arXiv:0902.2949v1 [astro-ph.SR] 17 Feb 2009 Hot And Cool: Bridging Gaps in Massive Star Evolution ASP Conference Series, Vol. xxx, 2009 C. Leitherer, Ph. D. Bennett, P. W. Morris & J. Th. van Loon, eds. Improving stellar parameter and abundance determinations of early B-type stars Mar´ ıa-Fernanda Nieva 1 & Norbert Przybilla 2 1 MPI for Astrophysics, Postfach 1317, 85741 Garching, Germany 2 Dr. Remeis Observatory, Sternwartstr. 7, 96049 Bamberg, Germany Abstract. In the past years we have made great efforts to reduce the sta- tistical and systematic uncertainties in stellar parameter and chemical abun- dance determinations of early B-type stars. Both the construction of robust model atoms for non-LTE line-formation calculations and a novel self-consistent spectral analysis methodology were decisive to achieve results of unprecedented precision. They were extensively tested and applied to high-quality spectra of stars from OB associations and the field in the solar neighbourhood, covering a broad parameter range. Initially, most lines of hydrogen, helium and carbon in the optical/near-IR spectral range were reproduced simultaneously in a con- sistent way for the first time, improving drastically on the accuracy of results in published work. By taking additional ionization equilibria of oxygen, neon, silicon and iron into account, uncertainties as low as ∼1% in effective tempera- ture, ∼10% in surface gravity and ∼20% in elemental abundances are achieved – compared to ∼5-10%, ∼25% and a factor ∼2-3 using standard methods. Several sources of systematic errors have been identified when comparing our methods for early B-type stars with standard techniques used in the nineties and also recently (e.g. VLT-FLAMES survey of massive stars). Improvements in automatic analyses are strongly recommended for meaningful comparisons of spectroscopic stellar parameters and chemical abundances (’observational con- strains’) with predictions of stellar and galactochemical evolution models. 1. Introduction Normal unevolved early OB-type stars of ∼8-20 M ⊙ are the objects with the simplest photospheric physics among the massive stars. They are unaffected by e.g. strong stellar winds like the hotter and more luminous stars or by convection and chromospheres like the cool supergiants. However, their spectral analysis turned out to provide inconclusive results in the past decades, i.e. too large uncertainties in basic stellar parameters and an overall enormous range in derived elemental abundances, posing a challenge to predictions of stellar and Galactochemical evolution models (see review by Przybilla 2008). In order to improve the quantitative analysis of these stars we have exhaus- tively updated the spectral modelling by constructructing robust model atoms for non-LTE line-formation calculations. In parallel, we have implemented a powerful self-consistent analysis technique, which brings numerous spectroscopic parameter and abundance indicators into agreement simultaneously. Our efforts have provided highly-promising results so far, i.e. a drastic reduction of statistical and systematic uncertainties in stellar parameters and 1