2010ApJ...722..605H

Two recent observing campaigns provide us with moderate dispersion spectra of more than 230 cluster and 370 field B stars. Combining them and the spectra of the B stars from our previous investigations (∼430 cluster and ∼100 field B stars) yields a large, homogeneous sample for studying the rotational properties of B stars. We derive the projected rotational velocity Vsin i, effective temperature, gravity, mass, and critical rotation speed V_crit for each star. We find that the average Vsin i is significantly lower among field stars because they are systematically more evolved and spun down than their cluster counterparts. The rotational distribution functions of V_eq/V_crit for the least evolved B stars show that lower mass B stars are born with a larger proportion of rapid rotators than higher mass B stars. However, the upper limit of V_eq/V_crit that may separate normal B stars from emission-line Be stars (where rotation promotes mass loss into a circumstellar disk) is smaller among the higher mass B stars. We compare the evolutionary trends of rotation (measured according to the polar gravity of the star) with recent models that treat internal mixing. The spin-down rates observed in the high-mass subset (∼9 M_☉) agree with predictions, but the rates are larger for the low-mass group (∼3 M_☉). The faster spin-down in the low-mass B stars matches well with the predictions based on conservation of angular momentum in individual spherical shells. Our results suggest that the fastest rotators (that probably correspond to the emission-line Be stars) are probably formed by evolutionary spin-up (for the more massive stars) and by mass transfer in binaries (for the full range of B star masses).