Strontium I, III, IV and V: Electron Impact Excitation Data for Kilonovae and White Dwarf Diagnostic Applications

TL;DR

This paper provides new atomic data for multiple ionisation stages of strontium, enabling improved spectral modeling and diagnostics for kilonovae and white dwarf stars, through advanced calculations of energy levels, transition probabilities, and collision strengths.

Contribution

It introduces comprehensive electron-impact excitation data for Sr I, III, IV, and V, using state-of-the-art computational methods, to enhance astrophysical spectral diagnostics.

Findings

Identified potential Sr IV and Sr V diagnostic lines at 1027.69nm and 1203.35nm.

New data sets enable synthetic spectra for ionisation stages of Sr in astrophysical environments.

Most diagnostic lines are ineffective due to LTE and coronal conditions in the studied regimes.

Abstract

Strontium (Sr) emissions have been observed across a wide range of astrophysical phenomena, from kilonovae (KNe) events to white dwarf (WD) stars. Precise and extensive atomic data for low ionisation stages of Sr is required for accurate theoretical modelling and to improve our understanding of evolutionary pathways. We calculated energy levels, Einstein A coefficients and electron-impact excitation collision strengths for Sr I, Sr III, Sr IV and Sr V at the temperature and density ranges of interest in KNe and WD research. We developed new target structures using the GRASP0 and AUTOSTRUCTURE packages. The energies and A-values arising from the new structures were found to be in good agreement with experimental and theoretical equivalents reported in the literature. Maxwellian averaged electron impact collision strengths were calculated using the R-matrix approach, as applied through the DARC and RMBP coding packages. These are presented in adf04 file format. The new data sets allowed us to construct synthetic spectra for the first five ionisation stages of Sr and probe possible density and temperature diagnostic lines. The synthetic spectra within the KNe regime revealed possible Sr IV and Sr V candidate lines at 1027.69nm and 1203.35nm respectively. These may provide useful benchmarks for determining the extent of Sr ionisation that can be reached in an evolving KNe event. Additional diagnostic lines were found to be poor across the Sr ion stages for both KNe and WD regimes due to most levels being in either coronal or Local Thermodynamic Equilibrium (LTE) conditions.