Muhammed Saleem, Hsin-Yu Chen, Daniel M. Siegel, Philippe Landry, Jocelyn S. Read, Kaile Wang

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.