Mergers Fall Short: Nonmerger 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, multiple lines of evidence, including the observations of r -process abundances as inferred from stellar spectra of Milky Way disk stars, suggest that additional channels may be 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 paper, we combine gravitational-wave observations from LIGO–Virgo–KAGRA with data from short gamma-ray bursts, Galactic pulsars, and Galactic [Eu/Fe] versus [Fe/H] abundance observations to assess the contribution of these mergers to r -process enrichment in the Galactic disk. Our analysis employs a unified, likelihood-based inference framework that consistently propagates uncertainties in merger rates, delay-time distributions, mass- and spin-dependent ejecta yields, and stellar abundance measurements. We find that neither neutron star–black hole mergers nor fast-merging binary neutron star populations can dominate the additional prompt enrichment required by the disk-star data unless one invokes extreme assumptions about merger rates or yields. Within the adopted model, these results strongly favor a substantial prompt enrichment component beyond the merger channels considered here.