Oxygen abundances in G- and F-type stars from HARPS

2.3 MB·English

by  S. Bertran de Lis

Astronomy&Astrophysicsmanuscriptno.oxygen cESO2015
(cid:13)
January26,2015
Oxygen abundances in G- and F-type stars from HARPS
Comparison of [OI] 6300Å and OI 6158Å
S.BertrandeLis1,2,E.DelgadoMena3,V.Zh.Adibekyan3,N.C.Santos3,4 andS.G.Sousa1,3,4
1 InstitutodeAstrofísicadeCanarias,E-38205LaLaguna,Tenerife,Spain
e-mail:[email protected]
2 UniversidaddeLaLaguna,Dept.Astrofísica,E-38206LaLaguna,Tenerife,Spain
5 3 CentrodeAstrofísicadaUniversidadedoPorto,RuadasEstrelas,4150-762Porto,Portugal
1 4 DepartamentodeFísicaeAstronomia,FaculdadedeCiênciasdaUniversidadedoPorto,Portugal
0
Received...;Accepted...
2

n
a ABSTRACT
J
Aims.Wepresentadetailedanduniformstudyofoxygenabundancefromtwodifferentoxygenlinesat6158Åand6300Åinalarge
3
sampleofsolar-typestars.Theresultsareusedtocheckthebehaviourofthesespectrallinesasoxygenabundanceindicatorsandto
2
studytheevolutionofoxygeninthickandthindiskpopulationsoftheGalaxy.

Methods.Equivalentwidthmeasurementswerecarriedoutforthe[OI]6158ÅandOI6300Ålines.LTEabundanceswereobtained
]
R fromthesetwolinesin610and535stars,respectively.Wewereabletomeasureoxygenabundancefrombothindicatorsin447stars,
enablingus,forthefirsttime,tocomparetheminauniformway.Carefulerroranalysishasbeenperformed.
S
Results.Wefoundthatoxygenabundancesderivedfromthe6158Åand6300Ålinesagreetowithin0.1dexin58%ofthestarsin
.
h oursample,andthisresultimprovesforhighersignal-to-noisevalues.Weconfirmanoxygenenhancementinstarsofthethickdisk,
p ashasalsobeenseenforotherα-elements.Thenewoxygenabundancesconfirmpreviousfindingsforaprogressivelinearriseinthe
- oxygen-to-ironratiowithaslopeequalto0.78fromsolarmetallicityto[Fe/H] -1.However,theslopewemeasuredissteeperthan
o theonefoundinpreviousstudiesbasedontheoxygentriplet.Below[Fe/H]= 0∼.6ourstarsshow[O/Fe]ratiosashighas 0.8,which
r − ∼
t canbeinterpretedasevidenceforoxygen overproduction intheGalacticthickdisk.Thesehighoxygenabundances donotposea
s problemtochemodynamicalmodelssincethereisarangeofparametersthatcanaccommodateourresults.
a
[ Keywords. stars:abundances–stars:atmospheres–stars:solar-type–Galaxy:abundances

1
v
51. Introduction OI triplet at 7771–4Å. The weak spectral lines of OI 6158Å
0 and [OI] 6363 Å are hardly used owing to their small equiva-
8After hydrogen and helium, oxygen is the most abundant ele- lent widths. The strong lines of the oxygen triplet are strongly
5ment in the Universe. As a matter of fact, oxygen is the only affected by deviations from local thermodynamic equilibrium
0
chemical element that has a unique production site. This has (LTE)(e.g.Caffauetal. 2008), whichdependonthe crosssec-
.
1beenusedbymanyauthorstoarguethatoxygenisthebesttracer tion of collisionsbetweenthe atomsof oxygenand neutralhy-
0ofgalacticchemicalevolution.Itisproducedbyalphaparticles drogen. The physical description of these collisions is still a
5during hydrostatic nucleosynthesis in massive stars. The inter- matter of scientific debate. Moreover, the forbidden line is of-
1stellarmediumisenrichedwithoxygenwhenmassivestarsex- tenblendedwithtelluricfeaturesandalsocontainsaNiIblend
:
vplodeas TypeII supernovae.(e.g.Wheeleretal. 1989; Maeder at6300.40Å(e.g.Lambert1978;AllendePrietoetal.2001)and
i2009;Stasin´skaetal.2012). a much weaker CN blend (e.g. Teskeetal. 2013). The Ni line,
X
withanuncertainoscillatorstrength,maycontributeupto60%
Despite the importance of deriving precise oxygen abun-
ardances in solar-type stars, this is usually not a trivial process. to the equivalent width (EW) of the 6300Å feature (see sec-
The number of spectral lines of atomic oxygen suitable for tion 4). In addition, a disagreement between the OI triplet and
abundance analysis in visual spectra is small. There is a large forbiddenlineabundancesinmetal-poorgiantscanreachupto
number of molecular lines of OH in the near-UV and near- 1dex(Israelianetal.2004).
IR, strong enough to be measured in main-sequence metal- There are many articles dedicated to oxygen measure-
poorstarsdownto[Fe/H] -3.5dex(Israelianetal.1998,2001; ments in solar-type stars. Several workshops and conferences
Boesgaardetal.1999).Ho∼wever,itisextremelydifficulttomea- have been held to review the status of oxygen measure-
sure abundances from molecular features as they are affected ments and address unresolved problems related to inconsis-
by several physical processes that are not well understood in tent abundances derived from different oxygen lines. Most
solar-type stars (UV opacities, etc.). The situation with atomic of the previous work on oxygen with large samples of stars
linesisnomorepromising.Seriousdisagreementsexistinboth (e.g.Bensbyetal.2004;Takeda&Honda2005;Ecuvillonetal.
solar-type main sequence and evolved sub-giant/giant stars in 2006; DelgadoMenaetal. 2010; Petigura&Marcy 2011;
any metallicity range. The well-knownspectral lines of atomic Ramírezetal.2013)werebasedontwolines:OItriplet 7774Å
∼
oxygensuitableforabundanceanalysisare[OI]6300Åandthe and[OI]6300Å.Ecuvillonetal.(2006)havecomparednear-UV
Articlenumber,page1of15
A&Aproofs:manuscriptno.oxygen
150 120 300
100 250
120
mberofstars6900 mberofstars6800 mberofstars125000
Nu Nu40 Nu100
30
20 50
0
-1.5 -1.0 -0.5 0.0 0.5 5200 5600 6000 6400 6800 7200 3.8 4.0 4.2 4.4 4.6 4.8 5.0
[Fe/H] Effectivetemperature Log(g)
Fig.1.DistributionofthefundamentalstellarparametersintheHARPSsampleforstarswitheffectivetemperatureshigherthan5200K.
OHlineswiththeOItripletand[OI]andfoundgoodagreement of the [O/Fe] trends, separation of the different populations of
betweenthe[O/H]ratiosfromforbiddenandOHlines,whilethe starsandcomparisonwithpreviousworkscanbefoundinSect.
