Terminal Velocities and the Bistability of Stellar Winds

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Abstract

The terminal velocity, υ∞, of the stellar winds of 117 stars of spectral types 0 through F has been measured from the spectra in the new Atlas of Ultraviolet P Cygni Profiles by Snow et al. For the 68 stars with the most accurate determination of υ∞ we studied the relation between υ∞ and the photospheric escape velocity, V/_esc. The ratio υ/_∞/ decreases from 2.58±0.20 in O-stars to 0.72±0.15 in A and F stars. There is a clear discontinuity in this ratio near T_eff ≃ 21,000 K at spectral type B1, where the ratio drops by almost a factor 2 from υ_∞/υ_esc ≃ 2.6 at the high-temperature side υ_∞/υ_esc ≃ 1.3 at the low-temperature side, and possibly a smaller discontinuity near T_eff ≃ 10,000 K. The discontinuity near 21,000 K is due to the change in the nature of the lines that produce the radiation pressure. At T_eff ≃ 21,000 K the radiation pressure is due to high- ionization lines which are largely optically thick, whereas at T_eff ≃ 21,000 K the radiation pressure is due to very large numbers of low-ionization lines, which are more optically thin, Near T_eff ≃ 21,000 K the winds can exist in two states: winds with high mass-loss rates and low velocities, and winds with low mass-loss rates and high velocities. This is the bistability of stellar winds for stars with T_eff ≃ 21,000 K. We present evidence that the bistability may produce a jump in the mass-loss rates of the stars near T_eff ≃ 21,000 K with the higher mass-loss rates at the low temperature side of the jump. We briefly discuss the possible role of bistability in the formation of disks around B[e]-supergiants. The observed values of υ/_∞ are used to derive empirical values of the force multiplier parameter α_emp that can be used to predict the terminal velocity of any early-type star with an accuracy of better than 5% with the method described in Appendix B, if the stellar parameters are known.

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