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Issue A&A
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

Letter

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 )

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 ( ${\hbox{$\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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© ESO 2005


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