LTE or non-LTE, that is the question

0.38 MB·English

by  Camilla J. Hansen

Astronomy&Astrophysicsmanuscriptno.Sr˙nlte˙arXiv2 (cid:13)c ESO2013
January17,2013
LTE or non-LTE, that is the question
The NLTE chemical evolution of Sr in extremely metal-poor stars
C.J.Hansen1,M.Bergemann2,G.Cescutti3,P.Francois4,A.Arcones5,A.I.Karakas6,K.Lind2,andC.Chiappini3
3
1 1 Landessternwarte,ZAH,Ko¨nigstuhl12,69117Heidelberg,Germany
0
2 2 Max-PlanckInstituteforAstrophysics,Karl-SchwarzschildStr.1,85741,Garching,Germany

n 3 Leibniz-Institutfu¨rAstrophysikPotsdam(AIP),AnderSternwarte16,D-14482Potsdam,Germany
a
J
4 GEPI,ObservatoiredeParis,CNRSUniversiteParisDiderot,PlaceJulesJanssen,92190Meudon,France

6
1 5 TUDarmstadt,Institutfu¨rKernphysikTheoriezentrum,Schlossgartenstr.2,64289Darmstadt,Germany

] 6 ResearchSchoolofAstronomy&Astrophysics,MountStromloObservatory,WestonCreekACT2611,Australia
R
S Received18/10/2012/Accepted24/12/2012
.
h
ABSTRACT
p
-
o Context.Strontiumhasprovenitselftobeoneofthemostimportantneutron-captureelementsinthestudyofmetal-poorstars.Thanks
tothestrongabsorptionlinesofSr,theycanbedetectedeveninthemostmetal-poorstarsandalsoinlow-resolutionspectra.However,
r
t westillcannotexplainthelargestar-to-starabundancescatterwederiveformetal-poorstars.
s Aims.HerewecompareGalacticchemicalevolution(GCE)predictionswithimprovedabundancesforSriandSrii,includingupdated
a
[ atomicdata,toevaluatepossibleexplanationsforthelargestar-to-starscatteratlowmetallicities.
Methods.Wehavederivedabundancesunderbothlocalthermodynamicequilibrium(LTE)andnon-LTE(NLTE)forstarsspanning

2 alargeintervalofmetallicities,aswellasabroadrangeofotherstellarparameters.Gravitiesandmetallicitiesarealsodeterminedin
v NLTE.WeemployedMARCSstellaratmospheresandMOOGfortheLTEspectrumsynthesis,whileMAFAGSandDETAILwere
7 usedtoderivetheNLTEabundances.WeverifiedtheconsistencyofthetwomethodsinLTE.
4 Results. WeconfirmthattheionisationequilibriumbetweenSrIandSrIIissatisfiedunderNLTEbutnotLTE,wherethedifference
1 between neutral and ionised Sr is on average ∼ 0.3dex. We show that the NLTE corrections are of increasing importance as the
4 metallicitydecreases.Forthestarswith[Fe/H]> −3,theSriNLTEcorrectionis ∼ 0.35/0.55dexindwarfs/giants, whiletheSrii
. NLTEcorrectionis<±0.05dex.
2
Conclusions.OnthebasisofthelargeNLTEcorrectionstoSri,SrishouldnotbeappliedasachemicaltracerunderLTE,whileit
1
isagoodtracerunderNLTE.Srii,ontheotherhand,isagoodtracerunderbothLTEandNLTE(downto[Fe/H]∼ −3),andLTE
2
isasafeassumptionforthismajorityspecies(iftheNLTEcorrectionsarenotavailable).However,theSrabundancefromSriilines
1
dependsondetermininganaccuratesurfacegravity,whichcanbeobtainedfromtheNLTEspectroscopyofFelinesorfromparallax
:
v measurements. Wecouldnotexplainthestar-to-starscatter(whichremainsunderbothLTEandNLTE)bytheuseoftheGalactic
i chemical evolution model, since Sryields to date have been too uncertain to draw firm conclusions. At least two nucleosynthetic
X
productionsitesseemnecessarytoaccountforthislargescatter.
r
a Keywords.stars:abundances–nuclearreactions,nucleosynthesis,abundances–Galaxy:evolution
1. Introduction 2008;Chiappinietal.2011;Frischknechtetal.2012).Thismay
modifythepresumptionofpurer-processpatternsinultrametal-
Strontium is one of the two neutron-capture elements (namely poor (UMP; −5 < [Fe/H] < −4 cf. Beers & Christlieb 2005)
strontium and barium - Ba) that intrinsically show very strong stars. For this reason, disentangling the nucleosynthetic origin
absorption lines even in metal-poor stars. At solar metallicity of Sr and Ba in metal-poorstars would help us understandthe
Srissynthesisedbyavarietyofnucleosyntheticsourcesinclud- formationandevolutionoftheearlyGalaxy.
ing the weak slow neutron-capture (s-) process that occurs in
Onlythe4077Å SrIIlineremainsdetectableinbothdwarfs
massivestars(e.g.Heiletal.2009;Pignatarietal.2010)andin
andgiantsbothinhigh-andlow-resolutionspectraofmetal-poor
AGB stars (Travaglio et al. 2004). In comparison, the produc-
stars.Studyingthislinethusprovidesuniqueinsightintothebe-
tionofBaisdominatedbythes-processoccurringinlow-mass
haviourofneutron-captureelementsatallmetallicitiesandspec-
AGBstars(Ka¨ppeleretal.1989;Bussoetal.1999;Snedenetal.
tralresolutions,rangingfromthelow-resolutionLAMOSTsur-
2008). This picture changes at low metallicity, where Ba may veyto the high-resolutionGaia-ESO survey1. Clearly, accurate
be formed by a main rapid neutron-capture process and Sr by
abundances are needed to fully comprehendthe chemical evo-
a chargedparticle process (Hoffmanet al. 1997).Additionally,
very metal-poorrapidly rotating massive stars mightbe signif- 1 The Srii line is detectable if a blue setting is used for Gaia-ESO
icant producersof Sr and Ba via the s-process (Pignatariet al. follow-upobservations.
1
C.J.Hansenetal.:Srabundancesinlate-typestars
lutionofSr.Thismeansthattheeffectsofnon-localthermody- 2000,and were kindlymade available to us by T. Gehren.The
namicequilibrium(NLTE)anddeviationsfromhydrostaticequi- spectrahavearesolutionof∼ 60000andanS/N of∼ 200near
librium (3D)must be taken into accountin elementabundance 5000Å.Moredetailsontheobservationsanddatareductioncan
calculations,if wewanttoextractthecorrectinformation from befoundinGehrenetal.(2004,2006).
thefuturesurveys’largeflowofdata.Clearly,suchcalculations We selected the stars according to the following criteria:
are a challenge,as was recentlydemonstratedforO (Gonza´lez 1) the observations cover the spectral range of the Sri 4607Å
Herna´ndez et al. 2010), Ca (Spite et al. 2012),Fe (Bergemann line, 2)accuratephotometryis available,and3)the starscover
et al. 2012a), and for Ba (Dobrovolskas et al. 2012), while a broad stellar parameter space to test the Sri and Srii abun-
Bonifacioetal.(2009)provide3Dcorrectionsforalargenumber dancebehaviouratdifferenttemperatures,gravities,andmetal-
ofelementsfordwarfs.Theestimatesof3DeffectsfortheSrii licities.Oursamplethusconsistsof21dwarf,sub-giant,andgi-
resonancelineshavebeenprovidedbyColletetal.(2007),who antstars.Herewehavedisregardedcarbonenhancedmetal-poor
performed LTE calculations with 3D radiative-hydrodynamics stars (CEMP-s/CEMP-no — with and without s-process over-
simulationsofstellarconvectionforthemetal-poorstars.The3D abundances).SincetheCEMP-sstarstendtohaveverylarges-
LTEcorrectionsareoftheorderof−0.15dex,withrespectto1D processabundances,weareslightlybiasedagainsthighSrabun-
LTE.TheNLTEabundancesofSrwerereportedinafewstudies, dancesatlowmetallicity.
suchasthoseofBelyakova&Mashonkina(1997),Mashonkina Forcomparison,weincludeextremelymetal-poorstarsfrom
et al. (1999), Andrievsky et al. (2011), and Bergemann et al.
