Rotation and abundance anomalies in blue horizontal-branch stars

Abstract

To address the puzzling photometric properties of horizontal branch stars in Galactic GCs, several different groups have undertaken detailed spectroscopic analyses of individual blue HB stars. Hotter BHB stars show strong metal enhancement and helium depletion, likely due to atomic diffusion, and slow rotation velocities, in contrast to the cooler BHB stars, which have a bimodal distribution of rotation speeds -- some of them much faster than expected -- but no anomalous abundances. I review the observational results to date, and discuss possible explanations and ramifications of these abundance and rotation characteristics.