NLTE (non-localthermodynamicequilibrium)tripletshowed a 5.Finally,modelsofoxygenproductionintheGalaxyandcom-
systematically lower abundance. Takeda&Honda (2005) have parisonwithobservationsispresentedinsection6.
usedthe6158ÅOIlinetoderiveoxygenabundancesin160FGK
dwarfsandsub-giants.Nevertheless,thequalityoftheirdatawas
2. Sampledescriptionandstellarparameters
nothighenoughtoreachconclusionsaboutthereliabilityofthis
line. In the past decade a huge efforthas been made to discover ex-
trasolarplanets.Severalradialvelocitysurveyshavecarriedout
Toalltheabove-mentionedwemustaddthatthesolaroxy-
a thoroughfollow-up of solar-type stars over several years, re-
genabundanceitselfisstilluncertain.Sincethefirstdetermina-
sulting in homogeneous homogeneous sets of high resolution
tionsofLambert(1978)(logǫ(O)=8.92)andAnders&Grevesse
spectraand largecataloguesof uniformstellar parameters(e.g.
(1989)(logǫ(O)=8.93),thevalueofsolaroxygenhasundergone
SWEET-Cat–Santosetal.2013).Besidesthewell-knownsuc-
severalrevisionsthatyieldedprogressivelylower values.In re-
cessofsuchprojectsfordiscoveringextrasolarplanets,theseob-
cent years Asplundetal. (2004) have recommendeda value of
servationsgiveus,forthefirsttime,theopportunitytostudythe
logǫ(O)=8.66 0.05basedonmeasurementsof[OI],OI,andIR
± atmospheresand the chemical compositions of solar-type stars
OHlines,whichwassupportedbytheSocas-Navarro&Norton
withanunprecedentedhigh-quality,homogeneousandextended
(2007)determination(logǫ(O)=8.63)usingtheinfraredtriplet.
dataset.
This value was reviewed in 2009 to log ǫ(O)=8.69 0.05 us-
± The sample of stars used in this work is a combination of
ing new 3D hydrodynamical models of the solar atmosphere
three subsamples from the HARPS planet search programme:
(Asplundetal. 2009). On the other hand Ayres (2008) deter-
HARPS-1(Mayoretal.2003),HARPS-2(LoCurtoetal.2010)
mined a significantly larger value (log ǫ(O)=8.81) based on a
and HARPS-4 (Santosetal. 2011). These stars are mostly
single snapshot of a 3D modelfor the [OI]6300line and treat-
slowlyrotatingchromosphericallyinactive,unevolvedsolar-type
ingthelog(gf)ofNiblendedlineasafreeparameter.Recently,
dwarfs and some sub-giants, with spectral types between F2
Caffauetal. (2008) presented a new determinationof the solar
andM0. Theindividualspectrawere reducedwith the HARPS
photosphericoxygenabundanceby analysing differentspectral
pipelineandthencombinedusingIRAF1 aftercorrectingforits
atlasesofthesolarfluxanddisk–centreintensity,makinguseof
radialvelocity.Thecombinedspectraforeachstarhavearesolv-
thelatestgenerationofCO5BOLD3Dsolarmodelatmospheres.
ingpowerR 115000andasignal-to-noise(S/N)between 40
Theyhavestudied the photosphericoxygenabundanceby con-
∼ ∼
and 2000.About90%ofthestarshaveanS/Nhigherthan100.
sideringonlylinesfromatomictransitions.Ignoringtheroleof
∼
collisionswithhydrogenatomsontheNLTElevelpopulationsof Thishigh-qualitydataallowedustomeasurethe6158Ålinein
oxygen,theyproposedlogǫ(O)=8.76 0.07.Theyhavestressed alargesampleofstarsandinvestigateitsreliabilityasanoxygen
thatthemeasurementofequivalentwid±thswithhighprecisionis abundanceindicator.
stillanimportantandopenissue.Strangelyenough,evenforthe We selected 762 stars from the HARPS samples with tem-
solar high-resolutionatlas (Kuruczetal. 1984), they concluded peratures higher than 5200K, for which our stellar parameters
that the placement of the continuum is not trivial, and that the aremostprecise(Tsantakietal. 2013).Stellar parameterswere
influenceofmanyblendsgivesrisetosignificantuncertainties. determinedbySousaetal.(2008,2011a,b)usingthesamespec-
tra that we studied. The typical precision uncertainties are of
Ourworkpresentsacompleteanduniformstudyoftheoxy- about30KforT ,0.06dexforlog(g),0.08kms 1 forξ,and
eff − t
genabundancesinalargesampleofstarsfromtheHARPSsur- 0.03 dex for [Fe/H]. Figure 1 shows the distribution of stellar
vey,usingtwodifferentabundanceindicators:thehighexcitation
parametersforourfinalsample.
lineat6158Åandtheoxygenforbiddenlineat6300Å.Thisis
the first systematic comparisonof these lines in a large sample
ofmain-sequencesolar-typestars.Thepaperisorganizedasfol- 3. Abundanceanalysis
lows:inSect.2,weintroducethesampleusedinthiswork.The
Oxygen abundances were derived from OI6158Å and
methodof the chemicalabundancedetermination,analysis and
[OI]6300Å spectral lines. There are two main issues that
uncertaintiescalculationswillbeexplainedinSect.3.Sect.4is
dedicatedtothecomparisonbetween6158Åand6300Åasoxy- 1 IRAF is distributed by National Optical Astronomy Observatories,
genabundanceindicators,wherewe providedetailsabouttheir operatedbytheAssociationofUniversitiesforResearchinAstronomy,
reliabilityindifferentrangesofstellarparameters.Adiscussion Inc.,undercontractwiththeNationalScienceFoundation,USA.
Articlenumber,page2of15
S.BertrandeLisetal.:Oxygenabundances
SiI [OI]+Ni FeI FeI CaI OI TiI FeI OI FeI
1.00
x
u
Fl0.90
d
e
aliz
m
or
N
0.80
HD213240 HD213240
Teff =5982 EW[OI]=6.2±1.8m˚A Teff =5982 EWOI =7.9±1.8m˚A
[Fe/H]=0.14 [O/H] =0.29 [Fe/H]=0.14 [O/H] =0.16
Logg =4.27 S/N =321 Logg =4.27 S/N =321
0.70
SiI [OI]+Ni FeI FeI CaI OI TiI FeI OI FeI
1.00
x
u
Fl0.90
d
e
aliz
m
or
N
0.80
HD98281 HD98281
Teff =5381 EW[OI]=2.8±0.5m˚A Teff =5381 EWOI =1.0±0.5m˚A
[Fe/H]=-0.26 [O/H] =-0.24 [Fe/H]=-0.26 [O/H] =-0.20
Logg =4.42 S/N =1316 Logg =4.42 S/N =1316
0.70
SiI [OI]+Ni FeI FeI CaI OI TiI FeI OI FeI
1.00
x
u
Fl0.90
d
e
aliz
m
or
N
0.80
SunAtlas SunAtlas
Teff =5777 EW[OI]=3.4±0.5m˚A Teff =5777 EWOI =3.7±0.5m˚A
[Fe/H]=-0.0 [O/H] =0.00 [Fe/H]=0.0 [O/H] =0.00
Logg =4.44 S/N =3800 Logg =4.44 S/N =3800
0.70
6298 6299 6300 6301 6302 6303 6155 6156 6157 6158 6159 6160
WL(˚A) WL(˚A)
Fig.2.SpectralregionscontainingoxygenlinesinHD213240,astarfromourdatabasewithanaverageS/N=300representativeforourcatalogue,
HD98281, for which the high S/N allows us to measure lines as small as 1 mÅ, and for the Kurucz Sun Atlas (Kuruczetal. 1984). Stellar
parameters,equivalentwidthoftheoxygenlines(Niblendcontributionremoved)andLTEabundancesareprovidedintheplot.Wenotethatthe
providedsignal-to-noiseratioisthenominalS/N.