Franc¸ois et al. (2007) and Bonifacio et al. (2009). For details
(2012a).Forthe Srii resonanceline, the NLTE abundancecor- abouttheobserveddata,werefertothesepublications.
rectionsarenotlarge,typicallywithin ±0.2dex2,however,they
aresensitivetovariationsinthestellarparameters(Bergemann
et al. 2012a). Full 3D NLTE calculations for Sr have not been 3. Methods
performedyet.
Using the NLTE technique presented in Bergemann et al. Inthiswork,NLTEeffectsareaccountedforinthedetermination
(2012a), we have now expanded the stellar sample in order to of basic stellar parameters (surface gravity and metallicity), as
study the chemical evolution of Sr in the Galaxy. We have de- wellasSrabundances.Wedescribetheanalysisindetailbelow.
rived NLTE Sr abundances and NLTE stellar parameters for a
sample of 21 stars, plus comparison samples (51 very metal-
3.1.Modelatmospheres
poorstarsfromFranc¸oisetal.2007andBonifacioetal.2009).
We also include the predictions of the homogeneous chemi- All calculationsin thisworkwere performedwith classical 1D
cal evolution model for the Galactic halo of Chiappini et al. LTE plane-parallelmodelatmospheres.We used the MAFAGS
(2008), computed with the most up-to-date Sr yields available (Grupp 2004a,b) and MARCS (Gustafsson et al. 2008) mod-
intheliterature.Giventhestilllargeuncertaintiesonthestellar els, which are both well-adapted to analysis of late-type stars.
yields, the goalof comparingthe data and the chemicalevolu- The model atmosphere codes adopt slightly different prescrip-
tionmodelis togivea firstimpressionofhowfartheavailable tions for the convectiveflux transportand backgroundopacity.
stellaryieldsarefromexplainingthedata.Acomparisonwithin- However, comparison of the model T(τ) relation showed that
homogeneouschemicalevolutionmodels,moresuitabletolow- these differences are very small, and reveal themselvesonly in
metallicityenvironments,isbeyondthescopeofthepresentpa- opticallythick layers,where the treatmentof convectionis im-
per,seeCescutti(2008);Cescutti&Chiappini(2010).Thepaper portant(seeFig.1).AcomparisonoftheMAFAGSandMARCS
is structuredasfollows.Sections2 and3 describethe observa- models was presented in Bergemann et al. (2012b), where we
tions, stellar parameters, and NLTE calculations, respectively. showedthatthedifferencesinFeabundancesobtainedwithdif-
Sections 4 and 5 present the results, yields, and the chemical ferentmodelatmospherecodesareverysmall, typicallywithin
evolutionmodel.ConclusionscanbefoundinSect.6. 0.05dex.WecommentonthisfurtherinSect.3.3.
2. Sampleanddatareduction 3.2.Stellarparameters
A sub-sample of stars (marked ‘u’ in Sect. 4) was taken Stellar parameters for the selected sample of stars were taken
from(Hansenetal.2012,‘H’)and(Bergemann&Gehren2008;
from Hansen et al. (2012). These stars were observed with
UVES/VLT(Dekkeretal.2000,R ≥ 40000)between2000and Bergemannetal.2012b, –‘BG’;‘B’),givingpreferencetopa-
2002, and their spectra were reduced with the UVES pipeline rameters determined with IR photometry and parallaxes. The
(v. 4.3.0). The spectra have a signal-to-noiseratio, S/N, > 100 ‘H’,‘BG’,and‘B’ indicatethesourceofthetemperaturehere-
after.Abriefdescriptionofthesedatafollows.
at 3200Å. Two stars (see ‘h’ in the following) were observed
Theeffectivetemperatureswerederivedfromseveraldiffer-
withHIRES/Keck(R ∼ 60000,Vogtetal.1994).Theirspectra
entcolourindicesandcalibrationmethods(Alonsoetal. 1996,
areof similarqualityasthe UVESspectra,andtheyhavebeen
1999;Ram´ırez &Mele´ndez2005;Masanaetal. 2006;O¨nehag
retrieved from the HIRES reduced data archive. The pipeline-
etal.2009;Casagrandeetal.2010).Wechosetemperaturecal-
reduced data were wavelength-shifted,co-added,and had their
ibrationsthatfallinthemiddleoftheprobedcalibrationranges
continuanormalisedbeforeouranalysis.Forfurtherdetails we
(seeHansenetal.2012,fordetails).Thereddeningvalueswere
refertoHansenetal.(2012).
taken from Schlegel et al. (1998), and since all the reddening
Threestars(HD134169,HD148816,HD184448)wereob-
valuesaremuchlowerthan0.1mag(Bonifacioetal.2000),we
served with the FOCES echelle spectrographat the 2.2m tele-
have not applied their corrections to these values. Our chosen
scopeoftheCAHAobservatoryonCalarAlto,during1999and
stellarparametersarealso consistentwith the effectivetemper-
2 Forverymetal-poor(VMP;−3<[Fe/H]<−2)starsthecorrections atures (Teff) determined from the 1D fitting of Balmer profiles
arelessthanabout±0.08andslightlygreaterforextremelymetal-poor by Gehren et al. (2004, 2006). For the few stars (HD 19445,
stars([Fe/H]<−3). HD 142038, G 64-12) we have in common with Gehren et al.
2
C.J.Hansenetal.:Srabundancesinlate-typestars
to +0.3dex.Forthe extremelymetal-poor(EMP; −4 < [Fe/H]
8000 < −3) giant stars, we obtain a maximum NLTE correction of
5000, 2.2
7000 ∆NLTE[Fe/H]of+0.3dex.
−1.2
K) 6000 −2.4
T ( 5000
1.0
4000 0.8 giants
E dwarfs
3000 −4 −3 −2 −1 0 1 g gLT 0.6
o
log τ (500 nm) - lE 0.4
gNLT 0.2
g
8000 5400, 3.4 lo 0.0
7000 −1.2 -0.2
T (K) 6000 −2.4 1 2 log gLTE3 4 5
5000
0.4
4000
−4 −3 −2 −1 0 1 H]LTE 0.3 gdiwaanrtfss
log τ (500 nm) Fe/ 0.2
- [E
9000 NLT 0.1
6000, 4.2 H]
8000 Fe/ 0.0
−1.2 [
K) 7000 −2.4 -0.1
T ( 6000 -5 -4 -3 -2 -1 0
[Fe/H]
LTE
5000
Fig.2:ComparisonofLTEandNLTEsurfacegravities(top)and
4000
−4 −3 −2 −1 0 1 metallicities(bottom).
log τ (500 nm)
Fig.1: Comparison of MARCS (dotted) and MAFAGS (solid) Fortheotherstars,whichdonothaveknownparallaxes,the
modelatmospheresforselectedstellarparametersattwodiffer- initial estimate of logg and [Fe/H] was obtainedfromthe LTE
entmetallicities([Fe/H]=−1.2,−2.4). ionisationequilibriumofFe,andFeNLTEcorrectionswereap-
pliedtobothgravityandmetallicity.TocomputetheNLTEcor-
rectionforsurfacegravity,weusedtheapproximate∆ logg
NLTE
- ∆ [Fe/H] calculations from Lind et al. (2012, their Sect.
