Stellar rotation in young clusters. II. Evolution of stellar rotation and surface helium abundance

We derive the effective temperatures and gravities of 461 OB stars in 19 young clusters by fitting the H gamma 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 lambda lambda 4026, 4387, 4471 lines. A large number of helium peculiar stars are found among cooler stars with T-eff < 23, 000 K. The analysis of the high-mass stars (8.5 M circle dot < M < 16 M circle dot) shows that the helium enrichment process progresses through the main-sequence (MS) phase and is greater among the faster rotators. This discovery supports the theoretical claim that rotationally induced internal mixing is the main cause of surface chemical anomalies that appear during the MS phase. The lower mass stars appear to have slower rotation rates among the low-gravity objects, and they have a large proportion of helium peculiar stars. We suggest that both properties are due to their youth. The low-gravity stars are probably pre-main-sequence objects that will spin up as they contract. These young objects very likely host a remnant magnetic field from their natal cloud, and these strong fields sculpt out surface regions with unusual chemical abundances.