ROSSELAND AND PLANCK MEAN OPACITIES OF A ZERO-METALLICITY GAS

We calculate continuum Planck and Rosseland mean opacities of a zero-metallicity gas over a range of densities (10(-12) to 0.5 g cm-3) and temperatures (1000-7000 K) typical of accreting low-mass protostars. We formulate and solve a set of equilibrium equations for the creation and destruction of the major chemical species, extending previous LTE analyses by considering the effect of arbitrary dilutions of a radiation field with color temperature the same as the matter temperature. We analyze the individual species' contributions to the monochromatic opacities. We find that the mechanism of collision-induced absorption by H2 dominates the opacities at high density and low temperature. To facilitate complete coverage of our parameter regime, we extend current theoretical results on collision-induced opacity in a phenomenological fashion. We find, in general agreement with previous work, that there are four different mechanisms which control the Rosseland mean continuum opacity: collision-induced absorption by H2; bound-free absorption by H-; Rayleigh scattering by H, H2, and He; and Thomson scattering. We explicitly display the variation of the species abundances and the opacities with the density, temperature, and dilution of the radiation field. The H- abundance is especially sensitive to the lifting of the LTE assumption used in previous analyses. In addition, H3+ can dominate the positive ionization fraction and, indirectly, increases the H- abundance over previous estimates. Our new results produce significant differences in the Rosseland mean opacity (1) at high densities and low temperatures, where collision-induced absorption is important, and (2) at high densities and intermediate to high temperatures, where H- abundance has been more accurately calculated.