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Probing the origin of r-process elements in the universe represents amulti-disciplinary challenge. We review the observational evidence that probethe properties of r-process sites, and address them using galactic chemicalevolution simulations, binary population synthesis models, and nucleosynthesiscalculations. Our motivation is to define which astrophysical sites havesignificantly contributed to the total mass of r-process elements present inour Galaxy. We found discrepancies with the neutron star (NS-NS) mergerscenario. 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 whileaccounting for the delay-time distribution (DTD) of coalescence times($\propto~t^{-1}$) derived from short gamma-ray bursts and population synthesismodels. Steeper DTD functions ($\propto~t^{-1.5}$) or power laws combined witha strong burst of mergers before the onset of Type~Ia supernovae can reproducethe [Eu/Fe] trend, but this scenario is inconsistent with the similar fractionof short gamma-ray bursts and Type~Ia supernovae occurring in early-typegalaxies, and reduces the probability of detecting GW170817 in an early-typegalaxy. One solution is to assume an extra production site of Eu that would beactive 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 theEu production in the early universe, before the onset of Type~Ia supernovae.Rare classes of supernovae could be this additional r-process source, buthydrodynamic simulations still need to ensure the conditions for a robustr-process pattern.

Neutron Star Mergers Might not be the Only Source of r-Process Elements in the Milky Way