Suppression of Dielectronic Recombination Due to Finite Density Effects

TL;DR

This paper introduces a model for density-dependent dielectronic recombination rates, showing how finite density suppresses DR and affects plasma ionization states, with implications for astrophysical environments.

Contribution

The authors develop a general suppression formula for DR rates that incorporates density effects, improving plasma ionization models at various densities.

Findings

Denser plasmas show significantly more ionization due to DR suppression.

The model impacts predictions of ionization balance in cosmic gases like nova shells and accretion disks.

Density effects on DR are crucial for accurate astrophysical plasma modeling.

Abstract

We have developed a general model for determining density-dependent effective dielectronic recombination (DR) rate coefficients in order to explore finite-density effects on the ionization balance of plasmas. Our model consists of multiplying by a suppression factor those highly-accurate total zero-density DR rate coefficients which have been produced from state-of-the-art theoretical calculations and which have been benchmarked by experiment. The suppression factor is based-upon earlier detailed collision-radiative calculations which were made for a wide range of ions at various densities and temperatures, but used a simplified treatment of DR. A general suppression formula is then developed as a function of isoelectronic sequence, charge, density, and temperature. These density-dependent effective DR rate coefficients are then used in the plasma simulation code Cloudy to compute ionization balance curves for both collisionally ionized and photoionized plasmas at very low (ne = 1 cm^-3) and finite (ne=10^10 cm^-3) densities. We find that the denser case is significantly more ionized due to suppression of DR, warranting further studies of density effects on DR by detailed collisional-radiative calculations which utilize state-of-the-art partial DR rate coefficients. This is expected to impact the predictions of the ionization balance in denser cosmic gases such as those found in nova and supernova shells, accretion disks, and the broad emission line regions in active galactic nuclei.