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Astron. Astrophys. 350, 181-196 (1999)

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On the nature of the bi-stability jump in the winds of early-type supergiants

J.S. Vink 1, A. de Koter 2,3 and H.J.G.L.M. Lamers 1

1 Astronomical Institute, Utrecht University, P.O.Box 80000, 3508 TA Utrecht, The Netherlands
2 Astronomical Institute `Anton Pannekoek', University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
3 Advanced Computer Concepts, Code 681, Goddard Space Flight Center, Greenbelt, MD 20771, USA

Received 26 April 1999 / Accepted 28 July 1999

Abstract

We study the origin of the bi-stability jump in the terminal velocity of the winds of supergiants near spectral type B1. Observations show that here the ratio [FORMULA] drops steeply from about 2.6 at types earlier than B1 to a value of [FORMULA]=1.3 at types later than B2. To this purpose, we have calculated wind models and mass-loss rates for early-type supergiants in a [FORMULA] grid covering the range between [FORMULA] and [FORMULA]. These models show the existence of a jump in mass loss around [FORMULA] for normal supergiants, with [FORMULA] increasing by about a factor five from [FORMULA] to 22 500 K for constant luminosity. The wind efficiency number [FORMULA] also increases drastically by a factor of 2 - 3 near that temperature.

We argue that the jump in mass loss is accompanied by a decrease of the ratio [FORMULA], which is the observed bi-stability jump in terminal velocity. Using self-consistent models for two values of [FORMULA], we have derived [FORMULA] = 2.4 for [FORMULA] = 30 000 K and [FORMULA] = 1.2 for [FORMULA] = 17 500 K. This is within 10 percent of the observed values around the jump.

Up to now, a theoretical explanation of the observed bi-stability jump was not yet provided by radiation driven wind theory. To understand the origin of the bi-stability jump, we have investigated the line acceleration for models around the jump in detail. These models demonstrate that [FORMULA] increases around the bi-stability jump due to an increase in the line acceleration of Fe IIIbelowthe sonic point . This shows that the mass-loss rate of B-type supergiants is very sensitive to the abundance and the ionization balance of iron.

Furthermore, we show that the elements C, N and O are important line drivers in the supersonic part of the wind. The subsonic part of the wind is dominated by the line acceleration due to Fe. Therefore, CNO-processing is expected not to have a large impact on [FORMULA] but it might have impact on the terminal velocities.

Finally, we discuss the possible role of the bi-stability jump on the mass loss during typical variations of Luminous Blue Variable stars.

Key words: radiative transfer – stars: early-type – stars: mass-loss – stars: supergiants – stars: winds, outflows

Send offprint requests to: Jorick S. Vink (j.s.vink@astro.uu.nl)

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© European Southern Observatory (ESO) 1999

Online publication: September 24, 1999
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