NLTE
(2004, 2006), they derive Teff = 5985, 5773, and 6407 K, re- 3).ThechangesinloggduetoNLTEeffectsaresignificantand
spectively,withanrmsoffsetof10Kfromourvalues.Thiscon- haveacleareffectonthe[Sr/Fe]ratiosderivedfromthegravity-
firmstheagreementbetweentheBalmerTeff scalefromGehren sensitiveSriilines,comparedtothechangesinmetallicity.The
et al. (2004, 2006) and the method described in Alonso et al. NLTE gravity correctionsreach up to ∼ +0.8 dex for the most
(1996)thatweusedhere.BalmerlineTeff’s(1D)fromthesame metal-poorgiantsinoursample.TheinfluenceofTeff onSriiis
referenceagreewiththevaluesweadoptedforHD134169,HD minor.Bychangingthetemperaturewithitsuncertainty,theSrii
148816,andHD184448.TheparametersforHD122563andG abundancechangesby0.01-0.05dex,whilethechangeinSriis
64-12arethosefromBergemannetal.(2012b). muchlarger(seeSect.4.1).TheNLTEandLTEstellarparame-
Forthestarswithparallaxmeasurements,thesurfacegravity tersarecomparedinFig.2.
wascalculatedusingtheclassicalformulathatrelatesmass,tem-
perature, magnitude and parallax to gravity. Masses and bolo- As seen from Fig. 3 the temperaturesfor seven stars taken
metric corrections were taken from Nissen et al. (1997, 2002, fromthreedifferentsources(Casagrandeetal.2010;Bergemann
2007). Metallicities were then initially estimated in LTE, and etal.2012a;Hansenetal.2012)agreewithin40Kinmostcases,
theeffectsofNLTEweretakenintoaccountbyapplyingNLTE andforafewstars(e.g.HD106038)thedifferencebetweenthe
abundancecorrectionsto Fe. Thesecorrectionswere computed twoIRFMmethods(Alonsoetal.1996;Casagrandeetal.2010)
for the adopted Fei line list by interpolation in the Fe NLTE is170K.Thisdifferenceiswithinthecombinederrors,ifwein-
gridpresentedbyLindetal.(2012).Wenotethatforstarswith cludesystematicerrors,aswellastheuncertaintyon E(B−V).
metallicity[Fe/H]>−2,thesystematicdifferencebetweenLTE Thedwarfsfromthecomparisonsample(Bonifacioetal.2009)
abundancesofFeiandFeiiisnotlarge(≤0.1dex).TheSrNLTE had their temperatures estimated from Hα line profile fitting,
correctionsalsostaywithin0.05−0.07dex(seeSect.4)consis- whichyieldedvaluesingoodagreementwiththosedetermined
tentwith the resultsin Bergemannet al. (2012a).The effectof from Alonso et al. (1996) calibrations. We refer the reader to
NLTE becomes very important for Fe in very metal-poorstars Bonifacioetal.(2009)andSbordoneetal.(2010)forfurtherde-
with [Fe/H] < −2.5. In particular, most of the stars from our tailsonHαlineprofilefitting.Thegiantsfromthesecondcom-
comparisonsamples(Franc¸oisetal.2007;Bonifacioetal.2009) parisonsample(Franc¸oisetal.2007)hadtheirtemperaturesde-
are subject to NLTE metallicity correctionsranging from +0.2 terminedbytheuseofbroadrangephotometrycalibrationsfrom
3
C.J.Hansenetal.:Srabundancesinlate-typestars
HD 106038, Sr II
1.0
0.8
x
u
ve Fl 0.6
Relati 0.4
0.2 C6 = −32
C6 = −31.65
0.0
4077.2 4077.4 4077.6 4077.8 4078.0
λ [A]
HD 122563, Sr II
1.0
0.8
Fig.3: Comparison of effective temperatures determined with
different methods for seven stars. The legend indicates the ux
original paper (Casagrande et al. 2010; Hansen et al. 2012; ve Fl 0.6
Bergemannetal.2012a, –yellow’*’,bluediamond,andgreen Relati 0.4
square, respectively) from which the temperatures have been
taken.
0.2 C6 = −32
C6 = −31.65
0.0
Alonsoetal. (1999).Thisisthesamemethodwe appliedhere, 4077.2 4077.4 4077.6 4077.8 4078.0
λ [A]
and the differences/offsetsbetween ourtemperaturedetermina-
tionsandthosemadeforthecomparisonsamplesareminimal.
Fig.4: Effect of elastic H I collisions on the 4077 Å lines ex-
ThemicroturbulencevelocitywasfixedbyrequiringthatFei
pressedthroughdifferentC values(reddashedandblacksolid
lines yield the same abundances regardless of their equivalent 6
line) - compared to observations (black dots) of a dwarf (HD
width. In ourparameterspace, the NLTE correctionsto micro-
106038)andametal-poorgiant(HD122563)star.
turbulence(Lindetal.2012)aresmallerthantheformaluncer-
taintyofξ (±0.15km/s)andwerenotconsideredhere.
t
Themainerrorinourphotometrictemperaturescomesfrom
reddening (. ±0.05mag). For Balmer lines, the temperature TheLTEandNLTEabundancesofSrintheselectedmetal-
error is largely internal and is determined from the profile fit- poorstarsweredeterminedasfollows.TheLTEabundancesare
ting. This applies to the six stars marked by a ‘B’ in the fol- synthesised with MOOG, while NLTE synthesised abundances
lowing. The error in the adopted logg values is dominated by are derivedusing the SIU code (Reetz 1999),where the NLTE
that of parallaxes (. ±1.0”). The errors in [Fe/H] and mi- departurecoefficientsarecomputedwithDETAIL.Inbothcases
croturbulence are assumed to be 0.1(5)dex and 0.15km/s, re- weapplythesameatomicdata.WenotethattheLTEabundances
spectively. After propagating all stellar parameter uncertain- from SIU and MOOG agree within 0.1dex. This difference is
ties, we adopted a common set of uncertainties for our stars a combinationof local continuumplacement(which can be up
of (T /logg/[Fe/H]/ξ): ±100K/0.2dex/0.1dex/0.15km/s. For to 0.05dex when set locally by eye) and of the different syn-
eff
the most metal-poor stars, we find slightly higher values: thesis codes, since the model atmospheres are very similar, as
±100K/0.25dex/0.15dex/0.15km/s.Theseerrorsareinternalto seen from Fig. 1, they almost do not contribute to this differ-
ourmethod.ThedifferencesbetweenLTEandNLTEstellarpa- ence.Onlyverydeepintheatmosphere(closeto τ = 1)dothe
rametershighlightsthatsystematicerrorsinLTEarelargerthan MARCS and MAFAGS models differ due to a more efficient
theinternalerrors. transportofconvectivefluxintheMARCSmodels,whichleads
to a cooler atmosphere than in MAFAGS. For 18 stars, with a
four-staroverlapwiththeSIUanalysis,wefirstdetermineLTE
3.3.Srabundancedeterminations
abundancesbyspectrumsynthesiswiththeMOOGcode.Toob-
tainNLTEabundances,wethenapplytheNLTEcorrectionscal-
The NLTE statistical equilibrium calculations for Sr were per-
culatedwithDETAILandMAFAGS.Wedonotperformdiffer-
formed with the revised version of the DETAIL code (Butler
entialabundanceanalysiswithrespecttotheSun.
