Global View on Stellar Opacities

After summation over the various elements of the stellar medium, the total absorption coefficient at a given frequency is obtained by summing all coefficients for the various processes of absorption at frequency v,

Kv = [Kbf(v) + Kff (v) + Kbb(v) + ...] (l - e-hV/kT^j + KeS + ... (8.41)

The atomic opacities are corrected by a term which accounts for the stimulated or induced emission, i.e., the process by which an electron on an energy level goes down to a lower energy level due to the income of a photon with the same energy as the energy difference between the two levels. The induced emission is correlated to the direction of the incident photons, thus the absorption at the considered frequency is reduced. The factor of reduction derived from the study of Einstein's coefficient is (1 - e-hv/kT) [498]. The dots represent the other opacity sources, such as Compton diffusion, Raman and Rayleigh diffusion, molecular absorption, pair formation, etc., accounted for in the opacity tables. The induced emission does not act on scattering processes.

The terms depending on the frequency must be averaged with expression (3.22) to give the Rosseland mean opacity. This mean gives a particular high weight to the values of Kv such that hv/(kT) ~ 4.

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