Comprehensive nucleosynthesis analysis for ejecta of compact binary mergers

Abstract

We present the first comprehensive study of r-process element nucleosynthesis in the ejecta of compact binary mergers (CBMs) and their relic black hole (BH)–torus systems. The evolution of the BH–accretion tori is simulated for seconds with a Newtonian hydrodynamics code including viscosity effects, pseudo-Newtonian gravity for rotating BHs, and an energy-dependent two-moment closure scheme for the transport of electron neutrinos and antineutrinos. The investigated cases are guided by relativistic double neutron star (NS–NS) and NS–BH merger models, producing ∼3–6 M⊙ BHs with rotation parameters of ABH ∼ 0.8 and tori of 0.03–0.3 M⊙. Our nucleosynthesis analysis includes the dynamical (prompt) ejecta expelled during the CBM phase and the neutrino and viscously driven outflows of the relic BH–torus systems. While typically ∼20–25 per cent of the initial accretion-torus mass are lost by viscously driven outflows, neutrino-powered winds contribute at most another ∼1 per cent, but neutrino heating enhances the viscous ejecta significantly. Since BH–torus ejecta possess a wide distribution of electron fractions (0.1–0.6) and entropies, they produce heavy elements from A ∼ 80 up to the actinides, with relative contributions of A ≳ 130 nuclei being subdominant and sensitively dependent on BH and torus masses and the exact treatment of shear viscosity. The combined ejecta of CBM and BH–torus phases can reproduce the solar abundances amazingly well for A ≳ 90. Varying contributions of the torus ejecta might account for observed variations of lighter elements with 40 ≤ Z ≤ 56 relative to heavier ones, and a considerable reduction of the prompt ejecta compared to the torus ejecta, e.g. in highly asymmetric NS–BH mergers, might explain the composition of heavy-element deficient stars.