& Giddings 1985). The new model atom of Sr and other re-
latedaspectsoftheNLTEcalculationsaredescribedindetailin In particular, to remain consistent with our previousanaly-
Bergemannetal.(2012a).TheNLTEeffectsontheSrilinesare sis, we use the van der Waals broadening and gf-values from
primarily caused by over-ionisation, which leads to systemati- Bergemannetal.(2012a)(seeTable1).Accordingtothisstudy
cally higherNLTE abundancescomparedwith LTE, especially thedampingconstantsfortheSriilinescouldbesomewhatun-
formoremetal-poorandhotterstars.Incontrast,deviationsfrom certain.VariationinlogC6 by±0.35(∼ 20%)leadstoachange
LTEintheSriilinesarelargelydrivenbystronglinescattering. inabundancesby∓0.15dexforthe4077Å Sriilineinthemetal-
As a consequence,the differencesbetween the LTE and NLTE poorstarswith[Fe/H]> −1.5(seeFig.4),butonlyhasaminor
abundancesmaybepositiveornegative,dependingontempera- effect at very low metallicity (∼ ∓0.05 dex). The accuracy of
ture,gravity,andmetallicityofastar. thegf-valueforthe4607Å Srilinewascriticallyevaluatedby
4
C.J.Hansenetal.:Srabundancesinlate-typestars
Table1:AtomicdataforSrIandII HD 122563, Sr II
1.0
Element λ χ loggf log(γ/N ) logC
H 6
Å eV radcm3s−1 cm6s−1
0.8
SrI 4607.33 0.0 0.283 -7.53 -31.2
SrII 4077.71 0.0 0.158* -7.81 -32.0 ux
ve Fl 0.6
Notes. (∗) Total log gf value. Further details and hfs splitting can be Relati 0.4
foundinBergemannetal.(2012a).
0.2 with blend
no blend
HD 106038, Sr I
0.0
4215.3 4215.4 4215.5 4215.6 4215.7
λ [A]
1.00
Fig.6: Comparison of synthetic profiles (lines) of the Srii line
x to observationsof HD122563(black dots).The syntheses have
u
ve Fl 0.95 been computedwith (black solid line) and without(red dashed
Relati 0.90 line)theFeiblendsforthe4215Å Sriiline.
[Sr/Fe] = 0.2
[Sr/Fe] = 0.3
0.85 [Sr/Fe] = 0.4
W 0.4 giants
E dwarfs
4606.8 4607.0 4607.2 4607.4 4607.6 4607.8 ]
λ [A] Fe
r/ 0.2
S
[
Fig.5:Syntheticspectra(solidlines)ofthe4607Å Srilineplot- −
tedontopoftheobservedspectrumofHD106038(dots). yn 0.0
s
]
e
F
r/ −0.2
S
[
the NIST database. The uncertainties are less than one percent
−4 −3 −2 −1 0
andyieldaccurateabundancesevenformetal-richstars(seeFig.
[Fe/H]
5),whereastheSriiresonancelinesat4077and4215Åaretoo
blended and strong to give any reliable information about the
solar Sr abundance(see Fig. 6). We deriveourSri abundances Fig.7:DifferenceinSrabundancesbetweenEWandsynthesis.
fromthe4607Å lineandtheSriiabundancesfromthe4077Å Dwarfs/giantsareshownassmallred/bigbluefilledcircles,re-
spectively.
line,sinceallourstarshavesub-solarmetallicities.
We tested the Abfind package in MOOG, which uses
the measured equivalent widths (EW) to compute abundance
by the curve-of-growth method. We find, however, that this linesat4215.42and4216.18Å.Theformerisratherstrongand
method yields abundances in slight disagreement (±0.04 < clearly distorts the shape of the Srii profile, as directly seen in
[Sr/Fe] < ±0.25) with the results obtained by the full the very high-resolution spectra. Figure 6 shows that even for
EW−syn
profile fitting using the Synth (synthesis) package of the same theverymetal-poorgiantHD122563with[Fe/H]NLTE = −2.5,
code(seetheonlinematerial).Dependingonthelineproperties, theabundanceisover-estimatedby0.14dex(whensynthesised)
in particular equivalent width, the abundance is either over- or ifthisblendisnottakenintoaccount.Therefore,theabundance
under-estimated (Table 2). This expected; the Sri line is very determinedfromthe4215Å Sriilineissubjecttoasystematic
weak in metal-poorstars, and both Srii lines are strong, sensi- uncertainty,andtheblendissensitivetoTeff,logg,andmetallic-
tivetodamping,andaffectedbyblends.Forexample,theEWs ity.
determined by fitting Voigt and Gaussian profiles in IRAF (’/’ On these grounds, we do not include the 4215 Å Srii line
separatedentriesin the’EW’ columnsin Table 2)differgener- intheabundancecalculations.Furthermore,forthecomparison
allyby5–10%(inafewcases,liketheVMPdwarfHD106038, samplesfromFranc¸oisetal.(2007)andBonifacioetal.(2009),
thedifferenceisapproximatelyafactorofthreelarger).Asare- onlymeasurementsforthe4077Å Sriilineareavailable.Tobe
sult,abundancesderivedusingtheEWsmaybediscrepantbyup consistent and to retain the full sample size, we therefore only
to0.4dex(Fig.7).However,exceptforonecase(theEMPsub- usethe4077Sriiline.
giantHD 140283),thelargestdifferencebetweenEWandsyn- TheSrilineisgenerallyweak,andregardlessoftheprofile
thesisdeterminedabundancesare oftheorderof ±0.1dex(see fittedtothisline,theEW-convertedabundancestendtobelarger
onlinematerial).Thisvaluemightbeslightlyover-estimateddue than the synthesised abundances. This might be due to an iron
tolocalcontinuumplacement. blendin the red wing of this neutralstrontium line (see Fig. 5,
Blends influence abundance determinations in both the whereastrongerFelineblendsintotheredwingoftheSriline).
metal-poorand metal-rich parts of our sample. Features in the TheLTEabundancesderivedforoursamplewerecalculated
blueandredwingsofthe4077Sriiline,∼0.3Åawayfromthe withLTEstellarparameters(asdescribedinSect.3.2)usingthe
linecentre,onlyvanishinthespectraofmostmetal-poorstars, 1D LTE synthetic spectrum code MOOG to synthesise spectra
[Fe/H]<−2.Theseblendsareduetolanthanum,chromium,and forthesestars.TheEWsforthedwarfcomparisonsamplewere
dysprosium.The Srii lineat 4215Å is blendedbythe two Fei takenfromBonifacioetal.(2009).WemeasuredtheEWsforthe
5
C.J.Hansenetal.:Srabundancesinlate-typestars
Table2:ExampleofLTE[Sr/Fe]abundancesforadwarf,sub-giant,andgiantfromthemeasuredequivalentwidths(EW)andfrom
synthesis(synt).ThemeasuredGauss/VoigtEWs(G/V)aregiveninmÅ,togetherwiththestellarparameters.
