The Projected Rotational Velocity Distribution of a Sample of OB stars from a Calibration Based on Synthetic He i Lines

We derive projected rotational velocities ( v sin i ) for a sample of 156 Galactic OB star members of 35 clusters, H II regions, and associations. The He I lines at 4026, 4388, and 4471 Å were analyzed in order to define a calibration of the synthetic He I full widths at half-maximum (FWHMs) versus stellar v sin i . A grid of synthetic spectra of He I line profiles was calculated in non-LTE using an extensive helium model atom and updated atomic data. The v sin i values for all stars were derived using the He I FWHM calibrations, but also, for those target stars with relatively sharp lines, v sin i values were obtained from best-fit synthetic spectra of up to 40 lines of C II, N II, O II, Al III, Mg II, Si III, and S III. This calibration is a useful and efficient tool for estimating the projected rotational velocities of O9-B5 main-sequence stars. The distribution of v sin i for an unbiased sample of early B stars in the unbound association Cep OB2 is consistent with the distribution reported elsewhere for other unbound associations.