A cosmic abundance standard chemical homogeneity of the solar neighbourhood & the ISM dust-

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

DRAFTVERSIONSEPTEMBER14,2008

APreprinttypesetusingLTEXstyleemulateapjv.03/07/07

ACOSMICABUNDANCESTANDARD:

CHEMICALHOMOGENEITYOFTHESOLARNEIGHBOURHOOD&THEISMDUST-PHASECOMPOSITION1

NORBERTPRZYBILLA2,M.FERNANDANIEVA2,3

AND

KEITHBUTLER4

DraftversionSeptember14,2008

arXiv:0809.2403v1 [astro-ph] 14 Sep 2008

ABSTRACT

ArepresentativesampleofunevolvedearlyB-typestarsinnearbyOBassociationsandthe eldisanalysedtohighprecisionusingNLTEtechniques.Theresultingchemicalcompositionisfoundtobemoremetal-richandmuchmorehomogeneousthanindicatedbypreviouswork.Armsscatterof～10%inabundancesisfoundforthesample,thesameasreportedforISMgas-phaseabundances.Acosmicabundancestandardforthepresent-daysolarneighbourhoodisproposed,implyingmassfractionsforhydrogen,heliumandmetalsofX=0.715,Y=0.271andZ=0.014.Goodagreementwithsolarphotosphericabundancesasreportedfromrecent3Dradiative-hydrodynamicalsimulationsofthesolaratmosphereisobtained.Asa rstapplicationweusethecosmicabundancestandardasaproxyforthedeterminationofthelocalISMdust-phasecomposition,puttingtightobservationalconstraintsondustmodels.

Subjectheadings:stars:abundances—stars:early-type—stars:fundamentalparameters—ISM:abundances

—dust,extinction—solarneighbourhood

1.INTRODUCTION

TheSunisuniqueamongthestarsbecauseindependentin-dicatorsallowitschemicalcompositiontobeconstrainedwithaprecisionunmatchedforanyotherstar.Thiscanbedonebyspectroscopicanalysisofitsphotosphereandbymeasure-mentofsolarwindandsolarenergeticparticles.SolarnebulaabundancescanbedeterminedfromCIchondrites,whichareunalteredsincetheformationofthesystem.ThewealthofinformationestablishedtheSunastheprincipalstandardforthechemicalcompositionofcosmicmatter(e.g.Grevesse&Sauval1998,GS98;Holweger2001;Asplundetal2005,AGS05).However,isa4.6Gyroldstarindeedrepresentativeofthecosmicmatterinitsneighbourhood5atpresent?

IdealindicatorsforpristineabundancesareunevolvedearlyB-starsofspectraltypesB0–B2.Slowlyrotatingstarsarepre-ferredastheirphotospheresshouldbeessentiallyunaffectedbymixingofCN-processedmaterial2000).TheatmospheresofearlyB-starsarealsounaf-fectedbyatomicdiffusionthatgivesrisetopeculiaritiesofmetalabundancesinmanylater-typestars(e.g.Smith1996).Amajorpracticaladvantageisalsotheirrelativelysimplephotosphericphysics,whichisrepresentedwellbyclassicalmodelatmospheres,unaffectedbycomplicationssuchasstel-larwindsorconvection.

Asaconsequence,earlyB-starsinthesolarneighbour-hoodweresubjectofseveralNLTEstudiesinthepast(e.g.Gies&LambertKilian1994;Da onetal.1999,2004,2005).Overall,theyfoundawiderangeofabundances,byaboutafactor～10,andanaveragemetallicityofonly～2/3solar(GS98).Hence,theimpressionarosethatthesolarneighbourhoodischemicallyhighlyheterogeneous,andthe

Electronicaddress:przybilla@sternwarte.uni-erlangen.de

1BasedonobservationsobtainedattheEuropeanSouthernObservatory,proposal074.B-0455(A).

2Dr.Remeis-Observatory,Sternwartstr.7,D-96049Bamberg,Germany3MPIforAstrophysics,Postfach1317,D-85741Garching,Germany4UniversityObservatory,Scheinerstr.1,D-86179Munich,Germany

5Weconsidertheregionatdistancesshorterthan～1kpc(and±500pcinGalactocentricdirection)assolarneighbourhoodinordertominimizebiasduetoGalacticabundancegradients,seeFig.1foraschematicoverview.