Star 4077EW EWG/V 4077synt 4607EW EWG/V 4607synt T logg [Fe/H] ξ
eff LTE
[mÅ] [mÅ] [K] [km/s]
HD106038 0.28/0.65 186.1/271.0 0.5 0.43/0.47 13.1/14.1 0.33 5950 4.33 −1.48 1.1
HD140283 −0.44/−0.33 75.8/79.8 −0.15 – – – 5777 3.70 −2.58 1.5
HD122563 −0.34/−0.09∗ 158.8/184.0 −0.05 −0.54∗ 2.9 <−0.6 4665 1.65 −2.50 1.8
Notes. (∗)Valueuncertain
remainingstars. The LTE Sr II abundancesfor the comparison
samples (dwarfs and giants) were redetermined with MARCS 1.0
models and MOOG, using the solar abundance, from Anders e] 0.8 LTE
&LTGEraebvuenssdean(1ce9s89w)e,wcahliccuhlawteedhaagvreeeadwoipthteind0f.o0r5th-i0s.1studdeyx.wTihthe Sr I/F 00..46
those published in (Franc¸ois et al. 2007, F07) and (Bonifacio e] − [ 0.2
getreaalT.teh2re0t0hc9ao,nrBr0e0c.29ti)do,enaxnf.doronNlLyTfoEreaffheacntdsfuislomfosstatrcsoinsvtheneideinfftleyrepnecre- [Sr II/F −−000...420 nnoonn−−LLTTEE,, gdiwanartfss
formed by differentiating LTE and NLTE curves-of-growth at
−3 −2 −1 0
a given line strength. The method we adopted thus relies on
[Fe/H]
the determination of EWs and subsequent translation to LTE
andNLTEabundances.Fromthecomparisonsample(the“First
1.0
Stars” samples), we only have these EWs. When determin-
0.8 LTE
ingEWsbyprofilefittingwitha singlecomponent,unresolved e]
bElWen-dbsasmedayabpulnadyanaceroslaen,dsocowmepadreerdivthedembotothsyLnTthEesainseddNabLuTnE- − [Sr I/F 00..46
dancesto assess the impact the single-line assumptionhave on e] 0.2
tbhleenfidnsaalnadbpurnodfialnecfietst.inFgo,rwaellhtahveetmesatisntraeilnaeteddotnoeasteotmoficstdealltaar, [Sr II/F −00..20 non−LTE, dwarfs
parametersforeach star (those listed in Table 3). Thus, the re- −0.4 non−LTE, giants
sultingdifferenceinabundanceisanexpressionofuncertainties
1 2 3 4 5
intheatomicdata,unknownblends,andcontinuumplacement. log g
From Fig. 7 we estimate that the Sr abundances from the
metal-rich stars might be overestimated when using the EW
Fig.8: Sr LTE (open circles) and NLTE abundance differences
method, while Sr in the metal-poorstars will be overestimated
(filledbigblue/smallredcirclesforgiants/dwarfs)asafunction
with<0.32dex.Anotherpartoftheover-/under-estimationcan
of[Fe/H](toppanel)andlogg(bottompanel).
beassignedtothelineprofilefittingandcontinuumplacement.
line-to-line scatter. If the gravities are derived from parallaxes,
4. Results:LTEvsNLTE
theirvalueswill bethe same in bothLTE and NLTE;however,
TheresultsforoursamplearesummarisedinTable3,whichpro- when derived from ionisation equilibrium,the NLTE corrected
videsthemeanofsynthesisedabundancesforSriandSrii.Their loggcorr will differfromthe LTE Fei based logg. Forourstel-
differencesareshowninFig.8asafunctionofstellar[Fe/H]and larsample,eventhoughtheline-to-linescatterisclearlysmaller
logg.TheLTEandNLTElinebylineabundancesfrombothEW underNLTE,thestar-to-starscatteralmostremainsthesameun-
andsynthesisarealsogivenintheonlineTableA.1(foroursam- derbothLTEandNLTE.Theresultsforthecomparisonsamples
ple). (the‘FirstStars’samples–F07,andB09)areshowninTable4.
Figure9showsthe[Sr/Fe]ratiosasafunctionof[Fe/H].In
TheLTEapproximationfailstoestablishtheionisationbal-
anceof SriandSrii.Figure8showsthattheoffsetbetweenthe thefigure,theerrorbarsarethetotal,propagateduncertainties,
computedasdescribedbelow.TheexceptionsareHD3567,HD
twoionisationstagesisabout0.2dexfordwarfs,butitincreases
up to 0.5 dex for giants. This difference is mainly caused by 19445, HD 122563, and HD 126587 with upper limits on the
the progressively increasing systematic error in the LTE abun- abundancefromthe4607ÅSriline.
danceinferredfromtheSriline,whichshowsNLTEabundance
correctionsof up to 0.5 dex at low metallicity and low gravity
4.1.Uncertainties
(onlineTableA.1).AlthoughtheNLTEeffectsontheresonance
Sriilinesarenotsignificantlypronounced,theydependonstel- A number of test calculations varying the input parameters in
larparameters,particularlyonthe[Fe/H]or,equivalently,onthe the spectrum synthesis were performedfor the two representa-
Srabundanceitself(seediscussioninSect.3.3).InTable3,we tivestarswiththesamemetallicity:HD106038(dwarf)andHD
seethattheLTEabundancesobtainedfromtheSriilinescanbe 74462(giant).Inparticular,weareinterestedinthesensitivityof
over-orunder-estimatedbyupto0.1dex,whichmayintroduce theabundancestothemodelatmosphereparameters:T ,logg,
eff
a spurioussystematic trendor, morelikely give rise to a larger [Fe/H],andmicroturbulence(Table5).Asseenfromthistable,
6
C.J.Hansenetal.:Srabundancesinlate-typestars
Table3:StellarparametersandderivedSrabundances±standarddeviationfortheselectedsampleofstars.Strontiumabundances
with superscript(1) are derivedonlyfrom the 4077Å Srii line. Parameterswith subscripthave been correctedforNLTE effects.
Starswithan‘s’superscripthavehadtheirabundancesderivedinSIU.
Star π±σ T logg logg [Fe/H] [Fe/H] ξ [Sr/Fe] [Sr/Fe] Comment
eff corr. LTE NLTE LTE NLTE
[mas] [K] km/s
HD3567 9.57±1.38 6035 4.08 4.08 −1.33 −1.29 1.5 −0.03±0.8x 0.06±0.8x [u,H]
HD19445 25.85±1.14 5982 4.38 4.38 −2.13 −2.10 1.4 0.13±0.8x 0.16±0.8x [u,H]
HD106038 9.16±1.50 5950 4.33 4.33 −1.48 −1.45 1.1 0.42±0.12 0.45±0.21 [u,H]
HD121004 16.73±1.35 5711 4.46 4.46 −0.73 −0.71 0.7 0.18±0.04 0.26±0.17 [u,H]
HD122196 9.77±1.32 6048 3.89 3.89 −1.81 −1.75 1.2 0.24(1) 0.19(1) [u,H]
HD134169 16.80±1.11 5930 3.98 3.98 — −0.86 1.8 −0.05±0.14 −0.06±0.13 [f,s,BG]
HD140283 17.16±0.68 5777 3.70 3.70 −2.58 −2.38 1.5 −0.15(1) −0.37(1) [u,H,B]
HD148816 24.34±0.90 5880 4.07 4.07 — −0.78 1.2 −0.13±0.18 −0.13±0.17 [f,s,BG]
HD184448 19.16±0.63 5765 4.16 4.16 — −0.43 1.2 0.00±0.21 −0.01±0.21 [f,s,BG]
G64-12 0.57±2.83 6464 4.30 4.30 −3.24 −3.12 1.5 0.00(1) 0.17(1) [u,B]
G64-37 2.88±3.10 6494 3.82∗ 4.23 −3.17 −3.00 1.4 0.08(1) 0.17(1) [u,H]
HD122563 4.22±0.35 4665 1.65 1.65 −2.60 −2.50 1.8 −0.23±0.8x −0.12±0.8x [u,B]
HD175305 6.18±0.56 5100 2.70 2.70 −1.38 −1.34 1.2 −0.12±0.32 0.04±0.1 [h,H]
BD-133442 – 6450 4.20 4.42 −2.56 −2.47 1.5 0.30(1) 0.21(1) [u,H]
CS30312-059 – 5021 1.90 2.41 −3.06 −2.89 1.5 0.50(1) 0.31(1) [h,H]
CS31082-001 – 4925 1.51 2.05 −2.81 −2.63 1.4 0.70(1) 0.60(1) [h,H]
HD74462 – 4590 1.84 1.98 −1.48 −1.43 1.1 −0.25±0.35 −0.14±0.03 [h,H]
HD126238 – 4900 1.80 2.02 −1.92 −1.85 1.5 −0.17±0.24 0.01±0.06 [u,H]
HD126587 – 4950 1.90 2.36 −3.01 −2.86 1.65 0.23±0.8x 0.22±0.8x [u,H]
HE0315+0000 – 5050 2.05 2.47 −2.81 −2.67 1.7 0.39(1) 0.23(1) [u,H]
HE1219-0312 – 5100 2.05 2.58 −2.99 −2.81 1.65 0.29(1) 0.12(1) [u,H]
Notes. (∗)Valuefromionisationequilibrium
(B,BG,H)Temperaturefrom(Bergemannetal.2012b,B),(Bergemann&Gehren2008,BG),and(Hansenetal.2012,H),respectively.
