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Thermal Equilibrium

Figure 12 shows the results of a thermal balance calculation of an optically thin photoionized gas as a function of density and ionization parameter. The curves correspond to ionization parameters log($\xi $)=4,3,2,1 for the same power law ionizing spectrum used previously. This demonstrates that the net effect of higher densities is an increase in temperature at the highest densitied and lowest temperatures, and a decrease in temperature at lower density and higher temperature, for the reasons listed in the previous section. We emphasize that the quantitative value of the temperature, particularly for high ionization parameters and/or temperatures greater than 10$^7$ K or so, depends on the detailed shape of the ionizing spectrum over all energies, owing to the possible importance of Compton heating and cooling (the spectral dependence of the effects of stimulated recombination, again, have been excluded from these results).

Tim Kallman 2018-12-14