Inelastic e+Mg collision data and its impact on modelling stellar and supernova spectra

Abstract

Results of calculations for inelastic e+Mg effective collision strengths for the lowest 25 physical states of Mg i (up to 3s6p 1 P), and thus 300 transitions, from the convergent close-coupling (CCC) and the B-spline R-matrix (BSR) methods are presented. At temperatures of interest, ∼5000 K, the results of the two calculations differ on average by only 4%, with a scatter of 27%. As the methods are independent, this suggests that the calculations provide datasets for e+Mg collisions accurate to this level. Comparison with the commonly used dataset compiled by Mauas et al. (1988, ApJ, 330, 1008, covering 25 transitions among 12 states, suggests the Mauas et al. data are on average ∼57% too low, and with a very large scatter of a factor of ∼6.5. In particular the collision strength for the transition corresponding to the Mg i intercombination line at 457 nm is significantly underestimated by Mauas et al., which has consequences for models that employ this dataset. In giant stars the new data leads to a stronger line compared to previous non-LTE calculations, and thus a reduction in the non-LTE abundance correction by ∼0.1 dex (∼25%). A non-LTE calculation in a supernova ejecta model shows this line becomes significantly stronger, by a factor of around two, alleviating the discrepancy where the 457 nm line in typical models with Mg/O ratios close to solar tended to be too weak compared to observations.

Non-LTE analysis of K I in late-type stars

“…The data presented there are given as matrices, following the ordering of indexes in Table 1 (i.e., the transition 1-2 corresponds to element (1,2)), one matrix for each temperature. The data from the CCC and BSR methods are generally in very good agreement, with the location (offset) and scale (scatter) of the ratio Υ i j (CCC)/Υ i j (BSR), assuming a log-normal distribution, 1.03 and 0.17, respectively (see Barklem et al (2017)). This indicates a mean offset of only 3% and scatter of 17%.…”

“…In non-LTE applications, the rate coefficient is required, which is calculated by folding the cross sections σ produced in the CCC and BSR calculations with the velocity distribution, assumed here to be the Maxwell distribution. The relevant equations are given in Barklem et al (2017), and the data presented here are similar in form. The effective collision strengths Υ i j from the CCC and BSR methods are calculated for transitions between the 15 lowest-lying states of K, which includes all states up to 6d at 3.93 eV; this is the complete set of low-lying states that are included in both calculations.…”

“…In this study, we present new data based on two modern close-coupling methods, namely the convergent close coupling method (CCC) and the B-spline R-matrix method (BSR). Osorio et al (2011) and Barklem et al (2017) demonstrated that the rate coefficients calculated via these two methods tend to agree better than a factor of two for lithium and magnesium.…”

Carbon, oxygen, and iron abundances in disk and halo stars

“…Barklem 2016a) not least in the area of ab initio calculations for inelastic collisions with electrons (e.g. Barklem et al 2017) and with atomic hydrogen (e.g. Barklem 2016b;Belyaev et al 2018).…”

Comparison of experiment and theory for superelastic electron-collision studies from laser-aligned magnesium

“…Here we are interested in the e-Mg scattering system. The CCC method has already been extensively applied to this problem to study the 3 1 P 1 optical excitation function [67], compare with measurement of ACPs at 20 eV [9,10], resolve convergence problems at low energies [10], address discrepancy with experiment for the total ionization cross section [68], and for astrophysical modeling [69]. Consequently, we only give a brief overview of the essential theoretical concepts.…”