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

TL;DR

This study uses the FASTWIND code to analyze Galactic B-supergiants, revealing that their temperature scale should be revised downward and providing new insights into wind properties and mass loss rates across the bi-stability region.

Contribution

It offers the first detailed spectroscopic analysis of B-supergiants using NLTE models, revising temperature scales and examining wind parameters in comparison with theoretical predictions.

Findings

Temperature scale of B-supergiants is 10-20% lower than previously thought.

Wind terminal velocities decrease over the bi-stability region, with limits at lower temperatures.

Mass loss rate varies less than predicted, indicating a complex wind behavior.

Abstract

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. As a result we find that 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 percent, the latter value being appropriate for stronger winds. In fair accordance with recent results, our sample furthermore indicates a gradual decrease in wind terminal velocities over the bi-stability region, where the limits of this region are located at lower temperatures than the predicted ones. Introducing a distance-independent quantity Q' related to wind-strength, we also show that this quantity is a well defined, monotonically increasing function of Teff outside this region. Inside and from hot to cool, the mass loss rate changes by a factor (in between 0.4 and 2.5) which is (much) smaller than the predicted factor of 5. All this indicates that the decrease in wind terminal velocity over the bi-stability region is not over-compensated by an increase of mass loss rate, as frequently argued (provided the wind-clumping properties on both sides of this region do not differ substantially).