Fundamental Parameters of B Supergiants

To understand the chemical evolution of galaxies, we need a deep understanding of the evolution of massive stars and their role in processing and delivering chemical elements. B-type supergiants represent a substantial population in this context. We present here initial results from non-LTE, line-blanketed stellar atmosphere modelling [1] of a large sample of B0-B5 supergiants (see Fig. 1). We focus in this report on revisions to the effective temperature scales; finding reasonable agreement with the temperatures of [2] for B1-B5 supergiants. For early type supergiants (B0-B1), the less luminous Ib's are hotter than Ia's as expected (excluding HD190603 (B1.5 Ia+) which is a hypergiant). The temperature discrepancy between B0 Ia's &, B0 Ib's is up to 2500 K, decreasing to 1000 K for B0.5 and remains as 1000 K or less for later B type supergiants. Whilst some early B type supergiants are slightly cooler compared to the Humphreys et al. temperatures [2], a few B0.5 Ia & Ib stars appear to be slightly hotter. This discrepancy requires further investigation, since recent OB supergiant revised temperature scales have shown a trend for temperatures to be slightly cooler [3] than previously thought from [2].Moreover, to improve our understanding of the evolution of massive stars and the galaxies that harbour them, we require more precise constraints on the effects of mass loss & rotation on their evolution. Mass loss rates have also been derived from H alpha profiles for the sample of B supergiants presented here (using the same non-LTE, line blanketed stellar atmosphere model [1]) and will be presented in [4], along with further details of this work. Estimates of CNO abundances from the non-LTE models are however included here (see Fig.2) and will be compared to predicted CNO abundances from evolutionary tracks. This will allow us to determine how much CNO processing has occurred in these stars and if they have gone through a "blue loop" i.e. evolved from a blue to red supergiant phase (during which the star undergoes convective mixing of its core materials) then returned to blue supergiant status. Such an effect is predicted for some massive stars by stellar evolution models [5]. Early indications suggest that the B supergiants in this sample are partially processed and therefore have not gone through a red supergiant phase.