ar X iv : a st ro - p h / 03 07 17 8 v 1 9 J ul 2 00 3 Chemical Abundances and Rotation Velocities of Blue Horizontal - Branch Stars in Six Globular Clusters

High-resolution spectroscopic measurements of blue horizontal-branch stars in six metal-poor globular clusters —M3, M13, M15, M68, M92, and NGC 288 — reveal remarkable variations in photospheric composition and rotation velocity as a function of a star’s position along the horizontal branch. For the cooler stars (Teff < 11200 K), the derived abundances are in good agreement with the canonical cluster metallicities, and we find a wide range of v sin i rotation velocities, some as high as 40 km s. In the hotter stars, however, most metal species are strongly enhanced, by as much as 3 dex, relative to the expected cluster metallicity, while helium is depleted by 2 dex or more. In addition, the hot stars all rotate slowly, with v sin i < 8 km s. The anomalous abundances appear to be due to atomic diffusion mechanisms — gravitational settling of helium, and radiative levitation of metals — in the non-convective atmospheres of these hot stars. We discuss the influence of these photospheric metal enhancements on the stars’ photometric properties, and explore possible explanations for the observed distribution of rotation velocities. Subject headings: stars: horizontal-branch, stars: rotation, stars: abundances, globular clusters: general, globular clusters: individual (NGC 288, NGC 4590, NGC 5272, NGC 6205, NGC 6341, NGC 7078) Based on observations obtained at the W.M. Keck Observatory, which is operated jointly by the California Institute of Technology and the University of California. Present address: McDonald Observatory, RLM 15.308, University of Texas at Austin, Austin TX 78712, email [email protected]