Plasma and Fusion Research

Volume 21, 2401005 (2026)

Regular Articles

Benchmarking the Plane-Wave Born and Distorted Waves Approximations for Electron-Impact Collision Strength Computations: the Sample Case of Sr II

Jérôme DEPRINCE^1,2), Lucas MAISON¹⁾

1): Université de Mons (UMONS), Mons 700, Belgium
2): Université Libre de Bruxelles (ULB), Brussels 1050, Belgium

(Received 29 August 2025 / Accepted 30 October 2025 / Published 31 March 2026)

Abstract

The discovery of gravitational waves (GW170817) and the associated kilonova (AT2017gfo) from a neutron star merger in 2017 confirmed the latter as key sites for heavy element production through the r-process. Subsequent observations, including late-time spectra with JWST, have highlighted the need for accurate modeling of kilonova ejecta. In the photospheric phase, atomic level populations can be estimated under LTE using Boltzmann and Saha relations, but about a week after the merger the ejecta enters the nebular phase where non-LTE effects dominate. Modeling nebular spectra therefore requires a detailed treatment of radiative and collisional processes that affect the population of atomic levels. This work focuses on electron-impact excitation in Sr II, a heavy ion relevant for kilonova spectra. Two computational approaches are employed: the Plane-Wave Born approximation within the pseudo-relativistic Hartree-Fock method, and a Distorted Waves method using AUTOSTRUCTURE. The resulting collision strengths are compared against reference R-matrix data to evaluate the accuracy of these approximations and their suitability for large-scale applications to all heavy elements. In addition, radiative parameters for forbidden transitions are computed. These results provide an essential benchmark of approximations that could be used to compute atomic data for nebular-phase kilonova modeling.

Keywords

kilonova, strontium, collision strengths, forbidden transitions, electron-impact excitation, nebular-phase, non-local thermodynamic equilibrium, neutron star merger, plane-wave Born, distorted waves

DOI: 10.1585/pfr.21.2401005

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