The ionization balance of a non-equilibrium plasma

Abstract

Most of the quantitative information we have about the cosmos comes from spectroscopy. Whether it is the cosmic expansion, deviations within the CMB, the chemical evolution of galaxies, or the events that occur during a supernova explosion, our knowledge can be traced back to the analysis of a spectrum of some sort.The gas in a stellar atmosphere is often dense enough for equilibrium thermodynamic to apply. In this case a local temperature is meaningful and the excitation and ionization of the gas will follow from thermodynamic principles. The text Mihalas (1978) gives much of the underlying physics for this "local thermodynamic equilibrium" (LTE) case.The LTE limit is seldom achieved in emission-line objects. Emission lines are commonly observed when energy is deposited into a gas by some external process, perhaps mechanical or by exposure to ionizing radiation, so that the gas becomes hot. If the column density is small enough for the gas to be optically thin to continuous absorption, an emission-line spectrum will result.Complications are introduced because of the low density of most emission-line gas. Collisions between various constituents are too slow for different species to share their energy and for kinetic equilibrium to dominate. This means that a variety of processes, some with a vast range of energies, determine the properties of the gas. An atom may be irradiated by the CMB, a true blackbody with a low temperature; starlight, with typical energies of 10 4 K to 10 5 K but highly diluted; and by emission from neighboring atoms. Cosmic rays, with relativistic energies, are often present. Collisions between various constituents of the gas are not fast enough to bring them into statistical equilibrium. Even if a temperature is defined by a Boltzmann equation, that temperature would be different for each ion and level. The result is that the level populations and the observed spectrum are quite sensitive to microphysical details. This is why the spectrum reveals so much about the intrinsic properties of the gas, such as its composition, heating, temperature, and pressure. But this is a two-edged sword. We need to understand the microphysical processes that govern the ionization, excitation, and chemistry of the gas to harvest the information present in a spectrum.The Arnaud & Rothenflug (1985, hereafter AR85) paper, and the supplement for Fe by Arnaud & Raymond (1992, hereafter AR92), did much of the difficult work needed to assemble the data base that is essential for understanding the properties of a low-density gas. These two papers have over 1300 citations at the time of this writing, placing them in the highest "renowned" citation category recognized by SPIRES. A good part of these citations are because of the quality and comprehensiveness of the work. But another reason is that basic atomic rates have universal value. The data derived by AR85 can be applied to regions ranging from the solar corona to the intergalactic medium. Such atomic data are among the foundations for how we understand the uni...

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