequate for the study of giants. It is important to men-
with a low star-to-star scatter of only 0.06 dex (Fig. 11). This tion that Fulbright et al. (2007) also included 17 nearby disk
scatter is higher than for the more metal-poor stars (σ = 0.03 giants (mostly from the thin disk) in order to check whether
dex), but fully explained by the higher uncertainties in the theyfollowthesameabundancepatternasdiskdwarfs,butmost
analysis of the more crowded spectra of the metal-rich stars. of their giants were observed at a lower resolving power, R
Nevertheless,wedonotdiscardthattheremaybesomecontam- ∼ 30,000, implying higher errors in the abundances obtained
inationinthemetal-richsamples.Wedemonstratethusthatatall fromthecrowdedspectraofmetal-richgiantstars.Furthermore,
probedmetallicities(−1.5<[Fe/H]<+0.5),boththebulgeand their comparison sample of disk dwarfs included several stud-
thickdiskstellarpopulationshaveastrikingchemicalsimilarity. ies which may have different systematic offsets between them.
TheAlabundancesofthemostmetal-richbulgestars([Fe/H] For example the study by Reddy et al. (2003) used Stro¨mgren
&0)seemenhanced([Al/Fe]∼+0.25),butNaseemssolar(al- photometryto estimate effectivetemperaturesusingthe (b−y)
thoughwith a large scatter) at these metallicities, as illustrated calibrationbyAlonsoetal.(1996).However,asrecentlyshown
in Fig.12-13. Thus,the enhancementin Alinmetal-richbulge byMele´ndezetal.(2010,inpreparation)usinguvby-βphotome-
giants is probably not related to the Al-Na correlation seen in tryofsolartwins,thiscalibrationhasazero-pointerrorof130K.
globularclusters,butmostlikelyduetothefactthatthetwoAli In turn, this implies abundancevariations (∆[X/H]) from -0.12
lines employed are blended and are more difficult to deblend dex(ObasedonOItriplet,whichwasthemainabundanceindi-
in the relatively moderate S/N spectra of the metal-rich bulge catorusedbyReddyetal. 2003)to+0.13dex(Ti).Yet, dueto
giants. This is reinforcedby recent studies of metal-rich bulge a compensatingchangein iron, for mostelements studied here
dwarfsthroughmicrolensing(Cohenetal.2008;Johnsonetal. (exceptfor[O/Fe]thatisaffectedby-0.22dex;basedontheOI
2008; Bensby et al. 2009a), which find [Al/Fe] ∼ +0.10 dex. triplet),the∆[X/Fe]rangesfrom-0.08dex([Si/Fe])to+0.03dex
Thus,ourAlabundancesforbulgestarswith[Fe/H]&0arethus ([Ti/Fe]).Ontheotherhand,Bensbyetal.(2003,2004)adopted
likely affected by systematic errors, which should be borne in spectroscopictemperatures,thereforenotonlytheirTeffbutalso
mindincomparisonswithchemicalevolutionmodels. their[X/Fe]abundanceratiosarelikelymoreaccurateforcom-
parisonofthesereddenedbulgeregions.
Thus, the main reason why our conclusions regarding the
5. Discussion
abundancetrendsinthebulgeincomparisonwiththethickand
Our previous homogeneous abundance analysis of OH lines thindiskdifferfromthefindingsofpreviousstudies(e.g.Zoccali
in high resolution infrared Gemini+Phoenix spectra was the etal.2006;Fulbrightetal.2007;Lecureuretal.2007)isthatwe
first to show that the Galactic bulge and the local thick disk perform a strictly differential analysis of red giants with very
have indistinguishable[O/Fe] trendsup to at least metallicities similarparametersforallpopulationsratherthanrelyingonei-
[Fe/H]≈ −0.3wherethethickdiskisunambiguouslyidentified ther literature data or using dwarf stars for the disk samples.
(Mele´ndezetal.2008).Inthepresentworkwehaveanalyzedthe
8 similar problems were already recognized by Gratton & Sneden
forbiddenoxygenlinesanddemonstratedthatthoseindeedgive
(1990)intheirabundanceanalysisofµLeo,whichwasultimatelyrel-
consistentresultswiththeOHlines.Importantly,wealsoextend
ativetotheSunsincetheyusedsolargf-values.Theyremarkthattheir
the conclusions of Mele´ndez et al. (2008) to other α-elements
procedure might introduce inconsistencies due to the different atmo-
(Mg,Si,Ca,Ti).Theα-elementsinthebulgeandlocalthickdisk sphericstructuresofµLeoandtheSun.Furthermore,independentlyof
starsarethesameintherange−1.5<[Fe/H]<−0.3,showinga theadopted gf-values, bydefinition thesolar abundances areneed to
verylowstar-to-starscatterin[α/Fe]ofonly0.03dex.Similarly, obtain[X/Fe].