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,
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