The Gaia-ESO Survey: radial metallicity gradients and age-metallicity relation of stars in the Milky Way disk

1.4 MB·

by  M. Bergemann

Astronomy&Astrophysicsmanuscriptno.ms-rw-mb˙10 (cid:13)c ESO2014
April10,2014
The Gaia-ESO Survey: radial metallicity gradients and
⋆
age-metallicity relation of stars in the Milky Way disk
M.Bergemann1,G.R.Ruchti2,A.Serenelli3,S.Feltzing2,A.Alves-Brito4,23,M.Asplund4,T.Bensby2,P.
5 5 1 5 1 4 1 5 2
Gruiters ,U.Heiter ,A.Hourihane ,A.Korn ,K.Lind ,A.Marino ,P.Jofre ,T.Nordlander ,N.Ryde ,C.C.
1 1 6 10 11 12 13
Worley ,G.Gilmore ,S.Randich ,A.M.N.Ferguson ,R.D.Jeffries ,G.Micela ,I.Negueruela ,T.
Prusti14,H-W.Rix15,A.Vallenari16,E.J.Alfaro21,C.AllendePrieto7,A.Bragaglia16,S.E.Koposov1,8,A.C.
Lanzafame24,E.Pancino17,9,A.Recio-Blanco18,R.Smiljanic19,20,N.Walton1,M.T.Costado21,E.Franciosini6,
4 18 17 18 6 6 1 6 6
V.Hill ,C.Lardo ,P.deLaverny ,L.Magrini ,E.Maiorca ,T.Masseron ,L.Morbidelli ,G.Sacco ,G.
1
1 22
0 Kordopatis ,andG.Tautvaisˇiene˙
2

r 1 Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0HA, Cambridge, UK e-mail:
p
[email protected]
A
2 LundObservatory,DepartmentofAstronomyandTheoreticalPhysics,Box43,SE-22100Lund,Sweden
3 InstituteofSpaceSciences(IEEC-CSIC),CampusUAB,Fac.Cie`ncies,TorreC5parell2,08193,Bellaterra,Spain
9
4 Research School of Astronomy & Astrophysics, Mount Stromlo Observatory, The Australian National University, ACT 2611,

Australia
]
A 5 DepartmentofPhysicsandAstronomy,DivisionofAstronomyandSpacePhysics,Ångstro¨mlaboratory,UppsalaUniversity,Box
516,75120Uppsala,Sweden
G
6 INAF-OsservatorioAstrofisicodiArcetri,LargoE.Fermi5,50125,Florence,Italy
. 7 InstitutodeAstrof´ısicadeCanarias,E-38205LaLaguna,Tenerife,Spain
h
8 MoscowMVLomonosovStateUniversity,SternbergAstronomicalInstitute,Moscow119992,Russia
p
9 ASIScienceDataCenter,ViadelPolitecnicoSNC,00133Roma,Italy
-
o 10 InstituteofAstronomy,UniversityofEdinburgh,BlackfordHill,EdinburghEH93HJ,UnitedKingdom
r 11 AstrophysicsGroup,ResearchInstitutefortheEnvironment,PhysicalSciencesandAppliedMathematics,KeeleUniversity,Keele,
st StaffordshireST55BG,UnitedKingdom
a 12 INAF-OsservatorioAstronomicodiPalermo,PiazzadelParlamento1,90134,Palermo,Italy
[ 13 DepartamentodeF´ısica,Ingenier´ıadeSistemasyTeor´ıadelaSen˜al,UniversidaddeAlicante,Apdo.99,03080,Alicante,Spain
14 ESA,ESTEC,Keplerlaan1,PoBox2992200AGNoordwijk,TheNetherlands
3 15 Max-PlanckInstitutfu¨rAstronomie,Ko¨nigstuhl17,69117Heidelberg,Germany
v 16 INAF-PadovaObservatory,Vicolodell’Osservatorio5,35122Padova,Italy
7 17 INAF-OsservatorioAstronomicodiBologna,viaRanzani1,40127,Bologna,Italy
3 18 Laboratoire Lagrange (UMR7293), Universite´ de Nice Sophia Antipolis, CNRS,Observatoire de la Coˆte d’Azur, CS 34229,F-
4
06304Nicecedex4,France
4 19 DepartmentforAstrophysics,NicolausCopernicusAstronomicalCenter,ul.Rabian´ska8,87-100Torun´,Poland
1. 20 EuropeanSouthernObservatory,Karl-Schwarzschild-Str.2,85748GarchingbeiMu¨nchen,Germany
0 21 InstitutodeAstrof´ısicadeAndaluc´ıa-CSIC,Apdo.3004,18080Granada,Spain
4 22 InstituteofTheoreticalPhysicsandAstronomy,VilniusUniversity,Gosˇtauto12,LT-01108Vilnius,Lithuania
1 23 InstitutodeFisica,UniversidadeFederaldoRioGrandedoSul,Av.BentoGoncalves9500,PortoAlegre,RS,Brazil
: 24 DipartimentodiFisicaeAstronomia,SezioneAstrofisica,Universita´diCatania,viaS.Sofia78,95123,Catania,Italy
v
i Receiveddate/Accepteddate
X
r ABSTRACT
a
Westudytherelationshipbetweenage,metallicity,andα-enhancementofFGKstarsintheGalacticdisk.Theresultsarebasedupon
the analysis of high-resolution UVES spectra from the Gaia-ESO large stellar survey. We explore the limitations of the observed
dataset,i.e.theaccuracyofstellarparametersandtheselectioneffectsthatarecausedbythephotometrictargetpreselection.Wefind
thatthecolourandmagnitudecutsinthesurveysuppressoldmetal-richstarsandyoungmetal-poorstars.Thissuppressionmaybe
ashighas97%insomeregionsoftheage-metallicityrelationship.Thedatasetconsistsof144starswithawiderangeofagesfrom
0.5Gyrto13.5Gyr,Galactocentricdistances from6kpcto9.5kpc, andverticaldistances fromtheplane0 < |Z| < 1.5kpc. On
thisbasis,wefindthati)theobservedage-metallicityrelationisnearlyflatintherangeofagesbetween0Gyrand8Gyr;ii)atages
olderthan9Gyr,weseeadecreasein[Fe/H]andaclearabsenceofmetal-richstars;thiscannotbeexplainedbythesurveyselection
functions;iii)thereisasignificantscatterof[Fe/H]atanyage;andiv)[Mg/Fe]increaseswithage,butthedispersionof[Mg/Fe]
atages>9Gyrisnotassmallasadvocatedbysomeotherstudies.Inagreementwithearlierwork,wefindthatradialabundance
gradientschangeasafunctionofverticaldistancefromtheplane.The[Mg/Fe]gradientsteepensandbecomesnegative.Inaddition,
weshowthattheinnerdiskisnotonlymoreα-richcomparedtotheouterdisk,butalsoolder,astracedindependentlybytheages
andMgabundancesofstars.
Keywords.Stars:abundances-Stars:fundamentalparameters-Galaxy:solarneighbourhood-Galaxy:disk-Galaxy:formation-
Surveys
1
1. Introduction spectra will be acquired in medium-resolution mode (R ∼
16000), with stars probing distances up to 15 kpc. For the
One of the main quests in modern astrophysics is to under-
analysis of the spectra, several state-of-the-art spectrum anal-
stand the formation history of the Milky Way. This requires
ysis codes are used (Gilmoreetal. 2012, Randichetal. 2013,
observational datasets that provide full phase-space informa-
Smiljanic et al. in prep). In addition, sophisticated methods
tion, ages, and element abundances for a large number of
based on Bayesian inference are now available that combine
starsthroughouttheGalaxy.Thedistributionfunctionsofthese
in a systematic way observational information and stellar evo-
quantities present major constraints on the models of the for-
lutiontheory(Pont&Eyer2004;Jørgensen&Lindegren2005;
mation and evolution of the Milky Way (e.g. Burkertetal.
Schoenrich&Bergemann2013;Serenellietal.2013).
