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

Abstract

Expulsion of neutron-rich matter following the merger of neutron star 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 discs 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 modelled as a lightbulb that accounts for the disc geometry and moderate optical depth effects. Over several viscous times (~1 s), a fraction of ~10 per cent of the initial disc mass is ejected as a moderately neutron-rich wind (Ye ~ 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 disc 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 {greater-than or approximate} 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, disc outflows contain a small residual fraction ~1 per cent of helium, which could produce a unique spectroscopic signature. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.