A method for solving the line-formation problem using the full comoving-frame formulation of the radiative-transfer equation is presented for the case of spherically symmetric atmospheres expanding with arbitrarily large velocities. A stable differencing scheme and a frequency-by-frequency elimination procedure are developed to solve the partial differential equations that describe the radiation field in the comoving frame. It is noted that this method allows computation of the radiation field from a given model atmosphere which must specify the depth dependence of all the relevant physical variables. Numerical results obtained for several models involving line formation by two-level atoms, electron scattering, and continuous absorption are discussed which simulate situations in the stellar winds of hot stars and similar objects. The force exerted by radiation on the gas is examined in a number of situations, and flux profiles are described for very high-velocity flows with very weak or nonexistent continuum and electron-scattering opacities. It is concluded that the mechanism proposed by Noerdlinger and Rybicki (1974) for the destruction of radially driven envelopes in planar geometries becomes inoperative even in the case of slightly extended spherical configurations.
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Mihalas et al. (1975) studied this question.