1992; Pagel&Tautvaisiene 1995; Velazquez&White 1999;
Inthisstudy,weusethefirstresultsobtainedwiththeGaia-
Houetal. 2000; Chiappinietal. 2001; Brooketal. 2004,
ESOsurveytostudytherelationshipbetweenageandmetallic-
2005; Naab&Ostriker 2006; Scho¨nrich&Binney 2009b;
ityintheGalacticdisk.Wenotethatthisisanexploratorystudy
Kubryketal.2013;Minchevetal.2013;Rix&Bovy2013).
andmoredefinitiveresultswillbeavailablefromthenextGaia-
Inthiscontext,theprimeobservablesaretheradialmetallic-
ESO data releases with larger samples. Furthermore, there are
ity profilesand the age-metallicityrelation in the Galactic disk
othersurveypapersinpreparationthatwillalsoexplorethedisk
(seee.g.thereviewbyFeltzing&Chiba2013).Foralongtime,
and other components in the Milky Way (Recio-Blanco et al.,
the mere existence of any age-metallicity relation has been a
Mikolaitisetal.,Rojas-Arriagadaetal.,inprep.).Intheupcom-
matterofgreatdebate.ThestudybyTwarog(1980),whichcon-
ingpapers,wewillalsoaddressotheraspectsoftheGalacticdisk
cludedthatagesandmetallicitiesofstarsinthediskareuniquely
population, including the now-controversial dichotomy of the
correlated, was the first important step in putting this discus-
disk(Bovyetal.2012a),byderivingthekinematicsofstarsand
sion ontoa modernfooting.Edvardssonetal. (1993),however,
adding individual abundances of different chemical elements.
was able to show that there is no strong correlation. In addi-
We apply the Bayesian method developed by Serenellietal.
tion, Feltzingetal. (2001), and later the Geneva-Copenhagen
(2013) to derive stellar ages. We explore the limitations of the
Survey (Nordstro¨metal. 2004; Holmbergetal. 2007, 2009;
observeddatasetsandperformadetailedanalysisofthederived
Casagrandeetal. 2011), found a large scatter in metallicity
stellarproperties(mass,ages,metallicity).Finally,weusethese
at all ages, and confirmed the tentative existence of an old
datathatspanafullrangeofagesfrom1Gyrto14Gyr,tostudy
metal-richpopulation.Themostrecentin-depthanalysisofthe
therelationshipbetweenelementalabundancesandagesofstars,
age-metallicity relation in the solar neighbourhood is that by
anddiscusstheradialabundanceprofilesintheGalacticdisk.We
Haywoodetal.(2013)whocometosimilarconclusions.
alsodiscussthecorrelationbetweenMgabundanceandagesof
Combined radial and vertical element abundance profiles
stars.
providethesecondmajorobservationalconstrainttotheGalaxy
Thepaperisstructuredasfollows.InSection2observations
formation models. Attempts have been made to quantify the
anddata reductionare described.Section 3 presentsthe details
metallicity distribution function and abundance gradients of
of stellar evolutionanalysis and varioustests applied to the in-
the disk using data from recent large-scale spectroscopic sur-
put datasets. The sample selection bias is discussed in Section
veys, such as SEGUE/SDSS (e.g. Leeetal. 2011; Bovyetal.
4. Finally, Section 5 describes the results in the context of the
2012b,c;Schlesingeretal.2012;Chengetal.2012),RAVE(e.g.
Galacticevolution,withthefocusontheage-abundanceandspa-
Boecheetal. 2013), and APOGEE (e.g. Andersetal. 2013;
tialdistributionofstarsintheMilkyWaydisk,andtheconclu-
Haydenetal. 2013), as well as smaller samples of stars with
sionsaredrawninSection6
high-resolutionobservations(e.g.Bensbyetal.2003;Fuhrmann
2008;Ruchtietal.2011;Bensbyetal.2014).However,thebulk
of the data has so far come from the traditional techniques,
suchasOBstars,Cepheids,andHIIregions(Andrievskyetal. 2. Observationsandstellarparameters
2002b,a; Le´pineetal. 2011; Lemasleetal. 2013). These pop-
Thisworkmakesuseofthehigh-resolutionUVESobservations1
ulations are young, < 1 Gyr, only providing a snapshot of
taken within the first half year of observations with the Gaia-
the present day abundance pattern. Open clusters and plane-
ESOsurvey(Jan.2012-June2012),whichcomprises576stars.
tary nebulae also provide information about older populations
The details on the data reduction will be provided in Sacco et
(Frieletal. 2002; Stanghellini&Haywood 2010; Yongetal.
al. (in prep). Of those, 348 are field stars while the others are
2012;Heiteretal.2014):openclustersupto8−9Gyrandplane-
members of open or globular clusters. The distribution of the
tarynebulaeupto6Gyr(Macieletal.2010).However,inorder
observedfieldsampleintheNIRcolour-magnitudediagramand
to fully study the evolution and build-up of radial and vertical
the selection box is shown in Fig. 1. The UVES solar neigh-
abundancedistributionsintheGalacticdisk,weneedtracersof
bourhoodtargetswerechosenaccordingtotheircolourstomax-
stellarpopulationsofallages,fromtheearliestepochofGalaxy
imise the fraction of un-evolved FG stars within 2 kpc in the
formationtothepresentday.
solarneighbourhood.Thesurveyselectionboxwasdefinedus-
With the data taken with the Gaia-ESO Survey, which
ingthe2MASSphotometry:12 < J < 14and0.23 < J −K <
form the basis of our work, we are in a good position to ad-
0.45+0.5E(B−V);iftherewerenotenoughtargets,therededge
dress these two fundamental problems and set the stage for
wasextended2.Withthesecriteria,wearepredominantlyselect-
the larger datasets coming in the future releases from the sur-
ingFGstarswithmagnitudesdowntoV = 16.5(Gilmoreetal.
vey. The Gaia-ESO Survey was awarded 300 nights on the
inprep.).
Very Large Telescope in Chile. In the high-resolution (R =
λ/∆λ ∼ 47000) mode it will acquire spectra for about 5000
fieldstars,probingdistancesupto2kpcfromtheSun;100000 1 ResolutionR∼47000
2 ThetargetsselectedbeforeApril(2012)hadthebrightestcutonJof
⋆ Based on observations made with the ESO/VLT, at Paranal 11insteadof12.Ifthenumberofobjectsinthefieldwithintheboxwas
Observatory, under program 188.B-3002 (The Gaia-ESO Public lessthanthenumberofUVESfibers,thenthered-edgeofthecolour-
SpectroscopicSurvey). boxwasshiftedtohaveenoughtargetstofillthefibers.Inaddition,very
M.Bergemannetal.:Agesandabundancesofstarsinthedisk
Finally, we applied the correction for the dust layer, such that
9 thereddeningtoastaratdistanceDandGalacticlongitudebis
10 reduced by a factor 1−exp(−| Dsinb |/h) where h = 125 pc.
J2MASS1121 TclhoisserteodtuhcetiGonalaaffcteicctspltahnees.tarsthatarerelativelynearbyandlie
13 Wecomparetheresultsonlyforasubsampleofthestarswith
14 E(B−V)<0.05.Fig.2showsthedifferencesbetweenthespec-
0.2 0.4 0.6 0.8 1.0 1.2 1.4
J2MASS − K2MASS troscopicandphotometrictemperaturesforthecalibrationfrom
Gonza´lezHerna´ndez&Bonifacio (2009) (top panel) and for
9
10 thecalibrationbyCasagrandeetal. (2010) (bottompanel).The
11 calibrations provide very similar results; the Casagrandeetal.
J2MASS1132 a(2p0p1e0a)rctoalbiberaintioangriesem50enKtownaramveerr.agTeh,ebbuottthhetermeipsearastuyrsetesmcaalteics
14 driftathigherTeff. The originof thisdriftis notclear:it could
15 be caused by reddening, uncertainties in theoretical photomet-
12.0 12.5 13.0 13.5 14.0 14.5 15.0 15.5
VAPASS ricmagnitudes,orbyspectroscopicuncertainties.Forthisstudy,
we have decided to keep all stars. The difference between our
Fig.1. The distribution of the observed UVES data sample in
scale andthatgivenbyIRphotometryis∆T = −20±170K
the 2MASS (top) colour-magnitudeplane and as a function of eff
(calibrationbyGonza´lezHerna´ndez&Bonifacio2009).Thein-
V magnitudes from APASS (bottom). The photometric box is
trinsicuncertaintyoftheJ−KIRFMcalibrationis150K,which
shown(seetext).
maycontributetothewideapparentspread.
