High S / N Echelle spectroscopy in young stellar groups ? II . Rotational velocities of early - type stars in Sco OB 2

We investigate the rotational velocities of early-type stars in the Sco OB2 association. We measure vsini for 156 established and probable members of the association. The measurements are performed with three different techniques, which are in increasing order of expected vsini: 1) converting the widths of spectral lines directly to vsini, 2) comparing artificially broadened spectra of low vsini stars to the target spectrum, 3) comparing the He i λ4026 line profile to theoretical models. The sample is extended with literature data for 47 established members of Sco OB2. Analysis of the vsini distributions shows that there are no significant differences between the subgroups of Sco OB2. We find that members of the binary population of Sco OB2 on the whole rotate more slowly than the single stars. In addition, we find that the B7–B9 single star members rotate significantly faster than their B0–B6 counterparts. We test various hypotheses for the distribution of vsini in the association. The results show that we cannot clearly exclude any form of random distribution of the direction and/or magnitude of the intrinsic rotational velocity vector. We also investigate the effects of rotation on colours in the Walraven photometric system. We show that positions of B7–B9 single dwarfs above the main sequence are a consequence of rotation. This establishes the influence of rotation on the Walraven colours, due primarily to surface gravity effects.