Delayed outflows from black hole accretion tori following neutron star binary coalescence

TL;DR

This study models the long-term evolution of black hole accretion disks formed after neutron star mergers, revealing that they eject neutron-rich material capable of producing heavy r-process elements, thus contributing to nucleosynthesis.

Contribution

It provides the first detailed hydrodynamical simulations of viscous disk outflows post-merger, including electron fraction evolution and neutrino effects, highlighting their role in heavy element synthesis.

Findings

Approximately 10% of disk mass is ejected as neutron-rich wind.

Ejecta predominantly produce heavy r-process elements with A > 130.

Disk outflows contain about 1% helium, potentially affecting spectra.

Abstract

Expulsion of neutron-rich matter following the merger of neutron star (NS) binaries is crucial to the radioactively-powered electromagnetic counterparts of these events and to their relevance as sources of r-process nucleosynthesis. Here we explore the long-term (viscous) evolution of remnant black hole accretion disks formed in such mergers by means of two-dimensional, time-dependent hydrodynamical simulations. The evolution of the electron fraction due to charged-current weak interactions is included, and neutrino self-irradiation is modeled as a lightbulb that accounts for the disk geometry and moderate optical depth effects. Over several viscous times (~1s), a fraction ~10% of the initial disk mass is ejected as a moderately neutron-rich wind (Y_e ~ 0.2) powered by viscous heating and nuclear recombination, with neutrino self-irradiation playing a sub-dominant role. Although the properties of the outflow vary in time and direction, their mean values in the heavy-element production region are relatively robust to variations in the initial conditions of the disk and the magnitude of its viscosity. The outflow is sufficiently neutron-rich that most of the ejecta forms heavy r-process elements with mass number A >130, thus representing a new astrophysical source of r-process nucleosynthesis, distinct from that produced in the dynamical ejecta. Due to its moderately high entropy, disk outflows contain a small residual fraction ~1% of helium, which could produce a unique spectroscopic signature.