The stellar parameters, T , logg, [Fe/H], and abundances
eff
Gonzalez−Hernandez & Bonifacio (2009)
for the UVES spectra are determinedas follows. The observed
500
spectra were processedby 13 research groupswithin the Gaia- Teff
ESOsurveycollaborationwiththesamemodelatmospheresand -Ks 0
linelists, butdifferentanalysismethods,e.g.fullspectrumchi- T-J
−500
square minimisation, using pre-computedgrids of modelspec-
tra or calculating synthetic spectra on the fly, or an analysis
5000 5500 6000 6500 7000
basedonequivalentwidths.Themodelatmospheresare1DLTE Teff
spherically-symmetric(logg < 3.5)andplane-parallel(logg ≥
3.5) MARCS models (Gustafssonetal. 2008). The determina-
Casagrande et al. (2010)
tionofthefinalsetofstellarparametersbasedonallthoseanaly- 500
sesisdescribedinSmiljanicetal.(inprep).Inshort,thefinalpa- Teff
rameterhomogenisationinvolvesamulti-stageprocess,inwhich -Ks 0
both internal and systematic uncertainties of different datasets T-J
−500
arecarefullyevaluated.Variousconsistencytests,includingthe
analysisofstellarclusters,benchmarkstarswithinterferometric 5000 5500 6000 6500 7000
Teff
andastroseismicdata(Jofreetal.2013,Blanco-Cuaresmaetal.
Fig.2.Comparisonofspectroscopictemperaturesandphotomet-
2014, Heiter et al. in prep.), have been used to assess each
ricT temperaturesforasubsampleofstars(SeeSection2.1).
group’s performance. The final parameters are taken to be the J−K
medianofthe multipledeterminations,andthe uncertaintiesof
stellarparametersaremedianabsolutedeviations,whichreflect
the method-to-method dispersion (see Section 5 for more de-
tails). This dataset is available internally to the Gaia-ESO col- 2.2.NLTEeffects
laboration,andwillbereferredtoasiDR1.
One important problem that needs to be addressed is the
influence of the LTE approximation on the determination
2.1.Comparisonwithphotometry of stellar parameters. As shown by Bergemannetal. (2012)
and Lindetal. (2012), non-local thermodynamic equilibrium
The spectroscopic T scale can be tested using the Infra-Red
eff (NLTE) effects change surface gravities and metallicities of
FluxMethod(IRFM; Blackwelletal.1980).Wecomputedthe
stars,inawaysuchthatloggand[Fe/H]obtainedfromFelines
effective temperatureT using the colour - temperaturecal-
J−Ks
are systematically higher. However, these studies also showed
ibrations from Gonza´lezHerna´ndez&Bonifacio (2009) as de-
that the differences are typically small at metallicity [Fe/H] >
scribed in Ruchti et al. (2011). The J, H, and K magnitudes
S −1.
weretakenfrom2MASSandreddeningwasestimatedinaiter-
WehaveutilisedtheSpectroscopyMadeEasy(SME)spec-
ativeprocedurestartingfromthatfoundwiththeSchlegeletal.
(1998) dust maps. Those stars with E(B−V) > 0.1 were then tral synthesis code (Valenti&Piskunov 1996) to study the dif-
ferencebetweenstellar parametersderivedinLTEversusthose
correctedaccordingtotheprescriptiondescribedinBonifacioet
derivedinNLTE.SMEhasbeenrecentlyupgradedbythemod-
al.(2000):
uletosolveforNLTElineformationusingtheFegridsdescribed
E(B−V) =0.035+0.65∗E(B−V) . in Lind et al. (2012).A complete description of the new mod-
corrected Schlegel
ulewillbepresentedelsewhere.ApplicationoftheNLTESME
redobjectswereselectedwithacolour-dependentJcuttopreventlow module to the complete iDR1 UVES dataset analysed in this
S/Nintheoptical. work revealed that the NLTE effects are minor. For complete-
3
M.Bergemannetal.:Agesandabundancesofstarsinthedisk
Rel. star number density Completeness [%]
0.0 0.2 0.4 0.6 0.8 1.0 100 80 60 40 20 0
(a) (a) with J−K and logg cuts
0.4 0.4 0.4
1
0.2 0.2 0.2 %
1
0
0.0 0.0 0.0 %
H] −0.2 H] −0.2 H] −0.2 20%
e/ e/ e/
F F F
[ −0.4 [ −0.4 [ −0.4
−0.6 −0.6 −0.6
−0.8 −0.8 −0.8 40%
−1.0 −1.0 −1.0
0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14
age (Gyr) age (Gyr) age (Gyr)
Rel. star number density Completeness [%]
0.0 0.2 0.4 0.6 0.8 1.0 100 80 60 40 20 0
(b) (b) with J−K and logg cuts
0.4 0.4 0.4
1
0.2 0.2 0.2 %
1
0
0.0 0.0 0.0 %
H] −0.2 H] −0.2 H] −0.2 20%
e/ e/ e/
F F F
[ −0.4 [ −0.4 [ −0.4
−0.6 −0.6 −0.6
−0.8 −0.8 −0.8 40%
−1.0 −1.0 −1.0
0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14
age (Gyr) age (Gyr) age (Gyr)
Fig.3.StellarnumberdensitycomputedfromtheevolutiontracksassumingonlyIMF(a,top)andIMF+SFR(b,bottom)without
colourcuts.Middlepanels:withthecolourcut(0.23< J −K <0.45)andstellarparameterlimits(3.5 ≤ logg ≤ 4.5)tosimulate
theselectioneffectofthesurveyandourstellar sample.Right:the suppressionfactor,definedastheratioof starswithcutto the
numberofstarswithoutcut,i.e.thelargerthenumberthemorestarsaresuppressed(seetext).
ness,wesummarisehereourresultsfordwarfsandgiants,even The differences between our LTE and NLTE results are
though the latter are not used in the final analysis of the age- wellwithintheuncertaintiesofthestellarparametersandabun-
metallicityrelationandabundancedistributions. dances.WethusconcludethattheeffectofNLTEinthestudied
rangeofstellarparameterswillnotsignificantlybiastheresult-
The NLTE excitation equilibrium of FeI lines results in
ingageandabundancedistributions.
somewhatlowereffectivetemperaturesforgiants,withstronger
effects toward higher effective temperatures.The effect on sur-
face gravity is more complex. The direct effect of higher FeI 3. Determinationofagesanddistances
abundancesresultsin higherlogg.Atthe same time,thelower
Teff counteractthedirecteffectonFeIabundances,andsologg Theagesandabsolutemagnitudesofstarsweredeterminedus-
comes out with slightly lower values. The net result is that ingtheBayesianpipelineBeSPP3 developedbySerenellietal.
[Fe/H] stays essentially constant. For stars with logg < 3.5, (2013). The grid of stellar evolutionary tracks has been com-
themeandifferencebetweentheNLTEandLTEparametersis: puted with the GARSTEC code (Weiss&Schlattl 2008). In
−0.25±17K(Teff),−0.05±0.06dex(logg),−0.01±0.02dex GARSTEC, nuclear reaction rates are those recommended by
([Fe/H]). Adelbergeretal. (2011), OPAL opacities by Iglesias&Rogers
For dwarfs, the effect is reversed. Surface gravities and (1996), complemented at low temperatures by those from
Ferguson et al. (2005), the FreeEOS equation of state from
metallicitiesincrease,althoughthedifferencewithrespecttoan
Cassisietal. (2003). Convection is treated with the standard
LTEanalysisisverysmall.Forstarswith3.5< logg< 4.5,the
meanbiasinloggand[Fe/H]is0.03±0.04dexand0.01±0.01 mixinglength theory(αMLT = 1.811,froma solar calibration),
and overshootinghas been included as a diffusive mixing pro-
dex,respectively.ThereisnoeffectonT .
eff
cess(Freytagetal.1996).MoredetailscanbefoundinWeiss&
SME is notyetable to computeNLTE abundancesforMg, Schlattl(2008)andSerenellietal.(2013).
thusarealisticassessmentoftheNLTEeffectcannotbedoneyet.
