Rotation velocities of red and blue field horizontal-branch stars.
Abstract (from CDS):
We present measurements of the projected stellar rotation velocities (vsini) of a sample of 45 candidate field horizontal-branch (HB) stars spanning a wide range of effective temperatures, from red HB stars with Teff≃5000K to blue HB stars with Teffof 17,000K. Among the cooler blue HB stars (Teff=7500-11500 K), we confirm prior studies showing that, although a majority of stars rotate at vsini<15km/s, there exists a subset of ``fast rotators'' with vsini as high as 30-35 km/s. All but one of the red HB stars in our sample have vsini<10km/s, and no analogous rotation bimodality is evident. We also identify a narrow-lined hot star (Teff≃16,000K) 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 vsini and explore how measurements of field HB star rotation may shed light on the issue of HB star rotation in globular clusters.