The ionization structure of early-B supergiant winds

Abstract

We present empirically determined ionization conditions for the winds of 106 luminous B0 to B5 stars observed by IUE. The UV wind lines are modelled to extract products of mass-loss rates (Mdot) times ionization fractions (Mdot q_i(w), where w = v/v_infty) for N V, C IV, Si IV, Si III, Al III and C II. We describe the general behaviour of the Mdot q_i(w) and their ratios, demonstrating that the wind ionization increases with distance from the star, contrary to recent findings for O star winds. Using empirical mass-loss rates (from H alpha observations) and model prescriptions, we derive mean q_i(w) values integrated over the wind, <q_i>. These <q_i> are quite small, never exceeding 15% for Al III or 2% for Si IV. This is surprising, since the <q_i> for these ions clearly peak within the observed spectral range. We conclude that the low <q_i> arise because the <Mdot q_i> are underestimated by the wind models, which assume that the outflows are smooth when they are, in fact, highly structured.