Our grid of stellar evolution models spans the mass range
We have checked other theoretical studies in the literature and 0.6 ≤ M/M ≤ 3.5withstepsof∆M = 0.01M andthemetal-
⊙ ⊙
foundthatthebiasis alsoverysmall.Forexample,forthe two licity range−5.0 ≤ [Fe/H] ≤ 0.5with stepsof ∆[Fe/H] = 0.1
most important lines of MgI, 5711 and 5183 Å, the difference for [Fe/H] ≤ 0 and ∆[Fe/H] = 0.05 for [Fe/H] > 0. Models
betweentheNLTEandLTEabundanceisnotlargerthan±0.05
dexforstarswithlogg>3.5(e.g.Shimanskayaetal.2000). 3 BellaterraStellarParameterPipeline
4
M.Bergemannetal.:Agesandabundancesofstarsinthedisk
with [Fe/H] ≤ −0.6 include 0.4 dex α-enhancement. A com- 4. Samplecompleteness
pletedescriptionof thepipelinecanbe foundin Serenellietal.
One of the mostdifficultbutfundamentaltasks is to assess the
(2013). In that paper, we showed that below logg ∼ 3.5, stel-
completeness of the observed dataset. Various selection func-
larmassandagecannotbedeterminedbecauseofthedegener-
tions (e.g. in colours, magnitudes, or even T and logg) will
acy of stellar evolutionary tracks in the T −logg plane (see eff
eff reduce the relative number of stars with certain properties. In
also AllendePrieto&Lambert 1999). Therefore, in this work
principle,accountingforthesampleselectionismathematically
weconsideronlystarswith3.5 < logg < 4.5.Duetothelogg
straightforward(e.g.Rix&Bovy2013);howeverthishasrarely
cut,54evolvedstarsareremovedfromoursample.
beenappliedrigorously.
Age was determined by fitting a normal distribution of
Toassesstheinfluenceofsurveyselectioncuts,wedesigned
the probability distribution function (PDF) in the HR-diagram
asimpletest.First,wecreateastellarpopulationwithaSalpeter
where the probability density is higher than 20% of the max-
initial mass function (IMF), a constant SFR, and a uniform
imum. The best-estimate age is taken to be the centre of the
metallicitydistribution.Theresultingdensityofsamplestarsis
Gaussian fit and the uncertainties are determined from the full
shown on the upper-left panel of Figure 3. The upper-central
PDF as 1 − σ confidence level. We also tested other statistics
panelofFigure3showsthedistributionretainedafterthecolour
usingasampleofsyntheticstarsgeneratedfromthetracks(see
cutusedfortheUVESsampleisapplied,0.23 < J−K < 0.45,
AppendixA),butfoundthatneithertheweightedmeannorthe
and with the cuts on surface gravity, 3.5 < logg < 4.5 (see
medianofthe underlyingage PDFisa satisfactoryapproxima-
Section2.1).Thedensityofstarsinthesampleishighestwhere
tion ofthe age when the PDFis broadandasymmetric.Means
J−K = 0.23,thatapproximatelycorrespondsto∼ 6500K,i.e.
or mediansin truncatedPDFs onthe youngside (or onthe old
thebluecutoftheUVESsample.Asthefigureshows,thecuts
side)alsoleadtoanoverestimation(underestimation)ofthetrue
reducetheoverallnumberofstars,withastrongereffectonthe
ageofthestar.
oldestmetal-richstars.Bytakingtheratioofthestellarnumber
The BeSPP pipeline has been tested in a variety of ways densities computed with (N ) and without (N ) the colour
cut nocut
in Serenelli et al. (2013). However, in this work we are inter- and gravity cuts, we can estimate what fraction of stars is re-
ested in how uncertainties on stellar parameters influence the tained in the sample. The sample completeness is thus defined
determination of ages. This test was not done in our previous as:
work, which focused on the relative changes of stellar ages f = N /N
cut nocut
causedstellarparametersderivedunderLTEorNLTE.Toassess
systematic biases, which could be introduced by the Bayesian anditisshownintheupper-rightpanelofFig.3,alongwiththe
method, we performed test simulations with mock samples of contoursofequalprobability.Theoldmetal-richstars,[Fe/H]>
stars drawn randomly from the evolutionary tracks. We have 0.2canbesuppressedalmostcompletely,butalsothefractionof
also checked for the possibility of a systematic mismatch be- oldstarswithsolarmetallicityquicklydeclines.
tween T and logg scales from stellar evolution models and Wehavealsoconsideredamoresophisticatedmodel,assum-
eff
fromspectroscopy.Fromthesetests(seeAppendixA),wefound ing an SFR varying with time and with [Fe/H] (see Appendix
thattheageestimateisrobustandun-biasedwhenitsuncertainty B). The results of this simulation are shown in the lower three
is either smaller than ∼ 30% or smaller than 2 Gyr. The latter panelsofFig.3.TheIMF+SFRdistributionofthestellarden-
conditionisrelevantforyoungerstarsthatnaturallytendtohave sity (the lower-left panel) is markedly different from the case
largerfractionalerrors. of a constant SFR. Because the SFR decreases with time, the
stellar number density always peaks at old ages. In particular,
the short e-folding time at the lowest [Fe/H] is evident by the
Completeness [%] almost complete absence of young (< 5 Gyr) metal-poor stars
([Fe/H] < −0.7). After applying the colour and gravity cuts
100 80 60 40 20 0 (low-middle panel), the notable difference with respect to the
0.4 D=0.6 kpc 1 0.4 D=1.0 kpc 1 constantSFRcase(topmiddlepanel),inadditiontotheabsence
0.2 % 0.2 % ofyoungmetal-poorstars,istheoveralllowerdensityatyounger
[Fe/H] −−000...042 20%10% [Fe/H] −−000...420 20% 20%10% laogweserf-orirgahntypa[Fneel/Ha]n.dTihteissaidmepnlteicaclomtoptlheetecnaesseswisisthhoawflnatinStFhRe
(upper-rightpanel).Thisresultcanbeunderstoodeasily.Stellar
−0.6 −0.6
number density is constructed by integrating for each age and
−0.8 −0.8
−1.0 40% −1.0 [Fe/H],overtheIMFandtheSFR.However,thecontributionof
0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 the SFR to the stellar numberdensity is only a functionof age
age (Gyr) age (Gyr)
and[Fe/H]andthereforecancelsoutwhencomputingthesam-
Fig.4. Same as Fig. 3 but also including the magnitude cut of
ple completenessas a functionof these two quantities.Adding
thesurvey:12< J <14 a spatial distribution to the discussion also does not affect this
conclusion,as long as it does not include a dependenceon the
stellarmass.
Distances were determined following Ruchtietal. (2011), Inthenextstep,wehaveimposedthemagnitudecutaccord-
using the absolute magnitudes as described above and the K ing to the surveyselection. Thiswas achievedby transforming
S
magnitudes from 2MASS. Reddening is described in Section theabsolutemagnitudesofsyntheticstarstotheapparentmag-
2.1.TheGalactocentricradialRdistanceandverticalZdistance nitudes for a range of distances, from 100 pc to 2 kpc (typ-
fromtheplanewerederivedusingthecombinationofeachstar’s ical of the observed sample). The distributions were then re-
distance from the Sun and Galactic (ℓ,b) coordinates.We note stricted to J magnitudes in the range from 12 to 14 mag. Fig
that we have assumed that the Sun lies at R = 8.3 kpc in the 4showstworepresentativecases,wherethedistanceis0.6and
plane.Uncertaintiesin bothR and Z were propagatedfromthe 1kpc(leftandrightpanel),alsoincludingthecolourandgravity
uncertaintyinthedistance. cuts as described above.Beyond 0.9 kpc, the density of young
5
M.Bergemannetal.:Agesandabundancesofstarsinthedisk
starswithsuper-andsubsolarmetallicitydrops,alsoatdistances 11..00 0.50
smaller than 0.6 kpc metal-rich stars are suppressed. In either
case,themostvulnerableregionistheupperrightcornerofthe 00..55 3%
30% 0.35
age-metallicityplot,thatcorrespondstooldmetal-richstars. 50%
00..00
Tosummarise,thecombinedeffectofthecolourandmagni- H]
tudelimitsintheGaia-ESOsurveycatalogueistounder-sample [Fe/ −−00..55 0.20
starsthatareeitheryoungandmetal-poor,oroldandmetal-rich. 50%
However,theextenttowhichthesestarsareunder-sampled,de- −−11..00 0.05
pends quantitatively on their distance and actual age distribu-
tion,whichisnotknownapriori.Furthermore,wehavenotac- −−11..55 −0.10
00 22 44 66 88 1100 1122 1144
countedfor the anisotropyof the inter-stellar reddening,which
age (Gyr) [Mg/Fe]
could have a differential effect on the typical characteristics of
stars observedwithin a givenphotometricbox as a functionof 0.6 0.30
spatialdirection.Therefore,weseethesetestsonlyasanumer-
ical illustration to help us understand the picture qualitatively, 0.4 0.00
andtheresultscannotbeusedtocorrecttheobservedsample.
