Inelastic Mg+H collision data for non-LTE applications in stellar atmospheres

Abstract

Rate coefficients for inelastic Mg+H collisions are calculated for all transitions between the lowest seven levels and the ionic state (charge transfer), namely Mg(3s 2 1 S, 3s3p 3 P, 3s3p 1 P, 3s4s 3 S, 3s4s 1 S, 3s3d 1 D, 3s4p 3 P)+H(1s) and Mg + (3s 2 S)+H − . The rate coefficients are based on cross-sections from full quantum scattering calculations, which are themselves based on detailed quantum chemical calculations for the MgH molecule. The data are needed for non-LTE applications in cool astrophysical environments, especially cool stellar atmospheres, and are presented for a temperature range of 500−8000 K. From consideration of the sensitivity of the cross-sections to various uncertainties in the calculations, most importantly input quantum chemical data and the numerical accuracy of the scattering calculations, a measure of the possible uncertainties in the rate coefficients is estimated.

The photospheric solar oxygen project

“…This would not be surprising, as it is well known that the classical treatment of collisions does not have a rigorous physical foundation. In fact, such nonuniform scaling has recently been demonstrated for Mg + H by Barklem et al (2012). In principle, S H should depend on temperature (see Barklem et al 2011) and, because the different triplet lines form at different atmospheric heights, S H could be expected to vary slightly from line to line.…”

Mg line formation in late-type stellar atmospheres

“…We wish to emphasize that with regard to hydrogen collisions in general, the accurate quantum (Barklem et al 2003;Barklem et al 2010Barklem et al , 2012 as well as the more approximate model calculations (Belyaev 2013;Belyaev et al 2014), based on a correct physical background, provide non-zero rate coefficients for both optically allowed and optically forbidden transitions, as well as for charge transfer processes, which generally have the highest rate coefficients among all hydrogen collision processes. Thus, the Drawin formula fails to provide reliable estimates for important inelastic processes in collisions with hydrogen.…”

“…Thus, the Drawin formula fails to provide reliable estimates for important inelastic processes in collisions with hydrogen. In the case of Mg, the quantum eight-channel calculations Belyaev et al (2012) provide 28 non-zero crosssections for the endothermic processes (excitation and ion-pair production), while the Drawin formula gives only five nonzero (overestimated) cross-sections for the same 28 transitions (see Barklem et al 2012). This is a general feature of the Drawin formula, as discussed in detail by Barklem et al (2011).…”

The Hamburg/ESO R-process Enhanced Star survey (HERES)

“…Table 5 presents the mean abundances, the number of used lines, N lines , and σ log ε , where N lines > 1. We also list the solar photosphere abundances, log ε , adopted from Lodders et al (2009) (2010) and Barklem et al (2012) were applied for Na+H and Mg+H collisions, respectively. For the remaining NLTE species, collisions with H i atoms were computed with the Drawinian rates (Drawin 1968(Drawin , 1969 scaled by a factor of S H = 0.1.…”