## On the nature of the bi-stability jump in the winds of early-type supergiants
^{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
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 drops steeply from about 2.6 at types earlier than B1 to a value of =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 grid covering the range between and . These models show the existence of a jump in mass loss around for normal supergiants, with increasing by about a factor five from to 22 500 K for constant luminosity. The wind efficiency number 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 , which is the observed bi-stability jump in terminal velocity. Using self-consistent models for two values of , we have derived = 2.4 for = 30 000 K and = 1.2 for = 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 Furthermore, we show that the elements C, N and O are important
line drivers in the Finally, we discuss the possible role of the bi-stability jump on the mass loss during typical variations of Luminous Blue Variable stars.
## Contents- 1. Introduction
- 2. What determines and ?
- 3. The method to predict
- 4. The model atmospheres
- 5. The predicted bi-stability jump
- 6. The origin of the bi-stability jump
- 7. Bi-stability and the variability of LBV stars
- 8. Summary, discussion, conclusions and future work
- Acknowledgements
- References
© European Southern Observatory (ESO) 1999 Online publication: September 24, 1999 |