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We derive the effective temperatures and gravities of 461 OB stars in 19young clusters by fitting the H-gamma profile in their spectra. We usesynthetic model profiles for rotating stars to develop a method to estimate thepolar gravity for these stars, which we argue is a useful indicator of theirevolutionary status. We combine these results with projected rotationalvelocity 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 bythe theories for the evolution of rotating stars. Furthermore, we find that themembers of binary stars also experience a marked spin down with advancedevolutionary state due to tidal interactions. We also derive non-LTE-correctedhelium abundances for most of the sample by fitting the He I 4026, 4387, 4471lines. A large number of helium peculiar stars are found among cooler starswith Teff < 23000 K. The analysis of the high mass stars (8.5 solar masses < M< 16 solar masses) shows that the helium enrichment process progresses throughthe main sequence (MS) phase and is greater among the faster rotators. Thisdiscovery supports the theoretical claim that rotationally induced internalmixing is the main cause of surface chemical anomalies that appear during theMS phase. The lower mass stars appear to have slower rotation rates among thelow 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 starsare probably pre-main sequence objects that will spin up as they contract.These young objects very likely host a remnant magnetic field from their natalcloud, and these strong fields sculpt out surface regions with unusual chemicalabundances.Comment: 50 pages 18 figures, accepted by Ap

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