Mergers Fall Short: Nonmerger Channels Required for Galactic Heavy Element Production

Abstract

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.