Fe] 0.2
g/ −0.30
M
5. Results [ 0.0
−0.60
5.1.Age-[α/Fe]andage-[Fe/H]relations
−0.2
−0.90
Themaingoalofthisworkistostudytherelationbetweenages
0 2 4 6 8 10 12 14
and abundances of stars in the Milky Way disk. As discussed age (Gyr) [Fe/H]
inSection3,weapplycutsonloggandonageerrors.We also
remove stars with uncertainties in Mg abundances larger than Fig.6. Top: Age-metallicity plot for the Milky Way disk. The
0.15dex,whichleavesuswith144starsinthefinalsample(Fig. contoursindicatetherelativesamplecompleteness,i.e.theper-
5). The mean uncertainties are 80 K in Teff, 0.15 dex in logg, centageofstarsthatwouldremaininthesampleduetotheGaia-
0.06in[Fe/H],and0.06in[Mg/Fe].Theresultsforallselected ESO survey selection functions, i.e. IR magnitude and colour
stars in the age-metallicity and age-[Mg/Fe] planes are shown cuts, and restrictions imposed on stellar parameters. Here the
in Fig. 6. The contours in the top panel of Fig. 6 indicate the magnitude cuts refer to the distance of 1 kpc. For clarity, this
relativesamplecompleteness;the percentageswerenormalised fractionwasnormalisedtoitspeakvalue.Bottom:thedistribu-
toitspeakvalue,whichis∼35%(Fig.3). tionofstarsinthe[Mg/Fe]-ageplane.
thattheolderstarsareprogressivelymoremetal-poor.Ourresult
1
doesnotsupporttheanalysisbyCasagrandeetal.(2011),which
−1.0
−0.00.5 isbasedonthephotometricmetallicitiesandagesofstarsinthe
0.3
Geneva-CopenhagenSurvey.Theauthorsfindnoage-metallicity
2
relation; the stars are homogeneouslydistributed in metallicity
in any age bin up to 12 Gyr (Casagrandeetal. 2011, their Fig.
16).While qualitatively,themeanmetallicityofthe samplefor
g
og 3 oldagescouldbeaffectedbyoursamplingbiasagainstoldand
l
metal-richstars,Fig.6showsthatthesuppressionrelativetothe
mostpopulatedpartoftheplotisnotlargerthan50−70%.The
4 factthatnometal-richstar isobservedwith age> 10Gyrmay
indicate that such stars are rare, if they exist at all in the solar
neighbourhood.
5 Figure6(bottompanel)shows[Mg/Fe]ratiosasafunction
7000 6000 5000 4000
of age, colour-coded with [Fe/H]. The oldest stars with ages
T (K)
eff > 12Gyr show[Mg/Fe],from0to 0.4dex,anda broadrange
of metallicity, from solar to [Fe/H] ∼ −1. There is little evi-
Fig.5. The selected UVES sample: (black open circles) - all
dencethattherelationtightensatagesgreaterthan9Gyrinour
iDR1 stars; (filled green circles) - stars that satisfy our selec-
tion criteria. 8 Gyr GARSTEC isochrones for different [Fe/H] sample,asadvocatede.g.byHaywoodetal.(2013)whouseda
subsample of 363 stars from the Adibekyanetal. (2012) sam-
arealsoshown(seeSection3).
ple of 1111 FGK stars. Bensbyetal. (2014) also found a knee
at 9 Gyr, with a clear increase in [Mg/Fe] with age (their Fig.
First, we turn to the discussion of the age-metallicity rela- 21),albeitwithanotablylargerscatteratages> 11Gyrthanin
tion(Fig.6,toppanel).Themainfeaturesdemonstratedinear- Haywoodetal.(2013).Itispossiblethatthelargerscatterinour
lier observational studies of the disk by Feltzingetal. (2001); sample at old ages is due to the fact that we include stars with
Haywoodetal. (2013);Bensbyetal. (2014) are clearlyvisible. relativelylargeageuncertainties.However,fromouranalysisof
Between 0 Gyr and 8 Gyr the trend is predominantly flat, and theselectioneffects,itistobeexpectedthatsomefractionofα-
thescatter in [Fe/H]is largeatanygivenage.Thisisthe most pooroldstarscouldbeartificiallysuppressedforthesamerea-
populatedlocusonthediagram,whichisunbiasedaccordingto sonsasdiscussedabove,akinto the[Fe/H]suppressionshown
our tests. After 8 Gyr, there is a clear decline in [Fe/H], such intheage-metallicityplot.Regardlessoftheseeffects,thetrends
6
M.Bergemannetal.:Agesandabundancesofstarsinthedisk
at old age, as seen by Haywoodetal. (2013) and Bensbyetal. 11..00 8000.00
(2014)fitwithinour[Mg/Fe]-agerelation.
Finally, one interesting feature of the age-metallicity rela-
tiondeservesacomment.Fig.7(toppanel)showstheobserved 00..55 3%
7125.00
stars in the age-metallicity plane colour-coded with their T . 30%
eff 50%
The obvious correlation with effective temperature is striking
00..00
yet it can be easily explained based on the similar considera- H]
tionsasinSection4. InthemiddleandbottompanelofFig.7, e/ 6250.00
F
we also show the maximum Teff to be expected in the [Fe/H] [ −−00..55
-ageplanebasedonstellarevolutionmodels,without(middle) 50%
and with (bottom) cuts on the photometry and stellar parame- 5375.00
ters. The colour and logg cuts affect the bottom left corner of −−11..00
theage-metallicityplot.Younghotstarsaresuppressedbecause
ofthebluecolourcutintheGaia-ESOUVESsampleselection.
−−11..55 4500.00
However,fromthecomparisonofthesetwoplotsweseethatthe 00 22 44 66 88 1100 1122 1144
trend for T ≤ 6500 K does not depend on selection effects. age (Gyr) Teff
eff
ThesegregationofthesampleaccordingtotheT isthussim-
eff
plytheconsequenceofstellarevolution:thehighest’observable’ Highest Teff
> 8000.00
temperatureofstarsdecreaseswithageandmetallicity,andnat-
0.4
urallyleadstotheT distributionshowninFig.7.Suchatrend
eff
mustbepresentinanysurvey,aslongastheprobedTeff rangeis 0.2 5500 K
nottoonarrow.
