Neutron star mergers versus core-collapse supernovae as dominant r-process sites in the early Galaxy

The astrophysical nature of r-process sites is a long-standing mystery and many probable sources have been suggested, among them lower-mass core-collapse supernovae (in the range 8-10 M_⊙), higher-mass core-collapse supernovae (with masses ≥20 M_⊙) and neutron star mergers. In this work, we present a detailed inhomogeneous chemical evolution study that considers for the first time neutron star mergers as major r-process sources, and compare this scenario to the ones in which core-collapse supernovae act as dominant r-process sites. We conclude that, due to the lack of reliable iron and r-process yields as a function of progenitor mass, it is not possible at present to distinguish between the lower-mass and higher-mass supernovae scenarios within the framework of inhomogeneous chemical evolution. However, neutron-star mergers seem to be ruled out as the dominant r-process source, since their low rates of occurrence would lead to r-process enrichment that is not consistent with observations at very low metallicities. Additionally, the considerable injection of r-process material by a single neutron-star merger leads to a scatter in [r-process/Fe] ratios at later times which is much too large compared to observations.