Chemical Abundances and Rotation Velocities of Blue Horizontal‐Branch Stars in Six Globular Clusters

High-resolution spectroscopic measurements of blue horizontal-branch stars insix metal-poor globular clusters -- M3, M13, M15, M68, M92, and NGC 288 --reveal remarkable variations in photospheric composition and rotation velocityas a function of a star's position along the horizontal branch. For the coolerstars (Teff < 11200 K), the derived abundances are in good agreement with thecanonical cluster metallicities, and we find a wide range of v sin i rotationvelocities, some as high as 40 km/s. In the hotter stars, however, most metalspecies are strongly enhanced, by as much as 3 dex, relative to the expectedcluster 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 anomalousabundances appear to be due to atomic diffusion mechanisms -- gravitationalsettling of helium, and radiative levitation of metals -- in the non-convectiveatmospheres of these hot stars. We discuss the influence of these photosphericmetal enhancements on the stars' photometric properties, and explore possibleexplanations for the observed distribution of rotation velocities.Comment: 77 pages, 27 figures, accepted for November 2003 publication in ApJ