Atomic Data for Non-Equilibrium Modeling of Kilonovae: The Ionization Properties of Te I - III

Abstract

Kilonovae, the electromagnetic transients produced from two merging neutron stars, exhibit evolving spectral signatures in ultraviolet, visible, and infrared radiation. Starting around one week post-merger, equilibrium assumptions describing the local ionization balance and atomic level populations in the ejecta come into question, and non-equilibrium modeling is required. In this non-equilibrium regime, interactions with non-thermal electrons are critical inputs to ionization balance models. With most databases storing rate coefficients, the necessary cross sections describing these interactions are generally unavailable. We report new level-resolved calculations of the ionization cross sections of a species tentatively identified in kilonovae, Te I - III, using the Flexible Atomic Code. Good agreement is found between the calculated cross sections and a limited number of available measurements. Particular attention is paid to diagnosing the accuracy of the above-threshold channels that contribute through excitation autoionization. Calculations in the configuration average approximation yield ionization cross sections close to both experimental and level-resolved theoretical values. The computed cross sections are combined with a Spencer-Fano non-thermal electron energy solver and subsequent ionization balance models to probe the impact of improved cross section datasets on ion fractions of Te I - IV at kilonova-like plasma conditions.