Ion Fractions and the Weak Wind Problem
Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, United Kingdom
Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, United Kingdom
Abstract
Some late-type O stars are observed to display anomalously weak winds. This issue and the uncertainty about the nature of wind clumping are challenges to line-driven wind theory and need resolving in order to fully understand hot stars. We describe the results from the computation of ion fractions for the various elements in O star winds using the non-LTE code CMFGEN, including parameterisations of microclumping and X-rays. Ion fractions can also be derived from fits to UV wind lines if a mass-loss rate is assumed. We discuss a project to fit unsaturated CIV lines in late O dwarfs, and show that the weak wind scenario could still be a major issue, with some values of M<qCIV> indicated to be more than an order of magnitude lower than theoretical predictions of mass-loss, and measurements from radio and optical diagnostics.