Sunanomalouslymetal-richcomparedtoyoungstars.

Both ndingsareproblematicintermsofGalacticchem-icalevolution.Dispersalofstellarnucleosynthesisproductsincreasesthemetallicityovertime(e.g.Chiappinietal.andhydrodynamicmixingtendstohomogenizetheinterstel-larmedium(ISM)locallyCharacteristictimescalesforhomogenizationareshort,rangingfrom106–108yrsonscalesof100-1000pcIncontrasttotheyoungstarstheinterstellargasshowsahighdegreeofchemicalhomogeneityinthesolarneighbour-hood(So a2004),withthermsscatterofmeanabundancesoftenbeinglessthan～10%.However,theISMgasphaseisnotsuitableasatracerforcosmicabundancesbecauseofse-lectivedepletionofelementsontodustgrains.Herewerein-vestigatetheconundrumofinhomogeneousstellarvs.homo-geneousISMgas-phaseabundancesinthesolarneighbour-hood,motivatedbyourprevious ndingofhomogeneousB-starabundancesforcarbon2008,NP08).

2.SAMPLEANALYSIS

Sixbrightandapparentlyslow-rotatingearlyBstarsinthesolarneighbourhood–randomlydistributedinOBassocia-tionsandinthe eld,andcoveringawiderangeofstellarpa-rameters–wereobservedinearly2005atESO/LaSilla,usingFEROSonthe2.2mtelescope.Spectrawithbroadwave-lengthcoverageandresolvingpowerλ/ λ≈48000wereobtained,atveryhigh-S/N(upto～800intheB-band).

Thequantitativeanalysisofthesamplestarswascar-riedoutfollowingthehybridNLTEapproachdiscussedbyNP07)andNP08.Inbrief,line-blanketedLTEmodelatmosphereswerecomputedwithAT-LAS91993)andNLTEline-formationcalculationswereperformedusingupdatedversionsofDETAILandSUR-FACEButler&State-of-the-artmodelatomswereadopted(seeTablewhichallowatmosphericparametersandelementalabundancestobeob-tainedwithhighaccuracy.

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

HR5285isexcludedfromthesiliconmeanabundance–asymmetricsil-iconlinepro les(otherspeciesareunaffected)indicatenon-radialpulsationswhichmayberelatedtothesiliconpeculiarity.

6

2006).Thewideabundancerangesfoundinpreviousworkre ecttheloweraccuracyoftheanalyses,whileshiftsoftheabundancedistributionsrelativetoeachotherre ectsystem-atics,withdifferenttemperaturescalesbeingthemostimpor-tantamongthese.

The ndingofchemicalhomogeneityforoursampleisinexcellentaccordancewithresultsfromtheanalysisoftheISMgas-phaseinthesolarneighbourhood(So a2004,andreferencestherein)andwiththeoryregardingtheef -ciencyofhydrodynamicmixingintheISM(Edmunds1975;Roy&Kunth1995).ExcellentagreementisalsoobtainedwithelementalabundancesintheOrionnebula(Estebanetal.2004,E04,seeTable2),withtheexceptionofC,whichmaybeaconsequenceoftheatomicdatausedintheOrionanal-ysis(seeNP08forthestellarcase)plusoverestimateddustcorrections.

Inthefollowingwebrie yinvestigatetheimpactofthiscosmicabundancestandardonimportanttopicsofcontem-poraryastrophysics.

4.THECOSMICABUNDANCESTANDARD,SOLARABUNDANCES

&THEDUST-PHASECOMPOSITION

Ingeneral,excellentagreementofourB-starabundanceswithsolarvaluesfromrecent3Dradiative-hydrodynamicalsimulationsofthesolaratmosphere(AGS05)isobtained.TheoxygenvaluefallsbetweenGS98andAGS05values(seealsoCaffauetal.2008)andneoniscompatiblewithGS98.As-sumingourOandNeabundancestobealsorepresentativefortheSun,thiscouldlargelyresolvethediscrepanciesbetweenhelioseismicconstraintsandthesolarinteriormodelbasedonabundancesofAGS05asreviewedbyBasu&Antia(2008).OurcosmicabundancestandardalsofacilitatesaprecisedeterminationofdustdepletioninthelocalISMforthepri-maryconstituents.TheamountofmaterialincorporatedintodustgrainsisdeterminedbythedifferencebetweenourB-starabundancesandtheISMgas-phaseabundances,seeTable2.Accordingly,acompositionpoorincarbonbutrichinoxygenandrefractoryelementsisindicated.

