Non-local Thermodynamic Equilibrium Stellar Spectroscopy with 1D and 〈3D〉 Models. II. Chemical Properties of the Galactic Metal-poor Disk and the Halo

Abstract

From exploratory studies and theoretical expectations it is known that simplifying approximations in spectroscopic analysis (LTE, 1D) lead to systematic biases of stellar parameters and abundances. These biases depend strongly on surface gravity, temperature, and, in particular, for LTE vs. non-LTE (NLTE) on metallicity of the stars. Here we analyse the [Mg/Fe] and [Fe/H] plane of a sample of 326 stars, comparing LTE and NLTE results obtained using 1D hydrostatic models and averaged 3D models. We show that compared to the 3D NLTE benchmark, all other three methods display increasing biases towards lower metallicities, resulting in false trends of [Mg/Fe] against [Fe/H], which have profound implications for interpretations by chemical evolution models. In our best 3D NLTE model, the halo and disc stars show a clearer behaviour in the [Mg/Fe] − [Fe/H] plane, from the knee in abundance space down to the lowest metallicities. Our sample has a large fraction of thick disc stars and this population extends down to at least [Fe/H] ∼ −1.6 dex, further than previously proven. The thick disc stars display a constant [Mg/Fe] ≈ 0.3 dex, with a small intrinsic dispersion in [Mg/Fe] that suggests that a fast SN Ia channel is not relevant for the disc formation. The halo stars reach higher [Mg/Fe] ratios and display a net trend of [Mg/Fe] at low metallicities, paired with a large dispersion in [Mg/Fe]. These indicate the diverse origin of halo stars from accreted low-mass systems to stochastic/inhomogeneous chemical evolution in the Galactic halo.

The AMBRE Project: Spectrum normalisation influence on Mg abundances in the metal-rich Galactic disc

“…The four saturated lines (MgIb triplet: 5167.3, 5172.7, 5183.6, and 5528.4 Å) and the intermediate-strength line 5711.09 Å seem to reproduce the thin-thick disc sequences more precisely, also showing a decreasing trend in [Mg/Fe] even at supersolar metallicities. This is in agreement with the analysis of NLTE effects on Mg abundances done by Bergemann et al (2017), where they highlight the robust behaviour of the strong lines 5172, 5183, 5528, and 5711 Å. The higher dispersion present on the abundance results for the weak non-saturated lines with respect to the strong saturated ones is due to different factors.…”

“…The selected lines in Table 1 have been widely used in the literature to determine both [Mg/Fe] and [α/Fe] abundances. Bergemann et al (2014Bergemann et al ( , 2017 analysed different approximations for radiative transfer and spectral line formation in model atmospheres, focused on their effect on Mg abundance determination using lines in the optical and infrared, among which there are four lines used in our analysis (5172, 5183, 5528, and 5711 Å). They find no significant differences between 1D LTE and 1D NLTE abundances, and for the lines in common with ours, they present a quite robust behaviour with respect to the full 3D NLTE calculations in cool FGK stars.…”

“…It is known to have a high number of measurable spectral lines in optical spectra. In addition, the ratio of the Mg abundance with respect to iron, [Mg/Fe], shows a large absolute separation of the Galactic thick-thin disc populations, along with a smaller scatter and a shallower trend with temperature and metallicity (Brewer et al 2016;Bergemann et al 2017;Ivanyuk et al 2017;Buder et al 2019), making this element possibly the best tracer.…”

NLTE Line Formation for Mg i and Mg ii in the Atmospheres of B–A–F–G–K Stars

“…Using a comprehensive model atom based on accurate collisional data, Osorio & Barklem (2016) calculated departure coefficients for the Mg i levels and LTE and NLTE equivalent widths for the Mg i lines in a grid of the MARCS atmospheric models (Gustafsson et al 2008). The magnesium NLTE abundances of extended samples of cool stars were determined by Andrievsky et al (2010) and Bergemann et al (2017b). Taking advantage of the NLTE line formation for not only Mg i but also Fe i-Fe ii, Zhao et al (2016) and Mashonkina et al (2017) showed that the Galactic dwarf and giant stars at [Fe/H] ≤ −0.8 and the very metal-poor ([Fe/H] < −2) giants in the classical dwarf spheroidal galaxies Sculptor, Ursa Minor, Sextans, and Fornax reveal a similar plateau at [Mg/Fe] ≃ 0.3.…”

The Laboratory Astrophysics Spectroscopy Programme at Imperial College London

“…Quantitative interpretation of stellar spectra relies on the existence and accuracy of atomic data such as wavelengths, transition probabilities, energy levels, and hyperfine structure constants. Well-defined linelists with reliable and consistent atomic data are also very important for the analysis of deviations from local thermodynamic equilibrium (non-LTE effects) when modelling stellar atmospheres [15][16][17][18]. Together with realistic stellar models and high-resolution stellar spectra, accurate atomic and molecular data are essential to obtain chemical abundances with the level of accuracy needed by Galactic Surveys (Figure 4).…”