Rotation Velocities of Red and Blue Field Horizontal-Branch Stars

Abstract

We present measurements of the projected stellar rotation velocities (v sin i) of a sample of 45 candidate field horizontal-branch (HB) stars spanning a wide range of effective temperature, from red HB stars with Teff ~ 5000 K to blue 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 of stars 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 our sample 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 enhanced photospheric metal abundances and helium depletion, similar to the abundance patterns found among hot BHB stars in globular clusters, and four other stars that may also belong in this category. We discuss details of the spectral line fitting procedure that we use to deduce v sin i, and explore how measurements of field HB star rotation may shed light on the issue of HB star rotation in globular clusters.