Comprehensive nucleosynthesis analysis for ejecta of compact binary mergers

TLDR

This study investigates r‑process nucleosynthesis in ejecta from compact binary mergers and their remnant black‑hole torus systems. The authors simulate the evolution of black‑hole accretion tori for seconds using a Newtonian hydrodynamics code with viscosity, pseudo‑Newtonian gravity, and a two‑moment neutrino transport scheme, guided by relativistic NS‑NS and NS‑BH merger models that produce ~3–6 M⊙ black holes and 0.03–0.3 M⊙ tori, and analyze both dynamical ejecta and neutrino/viscous outflows. They find that viscously driven outflows remove ~20–25 % of the torus mass, neutrino winds add at most ~1 % but enhance the ejecta, the torus ejecta span electron fractions 0.1–0.6 and entropies producing elements from A≈80 to the actinides with subdominant heavy nuclei, and the combined CBM and torus ejecta reproduce solar abundances for A>90, while variations in torus contributions could explain observed light‑to‑heavy element ratios and the composition of heavy‑element‑deficient stars.

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 Msun BHs with rotation parameters of A~0.8 and tori of 0.03-0.3 Msun. 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% of the initial accretion-torus mass are lost by viscously driven outflows, neutrino-powered winds contribute at most another ~1%, 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.