Radiation-driven winds of hot luminous stars. VIII. The bistable wind of the luminous blue variable P Cygni (B1 Ia+).

A systematic investigation of the basic properties of objects located close to the Humphreys-Davidson limit at the top of the HR diagram is carried out taking P Cygni as an example. A self-consistent stellar-wind code is utilized in calculations for a large grid of wind models necessary to represent the range of stellar parameters that could exemplify the uncertain evolutionary stage of P Cygni in its quiescent phase. On the basis of these calculations, constraints on the evolutionary stage and on the stellar parameters of this star are determined. It is shown that not only the acceleration of the wind but also the observed set of dynamical parameters is conveyed by radiation pressure due to electron scattering and to some 80,000 line transitions. It is concluded that the concept of radiation-driven cool winds is suitable for describing quantitatively not only the stationary-wind dynamics of objects at the top of the HR diagram, but also the variability due to shell ejection.