OPTICAL CORONAL EMISSION-LINES FROM EQUILIBRIUM AND COOLING PLASMAS

We have calculated the emission from gas at temperatures of T ∼ 106 K due to optical coronal lines that result from fine structure transitions within the ground states of highly ionized atoms. These lines include the [Fe X] λ6374 line, the [Fe XIV] λ5303 line, and many other lines from stages of ionization of S, Ca, Fe, and Ni. The calculations are valid in the limit of very low density, and apply to the diffuse gas found in supernova remnants, the interstellar medium in galaxies, and intergalactic gas. We consider two physical situations; first, the optical coronal line emission is calculated for a gas in ionization equilibrium for temperatures between 105 and 107 K. The results are plotted as a function of temperature for gasses with either a fixed density or a fixed pressure. Second, we calculate the emission of a gas cooling under its own radiation. We calculate the emission for gas cooling at either constant pressure (isobarically) or at constant density (isochorically). The effects of nonequilibrium ionization are included but are found to be insignificant. We find that the amount of optical coronal emission produced by a given amount of cooling gas is largely unaffected by changes in element abundances (providing the proportions of heavy elements remain the same).