make the automatic EW measurement of these lines prob-
220
[OI]+NiI6300˚A lematic. First, the two lines are placed in complex spectral
200
[OI]6300˚A regions close to some strong FeI lines and, in the case of the
180
OI6158˚A 6300Åtransition,also withtelluriclinesinitsvicinity(Fig.2).
160 Moreover, the strength of these lines in F- and G-type stars is
stars140 generallysmallerthan8mÅ(Fig.3).Altogether,thismakesthe
of120 continuumplacementandEWmeasurementaseriouschallenge
ber100 for any automatic code. Therefore, EW measurements were
m
Nu 80 carriedoutforbothlinesinadetailedindividualanalysisusing
thesplotpackagefromIRAF1.
60
40 Incombinedspectrathe sameatmosphericlinescanappear
20 atdifferentwavelengths,slightlyshiftedbetweenthemandwith
0 differentintensities.Toaccountforpossibleblendswithtelluric
0 2 4 6 8 10 12 14 16 18 20 22 24
lines, we made a thoroughobservationof the spectra in the re-
EW(m˚A)
gionoftheforbiddenline.Thisprocesswasdonebyidentifying
Fig.3.Distributionofmeasuredequivalentwidthsforeachspectralline. the atmospheric pattern, which is repeated along the region of
[OI]6300Å,inanearbywindowfreefromstellarlines.Thispat-
Articlenumber,page3of15
A&Aproofs:manuscriptno.oxygen
0.6 0.6
0.4 0.4
0.2 0.2
H] H]
O/6300 0.0 O/6158 0.0
[ [
-0.2 -0.2
-0.4 -0.4
-0.6 -0.6
-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
[Fe/H] [Fe/H]
Fig.4.[O/H]vs.[Fe/H]fordifferentindicators.Theyellowdotcorrespondstothesolarabundances.
Table1.Atomicparametersofthespectrallinestogetherwithourmea- Atlas(Kuruczetal.1984).Oncewemovetowardshotter/cooler
surements of equivalent width and abundance in the Sun. This solar
stars along the main sequence, one of these lines will become
valuewillbeusedasreferenceinthepresentanalysis.
strongerandtheotherweaker.ThankstothewiderangeofT
eff
and[Fe/H]ofthestarsinoursample(Fig.1),wehaveaninter-
λ log(gf) χlo Ref. EW logǫ(X) estingopportunitytocompareoxygenabundancesderivedfrom
(Å) (eV) (mÅ⊙) ⊙ two lines with very different formation physics. We note that,
for still unknown reasons, these lines provide different oxygen
OI 6158.171 -0.296 10.74 1 3.7 8.71
abundancesintheSun(Caffauetal.2008).Oneoftheimportant
[OI] 6300.304 -9.717 0.00 1 3.4 8.65
questionsthatwillbeaddressedinthisarticleiswhetherornot
NiI 6300.336 -2.110 4.27 2 2.0 6.25
thisdifferenceisseeninothersun-likestars.Ingeneral,wewant
tounderstandwhichoftheselinesismorereliableasanoxygen
References.(1)Caffauetal.(2008),(2)Johanssonetal.(2003)
abundanceindicatorandwhy.
It is well known that the forbidden line at 6300.30 Å is
tern can be later extrapolated to check if it is superimposed to severely blended with the NiI 6300.34Åline. The contribution
oftheNiIlineisnegligibleat[Fe/H]< 0.8,butbecomesvery
the6300Åline.Ontheotherhand,de-blendingwasrequiredfor
−
important at solar metallicities (see section 5). The EW of the
the6158ÅfeatureinordertoaccountforthepresenceoftheFeI
NiI line in our stars was estimated using the ewfind driver of
line at 6157.5Å. The red wing of this Fe line frequently over-
MOOGandNiabundancesfromAdibekyanetal.(2012),calcu-
lapswiththeoxygenline.Thespectralwindowscontainingboth
lated from the same spectra and stellar parametersused in this
oxygenspectrallinesare shownin Figure 2. For some still un-
work. The derived EW of the Ni line was subtracted from the
knownreasons,thenearbylineofoxygenat6156.75Åprovides
totalEWofthe6300Åfeaturetoobtainthecontributionofoxy-
very different abundances from those based on the 6158Å and gen.
6300Ålines.Infact,beingevenstrongerthanthelineat6158Å, Many authorsuse the so-called solar gf-valuesto carry out
it has been avoided in previousstudies(e.g. Caffauetal. 2008; differentialabundanceanalysis,meaningthattheyforcetheos-
Takeda&Honda2005). cillatorsstrengthsofdifferentspectrallinesofthesamechemi-
The Solar Atlas of Kurucz (Kuruczetal. 1984) has been calelementtoprovideauniqueabundancevalue.Herewetake
used to select continuum regions free from spectral lines. adifferentapproach.Wedonotmodifythegf-valuesoftheoxy-
We found that the continuum windows are 6156.94–6157.10Å genlinestoobtainthesameabundance.Inthisapproachweob-
and6158.80–6158.90ÅforOI 6158Å,and6298.20–6298.80Å, tain0.06dexdifferencebetweenthesolaroxygenderivedfrom
6297.15–6297.45Åand 6303.00–6303.30Åforthe [OI] 6300Å the6158Åand6300Ålines.Wewanttoinvestigatewhetherthis
line.Wehavealsoselectedtwostarsinoursample,withtemper- differenceexistsinothersun-likestarswithaqualityofspectra
aturesfarfromthatof theSun, in orderto verifythe reliability similartotheSolarAtlas.ThesolarabundanceslistedinTable1
of those regions at different T . This procedure guarantees a areusedasareferenceintheanalysis.
