Rosseland and Planck mean opacities of a zero-metallicity gas

Planck and Rosseland mean opacities of a zero-metallicity gas are calculated over a range of densities 10 to the {minus}12th to 0.5 g/cu cm and temperatures 1000-7000 K typical of accreting low-mass protostars. Equilibrium equations for the creation and destruction of the major chemical species are formulated and solved. The previous LTE analysis is extended by considering the effect of arbitrary dilutions of a radiation field with color temperature the same as the matter temperature. The individual species' contributions to the monochromatic opacities are analyzed. It is found that the mechanism of collision-induced absorption by H2 dominates the opacities at high density and low temperature. Four different mechanisms are found to control the Rosseland mean continuum opacity: collision-induced absorption by H2; bound-free absorption by H({minus}); Rayleigh scattering by H, H2, and He; and Thomson scattering. 59 refs.