Measuring the contribution of neutron star-black hole mergers to the production of heavy metals

Hsin-Yu Chen, Salvatore Vitale, Francois Foucart

Published 2021-07-06Version 1

The origin of the heavy elements in the Universe is not fully determined. Neutron star-black hole (NSBH) and neutron star-neutron star mergers may both produce heavy elements via rapid neutron-capture process (r-process). We use the recent detection of gravitational waves from NSBHs, improved measurements of neutron star equation-of-state, and the most modern numerical simulations of the ejected materials from binary collisions to investigate the production of heavy elements from binary mergers. As the amount of ejecta depends on the mass and spin distribution of compact objects, as well as on the equation-of-state of neutron stars, we consider various models for these quantities, informed by gravitational-wave and pulsar data. We find that even in the most favorable model, neutron star-black hole mergers are unlikely to account for more than 77% of the r-process elements in the local Universe. If black holes have preferentially small spins, this bound decreases to 35%. Finally, if black hole spins are small and there is a dearth of low mass ($<5M_{\odot}$) black holes in NSBH binaries, the NSBH contribution to r-process elements is negligible.