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

N. Markova$^1$, J. Puls$^2$

1 - Institute of Astronomy, National Astronomical Observatory, Bulgarian Academy of Sciences, P.O. Box 136, 4700 Smolyan, Bulgaria
2 - Universitaets-Sternwarte, Scheinerstrasse 1, D-81679 Muenchen, Germany

$Context.$ 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 downwards, 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 vinf over the bi-stability region, where the limits of this
region are located at lower Teff than the predicted ones. Introducing a distance-independent quantity $Q'$ related to wind-strength, we show that this quantity is a well defined, monotonically increasing function of Teff $outside$ this region. $Inside$ and from hot to cool, Mdot 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 vinf over the bi-stability region is $not$ over-compensated by an increase of Mdot, as frequently argued, provided that wind-clumping properties on both sides of this region do not differ substantially.

Reference: A&A, in press
Status: Manuscript has been accepted