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 ( T _ { eff } \sim 21kK ) from Lamers et al .
is rather a more gradual trend ( with large scatter ) from v _ { \infty } / v _ { esc } \sim 3.4 for B0–0.5 supergiants above 24kK to v _ { \infty } / v _ { esc } \sim 2.5 for B0.7–1 supergiants with 20kK \leq T _ { eff } \leq 24kK , and v _ { \infty } / v _ { esc } \sim 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 H \alpha 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 .