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Rosseland mean opacity plays an important role in theories of stellarevolution and X-ray burst models. In the high-temperature regime, when most ofthe gas is completely ionized, the opacity is dominated by Compton scattering.Our aim here is to critically evaluate previous works on this subject and tocompute exact Rosseland mean opacity for Compton scattering in a broad range oftemperatures and electron degeneracy parameter. We use relativistic kineticequations for Compton scattering and compute the photon mean free path as afunction of photon energy by solving the corresponding integral equation in thediffusion limit. As a byproduct we also demonstrate the way to compute photonredistribution functions in case of degenerate electrons. We then compute theRosseland mean opacity as a function of temperature and electron degeneracy. Wecompare our results to the previous calculations and find a significantdifference in the low-temperature regime and strong degeneracy. We find usefulanalytical expressions that approximate well the numerical results. We thenproceed to compute the flux mean opacity and show that in diffusionapproximation it is nearly identical to the Rosseland mean opacity. We alsoprovide a simple way for accounting for the true absorption in evaluation ofthe Rosseland and flux mean opacities.

Rosseland and flux mean opacities for Compton scattering