0.0
5750 K
5.2.Abundancegradientsasafunctionofradialandvertical
H] −0.2
distance e/
F
[ 7125.00
−0.4
Figure 8 shows how stars are distributed with the vertical dis-
tance from the plane |Z| and Galactocentricdistance R, colour-
−0.6
codedaccordingtotheirages.Themiddlepanelsshowthegradi-
entsof[Fe/H]and[Mg/Fe]asafunctionofR;thebottompanel −0.8 6250 K 6000 K
isthe[Mg/Fe]-[Fe/H]relation.Thethreeopenclustersthatare
6500 K
availableintheUVESiDR1,NGC6705,Trumpler20,andNGC −1.0
4815(Magrinietal.2013)arealsoplotted. 0 2 4 6 8 10 12 14
age (Gyr)
There are obvious differences in the age, metallicity, and 6250.00
Mg gradients with R. The younger stars with ages ≤ 7 Gyr
Highest Teff w/cuts
showanoutwarddecreaseinthemean[Fe/H],slightlystronger
5500 K
comparedtothepreviousstudiesofyoungdiskpopulations,i.e 0.4
Cepheids,OBstars,andHIIregions(Andrievskyetal.2002a,b,
Lemasleetal.2013).Usingonlystarsclosetotheplane,within 0.2
|Z| of 300 pc, we derive ∆[Fe/H]/∆R = −0.068 ± 0.014
dex/kpc or ∆[Fe/H]/∆R = −0.076 ± 0.010 dex/kpc (stars 0.0
with ages ≤ 7 Gyr). Analysing RAVE spectra of disk dwarfs, 5750 K 5375.00
Boecheetal.(2013)foundthat∆[Fe/H]/∆R=−0.065dex/kpc e/H] −0.2
F
for Z < 400 pc. For 300 < Z < 800 pc, we obtain [
max max −0.4
∆[Fe/H]/∆R = −0.114±0.009dex/kpc.Thesteepeningofthe
gradientissurprisinggiventheotherstudiesfoundflatteroreven
−0.6
positive radial metallicity gradients at larger vertical distances
from the plane (AllendePrietoetal. 2006,Haydenetal. 2013; −0.8 6250 K 6000 K
Andersetal. 2013, Katzetal. 2011), but this could reflect our
6500 K
smalldatasetorthefactthatthestudiesprobemuchlarger|Z|. −1.0 4500.00
0 2 4 6 8 10 12 14
Theoldstarsinoursampletendtohavelower[Fe/H]even age (Gyr) Teff
in the inner disk (Fig. 8, (b)), and our [Fe/H] gradient for old
starswith agesabove12Gyris∆[Fe/H]/∆R = 0.069±0.044, Fig.7. Top panel: the observed stars in the age-[Fe/H] plane,
positivebutwitha largeruncertaintyandpotentiallyconsistent colour-coded with their T . Bottom two panels: The highest
eff
with being flat. However, we also note that this apparent age possibleT foragivenageand[Fe/H]obtainedfromthestellar
eff
dependence of the radial metallicity gradient is also related to evolutiontrackswithout(middle)andwith(bottom)photometry
the factthatwe observeolderstarsatlargerdistancesfromthe andloggcuts.SelectedcurvesofconstantT aregivenforref-
eff
plane. As seen in the top (a) panel of Fig. 8, the majority of erence.ForT ≤6500K,thehighest’observable’valuesofT
eff eff
old stars are also located at |Z| > 800 pc and our sample does donotdependonthecutsimposedonthesampleandaresimply
not have directional isotropy: we probe larger distances in the theresultofstellarevolutioneffects.
innerdisk.Therefore,wecannotyetmakemoredefinitivestate-
ments about the gradients for old stellar population,or at high
|Z|.Nordstro¨metal.(2004)notedachangeofslopeoftheradial
[Fe/H]gradientinthedisk.Theyfindaprogressivelyflattening δ[Fe/H]/δR withincreasingage,whichevenbecomesposi-
mean
7
M.Bergemannetal.:Agesandabundancesofstarsinthedisk
11..55 14.00
(a)
11..00
11.20
00..55 Trumpler20
c) NGC6705 8.40
kp 00..00
Z ( −−00..55 NGC4815 5.60
−−11..00 2.80
−−11..55 0.00
66 77 88 99 1100
R (kpc)
14.00
0.5 (b)
11.20
0.0
H] 8.40
e/ −0.5
F 5.60
[
−1.0
2.80
−1.5 0.00
6 7 8 9 10
R (kpc)
14.00
0.4 (c)
11.20
0.2
e] 8.40
F
g/ 0.0
M 5.60
[
−0.2 2.80
Open Clusters
−0.4 0.00
6 7 8 9 10
R (kpc)
14.00
0.4 (d)
11.20
e] 0.2 8.40
F
g/
M 0.0 5.60
[
2.80
−0.2
0.00
−1.0 −0.5 0.0 0.5 age (Gyr)
[Fe/H]
Fig.8.Thefinalstellarsample.Toptobottom:a)thedistributionofstarsintheZ vsRplane;bandc)radialgradientsintheMW
diskfor[Fe/H]and[Mg/Fe];d)the[Fe/H]−[Mg/Fe]relation.Colourrepresentsageofastar.Thethreeyoungopenclusters,which
areavailableinUVESiDR1,areshownwithlargecrosses.
tiveforages> 10Gyr4.Itislikelythattheirresultsalsoreflect in our sample, the most populated bin in our sample, we infer
theeffectofobservingolderstarsatlarger|Z|fromtheplane. ∆[Mg/Fe]/∆R = −0.045± 0.011. It appears that the gradient
The radial [Mg/Fe] gradient is shown in the panel (c) of steepens further at even larger vertical distance, |Z| > 0.8 kpc.
Fig. 8 colour-coded with age. For the stars with |Z| ≤ 300 pc, Moreover,we findthatthegradientof[Mg/Fe]becomesnega-
the radial gradient in the sample mean of the [Mg/Fe] abun- tiveandverysteepforolderstars,i.e.∆[Mg/Fe]/∆R= 0.015±
danceisclosetozero,∆[Mg/Fe]/∆R=0.021±0.014.However, 0.014(ages≤7Gyr)and∆[Mg/Fe]/∆R=−0.071±0.029(ages
thegradientbecomesnegativeandsteepenswithincreasingdis- ≥ 12 Gyr). In words, the outer disk has less high-Mg and old
tance from the plane. For the stars with 0.3 < |Z| < 0.8 kpc starsthantheinnerdisk.Theseresultsareveryinteresting,and
theysupportandextendtheresultsoftheearlierstudies,includ-
ingBensbyetal.(2010,2011)andBovyetal.(2012c).Thefor-
4 Nordstrometal.(2004)studiedgradientsasafunctionofthemean
merfocussedontheinner(4<R<7kpc)andouter(9<R<13
radiusR
mean
8
M.Bergemannetal.:Agesandabundancesofstarsinthedisk
kpc) disk giants. Their main conclusion is that while the in- prediction,moredataisneeded.Thereforeweleavethisanalysis
ner disk giants chemically behave as the solar neighbourhood forthenextpaper,whichwillincludea muchlargernumberof
stars in terms of the ”two” disk components (low [α/Fe] thin- starsfromtheGaia-ESODR2dataset.
diskstarsvshigh[α/Fe]thick-diskstars),theouterdiskisonly
formedof ”thin-disk”-likestars. Bovyetal. (2012c) foundthat
6. Summary
theincidenceofα-enhancedstarsfallsoff stronglytowardsthe
outerdiskusingG-dwarfsfromtheSEGUEsurvey.Inthiswork, The main goal of our work is to study the relationship be-
wearealsoabletoadd,forthefirsttime,agestotheinner-outer tweenageandmetallicityandspatialdistributionofstarsinthe
disk picture.The scatter of [Fe/H], [Mg/Fe], and ages of stars Galacticdiskfromobservationalperspective.
towardstheinnerdiskislargerthanintheouterdisk.Thismeans The stellar sample includes several hundreds of stars ob-
thatthe chemicalevolutionmodelsfor the Galactic disk which served at high-resolution (with the UVES instrument at VLT)
predict smaller abundance scatter in the inner disk (Houetal. within the first six monthsof the Gaia-ESO survey. Stellar pa-
2000)aredisfavouredbyourresults. rameters and abundances were determined with 1D local ther-
Thebottom(d)panelofFig.8showsthe[Mg/Fe]-[Fe/H]re- modynamicequilibrium(LTE)MARCSmodelatmospheres,by
lation.Theresultsarenotun-expected:thesolar-metallicitystars combiningtheresultsobtainedbyseveraldifferentspectroscopic
arepredominantlyyoung.Atlowermetallicity,weseeyounger techniques. We then derived the mass and age of each star, as
stars with low Mg/Fe abundance ratios, but also very old Mg- well as its distance from the Sun, using the Bayesian method
enhanced stars. Whether these are two different components, developedin Serenelliet al. (2013).The results were validated
each with its own history, or a continuous distribution of stars againstastero-seismologyandtheIRFM method.We tookspe-
cannotbefirmlyestablishedyet. cialcaretoquantifythesurveyselectioneffectsintheinterpreta-
Ingeneral,ourresultsmayhaveseveralinterpretations(see tionofthefinalparameterdistributions.Forthis,weperformed
Feltzing&Chiba 2013, for a review of disk formation sce- aseriesoftestsusingamockstellarsamplegeneratedfromthe-
narios). Some observational studies (e.g. Juric´etal. 2008) de- oreticalstellarmodels,whichwassubjecttovariouscutsinthe
compose the disk into the thin and thick components, which space of observables.The outcomeof these tests allowedus to
formedindifferentepisodesofGalaxyformation.Modelswhich define the optimal dataset to address the science problems. In
assume different evolutionary history of the two disks (e.g. the final sample, we have 144 stars with a wide range of ages
Chiappinietal.1997,2001)arebroadlyconsistentwithobserva- (0.5−13.5Gyr),Galactocentricdistancesfrom6kpcto9kpc,
tions.Starsinthethindiskareyoung<8Gyrandα-poor,while and vertical distances from the plane 0 < |Z| < 1.5 kpc. The
thethickdiskisolder,metal-poorandα-enhanced(Bensbyetal. sampleisnotlarge,howeveritshouldbenotedthatthestudyis
2003;Kordopatisetal.2011,2013).Inthisinterpretation,atthe exploratory,i.e.layingoutthemethodandthepropertiesofthe
smallestdistancesfromtheplane,ourage-metallicityrelationis observed field sample of stars in the Gaia-ESO Survey. At the
dominatedbythethindisk.Intheintermediateverticaldistance end of the survey we expect to have a one order of magnitude
bin,ourdatawouldindicateapresenceofboththethinandthick increaseinthestellarsample.
disk. Ourfindingscanbesummarisedasfollows.
