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.