Atmospheric models for luminous blue variables

The photospheres and stellar winds of luminous blue variables are studied theoretically. Non-LTE calculations of radiation transfer in spherically extended, expanding atmospheres are combined with a hydrodynamics code for radiation-driven winds. Models are calculated which are typical for luminous blue variables in minimum and maximum states. The recombination of iron group elements from double to singly ionized stages, which occurs when the effective temperature falls below 10,000 K, can explain why the mass-loss rates increase when luminous blue variables approach their maximum states; the singly charged ions provide a much larger number of strong lines which can intercept the radiation pressure. However, the mass-loss increase by itself cannot account for the drop of the effective temperature in a self-consistent way. The corresponding increase of the wind opacity causes only a comparativley small change in the radius of continuum formation. It is concluded that the observed variability of the photospheric radius must be induced from deeper, subphotospheric regions.