Recombination rates, Resonance Strengths and Line Profiles of Dielectronic Satellite Lines of He-like Ca, Fe, Ni

Summary

Dielectronic satellite (DES) lines arising from the radiative decay of ions with excited cores are used as diagnostics of the plasma conditions found in astronomical objects. In collisional plasmas, the most common DES lines are due to an electron being dielectronically captured by a He-like ion, forming a 3-electron, Li-like ion in a doubly excited, autoionizing state (i.e. 1s2l2l’ (KLL), 1s2l3l’ (KLM), etc). These states can decay to bound levels by emitting a photon, completing a process known as dielectronic recombination (DR). The autoionizing KLL complex gives rise to 22 DES lines, labeled ‘a’ through ‘v’, via transitions of the type 1s2l2l’→1s²2 l. These lines are seen at energies lower than the He-like resonance line (1s2p(¹P⁰ ₁)→ 1s²(¹S₀) - which is also referred to as the ‘w’ line). The unified recombination method, which treats photoionization and electron-ion recombination in a manner which couples autoionizing states to the continuum, has been extended to study the DES lines of the highly charged ions Ca XVIII, Fe XXIV, and Ni XXVI. In contrast to obtaining a single energy point for a DES line (as is the case for existing theoretical approaches based on the isolated resonance approximation - IRA), the unified method: (i) provides detailed profiles of DES lines, including the blending of lines which occurs in nature, (ii) includes the background contribution of radiative recombination (RR), and (iii) provides a simple relation between the resonance strengths and recombination rates of narrow lines which is useful for astrophysical modeling.