The status of neutron-capture elements according to Gaia-ESO.

Sammanfattning: The aim of this thesis is to get a better understanding of the evolution of the Milky Way stellar disc by looking at the abundance ratios for neutron-capture elements in thin and thick disc stars. In this project the formation of our Galaxy’s thin and thick disc was studied by analysing 10 different neutron-capture elements in field stars measured by Gaia-ESO. The methodology to determine if this new data by Gaia-ESO was reliable was first to match the new data to existing done by Fulbright (2000), Koch & Edvardsson (2002), Yong et al. (2005), Carretta et al. (2011) and Battistini & Bensby (2016). By fitting synthesised spectra to match the observed spectra, the limit for the lowest signal-to-noise ratio for trustworthy measurements was determined and the calculated abundance ratio were compared to the measured ones to verify the new data. Abundance results for up to 1486 stars are presented for Y, Zr, Mo, Ba, La, Ce, Pr, Nd and Eu. For the abundance ratio, [x/Fe], against the metallicity, [Fe/H], the general trend showed decreasing abundance ratios with increasing metallicity. This suggests that many elements were, relatively to Fe, more abundant in the early Galaxy, when the r-process was more active because of type-II supernovae (SN II) stars. Y and Ba illustrated a more flat tendency which indicates that they are produced via the s-process mostly, from AGB stars that enrich the environment when the Galaxy is older. The abundance ratio, [x/Fe] as a function of age showed that a change in slope is seen around 8 Gyr for some elements. Thick disc stars show an overall decrease in abundance ratio with respect to younger ages, which is due to the reduction of SN II with time, that causes the slowdown in production of r-process elements. For the thin disc, the general trend is flat, this can be explained by the main enrichment from the AGB stars for the s-process is balanced out by the production of Fe from type-Ia supernovae (SN Ia).