eff
uniformcontinuumplacementinourtargets,whosespectraare Abundances from OI6158Å were obtained for 535 stars,
similartothoseofthereferencestars. while610starswereanalysedusing[OI]6300Åfromtheinitial
Oxygen abundances from two indicators were determined sampleof762stars.Thefinalresultsfrombothlinesareshown
according to a standard LTE analysis with the revised version in Figure 4. These are presented relative to the solar reference
oftheMOOG2013spectralsynthesiscode(Sneden1973),using abundance (Table 1), derived following the same procedure as
the abfind driver and a grid of Kurucz ATLAS9 plane–parallel fortherestofthesample.Tables2and3providetheabundances
modelatmospheres(Kurucz1993).Parametersofspectrallines from the OI6158Å and [OI]6300Å lines, respectively, together
consideredinouranalysisarelistedinTable1.NLTEcorrections withthestellarparameters,theequivalentwidthofthelinesand
forOI6158Åarenegligible(e.g.Caffauetal.2008)andwerenot itstotaluncertainty.
takenintoaccount.Ontheotherhand[OI]6300Åisnotaffected In Fig. 5 we plotoxygenabundancesagainststellar param-
by deviations from LTE. The two oxygen lines have very dif- eters. Such plots reveal systematic effects that could influence
ferentexcitationenergies,butverysimilarstrengthsintheSolar ourmeasurements.Wenoteaslightdependenceof[O /H]on
6300
Articlenumber,page4of15
S.BertrandeLisetal.:Oxygenabundances
0.6 0.6
0.3 0.3
H] H]
O/6300 -00..30 O/6300 -00..30
[ [
-0.6 -0.6
5200 5600 6000 6400 6800 7200 3.8 4.0 4.2 4.4 4.6 4.8 5.0
Teff Log(g)
0.6 0.6
0.3 0.3
H] H]
O/6158 -00..30 O/6158 -00..30
[ [
-0.6 -0.6
5200 5600 6000 6400 6800 7200 3.8 4.0 4.2 4.4 4.6 4.8 5.0
Teff Log(g)
Fig.5.Systematictrendsof[O/H]fordifferentindicatorswithtemperatureandsurfacegravity.Thesolidlineisthecubicbestfit.
Table2.OxygenabundancesfromtheOI6158Åline
Star T log(g) ξ [Fe/H] EW δEW [O/H] Gal.Pop.
eff t OI OI OI
(K) (cms 2) (kms 2) (mÅ) (mÅ)
− −
Sun 5777 4.44 1.00 0.00 3.7 0.3 0.00 thin
HD70889 6051 4.49 1.13 0.11 6.9 0.5 0.10 thin
HD21161 5923 4.24 1.14 0.09 7.5 2.8 0.18 thin
HD37226 6178 4.16 1.61 -0.12 7.4 0.8 -0.01 thin
HD125881 6036 4.49 1.10 0.06 5.8 0.6 0.04 thin
HD117105 5889 4.41 1.13 -0.29 3.0 0.7 -0.12 thin
... ... ... ... ... ... ... ... ...
Notes.ThefulltableisavailableattheCDS.
Table3.Oxygenabundancesfromthe[OI]6300Åline
Star T log(g) ξ [Fe/H] [Ni/H]a EW EW δEW [O/H] Gal.Pop.
eff t Ni [OI] [OI] [OI]
(K) (cms 2) (kms 2) (mÅ) (mÅ) (mÅ)
− −
Sun 5777 4.44 1.00 0.00 0.00 2.0 3.4 0.3 0.00 thin
HD48611 5337 4.51 0.69 -0.36 -0.37 1.2 1.9 0.8 -0.42 thin
HD70889 6051 4.49 1.13 0.11 0.09 1.8 3.3 0.4 0.10 thin
HD21161 5923 4.24 1.14 0.09 0.11 2.2 5.9 2.9 0.23 thin
HD130322 5365 4.37 0.90 -0.02 -0.03 2.5 5.1 1.0 0.09 thin
HD222422 5475 4.46 0.73 -0.12 -0.15 1.8 2.8 0.8 -0.16 thin
... ... ... ... ... ... ... ... ... ... ...
Notes.(a)[Ni/H]fromAdibekyanetal.(2012).ThefulltableisavailableattheCDS.
T at temperatures less than 5600K, for which lower [O/H] 3.1.Uncertainties
eff
abundancesare found.Although restricted to the low tempera-
There are several sources of random error involved in the pro-
ture end, a trend with opposite sign can also be noticeable for
cessofmeasuringchemicalabundances.Thesearerelatedeither
[O /H]. Nevertheless,if we onlyconsider stars with temper-
6158 tothemeasurementoftheEWortothecalculationoftheabun-
atureslargerthan 5400K (e.g.whenthe 6158Ålineis strong
∼ dances.Itisnoteasytodefinethecontributionofeacherrorand
enoughforreliablemeasurements)we willfindnotemperature
accountanalyticallyforthem.Inthepresentworkwehavecon-
dependenceforthe6158Åline.Ingeneral,giventhehighabun-
sidered three different contributions to the uncertainties in our
dancescatterobservedinthesegraphs,thesmalldependenceon
measurements.
T forcoolstarscanbeneglected.Nogravityeffectwasfound.
eff First,westudiedthesensitivityoftheabundancestothestel-
lar parameters: effective temperature T , metallicity [Fe/H],
eff
surfacegravitylog(g)andmicroturbulenceξ.Variationsofthese
t
parameters introduce small changes in the abundances which,
regardless of the parameter we vary, also depend on the T
eff
Articlenumber,page5of15
A&Aproofs:manuscriptno.oxygen
Table4.Averagestellarparametersforthethreesub-samplesofdiffer-
entTeff. 5 Continuum
Photometric
StellarParameters
T log(g) [Fe/H] ξ
eff t 4
(K) (kms 1)
−
˚A)
LowTeff 5292 4.35 -0.16 0.73 (m 3
SHoiglahrTTeff 65377404 44..5400 --00..1120 11..5090 W6158
eff δE 2
ofthestar(Adibekyanetal.2012).Therefore,wewillcarryon 1
ouranalysiswiththreedifferentsub-samplesbasedontheT :
eff
T < 5377, 5377 T 6177 and T > 6177. The middle group 0
comprises the st≤ars w≤ith T =T 400 K. We expect that 0 300 600 900 1200 1500 1800 2100
the sensitivity to the stellarefpfaram⊙et±ers behave similar within NominalS/N
eachrangeoftemperature.Theaveragevaluesofthestellarpa-
Fig.6.Contributiontothetotalequivalent widtherrorofOI6158Åof
rameters for each group are shown in Table 4. These parame- eachsourceofuncertainty.
terstogetherwiththeaverageoxygenabundanceoneachgroup
are used to computetheoreticalEWs of oxygenlines using the
ewfinddriverofMOOG.KeepingtheEWconstant,wethencal- andthe ǫ continuumplacements,whereǫ istheinverseofthe
±
culateabundancesbyvarying,onebyone,thestellarparameters S/Nperresolutionelement.Thewidthatthecontinuumlevelis
byanamountequaltotheiruncertainty(Table5).Amongallthe againequalto6σ.Alltogetheryieldsanerrorof 6σǫ.
