Physical Conditions of the Coronal Line Region in Seyfert Galaxies

The launch of the Infrared Space Observatory and new atomic data have openeda window to the study of high ionization gas in active galactic nuclei (AGN).We present the results of a large number of photoionization simulations of the``coronal line'' region in AGN, employing new atomic data from the Opacity andIron Projects. Our grid of line emission spans 8 orders of magnitude in gasdensity and 14 orders of magnitude in ionizing flux in an effort to identifythe optimal conditions in which these lines form. We show that coronal linesform at distances from just outside the broad line region to ~400L_{43.5}^{1/2}pc, in gas with ionization parameter -2.0 < log U(H) < 0.75, corresponding togas densities of 10$^2$ to 10$^{8.5}$ cm$^{-3}$, with electron temperatures~12,000K -- 150,000K. A large range of distances from the central sourceimplies significant line width variation among the coronal lines. We identifyseveral line ratios which could be used to measure relative abundances, and weuse these to show that the coronal line gas is likely to be dust free.Comment: 18 pages plus 6 postscript figures, to appear in the June ApJ