2007AJ....133.1092W

We present the results of a study aimed at assessing the differences in the distribution of rotation speeds N(vsini) among young (1-15 Myr) B stars spanning a range of masses 6 M_☉<M<12 M_☉and located in different environments: seven low-density (ρ<1 M_☉/pc³) ensembles that are destined to become unbound stellar associations, and eight high-density (ρ≫1 M_☉/pc³) ensembles that will survive as rich, bound stellar clusters for ages well in excess of 10⁸ yr. Our results demonstrate (1) that independent of environment, the rotation rates for stars in this mass range do not change by more than 0.1 dex over ages t∼1 to ∼15 Myr; and (2) that stars formed in high-density regions lack the cohort of slow rotators that dominate the low-density regions and young field stars. We suggest that the differences in N(vsini) between low- and high-density regions may reflect a combination of initial conditions and environmental effects: (1) the higher turbulent speeds that characterize molecular gas in high-density, cluster-forming regions; and (2) the stronger UV radiation fields and high stellar densities that characterize such regions. Higher turbulent speeds may lead to higher time-averaged accretion rates during the stellar assembly phase. In the context of stellar angular momentum regulation via ``disk-locking'', higher accretion rates lead to both higher initial angular momenta and evolution-driven increases in surface rotation rates as stars contract from the birth line to the zero-age main sequence (ZAMS). Stronger UV radiation fields and higher densities may lead to shorter disk lifetimes in cluster-forming regions. If so, B stars formed in dense clusters are more likely to be ``released'' from their disks early during their pre-main-sequence lifetimes and evolve into rapid rotators as they conserve angular momentum and spin up in response to contraction. By contrast, the majority of their brethren in low-density, association-forming regions can retain their disks for much or all of their pre-main-sequence lifetimes, are ``locked'' by their disks to rotate at constant angular speed, and lose angular momentum as they contract toward the ZAMS, and thus arrive on the ZAMS as relatively slowly rotating stars.