Rotation Velocities of Red and Blue Field Horizontal‐Branch Stars

We present measurements of the projected stellar rotation velocities (v sini) of a sample of 45 candidate field horizontal-branch (HB) stars spanning awide range of effective temperature, from red HB stars with Teff ~ 5000 K toblue HB stars with Teff of 17000 K. Among the cooler blue HB stars (Teff =7500-11500 K), we confirm prior studies showing that although a majority ofstars rotate at v sin i < 15 km/s, there exists a subset of "fast rotators"with v sin i as high as 30-35 km/s. All but one of the red HB stars in oursample have v sin i < 10 km/s, and no analogous rotation bimodality is evident.We also identify a narrow-lined hot star (Teff ~ 16000 K) with enhancedphotospheric metal abundances and helium depletion, similar to the abundancepatterns found among hot BHB stars in globular clusters, and four other starsthat may also belong in this category. We discuss details of the spectral linefitting procedure that we use to deduce v sin i, and explore how measurementsof field HB star rotation may shed light on the issue of HB star rotation inglobular clusters.Comment: 47 pages, 9 figures, 5 tables, accepted for publication in November 2003 ApJ