The r-process in the neutrino-driven wind from a black-hole torus

TL;DR

This study models r-process nucleosynthesis in neutrino-driven winds from black-hole accretion tori post-merger, showing they can produce significant r-process elements consistent with solar abundances, but likely are not the sole source.

Contribution

It introduces a simplified, time-dependent model of black-hole torus winds for r-process nucleosynthesis, highlighting their potential role in galactic chemical evolution.

Findings

Ejecta with modest entropy (~30) and expansion times (~100ms) dominate the outflow.

Nucleosynthesis results align with solar r-process abundances for Ye,min~0.2.

Estimated ejected r-process mass is ~10^-3 solar masses, suggesting a galactic event rate of ~2x10^-4 yr^-1.

Abstract

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 emnants of binary neutron star or neutron star -- black hole mergers. We consider a simplified, analytic, time-dependent evolution model of a 3M_sun central black hole surrounded by a neutrino emitting accretion torus with 90km 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 (~100ms) 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, Ye,min~0.2, is achieved. In the case of Ye,min~0.3, the production of r-elements beyond A~130 does not reach to the third peak but could be still 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_sun. If our model was representative, this demands a Galactic event rate of ~2x10^-4 yr^-1 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.