Suchstudieswereundertakenpreviously,usinge.g.abun-dancesoftheSun,ofBstarsandofyoungF&Gstars(e.g.

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

4PRZYBILLAETAL.

TABLE2

CHEMICALCOMPOSITIONOFDIFFERENTOBJECTCLASSESINTHESOLARNEIGHBOURHOODANDOFTHESUN

Elem.

cosmicstandardBstars–thiswork

Oriongas+dustb

Bstarsc

YoungF&Gstarsc

ISMgas

ISMdustd

Sune/f

unitsoflog(El/H)+12/atomsper106Hnuclei–computedfromaveragestarabundances(meanvaluesoverallanalyzedlinesperelement);bE04;cSM01;ddifferencebetweenthecosmicstandardandISMgas-phaseabundances,inunitsofatomsper106Hnuclei;e/fGS98/AGS05,photosphericvalues;gSo a(2004);hMeyeretal.(1997),correctedaccordinglytoJensenetal.(2007);iCartledgeetal.(2004);jCartledgeetal.(2006)

ain

Snow&Witt1996,SM01,seeTable2)asproxiesforthedeterminationofthedust-phasecomposition,howeverwithmixedsuccess.Inparticular,Bstarswererejectedasreli-ableindicatorsasthederivedabundancesofmaterialindustatthattimeweretoolowtoproducetheobservedinterstellarextinction.OurstudyrevivesBstarsasproxiesoftheISMdust-phasecomposition,andevenmoresobecauseoftheex-tremelylowabundancescattercomparedtoallotherstandardsconsideredsofar,exceptfortheSun.

Thepresentresultsimplytightobservationalconstraintsondustmodelsintermsofcarbonabundance.Theobservedpropertiesofdustgrains,asinferredfromtheinterstellarex-tinctionlaw,havetobeproducedbyarathersmallamountofcarbon,posingachallengetomostdustmodels(seee.g.Snow&Witt1995).Wecancarryoutanimportantconsis-tencycheck,followingCartledgeetal.(2006):theOpre-dictedtobeincorporatedingrainsfromtheobservedMg,SiandFedustabundancesandarudimentarydustmodelagreeswiththederivedOdustabundancewithinthemutual(small)uncertainties.Fortherudimentarydustmodelweassumesil-icatestobepredominantlyMgSiO3,withonlyasmallfrac-tionofFeboundinsilicatesandonlyasmallfractionbeingofolivine-likecomposition.TheremainingMgandFefrac-tionisconsideredtobeinoxideform(MgO,FeO,Fe2O3,

Asplund,M.,etal.2005,ASPConf.Ser.,336,25(AGS05)Basu,S.,&Antia,H.M.2008,Phys.Rep.,457,217Becker,S.R.1998,ASPConf.Ser.,131,137Becker,S.R.,&Butler,K.1988,A&A,201,232Becker,S.R.,&Butler,K.1990,A&A,235,326

Butler,K.&Giddings,J.R.1985,inNewsletterofAnalysisofAstronomicalSpectra,No.9(Univ.London)

Caffau,E.,Ludwig,H.-G.,Steffen,M.,etal.2008,A&A,488,1031Cartledge,S.I.B.,Lauroesch,J.T.,etal.2004,ApJ,613,1037Cartledge,S.I.B.,Lauroesch,J.T.,etal.2006,ApJ,641,327

Chiappini,C.,Romano,D.,&Matteucci,F.2003,MNRAS,339,63Cunha,K.,&Lambert,D.L.1994,ApJ,426,170

Cunha,K.,Hubeny,I.,&Lanz,T.2006,ApJ,647,L143Da on,S.,Cunha,K.,&Becker,S.R.1999,ApJ,522,950

Da on,S.,Cunha,K.,Becker,S.R.,&Smith,V.V.2001a,ApJ,552,309Da on,S.,Cunha,K.,Butler,K.,&Smith,V.V.2001b,ApJ,563,325Da on,S.,Cunha,K.,Smith,V.V.,&Butler,K.2003,A&A,399,525Draine,B.T.2003,ARA&A,41,241Edmunds,M.G.1975,Ap&SS,32,483

