Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance

Abstract

We derive the effective temperatures and gravities of 461 OB stars in 19 young clusters by fitting the Hγ profile in their spectra. We use synthetic model profiles for rotating stars to develop a method to estimate the polar gravity for these stars, which we argue is a useful indicator of their evolutionary status. We combine these results with projected rotational velocity measurements obtained in a previous paper on these same open clusters. We find that the more massive B stars experience a spin-down as predicted by the theories for the evolution of rotating stars. Furthermore, we find that the members of binary stars also experience a marked spin-down with advanced evolutionary state due to tidal interactions. We also derive non-LTE-corrected helium abundances for most of the sample by fitting the He I λλ4026, 4387, 4471 lines. A large number of helium peculiar stars are found among cooler stars with Teff<23,000 K. The analysis of the high-mass stars (8.5 Msolar