Inference of multi-channel r-process element enrichment in the Milky Way using binary neutron star merger observations

Observations of GW170817 strongly suggest that binary neutron star (BNS) mergers can produce rapid neutron-capture nucleosynthesis (r-process) elements. However, it remains an open question whether BNS mergers can account for all the r-process element enrichment in the Milky Way's history. Here we demonstrate that a BNS population model informed by multimessenger neutron star observations predicts a merger rate and per-event r-process element yield consistent with geophysical and astrophysical abundance constraints. If BNS mergers are to explain the r-process enrichment of stars in the Galaxy, we further show using a one-zone Galactic chemical evolution model that they have to merge shortly after the formation of their progenitors, with a delay time distribution of power-law index α≤ -2.0 and minimum delay time t_ min≤ 40 Myr at 90 tension with those inferred from short gamma-ray bursts (sGRBs) and those predicted by standard BNS formation models. However, we find that a two-channel enrichment scenario, where the second channel follows the star formation history, can account for both Galactic stellar and sGRB observations. Our results suggest that 40-90 was produced by a star-formation-tracking channel, rather than BNS mergers with sGRB-compatible delay times.