Austin, M.J.; (2011) The nature and structure of the winds of galactic O stars. Doctoral thesis , UCL (University College London).

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Abstract

O stars are highly important objects in their host galaxies due to the impact of their powerful stellar winds and strong flux output. The details of their physics and evolution as individual entities and as populations have important consequences for understanding the stars themselves and their environments. Two major issues are currently challenging the successful theory of line-driven winds; that the winds are clumped and not smooth or homogeneous, and that some objects appear to have weaker mass-loss than prescribed by theory; in the case of O supergiants by a factor of a few, and for late O dwarfs by perhaps and order of magnitude or more. The key goal of this thesis is to exploit sophisticated model atmosphere calculations to explore the effects of wind-clumping and X-rays due to wind shocks in O stars in the Milky Way. The consequences for the spectral line profiles produced and the ionization balance are explored in detail. The level to which clumping and X-rays affect the observations of different classes of O star is further constrained. IUE observations of weak-wind O stars (those at spectral types O6.5-9.5 V) are examined in light of the ion balance determinations, and comment is made about the current state of the weak-wind problem in the Galaxy. This involves empirical line-synthesis matches to Civ and Nv line profiles. The influence of X-rays is found to be key in the mass-loss issue through altering the ion balance. Finally the wind-formed sulphur line profiles of a small sample of O type supergiants are fitted using model atmospheres. Sulphur is important in these O stars since the model atmospheres predict the majority of it to be found in three adjacent ion stages that all have corresponding wind lines in the ultraviolet and far-ultraviolet. When moderate clumping in the winds is assumed, the mass-loss rates are found to be approximately in line with estimates based on density-squared diagnostics, such as H�-alpha.

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