Letter to the Editor

The ionization structure of early-B supergiant winds

¹ Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK e-mail: [rkp;scs]@star.ucl.ac.uk
² SGT, Inc., Code 681.0, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA e-mail: massa@taotaomona.gsfc.nasa.gov

Received: 18 October 2004
Accepted: 15 December 2004

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 times ionization fractions (, where ) for N v, C iv, Si iv, Si iii, Al iii and C ii. We describe the general behaviour of the 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 Hα observations) and model prescriptions, we derive mean values integrated over the wind, . These are quite small, never exceeding 15% for Al iii or 2% for Si iv. This is surprising, since the for these ions clearly peak within the observed spectral range. We conclude that the low arise because the are underestimated by the wind models, which assume that the outflows are smooth when they are, in fact, highly structured.