Production of the entire range of r-process nuclides by black hole accretion disc outflows from neutron star mergers

Abstract

We consider $r$-process nucleosynthesis in outflows from black hole accretion discs formed in double neutron star and neutron star -- black hole mergers. These outflows, powered by angular momentum transport processes and nuclear recombination, represent an important -- and in some cases dominant -- contribution to the total mass ejected by the merger. Here we calculate the nucleosynthesis yields from disc outflows using thermodynamic trajectories from hydrodynamic simulations, coupled to a nuclear reaction network. We find that outflows produce a robust abundance pattern around the second $r$-process peak (mass number $A \sim 130$), independent of model parameters, with significant production of $A < 130$ nuclei. This implies that dynamical ejecta with high electron fraction may not be required to explain the observed abundances of $r$-process elements in metal poor stars. Disc outflows reach the third peak ($ A \sim 195$) in most of our simulations, although the amounts produced depend sensitively on the disc viscosity, initial mass or entropy of the torus, and nuclear physics inputs. Some of our models produce an abundance spike at $A = 132$ that is absent in the Solar system $r$-process distribution. The spike arises from convection in the disc and depends on the treatment of nuclear heating in the simulations. We conclude that disc outflows provide an important -- and perhaps dominant -- contribution to the $r$-process yields of compact binary mergers, and hence must be included when assessing the contribution of these systems to the inventory of $r$-process elements in the Galaxy.