Actinide-Boosting r Process in Black-Hole–Neutron-Star Merger Ejecta

We examine nucleosynthesis in the ejecta of black-hole–neutron-star mergers based on the results of long-term neutrino-radiation-magnetohydrodynamics simulations for the first time. We find that the combination of dynamical and postmerger ejecta reproduces a solarlike r-process pattern. Moreover, the enhancement level of actinides is highly sensitive to the distribution of both the electron fraction and the velocity of the dynamical ejecta. Our result implies that the mean electron fraction of dynamical ejecta should be ≳0.05 in order to reconcile the nucleosynthetic abundances with those in r-process-enhanced, actinide-boost stars. Since the tidal ejecta preserve the neutron richness in the inner crust of premerging neutron stars, this result provides an important constraint for nuclear equations of state if black-hole–neutron-star mergers are responsible for actinide-boost stars. Published by the American Physical Society 2024