Igniting weak interactions in neutron star postmerger accretion disks

Abstract: The merger of two neutron stars or of a neutron star and a black hole typically results in the formation of a post-merger accretion disk. Outflows from disks may dominate the overall ejecta from mergers and be a major source of r-process nuclei in our universe. We explore the parameter space of such disks, their outflows, and r-process yields by performing three-dimensional general-relativistic magnetohydrodynamic (GRMHD) simulations with weak interactions and approximate neutrino transport. We discuss the mapping between initial binary parameters and the parameter space of resulting disks, chiefly characterized by their initial accretion rate. We demonstrate the existence of an ignition threshold for weak interactions at around $\sim 10^{-3} M_\odot\mathrm{s}^{-1}$ for typical parameters by means of analytic calculations and numerical simulations. We find a degenerate, self-regulated, neutrino-cooled regime above the threshold and an advection dominated regime below the threshold. Excess heating in the absence of neutrino cooling below the threshold leads to $\gtrsim 60\%$ of the initial disk mass being ejected in outflows, with typical velocities $\sim (0.1-0.2)c$, compared to $\lesssim 40\%$ at $\sim (0.1-0.15)c$ above the threshold. While disks below the threshold show suppressed production of light r-process elements, disks above the threshold can produce the entire range of r-process elements in good agreement with the observed solar system abundances. Disks below the ignition threshold may produce an overabundance of actinides seen in actinide-boost stars. As gravitational-wave detectors start to sample the neutron star merger parameter space, different disk realizations may be observable via their associated kilonova emission.