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 produce rapid neutron-capture nucleosynthesis (r-process) elements. However, it remains an open question whether these mergers can account for all the r-process element enrichment in the Milky Way's history. Here, we constrain the contributions of the BNS channel using astrophysical neutron star observations. The rate and mass distributions are constrained by LIGO/Virgo/Kagra through the latest catalog GWTC-3, the neutron star equation of state by gravitational-wave, radio, and X-ray observations, and the delay time distribution by short gamma-ray burst (GRB) host galaxy associations. We present a Bayesian framework to consistently combine these lines of observations with abundance data to quantify the contribution and uncertainties of single and multiple astrophysical enrichment sources. Whereas we obtain a distribution of per-event BNS r-process element yields consistent with geophysical and astrophysical abundance constraints, BNS-only enrichment scenarios are inconsistent with the observed r-process abundance trend of disk stars in the Galaxy. Using stellar abundance observations instead of the short GRB constraints, we infer a shorter delay time distribution with power-law index $\alpha\leq -2.0$ and minimum delay time $t_{\rm min}\leq 40$ Myr at 90\% confidence. Such delay times are in tension with those predicted by standard BNS formation models. Alternatively, we confirm that a two-channel scenario, in which the second channel tracks the star formation history without significant delay, can account for both Galactic stellar and short GRB observations. We show 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.