The AMBRE Project: r-process elements in the Milky Way thin and thick
  discs
  The chemical evolution of neutron capture elements in the Milky Way disc is
still a matter of debate. We aim to understand the chemical evolution of
r-process elements in Milky Way disc. We focus on three pure r-process elements
Eu, Gd, and Dy. Using high-resolution FEROS, HARPS, and UVES spectra from the
ESO archive, we perform a homogeneous analysis on 6500 FGK Milky Way stars,
thanks to the automatic optimization pipeline GAUGUIN. We present abundances of
Ba (5057 stars), Eu (6268 stars), Gd (5431 stars), and Dy (5479 stars). We
chemically characterize the thin and the thick discs, and a metal-rich
alpha-rich population. We find that the [Eu/Fe] ratio follows a continuous
sequence from the thin disc to the thick disc as a function of the metallicity.
In thick disc stars, the [Eu/Ba] ratio is found to be constant, while the
[Gd/Ba] and [Dy/Ba] ratios decrease as a function of the metallicity. These
observations clearly indicate a different nucleosynthesis history in the thick
disc between Eu and Gd-Dy. We also find that the alpha-rich metal-rich stars
are also enriched in r-process elements (like thick disc stars), but their
[Ba/Fe] is very different from thick disc stars. Finally, we find that the
[r/\alpha] ratio tends to decrease with metallicity, indicating that supernovae
of different properties probably contribute differently to the synthesis of
r-process elements and \alpha-elements. We provide average abundance trends for
[Ba/Fe] and [Eu/Fe] with rather small dispersions, and for the first time for
[Gd/Fe] and [Dy/Fe]. This data may help to constrain chemical evolution models
of Milky Way r- and s-process elements and the yields of massive stars.
Including yields of neutron-star or black hole mergers is now crucial if we
want to quantitatively compare observations to Galactic chemical evolution
models.