±
values,weselectasthefinalerrorthelargestdeviationfromthe Figure6showssignificanceofdifferentcontributionstothe
total uncertainty of the EW, which is similar for both oxygen
original oxygen value. 6158Å appears to be more sensitive to
lines.Wehaveconvertedtheerrorinabundanceduetouncertain-
T while6300Ålargervariationswithlog(g).
eff tiesinstellarparameterstothecorrespondingEWerrorusingthe
Another source of uncertainty is the statistical photometric
ewfinddriverofMOOGsynthesiscode.Eachcontributiontothe
errorduetothenoiseineachpixel.We canevaluatethepartial
EW is finally added quadraticallyto get the one-σ uncertainty.
derivativeoftheEWwithrespecttotheflux(Bohlinetal.1983;
Because of the non-lineardependenceof the abundanceon the
Cayrel1988)toobtainananalyticalsolutiontothiscontribution:
EW, asymmetric errors will arise when the EW error is propa-
gated to get abundanceuncertainties. For further details on the
EW = ∆λCi−Ii ,δEW = N ∂EW 2 =2.45ǫ√σ∆λ determinationof the abundanceuncertaintywe referthe reader
Ci ! vuti=1 ∂Ii ! totheappendix.
X X
Where C is the continuum flux level, I is the flux of each
i i 4. Comparisonbetweenindicators
measurement,∆λrepresentsthewavelengthincrementperpixel
( 0.01Å for HARPS spectra), ǫ is the r.m.s. relative photomet- AbundancesfromboththeOI6158Åand[OI]6300Ålineswere
ric accuracyin thecontinuumandσ isthe standardwidthover measuredin447starsfromtheinitialsampleof762.Thislarge
whichwesumsixtimesassumingagaussianprofile.Wedeter- numberofstarscoversawiderangeoftemperaturesandmetal-
mine the standard width theoretically as the quadratic sum of licities,whichallowsustocarryoutacomprehensivecompari-
different broadenings:instrumental(wavelength divided by the son of the performanceof these spectrallines as oxygenabun-
spectralresolutionoftheinstrument),macroturbulencebasedon danceindicators.Inthissectionwestudytheagreementbetween
the relation provided by Valenti&Fischer (2005), microturbu- chemicalabundancesobtainedfrombothspectrallines,andwe
lence,rotation(usinganaveragevalueof3kms 1)andthermal alsoinvestigatetheirreliabilityandusabilityrelatedtothestellar
−
broadening.Thelatteriscalculatedwith(Gray1992): parameters.
In section 3 we have already noted that the absolute oxy-
λ 2kT T gen abundances from the two lines in the Sun disagree, since
∆λD = c r m =4.301·10−7λsµ ploagrisǫo(nO6f1o5r8)alilsth0e.0s6tadrsexwhitihghceormtmhaonnlmogeaǫsu(Ore6m30e0n).tsTihseshcoowmn-
WherekisBoltzmann’sconstant,Trepresentstheeffectivetem- in Figure 7. The OI6158Å line provides higher abundances in
peratureandµistheatomicweightinatomicmassunits. starswith[O/H]similarorlowerthantheSun,while[OI]6300Å
Thelast andthe greatestcontributiontothe uncertaintiesis yieldshighervaluesforoxygen-richstars.Thissmalltrendcould
related to the measurementof the EW and its sensitivity to the beduetooverestimationoftheequivalentwidthwhenthelines
continuumplacement.Althoughthiserrorcanreachalargeper- aresmall.However,thetrendisnotsignificantasitfallswithin
centageoftheEW,itiscommonlyignoredbymanyauthors,re- the standard deviation of 1σ = 0.127 dex. On average, abso-
±
sultinginsignificantlyunderestimateduncertaintiesintheabun- lute oxygen abundances derived from the 6158Å spectral line
dances. In order to quantify this contribution, one should cal- are0.024dexhigherthan[OI]6300Å.Thisvalueisconsiderably
culate the difference between the areas of the gaussian fits by smallerthanthetypicaluncertaintiesoftheoxygenabundances.
placingthecontinuumindifferentpositions.GiventhehighS/N Noconclusioncanbedrawn,exceptthatonaveragebothspec-
of our spectra, an therefore the low uncertainty in the rms, the tral lines yield compatible values of oxygen abundance within
possible continuum placements will yield small variations be- theuncertainties.
tweentherespectiveGaussiandepths.Thus,theerrorinEWcan As we move towards stars with better quality spectra, i.e.
be approximated as the area of the rectangle between the best higherS/N,theabundanceuncertaintiesdecrease.Asexpected,
Articlenumber,page6of15
S.BertrandeLisetal.:Oxygenabundances
Table5.Abundancesensitivitiestothestellarparametersofbothoxygenindicatorsforthethreesub-samplesofdifferentT .
eff
∆T ∆log(g) ∆[Fe/H] ∆ξ
eff t
+30K -30K +0.06dex -0.06dex +0.03dex -0.03dex +0.08kms 1 -0.08kms 1
− −
∆OI6158 -0.03 +0.04 +0.02 -0.01 0.00 +0.01 0.00 +0.01
LowT
eff ∆[OI]6300 +0.01 0.00 +0.03 -0.02 +0.02 -0.01 0.00 0.00
∆OI6158 -0.02 +0.03 +0.02 -0.01 0.00 +0.01 0.00 0.00
SolarT
eff ∆[OI]6300 +0.01 0.00 +0.03 -0.02 +0.01 -0.01 0.00 0.00
∆OI6158 -0.02 +0.02 +0.02 -0.02 0.00 +0.01 0.00 0.00
HighT
eff ∆[OI]6300 +0.01 -0.01 +0.02 -0.02 +0.01 -0.01 0.00 0.00
ever,forstars with S/N>1000the averagedifferencein oxygen
9.4 abundances is 0.051 dex, which is compatible with previously
measureddifferencesin the Sun(Caffauetal. 2008).These de-
9.2
viations increase as we consider higher S/N ratios, indicating
9.0 apossiblediscordancebetweenbothindicators.Theagreement
between abundance indicators as a function of the S/N is pre-
O)6158 8.8 sented in figure 8. We have performed a weighted moving av-
ǫ( 8.6 erage(500steps, stepsize 50andweigthedwith1/σ2) thathas
g
lo 8.4 been smoothed by a factor of 0.4. The 1,2 and 3 standard de-
viationslevelsfromthemean,whicharerepresentedbyshaded
8.2 areas in the figure, clearly show how the dispersion decreases
+1σ
atbetterS/N,whilethemeanisshiftedtowardshighervaluesof
8.0
-1σ O6158Å.Thisresultmaysuggestthatthe0.06dexdisagreement
7.87.8 8.0 8.2 8.4 8.6 8.8 9.0 9.2 9.4 foundintheSunisnotunique,andmayhavesomephysicalex-
logǫ(O6300) planation.
