The temporal evolution of neutron-capture elements in the Galactic discs
  Important insights into the formation and evolution of the Galactic disc(s)
are contained in the chemical compositions of stars. We analysed
high-resolution and high signal to noise HARPS spectra of 79 solar twin stars
in order to obtain precise determinations of their atmospheric parameters, ages
($\sigma$$\sim$0.4 Gyr) and chemical abundances ($\sigma$$<$0.01~dex) of 12
neutron-capture elements (Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, and Dy).
This valuable dataset allows us to study the [X/Fe]-age relations over a time
interval of $\sim$10 Gyr and among stars belonging to the thin and thick discs.
These relations show that i) the $s$-process has been the main channel of
nucleosynthesis of $n$-capture elements during the evolution of the thin disc;
ii) the thick disc is rich in $r$-process elements which suggests that its
formation has been rapid and intensive. %; iii) a chemical continuity between
the thin and thick discs is evident in the abundances of Ba. In addition, the
heavy (Ba, La, Ce) and light (Sr, Y, Zr) $s$-process elements revealed details
on the dependence between the yields of AGB stars and the stellar mass or
metallicity. Finally, we confirmed that both [Y/Mg] and [Y/Al] ratios can be
employed as stellar clocks, allowing ages of solar twin stars to be estimated
with an average precision of $\sim$0.5~Gyr.
