Multipleindependentspectroscopicindicatorswereconsid-eredsimultaneouslyforthedeterminationoftheatmosphericparameters,effectivetemperatureTeffandsurfacegravitylogg:allStark-broadenedBalmerlinesand4–6ionization
A representative sample of unevolved early B-type stars in nearby OB associations and the field is analysed to high precision using NLTE techniques. The resulting chemical composition is found to be more metal-rich and much more homogeneous than indicated
2PRZYBILLAETAL.
TABLE1
STELLARPARAMETERS&ELEMENTALABUNDANCES
HR6165
HR3055
HR1861
HR2928
HR3468
HR5285
ε(He)a10.99±0.05(20)10.94±0.05(16)10.99±0.05(14)10.99±0.05(14)10.99±0.05(14)10.99±0.05(13)ε(CII)b8.27±0.14(13)8.35±0.08(10)8.32±0.10(19)8.28±0.08(18)8.36±0.10(17)8.32±0.08(20)ε(CIII)b8.31±0.11(17)
8.30±0.05(7)
8.36±0.03(11)
8.27±0.02(5)
8.47±0.04(2)
8.42±0.06(2)
ε(CIV)b8.34(2)8.45(2)···
···
···
···
ε(NII)c8.16±0.12(73)
7.77±0.08(23)
7.75±0.09(61)8.00±0.12(61)7.92±0.10(56)7.76±0.08(47)ε(OI)d···
···
8.82±0.03(3)8.83±0.05(5)8.82±0.03(7)8.79±0.05(7)ε(OII)e8.77±0.08(51)8.79±0.10(41)
8.74±0.11(52)8.74±0.09(46)8.80±0.09(40)8.71±0.05(45)ε(NeI)f8.12±0.05(2)···
8.12±0.08(9)8.11±0.09(9)8.05±0.09(10)8.07±0.07(14)
ε(NeII)f8.14±0.07(16)8.07±0.07(8)8.08±0.09(14)8.03±0.12(8)8.06±0.03(2)···
ε(MgII)g7.62±0.03(3)
7.60±0.01(2)
7.58±0.10(6)7.56±0.03(3)7.51±0.10(6)7.50±0.05(4)ε(SiII)h···
···
7.47±0.17(2)7.56±0.08(2)7.51±0.10(5)7.22±0.13(6)ε(SiIII)h7.50±0.08(8)7.48±0.08(6)7.46±0.11(9)7.52±0.11(8)7.53±0.17(7)7.29±0.05(9)
ε(SiIV)h7.50±0.04(10)
7.51±0.18(5)
7.50±0.08(3)
7.48±0.14(2)
7.50±0.04(2)
···ε(FeII)i···
···
···
···
7.38(1)7.38(1)ε(FeIII)j
7.38±0.12(17)7.49±0.12(5)7.44±0.09(33)7.48±0.10(30)7.42±0.12(36)7.40±0.09(32)
A representative sample of unevolved early B-type stars in nearby OB associations and the field is analysed to high precision using NLTE techniques. The resulting chemical composition is found to be more metal-rich and much more homogeneous than indicated
ACOSMIC
ABUNDANCESTANDARD
3
FIG.2.—Comparisonofchemicalabundancestudies(NLTE)ofB-typestarsinthesolarneighbourhood.Redbars:presentwork;fulllineandthickfullline(forthesamestarsasinourwork):Kilian(1992,1994);dotted:Cunha&Lambert(1994),Cunhaetal.(2006)forNe;short-dashed:Gies&Lambert(1992),ex-cludingbrightgiantsandsupergiants;long-dashed:Da onetal.(1999,2001a,b,2003);dot-dashed:Morel&Butler(2008);triple-dot-dashed:Lyubimkovetal.(2004,2005).Binwidthisσ/2oftheindividualstudies.IronabundanceswerederivedassumingLTEinallpreviouswork.Solarabundancesarealsoindicated(⊙,GS98&AGS05:lowervalues).ThepanelforcarbonisreproducedfromNP08forcompleteness.Seethetextfordetails.
andSo a&Meyer(2001,SM01,seeTable2)toderiveareferencecomposition,inevitablyresultinginsub-solaraver-agevaluesandalargermsscatter.Theformerdiscrepancyhassincebeenlargelyremovedfromare-evaluationofso-larabundances(AGS05).However,thestatusquointermsofGalactochemicalevolutioncanonlybeunderstoodbyin-vokingand ne-tuningextraprocessessuchasinfall/out owofmaterialandlocalretentionofsupernovaproductsbylargeamounts.
Ontheotherhand,oursampleofearlyB-starsimpliesahighdegreeofhomogeneityforelementalabundancesinthesolarneighbourhood,withascatterof～10%,andabsolutevaluesofaboutsolar(GS98and/orAGS05,seeFig.2andTable2)6.TheonlyexceptionisN,whichismostsensitivetomixingoftheatmosphericlayerswithCN-processedmate-rial(e.g.Maeder&Meynet2000).InthiscasethepristineNabundancemaybeindicatedbythe3objectswiththelowestvalue,implyingapristineN/Cratioof0.31±0.05(bymass;errorbaradjustedtore ectadditionaluncertainties).
Althoughoursampleissmall,weregarditasrepresentativefortheearlyB-starpopulationinthesolarneighbourhood.ThestarssampletherelevantportionoftheH-burningphaseoftheobjectsintheHRDintermsofTeffandlogg(seeFig.1).Theyalsosampleonehemisphereofthesolarneighbourhood(insetofFig.1),halfofthemlocatedinOBassociationsandtheotherhalfinthe eld.All6starswereanalyzedbyKilian(1992,1994),whichweregardoneofthemostaccuratepre-viousstudiesintermsofstellarparameterandabundancede-termination.Kilian’svaluesforthe6starstypicallyspantheentireabundancerangeinhersampleof21stars(seeFig.2).Wethereforealso ndachanceselectionofstarswithsimilarchemicalcompositionforoursampleunlikely.Thisissup-portedfurtherbyacontrolsampleof6BA-typesupergiants(BA-SGs,Fig.1),forwhichmeanvaluesofε(O)=8.80±0.02andε(Mg)=7.55±0.07werederivedusingthesameanalysismethodologyasappliedhere(Przybillaetal.2006;Firnstein