Radiative forces in expanding envelopes

The escape probability method introduced by Sobolev (1957) is applied to derive expressions for the radiative forces acting on atoms by the resonance scattering of radiation in envelopes which expand spherically with positive or negative radial-velocity gradients. In the latter case, it is shown that the contribution of the whole stellar envelope to the radiative force acting on individual atoms has the same effect as the one due to the stellar core radiation; i.e., a radial outward acceleration. Numerical applications to a two-level atom model illustrate the behavior of radiative forces in outward-accelerating and outward-decelerating stellar envelopes.