EDP Sciences
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Volume 430, Number 2, February I 2005
Page(s) L41 - L44
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:200400129

A&A 430, L41-L44 (2005)
DOI: 10.1051/0004-6361:200400129


The ionization structure of early-B supergiant winds

R. K. Prinja1, D. Massa2 and S. C. Searle1

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 )

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 ( ${\hbox{$\skew3\dot M$ }}\,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 ${\hbox{$\skew3\dot M$ }}\,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 qi(w) values integrated over the wind, $\langle{q_i}\rangle$. These $\langle{q_i}\rangle$ are quite small, never exceeding 15% for Al III or 2% for Si IV. This is surprising, since the $\langle{q_i}\rangle$ for these ions clearly peak within the observed spectral range. We conclude that the low $\langle{q_i}\rangle$ arise because the $\langle{\hbox{$\skew3\dot M$ }}\,q_i\rangle$ are underestimated by the wind models, which assume that the outflows are smooth when they are, in fact, highly structured.

Key words: stars: early-type -- stars: mass-loss -- ultraviolet: stars

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