Rotational velocity distributions of A-type stars

Using an homogeneous sample of v sin i values for A-type main sequence stars (Royer et al. 2002), the equatorial velocity (v) distributions are determined as function of spectral class, from B9 to F2. The chemically peculiar and binary stars are discarded. These distributions of "normal" stars are discussed in terms of stellar formation and evolution, in particular the remaining bimodality observed for the earliest spectral types of the sample. We show that late B and early A-type main-sequence stars have genuine bimodal distributions of true equatorial rotational velocities probably due to the phenomena of angular momentum loss and of redistribution the star underwent before reaching the main sequence. A striking lack of slow rotators is noticed among intermediate and late A-type stars. The bimodal-like shape of their true equatorial rotational velocity distributions could be due to evolutionary effects.