2000A&A...353..655K

In recent years, B stars have been established as reliable tracers of present day abundance patterns. They yield an independent set of abundances from He to Fe which can be cross-checked against other sources such as HII regions and cool supergiants. In this paper, we present purely spectroscopic analyses of 9 non-supergiant B stars (13<m_V<15), 5 of which located in the young populous clusters NGC 1818, 2004 (LMC) and NGC 330 (SMC), the other 4 being MC field stars. CASPEC spectra (4000Å<λ<5000Å) with R∼20000 and S/N∼100 are used to determine T_ eff_, logg, ξ and the abundances of oxygen and silicon simultaneously. Particular attention is paid to a consistent treatment of the metallicity of the underlying atmosphere. Kurucz ATLAS9 LTE atmospheres and DETAIL/SURFACE non-LTE line formation (H, He, C, N, O, Mg, Al, Si and Fe) are used throughout the analysis. While we find little difference between the abundances from cluster members and field stars, the behaviour of N is surprising: it is generally enriched with respect to the HII value by 0.6 to 1.0dex. In the case of NGC1818/D1 (the only spectroscopic main sequence star in our sample), this enrichment is coupled with a depletion of C and an enrichment of He, strongly suggesting that these abundances have already been altered through some kind of mixing with CNO-processed material.