LTE or non-LTE, that is the question. The NLTE chemical evolution of strontium in extremely metal-poor stars

Context. Strontium has proven itself to be one of the most important neutron-capture elements in the study of metal-poor stars. Th anks to the strong absorption lines of Sr, they can be detected eve n in the most metal-poor stars and also in low-resolution spe ctra. However, we still cannot explain the large star-to-star abundance sc atter we derive for metal-poor stars. Aims. Here we compare Galactic chemical evolution (GCE) predictions with improved abundances for Sri and Srii, including updated atomic data, to evaluate possible explanations for the larg e star-to-star scatter at low metallicities. Methods. We have derived abundances under both local thermodynamic equilibrium (LTE) and non-LTE (NLTE) for stars spanning a large interval of metallicities, as well as a broad range of other stellar parameters. Gravities and metallicities are also determined in NLTE. We employed MARCS stellar atmospheres and MOOG for theLTE spectrum synthesis, while MAFAGS and DETAIL were used to derive the NLTE abundances. We verified the consisten cy of the two methods in LTE. Results. We confirm that the ionisation equilibrium between Sr I and Sr II is satisfied under NLTE but not LTE, where the di fference between neutral and ionised Sr is on average ∼ 0.3 dex. We show that the NLTE corrections are of increasing importance as the metallicity decreases. For the stars with [Fe/H] > −3, the Sri NLTE correction is∼ 0.35/0.55 dex in dwarfs/giants, while the Srii NLTE correction is