  Probing the origin of r-process elements in the universe represents a
multi-disciplinary challenge. We review the observational evidence that probe
the properties of r-process sites, and address them using galactic chemical
evolution simulations, binary population synthesis models, and nucleosynthesis
calculations. Our motivation is to define which astrophysical sites have
significantly contributed to the total mass of r-process elements present in
our Galaxy. We found discrepancies with the neutron star (NS-NS) merger
scenario. Assuming they are the only site, the decreasing trend of [Eu/Fe] at
[Fe/H]\,$>-1$ in the disk of the Milky Way cannot be reproduced while
accounting for the delay-time distribution (DTD) of coalescence times
($\propto~t^{-1}$) derived from short gamma-ray bursts and population synthesis
models. Steeper DTD functions ($\propto~t^{-1.5}$) or power laws combined with
a strong burst of mergers before the onset of Type~Ia supernovae can reproduce
the [Eu/Fe] trend, but this scenario is inconsistent with the similar fraction
of short gamma-ray bursts and Type~Ia supernovae occurring in early-type
galaxies, and reduces the probability of detecting GW170817 in an early-type
galaxy. One solution is to assume an extra production site of Eu that would be
active in the early universe, but would fade away with increasing metallicity.
If this is correct, this extra site could be responsible for roughly 50% of the
Eu production in the early universe, before the onset of Type~Ia supernovae.
Rare classes of supernovae could be this additional r-process source, but
hydrodynamic simulations still need to ensure the conditions for a robust
r-process pattern.
