Collision and Radiative Rates for Infrared to Extreme Ultraviolet Lines of S iii

Abstract

Electron excitation collision strengths for a wide range of transitions giving rise to infrared, optical, ultraviolet, and extreme ultraviolet lines of S iii have been calculated using the B-spline Breit–Pauli R-matrix method. The term-dependent non-orthogonal orbitals have been employed for the accurate representation of target wave functions and the electron plus S iii target scattering system. The multiconfiguration Hartree–Fock method has been utilized for the calculation of 198 S iii fine-structure level energies belonging to the , , , and 3s3p 25s configurations. The transition probabilities between fine-structure levels have also been calculated and compared with available other calculations. The close-coupling expansion includes these 198 fine-structure levels of S iii in the electron collision calculations. The effective collision strengths are calculated at electron temperatures in the range of 103–106 K for all possible transitions between the 198 fine-structure levels. The present calculation includes a larger number of levels in the close-coupling expansion and improved target description than previous calculations and should be useful for the analysis of measured spectra from various astrophysical objects. Comparison with other calculations is used to assess likely uncertainties in the existing collision and radiative rates for S iii. The collision and radiative rates are estimated to be accurate to about 20% or better for most main transitions of astrophysical importance.

The MOSDEF Survey: [S iii] as a New Probe of Evolving Interstellar Medium Conditions*

“…Accordingly, we need to convert the line flux of one line to the total doublet flux. This approach is possible because the ratio [S iii]λ9531/λ9069 is fixed to a value of ≈2.5 according to the transition probabilities (Osterbrock & Ferland 2006;Tayal et al 2019). We show the [S iii]λ9531/λ9069 ratios for individual z ∼ 1.5 galaxies, the Stack1 ∼ 1.5 composite, and the z ∼ 0 comparison samples in Figure 2.…”

Electron Impact Excitation of S III: An Assessment

“…In Figures 5 and 6, we consider three other transitions, namely 3-10 (3p 2 3 P 2 -3s3p 3 ( 2 P) 3 P o 2 ), 3-11 (3p 2 3 P 2 -3s3p 3 ( 2 P) 3 P o 1 ) and 4-13 (3p 2 1 D 2 -3p3d 1 D o 2 ), which also have comparable ∆E, as listed in Table 1, but the f-values differ considerably between the MCHF and FAC calculations, as the transitions are weak. For such transitions, resonances often contribute towards the determination of Υ and this can clearly be noted by the humps seen in Figure 6 in the results of Tayal et al [1]. It may also be noted that the BSR values of Υ for the 4-13 transition are larger than our results with FAC because the corresponding f-value is also larger, whereas for the other two transitions the Υ values are similar, particularly towards the higher end of the temperature range, in spite of the large differences in the f-values.…”

“…In some R-matrix calculations, it is assumed that Ω values (for all types of transitions) converge within a few partial waves, particularly in the thresholds region, and we suspect this is the case with the calculations of Tayal et al [1] too. However, there are many transitions (mainly allowed ones but some forbidden also) for which this is not true.…”

“…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 references therein. 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.…”

“…Since the adopted wavefunctions of Tayal et al [1] are accurate, subsequent calculations for collisional data are also 'expected' to be equally accurate, particularly when: (i) they have included a large range of partial waves with angular momentum J ≤ 23.5, to ensure convergence of Ω for most forbidden transitions, (ii) have included the contribution of higher neglected partial waves through a top-up procedure, to ensure the convergence of Ω for the allowed ones as well, (iii) have considered a large range of energy, up to 12.6 Ryd, and have even extrapolated their values of Ω for higher energies to determine the values of Υ up to T e = 10 6 K, as accurately as possible, (iv) have resolved resonances in a very fine energy mesh of 0.0001 Ryd, to take care of almost all resonances without giving undue weightage to their widths, and finally (v) have performed relativistic calculations with the B-spline R-matrix (BSR) code. Therefore, there should be no reason to suspect their calculations of Υ for accuracy, which has been estimated to be about 20% for most transitions.…”