(u,f,h)Observedspectrafrom:uUVES/VLT, f FOCES/Calar-AltoGehrenetal.(2004,2006),hHIRES/Keck
(x) Weightedaverage,whichincludesanupperlimitthatwasgivenhalfweight.Thelargeuncertaintyisareciprocalsquare rootofthesummed
weights.
the Srii line at 4077 Å is very strong and mainly sensitive to Table 5: Uncertainties in the individual LTE Sr I and II abun-
logg and ξ. In comparison, the neutral Sr abundance from the dancesinagiant(HD74462)andadwarf(HD106038)star.
weaker4607Å lineis sensitiveto temperatureand[Fe/H],but
almostnotaffectedbyloggandξ. HD74462:[Sr/Fe] 0.1 -0.5
Parameter/line[Å] 4077 4607
SeveraloftheSriabundancespresentedintheonlineTable
T±100 ±0.03 ±0.17
A.1areonlyupperlimits, whichiswhywe appliedaweighted
logg±0.2 ±0.04 ±0.01
average and a corresponding reciprocal square root of the [Fe/H]±0.1 ±0.12 ±0.09
summed weights (see Taylor 1997). The final [Sr/Fe] abun- ξ±0.15 ±0.07 ±0.01
dancesandtheiruncertaintiesare shown in Table 3. The upper Propagateduncertainty ±0.15 ±0.19
limits of Sri were given half the weight of Srii. This overesti- HD106038:[Sr/Fe] 0.65 0.33
mates the uncertainty a bit. However, with only two measure- T±100 ±0.04 ±0.09
ments, this approach seems sensible. In some stars only Srii logg±0.2 ±0.17 ±0.01
couldbemeasured,sotheabundanceisbasedononlyonetrust- [Fe/H]±0.1 ±0.22 ±0.09
ξ±0.15 ±0.13 ±0.01
worthy line. To estimate the uncertainty in this case we made
independentmeasurementsofthe4077Sriiline,andfoundthe Propagateduncertainty ±0.3 ±0.13
derivedabundancestobeconsistenttowithin0.0-0.1dex.An
averagevalueof±0.05dexwasadoptedinsteadofthestandard
deviationotherwiseappliedtothestarswithtwodetectablelines.
minationin orderto deriveaccurateabundancesforSrII lines.
We summarise that the assumption of LTE especially in Furthermore,wenotethatthelargestar-to-starscatterfoundin
metal-poor, low-gravity stars will, in addition to not fulfilling LTE abundancestudiesremainsunderNLTE evenatextremely
Sr ionisation balance, also introduce a weak spurious trend of lowmetallicities.
Sr abundances with metallicity. The Sri resonance line would OuranalysisofthechemicalevolutionofSrinNLTEisdif-
consistentlyunder-estimatetheSrabundancetrend,whereasthe ferent from previous studies. Andrievsky et al. (2009) has al-
abundances obtained from the subordinate Srii lines would be readyperformedNLTE calculationsforSr; however,theirstel-
systematicallytoolargeby∼0.05dex.Whereastheabundances lar parameters were determined assuming LTE. These have a
derived from the Sri line are significantly affected by NLTE measurable impact on the Sr abundances. The difference in
line formation, Srii lines are less so. However, NLTE effects the overall trend of [Sr/Fe] with metallicity (Bergemann et al.
forFemustbeaccountedforinthespectroscopicgravitydeter- (2012a) and our Fig. 9) is detectable, and is best seen in Fig.
7
C.J.Hansenetal.:Srabundancesinlate-typestars
Table4:BasicparametersandEWcalculatedSrabundancesfromthe4077Å lineforthecomparisonstars.Toppartcontainsdwarf
stars fromthe comparisonsample, while lowerpartshowsgiantsfromthe secondcomparisonsample.Details andsubscriptsare
describedinTable3.
Star T logg logg [Fe/H] [Fe/H] ξ [Sr/Fe] [Sr/Fe]
eff corrected LTE NLTE LTE NLTE
[K] [km/s]
DwarfsampleFS,B09
BS16023-046 6364 4.50 4.69 −2.97 −2.90 1.3 −0.20 −0.17
BS16076-006 5199 3.00 3.29 −3.81 −3.70 1.4 0.67 -0.64
BS16968-061 6035 3.75 4.04 −3.05 −2.94 1.5 −1.59 −1.58
BS17570-063 6242 4.75 4.87 −2.92 −2.87 0.5 0.03 −0.02
CS22177-009 6257 4.50 4.67 −3.10 −3.03 1.2 −0.15 −0.12
CS22888-031 6151 5.00 5.09 −3.30 −3.26 0.5 0.05 0.09
CS22948-093 6356 4.25 4.53 −3.30 −3.19 1.2 −0.08 −0.01
CS22953-037 6364 4.25 4.48 −2.89 −2.80 1.4 −0.45 −0.46
CS22965-054 6089 3.75 4.06 −3.04 −2.92 1.4 −2.09 −2.17
CS22966-011 6204 4.75 4.87 −3.07 −3.02 1.1 0.88 0.95
CS29499-060 6318 4.00 4.26 −2.70 −2.60 1.5 −0.63 −0.70
CS29506-007 6273 4.00 4.27 −2.91 −2.80 1.7 −0.49 −0.50
CS29506-090 6303 4.25 4.46 −2.83 −2.75 1.4 0.33 0.27
CS29518-020 6242 4.50 4.65 −2.77 −2.71 1.7 0.08 0.05
CS29518-043 6432 4.25 4.53 −3.20 −3.09 1.3 — —
CS29527-015 6242 4.00 4.34 −3.55 −3.41 1.6 0.12 0.22
CS30301-024 6334 4.00 4.27 −2.75 −2.64 1.6 −0.41 −0.47
CS30339-069 6242 4.00 4.28 −3.08 −2.97 1.3 0.43 0.40
CS31061-032 6409 4.25 4.45 −2.58 −2.50 1.4 −0.48 −0.54
GiantsampleFS,F07
BD+17:3248 5250 1.40 1.97 −2.07 −1.88 1.5 0.00 −0.09
BD-18:5550 4750 1.40 1.95 −3.06 −2.88 1.8 −1.03 −1.12
BS16467-062 5200 2.50 3.07 −3.77 −3.58 1.6 −1.96 −1.99
BS16477-003 4900 1.70 2.29 −3.36 −3.16 1.8 0.06 −0.19
BS17569-049 4700 1.20 1.74 −2.88 −2.70 1.9 0.17 −0.18
CD-38:245 4800 1.50 2.33 −4.19 −3.91 2.2 −0.72 −0.82
CS22169-035 4700 1.20 1.79 −3.04 −2.84 2.2 -0.33 −0.30
CS22172-002 4800 1.30 2.14 −3.86 −3.58 2.2 −1.40 −1.60
CS22186-025 4900 1.50 2.10 −3.00 −2.80 2.0 0.61 0.41
CS22189-009 4900 1.70 2.33 −3.49 −3.28 1.9 −1.01 −1.07
CS22873-055 4550 1.00 1.48 −2.99 −2.83 2.2 −0.16 −0.30
CS22873-166 4550 1.00 1.47 −2.97 −2.81 2.1 0.07 −0.07
CS22878-101 4800 1.30 1.98 −3.25 −3.02 2.0 −0.41 −0.36
