A cosmic abundance standard: chemical homogeneity of the solar neighbourhood and the ISM dust-phase composition

A representative sample of unevolved early B-type stars in nearby OB associations and the field is analysed to unprecedented precision using NLTE techniques. The resulting chemical composition is found to be more metal-rich and much more homogeneous than indicated by previous work. A rms scatter of ~10% in abundances is found for the six stars (and confirmed by six evolved stars), the same as reported for ISM gas-phase abundances. A cosmic abundance standard for the present-day solar neighbourhood is proposed, implying mass fractions for hydrogen, helium and metals of X=0.715, Y=0.271 and Z=0.014. Good agreement with solar photospheric abundances as reported from recent 3D radiative-hydrodynamical simulations of the solar atmosphere is obtained. As a first application we use the cosmic abundance standard as a proxy for the determination of the local ISM dust-phase composition, putting tight observational constraints on dust models.Comment: 4 pages, 2 figures. Accepted for publication in ApJ Letter

Mixing of CNO-cycled matter in massive stars

Aims: We test predictions of evolution models on mixing of CNO-cycled products in massive stars from a fundamental perspective. Relative changes within the theoretical C:N:O abundance ratios and the buildup of helium are compared with observational results. Methods: A sample of well-studied Galactic massive stars is presented. High-quality optical spectra are carefully analysed using improved NLTE line-formation and comprehensive analysis strategies. The results are put in the context of the existing literature data. Results: A tight trend in the observed N/C vs. N/O ratios and the buildup of helium is found from the self-consistent analysis of main-sequence to supergiant stars for the first time. The catalytic nature of the CNO-cycles is confirmed quantitatively, though further investigations are required to derive a fully consistent picture. Our observational results support the case of strong mixing, as predicted e.g. by evolution models that consider magnetic fields or by models that have gone through the first dredge-up in the case of many supergiants.Comment: 6 pages, 6 figures. A&A, in pres

The VLT-FLAMES Tarantula Survey XV. VFTS 822: A candidate Herbig B[e] star at low metallicity

We report the discovery of the B[e] star VFTS 822 in the 30 Doradus star-forming region of the Large Magellanic Cloud, classified by optical spectroscopy from the VLT-FLAMES Tarantula Survey and complementary infrared photometry. VFTS 822 is a relatively low-luminosity (log L = 4.04 ± 0.25 L⊙) B8[e] star. In this Letter, we evaluate the evolutionary status of VFTS 822 and discuss its candidacy as a Herbig B[e] star. If the object is indeed in the pre-main sequence phase, it would present an exciting opportunity to spectroscopically measure mass accretion rates at low metallicity, to probe the effect of metallicity on accretion rates

Quantitative spectroscopy of Galactic BA-type supergiants. I. Atmospheric parameters

BA-type supergiants show a high potential as versatile indicators for modern astronomy. The focus here is on the determination of accurate and precise atmospheric parameters for a sample of 35 Galactic BA-type supergiants. Some first applications include a recalibration of functional relationships between spectral-type, intrinsic colours, bolometric corrections and effective temperature, and an exploration of the reddening-free Johnson Q and Str\"omgren [c_1] and beta-indices as photometric indicators for effective temperatures and gravities of BA-type supergiants. An extensive grid of theoretical spectra is computed based on a hybrid non-LTE approach. The atmospheric parameters are derived spectroscopically by line-profile fits to high-resolution and high-S/N spectra obtained at various observatories. Ionization equilibria of multiple metals and the Stark-broadened H and the neutral He lines constitute our primary indicators for the parameter determination, supplemented by (spectro-)photometry. Data on Teff, logg, helium abundances, microturbulence, macroturbulence and rotational velocities are presented. The interstellar reddening and the ratio of total-to-selective extinction towards the stars are determined. Our empirical spectral-type-Teff scale is steeper than reference relations, the stars are significantly bluer, and bolometric corrections differ significantly from established literature values. Photometric Teff-determinations based on the reddening-free Q-index are found to be of limited use for studies of BA-type supergiants because of large errors of typically +-5%+-3% (1sigma statistical, 1sigma systematic), compared to a spectroscopically achieved precision of 1-2%. The reddening-free [c_1]-index and beta on the other hand are found to provide useful starting values for further analyses, with uncertainties of +-1%+-2.5% in Teff, and +-0.04+-0.13dex in log g. [abriged]Comment: 18 pages, 18 figures; A&

Quantitative Spektroskopie von BA-Typ Überriesen in unserer Galaxis

Überriesen der Spektraltypen B und A zählen zu den visuell hellsten Objekten in unserer Galaxis. Im Rahmen dieser Studie wurden Fundamentalparameter und photosphärische Elementhäufigkeiten für 35 dieser Sterne spektroskopisch bestimmt. Die Ergebnisse erlauben Rückschlüsse sowohl auf die Evolution massereicher Sterne als auch auf den aktuellen Entwicklungsstand unserer Galaxie, beides im Rahmen moderner astrophysikalischer Modelle.Supergiants of spectral type B and A are among the visually brightest objects in galaxies. As such, they show high potential as versatile indicators for stellar and galactic physics. High quality spectra of 35 of these stars were analyzed in this study, adopting a state-of-the-art non-LTE technique. Fundamental stellar parameters and photospheric abundances of key elements were determined, which in turn could be compared to current models of stellar and Galactic evolution

Quantitative spectroscopy of OB stars: from dwarfs to supergiants

We discuss recent progress made in the spectral modelling of OB stars from the main sequence to evolved phases as BA-type supergiants. Non-LTE line-formation computations can now reproduce observed spectra over the entire optical and near-IR wavelength range with high confidence, facilitating stellar atmospheric parameters and elemental abundances to be determined at high accuracy and precision. An overview is given how the fundamental stellar parameters of single stars determined in our new approach compare to high-precision data derived from detached eclipsing massive binary stars. Finally, the observational constraints for a sample of Galactic objects are put in context with state-of-the-art evolution models for rotating massive stars