Non-local thermodynamic equilibrium stellar spectroscopy with 1D and 3D models - II. Chemical properties of the Galactic metal-poor disc and the halo

TL;DR

This study compares LTE and NLTE spectroscopic models for 326 stars, revealing biases affecting chemical abundance trends and providing clearer insights into the Galactic disc and halo's chemical evolution.

Contribution

It demonstrates the importance of using <3D>NLTE models for accurate stellar chemical properties, especially at low metallicities, improving interpretations of Galactic evolution.

Findings

<3D>NLTE models yield more accurate [Mg/Fe] and [Fe/H] trends.

Thick disc stars show constant [Mg/Fe] ~ 0.3 dex with low dispersion.

Halo stars exhibit diverse [Mg/Fe] ratios and a trend at low metallicities.

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.