SIMBAD references

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 v_∞/v_esc drops steeply from about 2.6 at types earlier than B1 to a value of v_∞/v_esc=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 T_eff grid covering the range between T_eff=12500 and 40000K. These models show the existence of a jump in mass loss around T_eff=25000K for normal supergiants, with {dot}(M) increasing by about a factor five from T_eff≃27500 to 22500K for constant luminosity. The wind efficiency number η={dot}(M)v_∞/(L_*/c) 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 v_∞/v_esc, which is the observed bi-stability jump in terminal velocity. Using self-consistent models for two values of T_eff, we have derived v_∞/v_esc=2.4 for T_eff=30000K and v_∞/v_esc=1.2 for T_eff=17500K. 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 {dot}(M) increases around the bi-stability jump due to an increase in the line acceleration of Fe III below the 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 {dot}(M)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.