1997A&A...320..196A

We investigate the effects of radiation pressure on the atmospheres of A, F and G-supergiants by calculating hydrodynamical model atmospheres for stars with 5500≤T_eff≤9500K. In the subsonic part of the wind, the radiation pressure by continuum and lines from Kurucz (1992, ATLAS 6 program) is taken into account. In the supersonic part of the wind, the radiation pressure is expressed in terms of the force multiplier formalism (Castor et al. 1975ApJ...195..157C) with the correction for the finite disk taken into account. The temperature structure is from the T(τ) relation of blanketed model atmospheres. The predicted mass loss rates of the A-supergiants agrees excellently with the observed values. However the predicted terminal velocities are about a factor 3 higher than observed. We discuss several possible causes for this discrepancy. The most likely one is a change in the force multiplier parameter α of the line radiation force from about 0.5 in the lower parts of the wind to a much smaller value of about 0.1 throughout most of the wind. This might be the result of a change in the ionization of the wind with distance, or a decoupling of the line driven ions in the wind from the ambient gas. The predicted mass loss rate of the G-type supergiant 22Vul, which is the only G-supergiant with a reliable mass loss rate, is a factor 10⁵ smaller than observed. This is probably due to the fact that G-supergiants have chromospheres, which were not taken into account in our model. Our models for F-supergiants could not be compared with observations because there are no reliable empirical mass loss rates or terminal wind velocities for normal F-supergiants. The F-supergiants ρCas and HR8752 have highly variable mass loss rates which obviously cannot be explained by our models. We conclude that mass loss from A-type supergiants is most likely due to a line driven wind but that the mass loss from G-supergiants is not. It is interesting to find the spectral type between F0 and G3 where the radiation driven wind models break down and to compare that with the type where the chromospheres become noticeable. The high opacity in the hydrogen ionization zone produces a net outward force in those layers. This gives rise to a pressure inversion in the subsonic part of the atmosphere, but does not lead to high mass loss rates.