Rotational velocities for B0-B3 stars in seven young clusters: Further study of the relationship between rotation speed and density in star-forming regions

We present the results of a study aimed at assessing the differences in the distribution of rotation speeds N(v sin i) among young (1-15 Myr) Bstars spanning a range of masses 6 M-circle dot < M < 12 M-circle dot and located in different environments: seven low-density (rho < 1 M-circle dot pc(-3)) ensembles that are destined to become unbound stellar associations, and eight high-density (rho >> 1 M-circle dot pc(-3)) ensembles that will survive as rich, bound stellar clusters for ages well in excess of 10(8) 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 similar to 1 to similar 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(v sin i) between low-and high-density regions may reflect a combination of initial conditions and environmental effects: (1) the higher turbulent speeds that characterizemolecular 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.