Departures of the electron energy distribution from a Maxwellian in hydrogen. II. Consequences

The formalism developed in the preceding paper is used to investigate the consequences of relaxing the a priori assumption of LTE for the free electrons in a hydrogen gas. The model problems considered are the thermal ionization equilibrium of hydrogen in the optically thin limit, the formation of a spectral line by a gas of two-level atoms, and the transfer of radiation through a gas of four-level hydrogen atoms. The effects of atom-atom collisions are included. The central result which emerges from these calculations is that, due to the dependence of the electron-atom collision rates on the atomic level populations and ionization fraction, an instability exists which can cause the self-consistent nonequilibrium state to differ appreciably from that predicted assuming a Maxwellian in those situations in which photoexcitation is unimportant. For example, we find that in the optically thin limit the electron-hydrogen collisional ionization rate is reduced a factor of order 104 relative to its Maxwellian value. As a result atom-atom collisions dominate the thermal ionization equilibrium of hydrogen for temperatures below approximately 10,000 K. We also find that differences between Maxwellian and self-consistent results generally decrease with increasing optical extent of the gas, but can remain significant for gases of large optical extent. Some astrophysical implications of these results are discussed