Searle, SC; (2006) Spectroscopic analysis of the winds and atmospheres of galactic B supergiant stars. Doctoral thesis , UCL (University College London).

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Abstract

Uncertainties in the post main sequence evolution of B supergiant stars exist because their evolution is controlled by variable mass loss from the star as well as rotation, binarity and convection processes in the core. The latter effect leads to surface enrichment as the products of nuclear burning (carbon, nitrogen and oxygen) are brought to the surface. However, current stellar evolution models fail to predict the correct amount of CNO processing in massive stars. Accurate mass loss rates are essential for underpinning the Wind-momentum Luminosity Relation (Kudritzki et al., 1999) and also for improving stellar evolution calculations. Existing discrepancies between B star mass loss rates obtained from observations and those from theoretical predictions (e.g. Vink et al. 2000) also emphasise the need for a better understanding of structured stellar winds. Recent improvements in stellar atmosphere models to include full nLTE effects & line blanketing also provide us with the mean to derive more accurate temperatures & luminosities, leading to a reduction in OB star temperature scales. An optical and ultraviolet quantitative spectroscopic analysis of the atmospheres and winds of Galactic B supergiants is presented here. Fundamental parameters such as temperature, luminosity, mass loss rate and CNO abundances are derived for individual stars using the non-LTE, line-blanketed model atmosphere code of Hillier & Miller (1998). We present detailed temperature scales for B supergiants and discuss their implications. Additionally we discuss the derived mass loss rates and CNO abundances for our sample of Galactic B supergiants and compare them to other results. Empirical analysis of the ionisation conditions of early B supergiant winds has also been carried out (based on SEI modelling) and compared to model predictions from the stellar atmosphere code of Hillier & Miller (1998). This allows us to undertake a critical comparison of observed and predicted ionisation behaviour in the wind, focusing on trends of the ionisation fraction, qi, with velocity, relative ionisation strengths and whether ions tend to increase or decrease further out in the wind. Values of Mqi (the product of the mass loss rate times the ionisation fraction) have been obtained from UV line synthesis modelling and, using our derived mass loss rates, values of qi are acquired from Mqi for our sample of Galactic B supergiants. Our studies show that values of qi are much lower than expected and furthermore none of the ions (e.g. Al III, Si IV, C IV, N V) are dominant in the wind, a surprising result since the highest values of qi occur in the B spectral range. We discuss our most recent findings and their implications for clumping and structure in the wind. Most importantly these results provide strong evidence for a downward revision of mass loss rates by at least an order of magnitude. This evidence demonstrates a clear need to review mass loss rate determinations and the role of clumping in massive star winds.

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