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

Abstract: 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 $\alpha\leq -2.0$ and minimum delay time $t_{\rm min}\leq 40$ Myr at 90% confidence.Such short delay times are in tension with those predicted by standard BNS formation models and those observationally inferred from samples of short gamma-ray bursts (sGRBs). 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 45-90% of the r-process abundance in the Milky Way today was produced by a star-formation-tracking channel, rather than BNS mergers with significant delay times.