Present-day cosmic abundances. A comprehensive study of nearby early B-type stars and implications for stellar and Galactic evolution and interstellar dust models

摘要

Aims. A sample of early B-type stars in OB associations and the field withinthe solar neighbourhood is studied comprehensively. Present-day abundances forthe astrophysically most interesting chemical elements are derived. Methods.High-resolution and high-S/N spectra of early B-type stars are analysed inNLTE. Atmospheric parameters are derived from the simultaneous establishment ofindependent indicators, from multiple ionization equilibria and the hydrogenBalmer lines. Results. Teff is constrained to 1-2% and logg to less than 15%uncertainty. Absolute values for metal abundances are determined to better than25% uncertainty. The synthetic spectra match the observations reliably overalmost the entire visual spectral range. Conclusions. A present-day cosmicabundance standard is established. Our results i) resolve the discrepancybetween a chemical homogeneous local gas-phase ISM and a chemicallyinhomogeneous young stellar component, ii) facilitate the amount of heavyelements locked up in the interstellar dust to be constrained precisely:carbonaceous dust is largely destroyed inside the Orion HII region, unlike thesilicates, and that graphite is only a minority species in interstellar dust -,iii) show that the mixing of CNO-burning products in the course of massive starevolution follows tightly the predicted nuclear path, iv) provide reliablepresent-day reference points for anchoring Galactic chemical evolution modelsto observation, and v) imply that the Sun has migrated outwards from the innerGalactic disk over its lifetime from a birthplace at a distance around 5-6 kpcfrom the Galactic Centre; a cancellation of the effects of Galactic chemicalevolution and abundance gradients leads to the similarity of solar andpresent-day cosmic abundances in the solar neighbourhood, with a telltalingsignature of the Sun's origin left in the C/O ratio. (ABRIDGED)