Electron Impact Excitation of S III: An Assessment

Kanti M Aggarwal

doi:10.3390/atoms7030078

Abstract

In a recent paper, Tayal et al. (Astrophys. J. Suppl. 2019, 242, 9) reported results for energy levels, radiative rates (A-values), and effective collision strengths ( Υ ) for transitions among the 198 levels of Si-like S III. For the calculations, they adopted the multi-configuration Hartree–Fock (MCHF) code for the energy levels and A-values and the B-spline R-matrix (BSR) code for Υ . Their reported results appear to be accurate for energy levels and A-values, but not for Υ . Through our independent calculations by adopting the flexible atomic code (FAC), we demonstrate that their reported results for Υ are underestimated, by up to a factor of two, and at all temperatures, particularly for the allowed transitions, but some forbidden ones as well. Additionally, for transitions involving the higher levels, the behaviour of their Υ results is not correct.

Highlights

Si-like S III is an important ion for the studies of a variety of astrophysical plasmas, such as H II
Many of its observed lines varying from infrared to extreme ultraviolet regions have been useful for electron density and temperature diagnostics, for which atomic data, including energy levels, radiative rates (A-values), collision strengths (Ω), and effective collision strengths (Υ), are required
These pseudo-resonances in the variation of Ω, which arise at energies above thresholds due to the orthogonality conditions imposed on the orbitals, included in the generation of wavefunctions, but not in the collisional calculations, need to be smoothed over, as shown in Figure 1 of Aggarwal and Hibbert [6] for three transitions of O III

Summary

Introduction

Si-like S III is an important ion for the studies of a variety of astrophysical plasmas, such as H II regions, planetary atmospheres, and stellar objects; see Tayal et al [1] and the references therein. Assessing atomic data for accuracy is a (very) difficult task [7], for Υ This is because the corresponding experimental results for most transitions of ions are almost non-existent, and a large calculation, as performed by Tayal et al [1], cannot be repeated, as it requires enormous computational resources, time, and expertise. Forbidden transitions are appreciably affected by the presence of numerous closed-channel (Feshbach) resonances (see, for example, Figure 1 of Tayal et al.), by more than an order of magnitude in some instances, and towards the lower range of electron temperatures For this reason, it is much more difficult to assess their accuracy. This code does not (automatically) calculate resonances, as the R-matrix does, but it should not affect the comparisons, as our main interest is in (strong) allowed transitions

Collision Strengths and Effective Collision Strengths

Findings

Conclusions