CS22885-096 5050 2.60 2.99 −3.78 −3.65 1.8 −1.44 −1.47
CS22891-209 4700 1.00 1.71 −3.29 −3.05 2.1 0.15 −0.01
CS22892-052 4850 1.60 2.14 −3.03 −2.85 1.9 0.41 0.20
CS22896-154 5250 2.70 3.03 −2.69 −2.58 1.2 0.32 0.18
CS22897-008 4900 1.70 2.30 −3.41 −3.21 2.0 −0.39 −1.20
CS22948-066 5100 1.80 2.46 −3.14 −2.92 2.0 0.46 0.25
CS22952-015 4800 1.30 2.02 −3.43 −3.19 2.1 −0.96 −1.07
CS22953-003 5100 2.30 2.68 −2.84 −2.71 1.7 0.13 −0.01
CS22956-050 4900 1.70 2.29 −3.33 −3.13 1.8 −0.47 −0.42
CS22966-057 5300 2.20 2.72 −2.62 −2.45 1.4 −0.33 −0.44
CS22968-014 4850 1.70 2.31 −3.56 −3.36 1.9 −1.81 −1.69
CS29491-053 4700 1.30 1.85 −3.04 −2.86 2.0 −0.15 −0.36
CS29495-041 4800 1.50 1.98 −2.82 −2.66 1.8 −0.23 −0.36
CS29502-042 5100 2.50 2.87 −3.19 −3.07 1.5 −1.99 −2.05
CS29516-024 4650 1.20 1.74 −3.06 −2.88 1.7 −0.47 −0.61
CS29518-051 5200 2.60 2.93 −2.69 −2.58 1.4 0.06 −0.07
CS30325-094 4950 2.00 2.47 −3.30 −3.14 1.5 −2.38 −2.47
HD2796 4950 1.50 1.95 −2.47 −2.32 2.1 −0.23 −0.24
HD186478 4700 1.30 1.74 −2.59 −2.44 2.0 0.05 −0.12
Notes. (FS) ThesynthesisedLTEabundancescanbefoundintheFirstStarspapers;DwarfsfromBonifacioetal.(2009,B09),andgiantsfrom
Franc¸oisetal.(2007,F07)
10. As discussed in Bergemannet al. (2012a),the modelatom of NLTE abundance corrections. For the resonance Srii line at
by Andrievskyet al. (2011)is less complete than ours, and we 4077Å,ourNLTEcorrectionsaremildlynegativeforanylogg
incorporatednew atomic data, which influences the magnitude andT at[Fe/H]= −3,whereasAndrievskyetal. (2011)ob-
eff
8
C.J.Hansenetal.:Srabundancesinlate-typestars
2
Syn, LTE 1
1
e] 0 0
F
[Sr/ −1 dwarfs Fe]
giants Sr/ −1
−2 comparison dwarfs [
comparison giants
−3
−4 −3 −2 −1 0 −2
NLTE
[Fe/H]
LTE
−3
−5 −4 −3 −2 −1 0
2 [Fe/H]
LTE
1
Fig.10: Difference in [Sr/Fe] ratios between NLTE (blue dia-
e] 0 monds)andLTE(opencircles).
F
[Sr/ −1 dwarfs
giants
−2 comparison dwarfs et al. (2012)and Lugaroet al. (2012) that extend down to low
comparison giants metallicitiesof[Fe/H]=-2.3.
−3
−4 −3 −2 −1 0 TheyieldsfromWanajoetal.(2011)describetheejectafrom
[Fe/H] low-mass (∼ 9M⊙) faint core-collapse electron-capture super-
novae(ECSN).These mayoccurfrequentlyevenatlowmetal-
licity (e.g. Langer 2012), and their yields can therefore not be
2
neglectedwhenconsideringtheevolutionofSr. Theamountof
NLTE
1 Sr injected into the interstellar medium (ISM) is, based on the
self-consistent two-dimensional ECSN model of Wanajo et al.
Fe] 0 (2011),1.79·10−4M⊙,whichisobtainedfromtheejectawithan
[Sr/ −1 dwarfs etinotnro(pYy)rabnegtwinegenfr0o.m4a1n0dt0o.5265.kTBh/ibsacroyrornesapnodndasntoeltehcetrsotannfdraarcd-
giants e
−2 comparison dwarfs yield(Wstand).LowYevaluesareexpectedinlow-massprogeni-
comparison giants torsduetotheirfastexplosions.Convectivebubblesexpandfast
−3
enoughtoinhibittheneutrinoreactionthatincreasestheelectron
−4 −3 −2 −1 0
fraction.However,owingtotheself-consistencyoftheexplosion
[Fe/H]
modelthere are no free parametersin the simulations once the
progenitormodel and physicsinputare chosen. The uncertain-
Fig.9: Upper figure: LTE Sr synthesised (Syn) abundances for tiesthereforestem fromthesystematicuncertaintiesassociated
F07, B09 (open circles), and our sample (filled circles - giants with the input to the SN model and possible resolution limita-
big, blue; dwarfs small, red). The figure in the middle shows tions, but mainly from the progenitor mass range we adopt in
theEW-basedabundancesforthesamestars.Lowerfigure:The theGCEmodels,whichiswhatgivesrisetoWlowandWhigh.
same samples but with NLTE corrected stellar parameters,and HerewehaveselectedtwomassintervalsforWlowandWhigh
NLTEcorrectedEW-basedSrabundances. thatarerepresentativeoftheSNmassusedfortheseSNmodel
calculations(seealsoLanger(2012);Nomoto(1987)).
Neutrino-driven winds following immediately after super-
tain large positive corrections for dwarfs and negative ∆NLTE
novaexplosions,alsofrommoremassiveprogenitors,willlike-
forgiants.
wise contribute to the amountof Sr in the ISM. The wind pre-
dictions we have incorporated here are based on the compu-
5. Discussion—ChemicalevolutionofSr tations presented in Arcones & Montes (2011). Here we have
tested the impact that wind parameters, such as entropy, elec-
Since we wish to assess the impact of the LTE assumption vs tron fraction, and expansion time scale, have on the neutron-
NLTE on the chemical evolution of Sr, we selected a handful capture nucleosynthesis in the wind. The effect of progenitor
of Sr yields, covering both s- and r-process contributions. We massandprogenitormetallicityremainsanopenquestion,since
probed how LTE vs NLTE abundances behave in a Galactic thesupernovamodelsstillhaveuncertaintiesthataretoolargeto
chemical evolution scheme. The yields will briefly be outlined constrain these quantities. Nonetheless, we note that since this
below. process is a primary process, the impact of metallicity is not
the most important factor when trying to constrain the yields.
