Theoretical investigation of transition data of astrophysical importance in neutral sulphur

TL;DR

This paper provides extensive theoretical atomic data for neutral sulphur, including oscillator strengths and transition rates, crucial for stellar spectral modelling and abundance analysis, using advanced relativistic quantum methods.

Contribution

It presents a large set of calculated atomic data for neutral sulphur using MCDHF and RCI methods, with improved accuracy assessments and fine-tuning techniques.

Findings

Approximately 16% of data have uncertainties within 10%.

Fine-tuning improved the accuracy classification of about 24% of transitions.

The data support more precise non-LTE modelling of stellar spectra.

Abstract

Accurate and comprehensive atomic data are essential for the modelling of stellar spectra. Uncertainties in the oscillator strengths of specific lines used for abundance analyses directly translate into uncertainties in the derived elemental abundances; incomplete or biased atomic data sets can impart significant errors in non-local thermodynamic equilibrium (non-LTE) modelling. Theoretical calculations of atomic data are therefore crucial to supplement the limited experimental results. In this work, we present extensive atomic data, including oscillator strengths, transition rates, and lifetimes for 1730 electric-dipole (E1) transitions among 107 levels in neutral sulphur (S I) using the multi-configuration Dirac-Hartree-Fock (MCDHF) and relativistic-configuration-interaction (RCI) methods. These levels belong to the configurations $\mathrm{3p^3np (n=3-7)}$, $\mathrm{3p^3nf (n=4,5)}$, $\mathrm{3s3p^5}$, $\mathrm{3p^3ns (n=4-7)}$, and $\mathrm{3p^3nd (n=3-6)}$. The accuracy of the computed transition rates is assessed by combining the comparison of the differences in transition rates between the Babushkin and Coulomb gauges with a cancellation-factor (CF) analysis. Approximately 16% of the ab initio results achieved an accuracy classification of A-B, corresponding to uncertainties within 10%, as defined by the Atomic Spectra Database of the National Institute of Standards and Technology (NIST ASD). Applying a fine-tuning technique was found to significantly improve the accuracy of the results in the Coulomb gauge, thereby improving the consistency between the Babushkin and Coulomb gauges; about 24% of the fine-tuned transition data are assigned to the accuracy classes A-B.