Dielectronic recombination of iron M-shell ions: Fe8+ and Fe7+

ABSTRACT Dielectronic recombination (DR) rate coefficients for Ar-like Fe$^{8+}$ and K-like Fe$^{7+}$ were measured by Schmidt et al. at the Heidelberg heavy ion storage ring TSR (test storage ring). Comparisons with theoretical DR results calculated with autostructure (AS) indicated some discrepancies at temperatures typical of collisionally ionized plasma, and large differences at photoionized plasma (PP) temperatures. The low-energy resonances are sensitive to the resonance energies and strong correlation effects, and theoretical calculations are challenging, especially for PP temperatures. In this work, we carry out large-scale AS calculations for Fe$^{8+}$ and Fe$^{7+}$ and improve our previous AS results by including more electron correlations and, importantly, applying semi-empirical corrections to the resonance energies. We obtain good agreement with experimental DR plasma rate coefficients down to a few thousand Kelvin. We discuss the limitations in obtaining accurate DR results for lower temperatures, also due to the lack of experimental energies for high-lying states. The good agreement we have obtained for Fe$^{8+}$ and Fe$^{7+}$ with experiment gives us confidence in using these DR rates for the modelling and interpretation of astrophysical spectra and in applying similar strategies of calculations and resonance energy corrections to DR rates for lower charge states of iron.