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. Thanks to the strong absorption lines of Sr, they can be detected even in the most metal-poor stars and also in low-resolution spectra. However, we still cannot explain the large star-to-star abundance scatter we derive for metal-poor stars. Aims. Here we compare Galactic chemical evolution (GCE) predictions with improved abundances for Sr I and Sr II, including updated atomic data, to evaluate possible explanations for the large 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 the LTE spectrum synthesis, while MAFAGS and DETAIL were used to derive the NLTE abundances. We verified the consistency 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 difference between neutral and ionised Sr is on average similar to 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 Sr I NLTE correction is similar to 0.35/0.55 dex in dwarfs/giants, while the Sr II NLTE correction is <+/- 0.05 dex. Conclusions. On the basis of the large NLTE corrections to Sr I, Sr I should not be applied as a chemical tracer under LTE, while it is a good tracer under NLTE. Sr II, on the other hand, is a good tracer under both LTE and NLTE (down to [Fe/H] similar to -3), and LTE is a safe assumption for this majority species (if the NLTE corrections are not available). However, the Sr abundance from Sr II lines depends on determining an accurate surface gravity, which can be obtained from the NLTE spectroscopy of Fe lines or from parallax measurements. We could not explain the star-to-star scatter (which remains under both LTE and NLTE) by the use of the Galactic chemical evolution model, since Sr yields to date have been too uncertain to draw firm conclusions. At least two nucleosynthetic production sites seem necessary to account for this large scatter.