Physical Parameters and Wind Properties of Galactic Early B Supergiants

We present optical studies of the physical and wind properties, plus CNO chemical abundances, of 25 O9.5–B3 Galactic supergiants. We employ non-LTE, line blanketed, extended model atmospheres, which provide a modest downward revision in the effective temperature scale of early B supergiants of up to 1–2kK relative to previous non-blanketed results. The so-called ‘bistability jump’ at B1 (Teff ∼ 21kK) from Lamers et al. is rather a more gradual trend (with large scatter) from v∞/vesc ∼3.4 for B0–0.5 supergiants above 24kK to v∞/vesc ∼2.5 for B0.7–1 supergiants with 20kK≤ Teff ≤24kK, and v∞/vesc ∼1.9 for B1.5–3 supergiants below 20kK. This, in part, explains the break in observed UV spectral characteristics between B0.5 and B0.7 subtypes as discussed by Walborn et al. We compare derived (homogeneous) wind densities with recent results for Magellanic Cloud B supergiants and generally confirm theoretical expectations for stronger winds amongst Galactic supergiants. However, winds are substantially weaker than predictions from current radiatively driven wind theory, especially at mid-B subtypes, a problem which is exacerbated if winds are already clumped in the Hα line forming region. In general, CNO elemental abundances reveal strongly processed material at the surface of Galactic B supergiants, with mean N/C and N/O abundances 10 and 5 times higher than the Solar value, respectively, with HD 2905 (BC0.7 Ia) indicating the lowest degree of processing in our sample, and HD 152236 (B1.5 Ia) the highest.