2012ApJ...746..180W

We examine r-process nucleosynthesis in the neutrino-driven wind from the thick accretion disk (or "torus") around a black hole. Such systems are expected as remnants of binary neutron star or neutron star-black hole mergers. We consider a simplified, analytic, time-dependent evolution model of a 3 M_☉ central black hole surrounded by a neutrino emitting accretion torus with 90 km radius, which serves as basis for computing spherically symmetric neutrino-driven wind solutions. We find that ejecta with modest entropies (∼30 per nucleon in units of the Boltzmann constant) and moderate expansion timescales (∼100 ms) dominate in the mass outflow. The mass-integrated nucleosynthetic abundances are in good agreement with the solar system r-process abundance distribution if a minimal value of the electron fraction at the charged-particle freezeout, Y_e, min_∼ 0.2, is achieved. In the case of Y_e, min_∼ 0.3, the production of r-elements beyond A ∼ 130 does not reach to the third peak but could still be important for an explanation of the abundance signatures in r-process deficient stars in the early Galaxy. The total mass of the ejected r-process nuclei is estimated to be ∼1x10^–3 M_☉. If our model was representative, this demands a Galactic event rate of ∼2x10^–4/yr for black-hole-torus winds from merger remnants to be the dominant source of the r-process elements. Our result thus suggests that black-hole-torus winds from compact binary mergers have the potential to be a major, but probably not the dominant, production site of r-process elements.