Bright OB stars in the Galaxy - IV. Stellar and wind parameters of early to late B supergiants

Context. B-type supergiants represent an important phase in the evolution of massive stars. Reliable estimates of their stellar and wind parameters, however, are scarce, especially at mid and late spectral subtypes. Aims. We apply the NLTE atmosphere code FASTWIND to perform a spectroscopic study of a small sample of Galactic B-supergiants from B0 to B9. By means of the resulting data and incorporating additional datasets from alternative studies, we investigate the properties of OB-supergiants and compare our findings with theoretical predictions. Methods. Stellar and wind parameters of our sample stars are determined by line profile fitting, based on synthetic profiles, a Fourier technique to investigate the individual contributions of stellar rotation and macro-turbulence and an adequate approach to determine the Si abundances in parallel with micro-turbulent velocities. Results. Due to the combined effects of line-and wind-blanketing, the temperature scale of Galactic B-supergiants needs to be revised downward, by 10 to 20%, the latter value being appropriate for stronger winds. Compared to theoretical predictions, the wind properties of OB-supergiants indicate a number of discrepancies. In fair accordance with recent results, our sample indicates a gradual decrease in v(infinity) over the bi-stability region, where the limits of this region are located at lower T-eff than those predicted. Introducing a distance-independent quantity Q' related to wind-strength, we show that this quantity is a well defined, monotonically increasing function of T-eff outside this region. Inside and from hot to cool, M changes by a factor (in between 0.4 and 2.5) which is much smaller than the predicted factor of 5. Conclusions. The decrease in v(infinity) over the bi-stability region is not over-compensated by an increase of M, as frequently argued, provided that wind-clumping properties on both sides of this region do not differ substantially.