THE GALACTIC DISTRIBUTION OF CHEMICAL-ELEMENTS AS DERIVED FROM B-STARS IN OPEN CLUSTERS .1. NGC-6231 - STELLAR PARAMETERS AND CHEMICAL ABUNDANCES

The chemical composition of the open cluster NGC 6231 has been analysed as part of a study, in which we examine the size of the galactic metallicity gradient based on spectroscopic analysis of young clusters in the galactic plane. Stellar parameters (effective temperature, gravity, microturbulence) and chemical abundances of 10 unevolved B-stars in NGC 6231 are given. Line blanketed LTE model atmospheres and NLTE line formation calculations were used for the abundance determination of C, N, O, Mg, Al, and Si; LTE line formation calculations were used for the abundance determination of Ne, S, and Fe. All elements except neon show an average underabundance of 0.1-0.3 dex with respect to the solar value, which is in good agreement with results obtained earlier for B-type field stars and B-stars in young associations of the solar neighbourhood. Among the cluster stars abundance differences of 0.2-0.5 dex may be observed, which appear to be incompatible with estimated abundance determination errors under the assumption of complete mixing of primordial matter from which the cluster stars were formed.