Large databases of metal-poor stars corrected for three-dimensional and/or non-local thermodynamic equilibrium effects

¹ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy
² INAF/Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy

^★ Corresponding author: ioanna.koutsouridou@unifi.it

Received: 22 February 2025
Accepted: 2 May 2025

Abstract

Early chemical enrichment processes can be revealed by the careful study of metal-poor stars. In our Local Group, we can obtain spectra of individual stars to measure their precise, but not always accurate, chemical abundances. Unfortunately, stellar abundances are typically estimated under the simplistic assumption of local thermodynamic equilibrium (LTE). This can systematically alter both the abundance patterns of individual stars and the global trends of chemical enrichment. The SAGA database compiles the largest catalogue of metal-poor stars in the Milky Way. For the first time, we provide the community with the SAGA catalogue fully corrected for non-LTE (NLTE) effects, using state-of-the-art publicly available grids. In addition, we present an easy-to-use online tool NLiTE that quickly provides NLTE corrections for large stellar samples. For further scientific exploration, NLiTE facilitates the comparison of different NLTE grids to investigate their intrinsic uncertainties. Finally, we compare the NLTE-SAGA catalogue with our cosmological galaxy formation and chemical evolution model, NEFERTITI. By accounting for NLTE effects, we can solve the long-standing discrepancy between models and observations in the abundance ratio of [C/Fe], which is the best tracer of the first stellar populations. At low [Fe/H] < −3.5, models are unable to reproduce the high measured [C/Fe] in LTE, which are lowered in NLTE, aligning with simulations. Other elements are a mixed bag, where some show improved agreement with the models (e.g. Na) and others appear even worse (e.g. Co). Few elemental ratios do not change significantly (e.g. [Mg/Fe], [Ca/Fe]). Properly accounting for NLTE effects is fundamental for correctly interpreting the chemical abundances of metal-poor stars. Our new NLiTE tool, thus, enables a meaningful comparison of stellar samples with chemical and stellar evolution models as well as with low-metallicity gaseous environments at higher redshift.