On the Force Associated with Absorption of Spectral Line Radiation

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Abstract
The body force on stellar material produced when radiation is absorbed or scattered by atoms, that is, the force derived from the radiative stresses, is studied for the case in which the radiation is produced by line emission of a two-level atom according to the mechanism of non-coherent scattering with complete redistribution. The force is considered for cases with either Doppler or Voigt absorption profiles in static atmospheres with and without the effect of an overlapping continuum, and also in rapidly expanding atmospheres. Approximate analytic results are obtained in several asymptotic regimes. It is suggested that for static atmospheres the force is represented to order-of-magnitude accuracy by the result for LTE with the damping wings and the overlapping continuum omitted. This gives a simple relation, that for optical depths which are greater than unity in the line, but less than unity in the continuum, the force varies as the inverse of the optical depth. The force due to the line alone is very small for optical depths greater than unity in the continuum. In rapidly expanding atmospheres the force is also represented by this formula, except that the optical depth takes the velocity gradient into account. A consequence of these relations is that the force due to an optically thick line is independent of the strength of the line.

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