Dielectronic recombination studies of ions relevant to kilonovae and nonlocal thermodynamic equilibrium plasma

Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany

^★ Corresponding author: suvamsingh18sep@gmail.com

Received: 9 April 2025
Accepted: 17 June 2025

Abstract

This study presents calculations of rate coefficients, resonance strengths, and cross sections for the dielectronic recombination (DR) of Y⁺, Sr⁺, Te²⁺, and Ce²⁺ – low-charge ions relevant to kilonovae and nonlocal thermodynamic equilibrium (non-LTE) plasmas. Using relativistic atomic structure methods, we computed DR rate coefficients under conditions typical of these environments. These DR rate coefficients and cross sections were calculated using the Flexible Atomic Code. The DR resonance features were identified by comparing theoretical resonance energies, estimated as the difference between National Institute of Standards and Technology excitation energies and Dirac binding energies, with dominant autoionizing states confirmed through an analysis of autoionization rates. Our results highlight the critical role of low-lying DR resonances in shaping rate coefficients at kilonova temperatures (~10⁴ K) and regulating charge-state distributions. Pronounced near-threshold DR resonances significantly influence the evolving ionization states and opacity of neutron star merger ejecta. Comparisons with previous studies emphasize the necessity of including high-n Rydberg states for accurate DR rate coefficients, especially for complex heavy ions with dense energy levels. Discrepancies with existing datasets underscore the need for refined computational techniques to minimize uncertainties. These results provide essential input for interpreting spectroscopic observations of neutron star mergers, including James Webb Space Telescope data. We also put forward suitable candidates for experimental studies, recognizing the challenges involved in such measurements. The data presented here have the potential to refine models of heavy-element nucleosynthesis, enhance plasma simulation accuracy, and improve non-LTE plasma modeling in astrophysical and laboratory settings.