The heavy-elements heritage of the falling sky
  Recent dynamical analysis based on Gaia data have revealed major accretion
events in Milky Way's history. Nevertheless, our understanding of the
primordial Galaxy is hindered because the bona fide identification of the most
metal-poor and correspondently oldest accreted stars remains challenging.
Contrary to alpha-elements, neutron-capture elements present unexplained large
abundance spreads for low metallicity stars, that could result from a mixture
of formation sites. We have analysed the abundances of yttrium, europium,
magnesium and iron in Milky Way satellite galaxies, field halo stars and
globular clusters. The chemical information has been complemented with orbital
parameters based on Gaia data. In particular, the orbit's average inclination
has been considered. The [Y/Eu] abundance behaviour with respect to the [Mg/Fe]
turnovers for satellite galaxies of different masses reveals that higher
luminosity systems, for which the [Mg/Fe] abundance declines at higher
metallicities, present enhanced [Y/Eu] abundances, particularly in the [Fe/H]
regime between -2.25 and -1.25 dex. In addition, the analysis has uncovered a
chemo-dynamical correlation for both globular clusters and field stars of the
Galactic halo, accounting for about half of the [Y/Eu] abundance spread. [Y/Eu]
under-abundances typical of protracted chemical evolutions, are preferentially
observed in polar-like orbits, pointing to a possible anisotropy in the
accretion processes. Our results strongly suggest that the observed [Y/Eu]
abundance spread in the Milky Way halo could result from a mixture of systems
with different masses. They also highlight that both nature and nurture are
relevant to the Milky Way formation, since its primordial epochs, opening new
pathways for chemical diagnostics of our Galaxy building up.
