[en] Advances in merged-beams instruments have allowed experimental studies of the mutual neutralization (MN) processes in collisions of both Li⁺ and Na⁺ ions with D⁻ at energies below 1 eV. These experimental results place constraints on theoretical predictions of MN processes of Li⁺ and Na⁺ with H⁻, important for non-LTE modeling of Li and Na spectra in late-type stars. We compare experimental results with calculations for methods typically used to calculate MN processes, namely the full quantum (FQ) approach, and asymptotic model approaches based on the linear combination of atomic orbitals (LCAO) and semiempirical (SE) methods for deriving couplings. It is found that FQ calculations compare best overall with the experiments, followed by the LCAO, and the SE approaches. The experimental results together with the theoretical calculations, allow us to investigate the effects on modeled spectra and derived abundances and their uncertainties arising from uncertainties in the MN rates. Numerical experiments in a large grid of 1D model atmospheres, and a smaller set of 3D models, indicate that neglect of MN can lead to abundance errors of up to 0.1 dex (26%) for Li at low metallicity, and 0.2 dex (58%) for Na at high metallicity, while the uncertainties in the relevant MN rates as constrained by experiments correspond to uncertainties in abundances of much less than 0.01 dex (2%). This agreement for simple atoms gives confidence in the FQ, LCAO, and SE model approaches to be able to predict MN with the accuracy required for non-LTE modeling in stellar atmospheres.

[en] We determine Mg abundances in six Gaia benchmark stars using theoretical one-dimensional (1D) hydrostatic model atmospheres, as well as temporally and spatially averaged three-dimensional ($⟨<!--\; ???\; -->3\mathrm{D}⟩<!--\; ???\; -->$) model atmospheres. The stars cover a range of $T_{\mathrm{e}\mathrm{f}\mathrm{f}}$ from 4700 to 6500 K, $\mathrm{l}\mathrm{o}\mathrm{g}g$ from 1.6 to 4.4 dex, and $[\mathrm{F}\mathrm{e}/\mathrm{H}]$ from −3.0 dex to solar. Spectrum synthesis calculations are performed in local thermodynamic equilibrium (LTE) and in non-LTE (NLTE) using the oscillator strengths recently published by Pehlivan Rhodin et al. We find that: (a) Mg abundances determined from the infrared spectra are as accurate as the optical diagnostics, (b) the NLTE effects on Mg i line strengths and abundances in this sample of stars are minor (although for a few Mg i lines the NLTE effects on abundance exceed $0.6\mathrm{d}\mathrm{e}\mathrm{x}$ in $⟨<!--\; ???\; -->3\mathrm{D}⟩<!--\; ???\; -->$ and $0.1\mathrm{d}\mathrm{e}\mathrm{x}$ in 1D, (c) the solar Mg abundance is $7.56\pm <!--\; ??\; -->0.05\mathrm{d}\mathrm{e}\mathrm{x}$ (total error), in excellent agreement with the Mg abundance measured in CI chondritic meteorites, (d) the 1D NLTE and $⟨<!--\; ???\; -->3\mathrm{D}⟩<!--\; ???\; -->$ NLTE approaches can be used with confidence to analyze optical Mg i lines in spectra of dwarfs and sub-giants, but for red giants the Mg i 5711 Å line should be preferred, (e) low-excitation Mg i lines are sensitive to the atmospheric structure; for these lines, LTE calculations with $⟨<!--\; ???\; -->3\mathrm{D}⟩<!--\; ???\; -->$ models lead to significant systematic abundance errors. The methods developed in this work will be used to study Mg abundances of a large sample of stars in the next paper in the series.

[en] Although true metal-free “Population III” stars have so far escaped discovery, their nature, and that of their supernovae, is revealed in the chemical products left behind in the next generations of stars. Here we report the detection of an ultra-metal-poor star in the Sculptor dwarf spheroidal galaxy AS0039. With [Fe/H]_LTE = −4.11, it is the most metal-poor star discovered in any external galaxy thus far. Contrary to the majority of Milky Way stars at this metallicity, AS0039 is clearly not enhanced in carbon, with [C/Fe]_LTE = −0.75, and A(C) = +3.60, making it the lowest detected carbon abundance in any star to date. Furthermore, it lacks α-element uniformity, having extremely low [Mg/Ca]_NLTE = −0.60 and [Mg/Ti]_NLTE = −0.86, in stark contrast with the near solar ratios observed in C-normal stars within the Milky Way halo. The unique abundance pattern indicates that AS0039 formed out of material that was predominantly enriched by a ∼20 M _⊙ progenitor star with an unusually high explosion energy E = 10 × 10⁵¹ erg. Therefore, star AS0039 represents some of the first observational evidence for zero-metallicity hypernovae and provides a unique opportunity to investigate the diverse nature of Population III stars.