Enrichment of the Galactic disc with neutron capture elements: Sr

The enrichment history of heavy neutron-capture elements in the Milky Way disc provides fundamental information about the chemical evolution of our Galaxy and the stellar sources that made those elements. In this work we give new observational data for Sr, the element at the first neutron-shell closure beyond iron, N = 50, based on the analysis of high-resolution spectra of 276 Galactic disc stars. The Sr abundance was derived by comparing the observed and synthetic spectra in the region of the Sr I 4607 Å line, making use of the local thermodynamic equilibrium (LTE) approximation. Non-local thermodynamic equilibrium (NLTE) corrections lead to an increase in the abundance estimates obtained under LTE, but for this line they are minor near solar metallicity. The average correction that we find is 0.151 dex. The star that is mostly affected is HD 6582, with a 0.244 dex correction. The behaviour of the Sr abundance as a function of metallicity is discussed within a stellar nucleosynthesis context, in comparison with the abundance of the heavy neutron-capture elements Ba (Z = 56) and Eu (Z = 63). Comparison of the observational data with current Galactic chemical evolution (GCE) models confirms that s-process contributions from asymptotic giant branch stars and massive stars are the main sources of Sr in the Galactic disc and the Sun, while different nucleosynthesis sources can explain the high [Sr/Ba] and [Sr/Eu] ratios observed in the early Galaxy.