Dielectronic recombination plasma rate coefficients of Na-, Mg-, and Al-like iron ions: The role of the 2(s + p) → 4l, nl′ and 3(s + p) → 5l, nl′ resonances

Dielectronic recombination (DR) is the major electron-ion recombination process in many astrophysical and laboratory plasmas. This process has been thoroughly studied for several charged species, in particular for M-shell iron ions. Previous investigations into the DR of such ions have mostly focused on the valence 3l -> 3l, nl' and the core 2(s + p) -> 3l, nl ' DR resonances, while neglecting the 2(s + p) -> 4l, nl ' and 3(s + p) -> 5l, nl ' DR resonances. These were predicted to contribute to the broad unresolved transition array detected in several active galactic nuclei at reasonably high temperatures and column densities. In this work, we systematically assessed the importance of 2(s + p) -> 4l, nl ' and 3(s + p) -> 5l, nl ' resonances in the DR of three M-shell iron ions, Fe15+, Fe14+, and Fe13+, with the multi-configuration Dirac-Hartree-Fock (MCDHF) method. Above the collisionally ionized plasma temperatures, these high-energy resonances contribute to the overall DR of Fe15+, Fe14+, and Fe13+ by up to 21%, 40%, and 22%, respectively.