CAOSP abstracts, Volume: 38, No.: 2, year: 2008

Abstract: We discuss the presence of magnetic peculiar stars in open clusters on the basis of photometric identification, in a sample that covers not only the solar neighbourhood but also wider galactic regions. Our study includes the possible influence of metallicity and/or other environmental properties. The compactness of a cluster is investigated as a parameter in Ap-star formation. For that purpose we use the classical p-m-r "richness" groups defined by Trumpler (1930), together with approximate cluster volumes derived from a recent study of cluster diameters by van den Bergh (2006). Basing our study on a sample of 80 galactic clusters with published Δa photometry, we notice (a) that the Trumpler richness groups correspond to distinctly separated mean cluster diameters (2.0/3.4/4.9 pc) and (b) that the resulting densities (Ap stars per pc³) point overwhelmingly to the p group as the principal domain of Ap stars. A special feature is the prevailing appearance of p-group Ap stars alone in the inner galactic quadrants, unaccompanied by parallel behaviour of the m and r groups. Whether or not this is a metallicity effect has to be substantiated by the including additional clusters to improve the statistics. It does seem to be the case in the Large Magellanic Cloud, according to a study of a photometric sample of Ap stars, though the frequency is clearly lower on account of the lower metallicity of this satellite galaxy.