Probing the origin of r-process elements in the universe represents a multidisciplinary challenge. We review the observational evidence that probes 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. When we assume that 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 (∝t ⁻¹) derived from short gamma-ray bursts (GRBs) and population synthesis models. Steeper DTD functions (∝t ^−1.5) or power laws combined with a strong burst of mergers before the onset of supernovae (SNe) Ia can reproduce the [Eu/Fe] trend, but this scenario is inconsistent with the similar fraction of short GRBs and SNe Ia occurring in early-type galaxies, and it reduces the probability of detecting GW170817 in an early-type galaxy. One solution is to assume an additional production site of Eu that would be active in the early universe, but would fade away with increasing metallicity. If this is correct, this additional site could be responsible for roughly 50% of the Eu production in the early universe before the onset of SNe Ia. 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.

In this review, I will discuss the comparison between model results and observational data for the Milky Way, the predictive power of such models as well as their limits. Such a comparison, known as Galactic archaeology, allows us to impose constraints on stellar nucleosynthesis and timescales of formation of the various Galactic components (halo, bulge, thick disk and thin disk).

In recent papers, Mannucci et al. and Mannucci, Della Valle & Panagia suggested, on the basis of observational arguments, that there is a bimodal

Context.The recentGaiaData Release 3 (DR3) represents an unparalleled revolution in Galactic archaeology, providing numerous radial velocities and chemical abundances for millions of stars as well as all-sky coverage.AimsWe present a new chemical evolution model for the Galactic disc components (high- and low-αsequence stars) designed to reproduce the new abundance ratios provided by the General Stellar Parametriser-spectroscopy module for theGaiaDR3 and constrained by the detailed star formation (SF) histories for both the thick and thin disc stars inferred from previousGaiareleases.Methods.Sophisticated modelling based on previousGaiareleases have found evidence for narrow episodes of enhanced SF inferred in recent time. Additionally,GaiaDR3 indicated the presence of young (massive) low-αdisc stars that show evidence of a recent chemical impoverishment in several elements. In order to reproduce these observables, we propose a new chemical evolution model in which the low-αsequence is generated by two distinct infall episodes. Hence, in this study we compareGaiaDR3 chemical abundances with the predictions of a three-infall chemical evolution model for the high- and low-αcomponents.ResultsThe proposed three-infall chemical evolution model nicely reproduces the main features of the abundance ratio [X/Fe] versus [M/H] (X=Mg, Si, Ca, Ti,α) ofGaiaDR3 stars in different age bins for the consideredαelements. Moreover, the most recent gas infall – which started ∼2.7 Gyr ago – allowed us to predict accurately predict theGaiaDR3 young population which has experienced a recent chemical impoverishment.Conclusions.We extended previous chemical evolution models designed to reproduce APOGEE and APOKASC data in order to predict newGaiaDR3 chemical abundances. To this aim, we proposed a three-infall chemical evolution model to better trace both (i) the young population inGaiaDR3 with evidence of chemical impoverishment and (ii) the SF history from previousGaiareleases.