Mergers Fall Short: Non-merger Channels Required for Galactic Heavy Element Production

Abstract: Since the discovery of the binary neutron star merger GW170817 and its associated kilonova, neutron star mergers have been established as a key production channel for $r$-process elements in the Universe. However, observations of $r$-process abundances as inferred from stellar spectra of Milky Way disk stars, together with chemical evolution modeling, suggest that additional channels are needed to fully account for the $r$-process element enrichment in the Milky Way. Neutron star-black hole mergers and fast-merging binary neutron star systems are among the leading alternative candidates. In this study, we combine gravitational-wave observations from LIGO-Virgo-KAGRA with data from short gamma-ray bursts, Galactic pulsars, and Galactic [Eu/Fe] vs [Fe/H] abundance observations to assess the contribution of these mergers to $r$-process enrichment in the Galactic disk. We find that neither neutron star-black hole mergers nor fast-merging binary neutron star populations can serve as the dominant additional channel without generating strong tension with existing observations and theoretical expectations. These results constrain the viable sources of Galactic $r$-process enrichment and underscore the necessity of non-merger production channels.