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 about 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 the 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 SrI 4607 A line, making use
of the LTE approximation. NLTE corrections lead to an increase of the abundance
estimates obtained under LTE, but for these lines 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 behavior 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). The comparison of the
observational data with the current GCE models confirm that the s-process
contributions from Asymptotic Giant Branch stars and from massive stars are the
main sources of Sr in the Galactic disc and in the Sun, while different
nucleosynthesis sources can explain the high [Sr/Ba] and [Sr/Eu] ratios
observed in the early Galaxy.