The different sensitivities of the two oxygen lines to stel-
Fig.7.ComparisonoftheabsoluteoxygenabundancesfromOI6158Å larparametersallowsustodefinewindowswhereeachofthese
with those obtained from [OI]6300 Å lines. Solid lines represent the lines can be used more or less easily. The equivalent width of
diagonalandthelinearfittothedata.Dashedlinesenclose68%ofthe
bothspectrallines issmaller than 8mÅatT >5800K.How-
dataaroundthemean(0.024dex). eff
ever, because of the high excitation energy of 6158Å, this line
becomes significantly stronger in hotter stars. Specifically, the
EWofOI6158ÅisalwayslargerthantheEWof[OI]6300Åin
0.4 allthestarswithT >6200K.
eff
0.3
ǫ(O)6300 00..12 01..90 6700
g 6400
o
O)-l6158 -00..01 W+ONi6300 00..78 6100
ǫlog( -0.2 /EO6300 0.6 5800Teff
W
-0.3 E
0.5
-0.4 5500
0 300 600 900 1200 1500 1800 2100 0.4
NominalS/N
Fig. 8. Difference between absolute oxygen abundances derived from 0.3 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 5200
theOI6158Åand[OI]6300Ålinesagainstnominalsignal-to-noiseratio. [Fe/H]
Weightedmovingaverage(boxcaraverage)ofdatapointsisshownas
asolid line, together withthe 1, 2, and 3 sigmadispersion regions in Fig.9.Fractionofoxygenintheequivalentwidthofthe6300Åfeature
differentgreyshades.Dashedlinesrepresentthesimplemovingaverage asafunctionofmetallicity.Dependenceontheeffectivetemperatureis
oftheabundanceerrorsforeachline.InthetoppaneltheSunhasbeen shownincolours.
shiftedtowardslowerS/Nforplottingpurposes.Thelowerpanelshows
thewidthofthe 1σregionforthedataaverageandtheuncertainties. Itisalso importanttonotetheuncertaintiesthataffecteach
± line. Because of the density of spectralfeaturesin the rangeof
6158Å,thelargestsourceoferrorintheequivalentwidthmea-
our measurements become more precise and we achieve a surementisthecontinuumplacement.Therefore,itismorecom-
smallerdispersioninthedifferencebetweenoxygenindicators. plicated to measure this line in cool stars, where in addition to
Specifically, while the standard deviation for the whole sam- many blends, the line itself is very weak. The 6300Å feature
ple was 0.127 dex, at high S/N it drops to 0.072 dex. How- canbeconsideredasproblematicinsolar-typestarsfortwoim-
Articlenumber,page7of15
A&Aproofs:manuscriptno.oxygen
portant reasons. The first is related to the presence of numer-
ous telluric lines often severely blended with the oxygen fea- δEW<25%
ture. Even more serious is the problem with the Ni absorption 0.6 δEW>25%
line(seeabove).Johanssonetal.(2003)providedalog(gf)value
0.4
with14%uncertainty.OurcalculationsshowthattheNilinecan
accountforupto60%oftheEWofthe6300Åabsorptionfea-
H] 0.2
ture in cool, metal-rich stars (see Fig 9). Thus, the abundance /
of Ni and its atomic parameters plays a key role in this topic. O6158 0.0
As an illustration, the error in the Ni log(gf) is introducing an [
uncertaintyintheoxygenEWmeasurementofHD142equalto -0.2
3.1+0.2mÅ,whichistranslatedinto[O/H]=0.2+0.03dex.Instars
0.1 0.00
wit−h medium to low S/N this contribution will−be smaller than -0.4
the two main sources of uncertainty considered in this work.
-0.6
Even though, the larger scatter found in figure 4 with 6300Å -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
couldbepartiallyexplainedbytheuncertaintyintheNilog(gf), [Fe/H]
incombinationwiththepossiblepresenceofundetectedtelluric
linesslightlyabovethelevelofthenoise. Fig.10.[O/H]vs.[Fe/H]forOI6158Å.Filled(open)circlesrepresent
data for which EW measurements have an uncertainty under (above)
The main and the most importantaspect of the 6158Å line
25%.Absoluteerrorbarsareover-plottedforourbestmeasurements.
is that it does not suffer from known blends. It is as strong as
the forbidden line in solar analogues and stronger than 6300Å
attemperatureshigherthan6200K.Ourresultssuggestthatthe
6158Åline can be usedas a reliableindicatorof oxygenabun-
0.6
dance in solar-type stars. However, we should bear in mind
that it may suffer non-negligible non-LTE effects in hot stars
0.4
(Sitnovaetal.2013).
0.2
H]
5. Oxygentrendsinthethinandthickdisks O/6158 0.0
[ -0.2
5.1.Observeddispersionof[O/H]
-0.4
TheabundancescatterseenintheGalactictrendsofmanychem-
Boxcaraverage
ical elementsis verysmall. Adibekyanetal. (2012) carriedout -0.6 ±δ[O/H]average
ahomogeneousstudyandfoundsmallabundancedispersionin
suchelementsas SiorTi. Theabundancespresentedherehave als 0.05
du 0.00
beenderivedfromthesamespectraandstellarparameters.Thus, Resi-0.05
using the same database and techniques as Adibekyanetal.
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
(2012) we would expectto get equallyprecise and trustworthy [Fe/H]
resultsforotherelements.However,thoseelementshavemany
more reliable spectral lines for abundance analysis, and there- Fig.11.Upperpanel:[O/H]vs.[Fe/H]forOI6158Å.Onlystarswith
fore is not surprising that we found a wider scatter in the case EWrelative error below 25% are considered. Solid line represent the
average oxygen abundance. The 1,2 and 3 sigma levels of dispersion
ofoxygen(fig.4).Evenso,scatterfoundinthepresentworkfor
aroundthemeanarepresentedasshadedareas.Dashedlineshowsthe
[O /H]iscomparabletothatofMg,whichwasderivedusing
6158 average uncertainties. For plotting purposes the average uncertainties
three spectral lines (Adibekyanetal. 2012). Petigura&Marcy
are shown around the mean abundance. Lower panel: Difference be-
(2011) suggested an astrophysical origin to explain the disper-
tweenthe 1σwidthofthescatteraroundthemeanandthe 1σwidth
sionfoundinoxygentrends. oftheunce±rtainties. ±
We considera sub-sampleof296stars, 55%of thesample,
forwhichwehaveerrorssmallerthan25%inEWinordertover-
ifywhethertheobservedabundancescatterofoxygenisdueto test is to find out which of the two contributionsto the scatter
thelackofprecisemeasurements.Wealsorestrictedtheanalysis dominatesoursample.Resultsofthetestaredepictedinfig.11.
totheOI6158line,sincewefoundinthisstudythatitismore Weareawarethatthistestisnotvalidattheedgesofthemetal-
trustworthy.We see in Fig. 10 that the scatter is still veryhigh licityrangeduetotheundersamplingofthe bins,whichmakes
in this‘mostprecise’sub-sampleat [Fe/H]< 0.2dex.On the unreliabletheprovidedmeanvalues.