Assuming a neutron-rich wind with the electron fraction con-
5.1.TheoreticalpredictionsofstellarSryields
strictedto0.4< Y <0.49(ascurrentlysuggestedbyMart´ınez-
e
Here we consider the weak r-process yields from Arcones & Pinedo et al. 2012;Roberts & Reddy 2012;Roberts 2012),we
Montes(2011)andWanajoetal.(2011),andthes-processyields cantrytolooselyconfinesomeparameterstorealisticrangesby
fromBisterzo et al. (2010),Frischknechtet al. (2012),and our settinganentropyintervalof50to150k /baryonandawindex-
B
ownAGByieldsbasedonthecalculationspresentedinKarakas pansiontimescalelimitedtoafewmilliseconds.Thesearetyp-
9
C.J.Hansenetal.:Srabundancesinlate-typestars
icalvaluesfoundinhydrodynamicalwindsimulations(Arcones Because the goal of this section is to check for the broad im-
et al. 2007;Fischer et al. 2010)and lead to Sr yields spanning pact of the different stellar yields on the chemical enrichment,
10−4 −10−7M , where the largestcontributioncomes from12 weadoptahomogeneouschemicalevolutionmodeltoguideour
⊙
– 25 M supernovae, and the smallest yield could be assigned discussion.Forthatweusethehalochemicalevolutionmodelof
⊙
to 8 – 12 M SN explosions. Generally speaking, the smaller Chiappinietal.(2008),whichwebrieflysummarisehere:
⊙
Sr yields can be assigned to low-mass progenitors,while more 1)theinfalllawoftheprimordialgasfollowsaGaussianfunc-
massiveonesproducelargerSryields. tion,
The first s-process yield we consider here are those of 2) the occurringoutflowfrom the system is proportionalto the
Bisterzo et al. (2010), who provide the yields from AGB stars starformationrate(SFR).
inthemassrange1.3−2M atdifferentmetallicities([Fe/H]):0, Thetimescalefortheformationofthehaloisfast(lessthan
⊙
-0.8,-1.6,and-2.6(S.Bisterzo,priv.comm.).Theseyieldshave 0.5 Gyr). The other crucial element for our study is iron, for
been calculated with the FRANEC (Frascati Phapson-Newton whichwe adoptthepredictionsfromWoosley& Weaver1995,
EvolutionaryCode)thatusesreactionratesfromtheKADoNiS (WW95). As shown in Cescutti & Chiappini (2010),this com-
and NACRE databases. The neutron-capture elements are cre- binationofparametersisabletoproduceasyntheticmetallicity
ated in 13C pockets and brought to the stellar surface during distribution function,which is in good agreementwith the one
thermal pulses. The AGB star experiences a mass loss in the observed in the Galactic halo (Lai et al. 2008; Scho¨rck et al.
range10−4−10−7M /yrwheretheintervalbetweensuccessive 2009).Asaresultweassumethatthismodelfollowsthecorrect
⊙
thermalpulseslastsforabout104−105 yearsdependingonthe timescaleforthechemicalenrichment.
AGBcoremass(e.g.,seemodeldatapublishedinKarakasetal. WehavenotrunindividualmodelsforeachsetofAGBnu-
2012 or Cristallo et al. 2011). The impact at low metallicity cleosynthesis prediction. The reason is that, even though our
([Fe/H]<−2.6),whichweareinterestedin,isminorandwould chemical evolution model calculates the enrichmentfrom low-
need to be extrapolated. However, yields from low-mass AGB massAGBstarsandSNeIa,onlythemoremassivestars(SNII)
starsareimportantathighermetallicities. canbeconsidereddriversoftheearlychemicalenrichmentinthe
We also use AGB yields based on the nucleosynthesis cal- halo.Thisisduetotheconsiderablelongertimescalesneededto
culations presented in Karakas et al. (2012) and Lugaro et al. evolve AGBs and SNe Ia. We have only tested the metal-poor
(2012).These calculationsuse reaction rates from the NACRE yields for Sr (i.e. from the metal-poor extension to the yields
and JINA databases, which includes the KADoNIS neutron- from Lugaroet al. 2012),for which we have predictionsup to
capturecrosssections.IncomparisontotheyieldsfromBisterzo 6M stars(comparedtothe2−3M fromBisterzoetal.2010;
⊙ ⊙
etal. 2010,thesespanarangefrom0.9M to6M at[Fe/H]= Cristalloetal.2011).ThemassiveAGBstarsaremorelikelyto
⊙ ⊙
-2.3;athighermetallicitieswecoveramassrangefrom1.25M contribute to the chemical enrichment during the formation of
⊙
to 6M at [Fe/H] = -0.15 and 5M to 8M at [Fe/H] = +0.14 thehalo,i.e.atlowermetallicities.Nevertheless,theenrichment
⊙ ⊙ ⊙
(slightly super solar). For the heaviest of these stars, the main producedbythe6M AGBstarsdonotinfluencetheoverallre-
⊙
neutronsourceisthe22Ne(α,n)25Mgreaction,anda13Cpocket sults.Onlywhenextremelylowyieldsfromevenmoremassive
hasbeenintroduced.However,forthelowestmassstars,wein- stars(SNII)areadopted(lowerthanWlow),isitpossibletosee
troducesome partialmixingofprotonsatthe deepestextent of theinfluenceoftheheavyAGByields.
eachthirddredge-upepisodeinordertoforma 13Cpocket(see Wehavecomputeddifferentchemicalevolutionmodelswith
detaileddiscussioninLugaroetal.2012).Formostmodelsthe differentnucleosynthesisassumptions,namely:
mass-lossrateusedontheAGBisthesemi-empiricallyderived
Vassiliadis&Wood(1993)masslossformulae.Oneofthema- – Case 1) Wanajo low (Wlow): 1.79·10−4M⊙ in the range
joruncertaintiesin AGB modellingis the mass-loss rate, espe- 9.5-10M⊙;
cially at the lowest metallicities, and for this reason we exper-
imentwith variationsto themassloss inthe intermediate-mass – Case 2) Wanajo high (Whigh): 1.79·10−4M⊙ in the range
AGBmodels.WerefertothediscussionsinKarakas(2010)and 8-10M⊙;
Karakasetal.(2012)formoredetails.
Thes-processyieldsofmassiverapidrotatingstarsarefrom – Case 3) Arcones & Montes low (AMlow): 10−7M⊙ mass
Frischknecht et al. (2012); these results are more recent than range8-25M⊙;
Pignatarietal.(2008),andFrischknechtetal.(2012)basedtheir
yield predictions on a stellar evolution code. The code used is – Case4)Arcones&Monteshigh(AMhigh):10−4M⊙ inmass
the Geneva stellar evolution code (GENEC), with reaction li- range8-25M⊙3;
brariesfromREACLIB, and neutroncapturesfromKADoNiS.
Moreover, the results by Frischknecht et al. (2012) are based – Case 5) Frischknecht low (Flow) with rotation and their
on the still favouredreaction rate for 17O(α,γ) by Caughlan & ’standard’ value for the reaction rate, generalising the pro-
Fowler (1988). We explore the uncertainty linked to this reac- ductionofthe25M⊙ intheirpapertoarangeof15-40M⊙;
tion rate decreasing it by a factor of 10. In this way we can
generate upper and lower limits for what we might expect as – Case 6) Frischknecht high (Fhigh) calculated as Flow but
Sryieldsfromthesemassiverotatingstars.Ingeneraltheabove- with a decreasedvalueforthe reaction 17O rate (producing
mentioneds-processyieldsareintherange10−6−10−9M .(For higherSryields)
⊙
comparisonthenon-rotatingstarsbarelyproduceanySr.)
In Fig. 11a) (top) we show the models Wlow and Whigh,
which differin the level of Sr enrichmentby the ECSN due to
5.2.TheimpactofdifferentstellaryieldsonGalactic the supernovamass used in the GCE model. The high level of
chemicalevolution(GCE)models Srproductioninanarrowrangeofmassesproducesinthecase
We now show a chemical evolution model computed with the 3 Arcones &Montes standard (Astand): 10−4M inmass range 12-
⊙
different assumptions for the stellar yields discussed above. 25M and10−7M inmassrange8-12M andM>25M .
⊙ ⊙ ⊙ ⊙
10