NLTE corrections for metal-poor stars

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 were unable to reproduce the high measured [C/Fe] in LTE, which are lowered in NLTE, aligning with the simulations. Other elements are a mixed bag, where some show improved agreement with the models (e.g. Na) and other 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.