While the discrete nature of the disk is one possibility, our First, the observed age-metallicity relation is fairly flat in
data can be also explained by other Galaxy formation mod- rangeofagesbetween0and8Gyr(orequivalentlyintheyoung
els, which do not explicitly divide the disk into subcompo- disk, for stars close to the plane). We also confirm the previ-
nents (Houetal. 2000; Rosˇkaretal. 2008; Rahimietal. 2013; ousresultsthatthereisasignificantscatterofmetallicityatany
Kubryketal. 2013; Minchevetal. 2013, 2014). For example, age.Asteepdeclinein[Fe/H]isseenforstarswithagesabove
the semi-analytical model by (Scho¨nrich&Binney 2009b, see 9 Gyr. The colour and magnitude cuts on the survey suppress
their Fig. 6), which assumes radial migration driven by tran- veryoldmetal-richstarsandyoungmetal-poorstars.However,
sient spiral arms, is consistent with our data both what con- thesuppressionrelativetothemostpopulatedlocusontheage-
cerns the age-metallicity relation shape and the [Fe/H] scatter metallicityrelationisnotlargerthan50−70%.Inoursample,no
at a given age. In accord with our results, this model also pre- solar-metallicity star is observedwith age > 10 Gyr. This may
dicts the apparent bimodality in the [Fe/H]-[α/Fe]plane, just indicate that such stars are rare, if they exist at all in the solar
as we observe (Fig. 8, bottom panel). This effect is not a con- neighbourhood.
sequence of a star formation hiatus (as e.g. in Chiappinietal. [Mg/Fe]ratioscorrelatewithage,suchthatα-richstarsare
2001), but the 1 Gyr timescale of SN Ia coupled with secular older,butthedispersionof[Mg/Fe]abundancesisnotsmallat
heatingandchurning(Scho¨nrich&Binney2009a,asimplean- anyage.Inparticular,theoldstarswithagesabove9Gyrhave
alytical explanation is given in their Appendix B). There exist a range of α-enhancement, from 0 to 0.4 dex, and metallicity
othertypesofmodelsthatalsoshowconsistencieswithourdata. from solar to [Fe/H] ∼ −1. This contrasts with the very tight
Forexample,hydrodynamicalN-bodysimulations(Brooketal. correlation of [Mg/Fe] and ages for old stars as suggested by
2012; Minchevetal. 2013) form a thick-disk like component Haywoodetal.(2013).However,thetrendsfoundbyHaywood
through early gas-rich mergers and radial migration driven by etal.(2013)andBensbyetal.(2014)overlapwithour[Mg/Fe]-
externalandinternalperturbations.Themostrecentsimulations agerelation.
byMinchevetal.(2014,theirFig.9)predictthatmono-agepop- We also show, in agreement with earlier observational and
ulationshaveradialabundancegradientsthatdonotchangewith theoretical work, that the radial abundance gradients (Fe, Mg)
verticaldistancefromtheplane.Thechangeofradialgradients change as a function of vertical distance from the plane, or
with|Z|isthuscausedbyadifferentmixofstellaragesatdiffer- the mean age of a population. At |Z| ≤ 300 pc, we find
ent altitudes. We cannotyet test this hypothesisbecause of the ∆[Fe/H]/∆R = −0.068±0.014and ∆[Mg/Fe]/∆R = 0.021±
sparsesamplingofstars.However,weconfirmthatthepositive 0.014. For the most populated |Z| bin in our sample, 300 ≤
gradientin[Mg/Fe]atlow|Z|changestonegativeatlargerverti- |Z| ≤ 800 pc we infer ∆[Mg/Fe]/∆R = −0.045± 0.011, and
caldistancefromtheplane,wealsoobserveasimilarstructurein ∆[Fe/H]/∆R = −0.114 ± 0.009. The picture is not too dif-
themono-agebins.Toputmorequantitativeconstraintsontheir ferent when separating stars according to their age: the gra-
9
M.Bergemannetal.:Agesandabundancesofstarsinthedisk
dient of [Mg/Fe] is close to zero for younger stars, but be- Holmberg,J.,Nordstro¨m,B.,&Andersen,J.2009,A&A,501,941
comes negative and very steep for the older stellar popula- Hou,J.L.,Prantzos,N.,&Boissier,S.2000,A&A,362,921
tion, i.e. ∆[Mg/Fe]/∆R = 0.015 ± 0.014 (ages ≤ 7 Gyr) Iglesias,C.A.&Rogers,F.J.1996,ApJ,464,943
Jofre,P.,Heiter,U.,Soubiran,C.,etal.2013,ArXive-prints
and ∆[Mg/Fe]/∆R = −0.071 ± 0.029 (ages ≥ 12 Gyr). The
Jørgensen,B.R.&Lindegren,L.2005,A&A,436,127
anisotropic distribution of stars in our sample, i.e. larger frac- Juric´,M.,Ivezic´,Zˇ.,Brooks,A.,etal.2008,ApJ,673,864
tionof starsobservedin the innerdisk, shouldbe keptin mind Katz,D.,Soubiran,C.,Cayrel,R.,etal.2011,A&A,525,A90
wheninterpretingthesegradients. Kordopatis,G.,Gilmore,G.,Wyse,R.F.G.,etal.2013,MNRAS,436,3231
Kordopatis,G.,Recio-Blanco,A.,deLaverny,P.,etal.2011,A&A,535,A107
Tosummarise,perhapsthemostimportantresultofthisanal-
Kubryk,M.,Prantzos,N.,&Athanassoula,E.2013,MNRAS,436,1479
ysisishowthepropertiesofthedominantstellarpopulationsin
Lee,Y.S.,Beers,T.C.,An,D.,etal.2011,ApJ,738,187
thediskchangeasweprobestarswithdifferentages,distances, Lemasle,B.,Franc¸ois,P.,Genovali,K.,etal.2013,A&A,558,A31
Mg abundances, and metallicities. We find more older, α-rich, Le´pine, J.R.D.,Cruz, P.,Scarano, S.J.,etal. 2011,Monthly Notices ofthe
andmetal-poorstarsintheinnerdisk.Intheouterdisk,starsare RoyalAstronomicalSociety,417,698
Lind,K.,Bergemann,M.,&Asplund,M.2012,MNRAS,427,50
onaverageyoungerandα-poor.Althoughourcurrentsampleis
Maciel,W.J.,Costa,R.D.D.,&Idiart,T.E.P.2010,A&A,512,A19
small,ourresultslendsupporttocurrentpicturesoftheforma- Magrini,L.,Randich,S.,Romano,D.,etal.2013,ArXive-prints
tionoftheGalacticdisk,suchastheinside-outformation.This Minchev,I.,Chiappini,C.,&Martig,M.2013,A&A,558,A9
initial dataset illustrates the potential of the Gaia-ESO Survey. Minchev,I.,Chiappini,C.,&Martig,M.2014,ArXive-prints
Naab,T.&Ostriker,J.P.2006,MNRAS,366,899
With upcomingdata releases, we will have a large database of
