Rotation and chemical abundances of Ap/Bp stars in the open cluster NGC 6475

Abstract

The connection between age, rotation and chemical abundance of magnetic Ap stars is poorly understood. Using open clusters, we are able to study samples of stars that are both co-eval and co-environmental. By determining rotation and chemical abundance for Ap star members of clusters with various ages, the variations of these properties as a function of age and environment can be derived. All four probable Ap star members of the open cluster NGC 6475, as well as one normal late B star, were studied using detailed spectrum synthesis of high resolution UVES-POP spectra. Probable cluster membership was confirmed for all five stars, however chemical abundance anomalies only appear to be present in spectra of three. Projected rotational velocity and chemical abundances for 21 elements ranging from C to Eu are presented for the 5 stars. In the three peculiar stars we find overabundances of Si, Cr, Mn, Fe and rare earths such as Nd, characteristic of Ap stars. The set of chemically peculiar stars show fairly homogeneous abundance tables, however notable differences exist for a few elements. There also exist appreciable differences in the v sin i and main sequence evolutionary stage of the chemically peculiar stars. This may hint at the underlying processes giving rise to the observed abundance anomalies. With this first detailed study of chemical abundances of a complete sample of magnetic Ap/Bp stars in an open cluster, we have initiated an exploration of the environmental and evolutionary influence on chemical peculiarity.