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

We examine the effects of low-temperature, or $\Delta$$n = 0$, dielectronicrecombination (DR) on the ionization balance of the Fe M-Shell (Fe IX throughFe XVI). Since $\Delta$$n = 0$ rates are not available for these ions, we havederived estimates based on the existing rates for the first four ionizationstates of the CNO sequence and newly calculated rates for L-shell ions of 3rdrow elements and Fe. For a range of ionization parameter and column densityapplicable to the intrinsic absorbers detected in {\it ASCA}, {\it Chandra},and {\it XMM-Newton} observations of Seyfert galaxies, we generated two gridsof photoionization models, with and without DR. The results show that theionization parameter at which the population of an Fe M-shell ion peakscanincrease in some cases by factor $>$ 2 when these rates are included. Moreimportantly, there are dramatic changes in the range in ionization parameterover which individual M-shell ions contain significant fractions of the totalFe (e.g. $>$ 10%) in the plasma. These results may explain the mismatch betweenthe range of Fe ionization states detected in the X-ray spectra of Seyferts,identified by the energies of the M-Shell Unresolved Transition Array, andthose predicted by photoionization models of the X-ray absorbers that reproducelines of second and third row elements. The results suggest that care should betaken in using 3rd and 4th row ions to constrain the physical conditions inphotoionized X-ray plasmas until accurate DR rates are available. Thisunderscores the importance of atomic physics in interpreting astronomicalspectroscopy.Comment: 10 pages, 5 figures (3 color), 2 tables. Accepted for publication in the Astrophysical Journa