Nordstro¨m,B.,Mayor,M.,Andersen,J.,etal.2004,Astronomy&Astrophysics,
spectrathathavebeenreducedandanalysedinasystematicand
418,989
homogeneousmanner.Wewillthusbeabletodistinguishamong Pagel,B.E.J.&Tautvaisiene,G.1995,MNRAS,276,505
themanytheoriesoftheformationofthediskoftheMilkyWay. Pont,F.&Eyer,L.2004,MNRAS,351,487
Rahimi,A.,Carrell,K.,&Kawata,D.2013,ArXive-prints
Randich,S.,Gilmore,G.,&Gaia-ESOConsortium.2013,TheMessenger,154,
References 47
Rix,H.-W.&Bovy,J.2013,A&ARev.,21,61
Adelberger, E. G., Garc´ıa, A., Robertson, R. G. H., et al. 2011, Reviews of Rosˇkar,R.,Debattista,V.P.,Quinn,T.R.,Stinson,G.S.,&Wadsley,J.2008,
ModernPhysics,83,195 ApJ,684,L79
Adibekyan,V.Z.,Sousa,S.G.,Santos,N.C.,etal.2012,A&A,545,A32 Ruchti,G.R.,Fulbright,J.P.,Wyse,R.F.G.,etal.2011,ApJ,737,9
AllendePrieto,C.,Beers,T.C.,Wilhelm,R.,etal.2006,ApJ,636,804 Schlegel,D.J.,Finkbeiner,D.P.,&Davis,M.1998,ApJ,500,525
AllendePrieto,C.&Lambert,D.L.1999,A&A,352,555 Schlesinger,K.J.,Johnson,J.A.,Rockosi,C.M.,etal.2012,ApJ,761,160
Anders,F.,Chiappini,C.,Santiago,B.X.,etal.2013,ArXive-prints Schoenrich,R.&Bergemann,M.2013,ArXive-prints
Andrievsky,S.M.,Bersier,D.,Kovtyukh,V.V.,etal.2002a,A&A,384,140 Scho¨nrich,R.&Binney,J.2009a,MNRAS,396,203
Andrievsky,S.M.,Kovtyukh,V.V.,Luck,R.E.,etal.2002b,A&A,381,32 Scho¨nrich,R.&Binney,J.2009b,MNRAS,399,1145
Bensby,T.,Alves-Brito,A.,Oey,M.S.,Yong,D.,&Mele´ndez,J.2010,A&A, Serenelli,A.M.,Bergemann,M.,Ruchti,G.,&Casagrande,L.2013,MNRAS,
516,L13 429,3645
Bensby,T.,Alves-Brito,A.,Oey,M.S.,Yong,D.,&Mele´ndez,J.2011,ApJ, Shimanskaya,N.N.,Mashonkina,L.I.,&Sakhibullin,N.A.2000,Astronomy
735,L46 Reports,44,530
Bensby,T.,Feltzing,S.,&Lundstro¨m,I.2003,A&A,410,527 Stanghellini,L.&Haywood,M.2010,ApJ,714,1096
Bensby,T.,Feltzing,S.,&Oey,M.S.2014,A&A,562,A71 Valenti,J.A.&Piskunov,N.1996,A&AS,118,595
Bergemann,M.,Lind,K.,Collet,R.,Magic,Z.,&Asplund,M.2012,MNRAS, Velazquez,H.&White,S.D.M.1999,MNRAS,304,254
427,27 Weiss,A.&Schlattl,H.2008,Ap&SS,316,99
Blackwell, D. E., Petford, A. D., & Shallis, M. J. 1980, Astronomy & Yong,D.,Carney,B.W.,&Friel,E.D.2012,AJ,144,95
Astrophysics,82,249
Blanco-Cuaresma,S.,Soubiran,C.,Jofre´,P.,&Heiter,U.2014,ArXive-prints
Boeche,C.,Siebert,A.,Piffl,T.,etal.2013,A&A,559,A59 Acknowledgements. This work was partly supported by the European Union
Bovy,J.,Rix,H.-W.,&Hogg,D.W.2012a,ApJ,751,131 FP7 programme through ERC grant number 320360. AS is supported by
Bovy,J.,Rix,H.-W.,Hogg,D.W.,etal.2012b,ApJ,755,115 the MICINN grant AYA2011-24704 and by the ESF EUROCORES Program
Bovy,J.,Rix,H.-W.,Liu,C.,etal.2012c,ApJ,753,148 EuroGENESIS (MICINN grant EUI2009-04170). The results presented here
Brook,C.B.,Gibson,B.K.,Martel,H.,&Kawata,D.2005,ApJ,630,298 benefited fromdiscussions heldduringGaia-ESOworkshops andconferences
Brook,C.B.,Kawata,D.,Gibson,B.K.,&Freeman,K.C.2004,ApJ,612,894 supported by the ESF (European Science Foundation) through the GREAT
Brook,C.B.,Stinson,G.S.,Gibson,B.K.,etal.2012,MNRAS,426,690 (GaiaResearchforEuropeanAstronomyTraining)ResearchNetworkProgram.
Burkert,A.,Truran,J.W.,&Hensler,G.1992,ApJ,391,651 Based on data products from observations made with ESO Telescopes at the
Casagrande, L.,Ramirez, I.,Melendez, J.,Bessell, M.,& Asplund, M.2010, LaSillaParanalObservatoryunderprogrammeID188.B-3002.Thisworkwas
Astronomy&Astrophysics,512,A54 partly supported by the European Union FP7programme through ERC grant
Casagrande,L.,Schoenrich,R.,Asplund,M.,etal.2011,VizieROn-lineData number 320360 and by the Leverhulme Trust through grant RPG-2012-541.
Catalog,353,09138 Weacknowledge thesupport from INAFand Ministero dell’ Istruzione, dell’
Casagrande,L.,Scho¨nrich,R.,Asplund,M.,etal.2011,A&A,530,A138 Universita`’ e della Ricerca (MIUR) in the form of the grant ”Premiale VLT
Cassisi,S.,Salaris,M.,&Irwin,A.W.2003,ApJ,588,862 2012”. GRR and SF acknowledge support by grant No. 2011-5042 from the
Cheng,J.Y.,Rockosi,C.M.,Morrison,H.L.,etal.2012,ApJ,752,51 SwedishResearchCouncil.WeacknowledgesupportfromtheSwedishNational
Chiappini,C.,Matteucci,F.,&Gratton,R.1997,ApJ,477,765 Space Board (Rymdstyrelsen). T.B. was funded by grant No. 621-2009-3911
Chiappini,C.,Matteucci,F.,&Romano,D.2001,ApJ,554,1044 fromTheSwedishResearchCouncil.
Edvardsson,B.,Andersen,J.,Gustafsson,B.,etal.1993,A&A,275,101
Feltzing,S.&Chiba,M.2013,NewARev.,57,80
Feltzing,S.,Holmberg,J.,&Hurley,J.R.2001,A&A,377,911 AppendixA: Tests
Freytag,B.,Ludwig,H.-G.,&Steffen,M.1996,A&A,313,497
Friel,E.D.,Janes,K.A.,Tavarez,M.,etal.2002,AJ,124,2693 To check for systematic biases caused by the method itself,
Fuhrmann,K.2008,MNRAS,384,173
weperformedMonte-Carlosimulationsbygeneratingarandom
Gilmore,G.,Randich,S.,Asplund,M.,etal.2012,TheMessenger,147,25
Gonza´lezHerna´ndez,J.I.&Bonifacio,P.2009,A&A,497,497 sample of synthetic stars assuming a Salpeter IMF, a constant
Gustafsson,B.,Edvardsson,B.,Eriksson,K.,etal.2008,A&A,486,951 starformationrate,andauniform[Fe/H]distributionovertime.
Hayden,M.R.,Holtzman,J.A.,Bovy,J.,etal.2013,ArXive-prints Thesampleis restrictedtothe low-massrange0.7 ≤ M/M ≤
⊙
Haywood,M.,DiMatteo,P.,Lehnert,M.,Katz,D.,&Gomez,A.2013,ArXiv 1.5 and −2.2 ≤ [Fe/H] ≤ 0.4, representative of the Gaia-ESO
e-prints
surveyUVESsample.Massand[Fe/H]arediscretisedtomatch
Heiter,U.,Soubiran,C.,Netopil,M.,&Paunzen,E.2014,A&A,561,A93
Holmberg,J.,Nordstro¨m,B.,&Andersen,J.2007,A&A,475,519 the values present in the grid. Along each track, Teff and logg
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