Dielectronic recombination of Fe¹³⁺: benchmarking the M-shell

We have carried-out a series of multi-configuration Breit–Pauli AUTOSTRUCTURE calculations for the dielectronic recombination of Fe 13+ . We present a detailed comparison of the results with the high-energy-resolution measurements reported recently from the Heidelberg heavy-ion Test Storage Ring by Schmidt et al . Many Rydberg series contribute significantly from this initial 3s 2 3p M-shell ion, resulting in a complex recombination 'spectrum'. While there is much close agreement between theory and experiment, differences of typically 50% in the summed resonance strengths over 0.1–10 eV result in the experimentally based total Maxwellian recombination rate coefficient being a factor of 1.52–1.38 larger than theory over 10 4 –10 5 K, which is a typical temperature range of peak abundance for Fe 13+ in a photoionized plasma. Nevertheless, the present theoretical recombination rate coefficient is an order of magnitude larger than that used by modellers to date. This may help explain the discrepancy between the iron M-shell ionization balance predicted by photoionization modelling codes, such as ION and CLOUDY, and that deduced from the iron M-shell unresolved-transition-array absorption feature observed in the x-ray spectrum of many active galactic nuclei. Similar data are required for Fe 8+ through Fe 12+ in order to remove the question mark hanging over the atomic data though.