The Effects of Low-Temperature Dielectronic Recombination on the Relative Populations of the Fe M-Shell States

Abstract

We examine the effects of low-temperature, or $\Delta$$n=0$, dielectronic recombination (DR) on the ionization balance of the Fe M-Shell (Fe IX through Fe XVI). Since $\Delta$$n=0$ rates are not available for these ions, we have derived estimates based on the existing rates for the first four ionization states of the CNO sequence and newly calculated rates for L-shell ions of 3rd row elements and Fe. For a range of ionization parameter and column density applicable to the intrinsic absorbers detected in {\it ASCA}, {\it Chandra}, and {\it XMM-Newton} observations of Seyfert galaxies, we generated two grids of photoionization models, with and without DR. The results show that the ionization parameter at which the population of an Fe M-shell ion peakscan increase in some cases by factor $>$ 2 when these rates are included. More importantly, there are dramatic changes in the range in ionization parameter over which individual M-shell ions contain significant fractions of the total Fe (e.g. $>$ 10%) in the plasma. These results may explain the mismatch between the range of Fe ionization states detected in the X-ray spectra of Seyferts, identified by the energies of the M-Shell Unresolved Transition Array, and those predicted by photoionization models of the X-ray absorbers that reproduce lines of second and third row elements. The results suggest that care should be taken in using 3rd and 4th row ions to constrain the physical conditions in photoionized X-ray plasmas until accurate DR rates are available. This underscores the importance of atomic physics in interpreting astronomical spectroscopy.

arXiv comment10 pages, 5 figures (3 color), 2 tables. Accepted for publication in the Astrophysical Journal

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