−
other hand, at highermetallicities the scatter is not remarkably Inordertoevaluatethe relativeimportanceofthe contribu-
reduced by selecting the best measurements, and only outliers tions to the scatter, in the lower panel of the figure we show
areremoved.Errorbarsareaboutthesizeofthedispersionand the differencebetween the widths of the 1σ regions.A nega-
itisthereforedifficulttodrawanyconclusionabouttheoriginof tiveresidualwouldimplyanoverestimatio±nofouruncertainties,
thescatter. whilea positiveresidualcouldbe dueto eitherarealscatteror
The observed scatter at a given metallicity is expected to toanunderestimationoftheerrors.Sincetheuncertaintieshave
havetwocontributions:arealstar-to-starscatter,andalsoadis- been derived with the same method for all stars, and there is
persion due to the uncertaintiesin our measurements.We have nodependencyonmetallicity,differencesinresidualsasafunc-
performedamovingaverage(250stepswith0.03dexwidth)to tionofmetallicityareexpectedtobecausedbyrealstar-to-star
deriveboth the average[O/H] with their respectivescatter, and dispersion.Athighmetallicitiestheobservedscatterandtheun-
themean δ[O/H]asafunctionofmetallicity.Thegoalofthis certaintiesarecomparable,anddonotallowustodetectthepos-
±
Articlenumber,page8of15
S.BertrandeLisetal.:Oxygenabundances
0.8 0.8
0.6 0.6
0.4 0.4
Fe] Fe]
/ /
O6300 0.2 O6158 0.2
[ [
0.0 0.0
Thindisk Thindisk
Thickdisk Thickdisk
-0.2 High-αmetal-rich -0.2 High-αmetal-rich
Halo Halo
-0.4 -0.4
-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
[Fe/H] [Fe/H]
Fig.12.[O/Fe]vs.[Fe/H]for[OI]6300ÅandOI6158Å.Starsfromdifferentpopulationsareseparatedaccordingtotheirchemicalcomposition.
siblepresenceofanastrophysicalscatter.However,thepositive
residualfoundat[Fe/H]<-0.2dexrevealsarealstar-to-starscat-
0.8
terthatcannotbeexplainedbyuncertainties.Theresultsofthis
testarenotsurprising,andtheyareinperfectagreementwiththe
0.6
factthatat[Fe/H]<-0.2dexthereisamixtureofstarsfromtwo
differentpopulationsoftheGalacticdisk(seebelowfordetails).
However,this expectedresults confirmthe effectivenessof this Fe] 0.4
/
methodtoshowupthepresenceofarealscatter.Inaddition,the O6158 0.2
satisfactoryagreementbetweenuncertaintiesandscatterathigh [
metallicitiessupportthevalidityofouruncertaintycalculations. 0.0
Giventhatthetestwasperformedusingasub-sampleofthebest Thindisk
measurements,thislastresultshouldconsequentlybeinterpreted -0.2 Thickdisk
withcaution.Asimilartestwiththewholesamplewouldshow High-αmetal-rich
-0.4
aslightoverestimationoftheuncertainties(negativeresiduals),
-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
duetotheexponentialgrowthoftheerrorsaswemovetowards [Fe/H]
lowerS/N.Therefore,weconcludethatalthoughouruncertainty
calculations are overestimated at very low S/N, they appear to Fig.13.[O/Fe]vs.[Fe/H]forthestarswithδEW<25%.Starsfrom
satisfactoryaccountforthe observedscatter atmediumtohigh differentpopulationsareseparatedaccordingtotheirchemicalcompo-
S/N. sition. Solid lines represent the fitted trend, weighted with the uncer-
tainties,foreachpopulation(thickandhαmrstarsarefittedasasingle
population).Absoluteerrorbarsareover-plotted.
5.2.Thinandthickdiskseparation
It is believedthat α elements, such as oxygen,are producedin cise abundancevalues(Fig.13), we cancheckthatnoneof the
massivestarsthatenrichthe interstellarmediumwhentheyex- thickdiskstars fallsbelowthe trendforthethin disk,although
plodeastypeIIsupernova.Bycontrast,ironpeakelementsare fewthindiskstarshave[O/Fe]higherthanexpectedforthispop-
mostlysynthesizedinSNeIaexplosions(e.g.Thielemannetal. ulation. In addition, both trends appear to be remarkablysepa-
2002).Theoxygenoverironratiocanprovidehintsonthefor- ratedatlowermetallicities.Thus,wecanconfirmwithourdata
mationoftheGalacticdiskanditschemicalhistory,buttheob- that oxygen, just like the other α elements, is enhanced in the
servedGalactic trendsarediffusedasa resultofthe mixtureof thickdiskstars.
stars from two different populations in the studied metallicity The high-αmetal-rich population(hαmr) was presented by
regime. Adibekyanetal. (2011, 2013) as a new population due to the
The chemical distinction between thin and thick disks was gap in metallicity found between the latter and the thick disk.