Esteban,C.,etal.2004,MNRAS,355,229(E04)

Firnstein,M.2006,DiplomaThesis,Univ.Erlangen-Nuremberg

FroeseFischer,C.,&Tachiev,G.2004,At.DataNucl.DataTables,87,1Giddings,J.R.1981,Ph.D.Thesis,UniversityofLondonGies,D.R.,&Lambert,D.L.1992,ApJ,387,673

Grevesse,N.,&Sauval,A.J.1998,SpaceSci.Rev.,85,161(GS98)

Grevesse,N.,Asplund,M.,&Sauval,A.J.2007,SpaceSci.Rev.,130,105Holweger,H.2001,AIPConf.Proc.,598,23

Jensen,A.G.,Rachford,B.L.,&Snow,T.P.2007,ApJ,654,955

Fe3O4),seee.g.Draine(2003)foradiscussionofobserva-tionalevidence.

Finally,wecombineourB-starabundanceswithdataforS,ClandArfromtheanalysisoftheOrionnebula(E04)andsolarmeteoriticvaluesforotherabundantrefractoryelements(withε(El) 5,AGS05)toderivemassfractionsforhydro-gen,heliumandthemetals.ValuesofX=0.715,Y=0.271,Z=0.014andZ/X=0.020characterizethepresent-daycos-micmatterinthesolarneighbourhood(tobecomparedtoprotosolarvaluesX0=0.7133,Y0=0.2735andZ0=0.0132,Grevesseetal.2007).Thesecombinedabundancesareourrecommendedvaluesforawiderangeofapplicationsrequir-inganaccurateknowledgeofthechemicalcompositionatpresent(e.g.foropacitycalculations),examplesbeingmod-elsofstar/planetformationorstellarevolution(inparticularofshort-livedmassivestars),orfortheempiricalcalibrationofGalactochemicalevolutionmodels.

WeexpressourdeepgratitudetoU.Heberforconstantsup-portoftheprojectandusefulcommentsonthemanuscript,andthankM.Asplundforstimulatingdiscussion.M.F.N.ac-knowledgessupportbyDFG(grantHE1356/45-1).

Kilian,J.1992,A&A,262,17Kilian,J.1994,A&A,282,867

Kurucz,R.L.1993,CD-ROM13(Cambridge:SAO)

Lyubimkov,L.S.,Rostopchin,S.I.,Lambert,D.L.2004,MNRAS,351,745Lyubimkov,L.S.,Rostopchin,S.I.,etal.2005,MNRAS,358,193Maeder,A.,&Meynet,G.2000,ARA&A,38,143

Meyer,D.M.,Cardelli,J.A.,&So a,U.J.1997,ApJ,490,L103Meynet,G.,&Maeder,A.2003,A&A,404,975Morel,T.,&Butler,K.2008,A&A,487,307

Morel,T.,Butler,K.,Aerts,C.,etal.2007,A&A,457,651Nieva,M.F.,&Przybilla,N.2006,ApJ,639,L39

Nieva,M.F.,&Przybilla,N.2007,A&A,467,295(NP07)Nieva,M.F.,&Przybilla,N.2008,A&A,481,199(NP08)Przybilla,N.2005,A&A,443,293

Przybilla,N.,&Butler,K.2001,A&A,379,955Przybilla,N.,&Butler,K.2004,ApJ,609,1181

Przybilla,N.,Butler,K.,Becker,S.R.,etal.2000,A&A,359,1085

Przybilla,N.,Butler,K.,Becker,S.R.,Kudritzki,R.P.2001,A&A,369,1009Przybilla,N.,Butler,K.,Becker,S.R.,Kudritzki,R.P.2006,A&A,445,1099Przybilla,N.,Nieva,M.F.,Heber,U.,Butler,K.2008,ApJ,684,L103Roy,J.-R.,&Kunth,D.1995,A&A,294,432Smith,K.C.1996,Ap&SS,237,77

So a,U.J.2004,ASPConf.Ser.,309,393

So a,U.J.,&Meyer,D.M.2001,ApJ,554,L221(SM01)Snow,T.P.,&Witt,A.N.1995,Science,270,1455Snow,T.P.,&Witt,A.N.1996,ApJ,468,L65

REFERENCES

本文来源：https://www.bwwdw.com/article/yn1m.html