Secular Outflows from 3D-MHD Hypermassive Neutron Star Accretion Disk Systems

Abstract: Magnetized hypermassive neutron stars (HMNSs) have been proposed as a way for neutron star (NS) mergers to produce high electron fraction, high velocity ejecta, as required by kilonova models to explain the observed light curve of GW170817. The HMNS drives outflows through neutrino energy deposition and mechanical oscillations, and raises the electron fraction of outflows through neutrino interactions before collapsing to a black hole (BH). Here we perform 3D numerical simulations of HMNS-torus systems in ideal magnetohydrodynamics, using a leakage/absorption scheme for neutrino transport, the nuclear APR equation of state, and Newtonian self-gravity, with a pseudo-Newtonian potential added after BH formation. Due to the uncertainty in the HMNS collapse time, we choose two different parameterized times to induce collapse. We also explore two initial magnetic field geometries in the torus, and evolve the systems until the outflows diminish significantly ($\sim 1 - 2$ $\mathrm{s}$). We find bluer, faster outflows as compared to equivalent BH-torus systems, producing $M\sim 10^{-3} M_\odot$ of ejecta with $Y_e \geq 0.25$ and $v \geq 0.25c$ by the simulation end. Approximately half the outflows are launched in disk winds at times $t\lesssim 500$ $\mathrm{ms}$, with a broad distribution of electron fractions and velocities, depending on the initial condition. The remaining outflows are thermally-driven, characterized by lower velocities and electron fractions. Nucleosynthesis with tracer particles shows patterns resembling solar abundances in all models. Although outflows from our simulations do not match those inferred from two-component modelling of the GW170817 kilonova, self-consistent multidimensional detailed kilonova models are required to determine if our outflows can power the blue kilonova.