first noticed by Fuhrmann (1998) and further investigated by In addition, a gap in the distribution of [α/Fe] at 0.17 was
Prochaskaetal. (2000), among others. We have classified the also noticed. The present work is based on the same sample
starsinoursamplebetweenthindisk,high-αmetal-poor(thick as Adibekyanetal. (2011), and therefore our data also show a
disk)andhigh-αmetal-richstarsaccordingtochemicalcriteria gap in metallicity. The separation in the [O/Fe] distribution is
described in Adibekyanetal. (2011). These authors show that not as clear as in [α/Fe] because both populations are mixed
diskpopulationsareclearlyseparatedintermsof[α/Fe],where around[O/Fe] 0.25,probablycausedbythelargererrorsinthe
‘α’ refers to the average abundance of Mg, Si, and Ti. The ra- determination∼of oxygen abundances compared to other α ele-
tioofoxygenoverironforthedifferentdiskpopulationsispre- ments. Our most precise sample show that all the stars from
sentedinFigure12.Thereisnoobviousdifferenceseenbetween the hαmr population fall above the average trend for thin disk
thethickandthethinpopulationsfrom[OI]6300,whichcanbe (Fig.13),andthuswecanconfirmthathαmrstarsareenhanced
attributed to the larger errors and scatter. We see a clear over- in [α/Fe] (where α refers here to oxygen). Recent studies by
abundanceofoxygeninthickdiskstarsoncetheOI6158Åline Bensbyetal.(2014)andHelmietal.(2014)alsoconfirmedthe
isconsidered.Moreover,usingthesub-samplewiththemostpre- alphaenhancementofametal-richpopulation,nevertheless,they
Articlenumber,page9of15
A&Aproofs:manuscriptno.oxygen
didnotobserveagapinmetallicitybetweenthemandthethick 2011; Bensbyetal. 2014), indicating that in the thin disk both
diskstars.Bensbyetal.(2014)arguedthatthehigh-αmetal-rich SN Ia and SN II contribute at a steady rate to the enrichment
familymightbeametalrichtailofthethickdisk.Withthisnew oftheinterstellarmedium.Incontrast,Ramírezetal.(2013)and
oxygen abundance determination we cannot draw conclusions Nissenetal. (2002, 2014) resultssuggesta flatteningof [O/Fe]
aboutthispopulation,i.e.whetherisdistinctorjustacontinua- at super-solar [Fe/H]. Our data also suggest that flattening in
tionofthethickdisktowardshighermetallicities. the[O/Fe]ratiomayappearatsolarmetallicities(Fig.12).The
Once the disk populationsare separated, we can revisit the “knee”isinappreciableinthecaseof[OI],andthus,forthemo-
issueabouttheoriginofthescatterintheobserved[O/H]trends. mentwecannotruleoutthatachangeofslopebelow[Fe/H]
∼
We repeat the test performedfor the best sample, but this time 0isreal.Thisapparentlackofagreementbetweenbothoxygen
independentlyforthethinandthick(includinghαmr)diskpop- indicators at high [Fe/H] can be attributed either to the scatter
ulations. The moving average for the oxygen abundances and inthe[OI]6300dataorbecausesomeblendmayappearathigh
δ[O/H]uncertaintiesareshowninfigure14,togetherwiththe metallicitiesneartheOI6158Åline,leadingtoanoverestimation
±
residuals of the 1σ width regions. Given the small residuals oftheEW.However,althoughfurtherworkneedstobedoneto
±
foundbetweenuncertaintiesandscatterforthickandhαmrpop- study the linelist near 6158Å spectral line, neither the FWHM
ulation,andtakingintoaccountthereducednumberofstarsthat orthe symmetryof the line seem to suggestthe presenceofan
we have in this group, no conclusion can be drawn about the unknownblend.
existence of a real star-to-star dispersion. In contrast, thin disk
Inordertostudythebehaviourof[O/Fe]towardsthemetal-
starsseemtoshowapositiveresidualat-0.2<[Fe/H]<0.3,which
poor regime in Fig. 15 we compare our results of OI6158Å
reaches 0.03dexat [Fe/H] 0.1.This residual may reveala real
∼ withthoseofRamírezetal.(2013).Theseauthorsderiveoxygen
astrophysical scatter, since uncertainties are unlikely to be un-
abundancesfrom the infrared triplet and correct them for non-
derestimated.Ontheotherhand,itcanbecheckedthatthethick
LTE effects, but neglect inelastic collisions with hydrogen.We
and thin disk populationsappear to be separated at a 1σ level,
alsomakeacomparisonwiththeresultsofBensbyetal.(2014)
thusconfirmingtheenrichmentofα-elementsinthethickdisk.
and Nissenetal. (2014), both of them based on a combination
In addition,the enhancementof the hαmr populationis clearly
of samples for which oxygen was measured using [OI]6300
revealed.
and the OI IR triplet, and only the OI IR triplet, respectively.
Nissenetal.(2014)correctedoxygenabundancesfordepartures
fromLTEaccountingforHcollisions,asdescribedbytheclas-
sical Drawin formula (Drawin 1968), and scaled by S =0.85.
H
0.6
Bensbyetal. (2014) used for this purpose an empirical correc-
tionderivedbycomparingabundancesof60starsfrom[OI]6300
0.4
andOI774indicators.
0.2 While our results mostly agree with those of Bensbyetal.
H] (2014) and Nissenetal. (2014), we find clear disagreementfor
O/6158 0.0 the metal-poor stars of Ramírezetal. (2013). Fig. 15 shows at
[ -0.2 [Fe/H]<-0.5howthe[O/Fe]risebecomessteeperandweobtain
muchlargervaluesfor[O/Fe]thanRamírezetal.(2013).Onthe
-0.4 Thindisk other hand, despite the similar slope, our results reach higher
Thickdisk
[O/Fe]ratiosthanNissenetal.(2014) andBensbyetal. (2014)
-0.6 hαmrstars
at [Fe/H] -0.8 dex. Although the most metal-poor stars in our
∼
als 0.05 samplehave weak OI6158Åoxygenlines, with EW between2
du 0.00
Resi-0.05 and4mÅ,wecansafelyruleoutthepossibilitythatourvalues
areoverestimatedbecauseofthepoorcontinuummeasurements
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
[Fe/H] and/orothersourcesoferrors.Toillustratethispoint,inFig.16
weshowoneofthetargetsinoursampleHD22879withoneof
Fig.14.Upperpanel:[O/H]vs.[Fe/H]forOI6158Å.Onlystarswith thehighest[O/Fe]=0.81and[Fe/H]= 0.82.Thecombinedspec-
EW relative error below 25% are considered. Stars are sepparated in trum of this star has S/N=1000 and−our precise measurements
different disk populations according to a chemical criteria. Solid line
of OI6158Å EW, which perfectly match the synthetic spectra,
represent the average oxygen abundance. The 1σ level of dispersion
clearlyruleoutanEWweakerthan2.5mÅ.Thus,ourbestvalue
aroundthemeanispresentedasashadedarea.Dashedlineshowsthe
average uncertainties. For plotting purposes the average uncertainties forthisstaris[O/Fe]=0.81.Thesehighoxygenoverironratios
are shown around the mean abundance. Lower panel: Difference be- arealsofoundwith[OI]measurements,asanexampleHD25704
tweenthe 1σwidthofthescatteraroundthemeanandthe 1σwidth is a star with [Fe/H]=-0.83 and [O/Fe]=0.69 dex. Finally, it is
± ±
oftheuncertainties. noteworthy to mention that the observed trend towards higher
valuesof[O /H]atlowtemperatures(Fig.5)donotaffectthe
6158
observed[O/Fe] trendsince noneof the high [O/Fe] stars have
T <5600K.
5.3.Comparisionwithpreviousstudies eff
Giventhequalityoftheobservationsandmeasurementspre-
Several studies have addressed the behaviour of oxygen over sented in these studies, the simplest way to explain the dis-
iron in the thin and thick disk populations. In general, all of crepancy with other works rely on the different non-LTE cor-
them agree that thick disk stars show an enhancementof α el- rectionsappliedtotheabundancecalculations.Differenceswith
ements, being this caused by different ratios of SNII/SNIa in Ramírezetal. (2013) arise becausethe inelastic collisionswith
bothdisks.Someofthemfoundashallowdeclineinthe[O/Fe] HatomswereneglectedintheirOIIRtripletcalculations,which
trendathigh[Fe/H]forthindiskstars(e.g.Bensbyetal.2004; wouldhaveshiftedthetrendtowardstheLTEresults.Actually,
Ramírezetal. 2007; Takeda&Honda 2005; Petigura&Marcy calculationsofnon-LTEbyFabbianetal.(2009)takingintoac-
Articlenumber,page10of15