Secular outflows from 3D MHD hypermassive neutron star accretion disc systems
Abstract
Magnetized hypermassive neutron stars (HMNSs) have been proposed as a way for neutron star mergers to produce high electron fraction, highvelocity 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 HMNStorus systems in ideal magnetohydrodynamics, using a leakage/absorption scheme for neutrino transport, the nuclear APR equation of state, and Newtonian selfgravity, with a pseudoNewtonian potential added after BH formation. Due to the uncertainty in the HMNS collapse time, we choose two different parametrized 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 BHtorus systems, producing M ~ 10^{3} M_{⊙} of ejecta with Y_{e} ≥ 0.25 and v ≥ 0.25c by the simulation end. Approximately half the outflows are launched in disc 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 twocomponent modelling of the GW170817 kilonova, selfconsistent multidimensional detailed kilonova models are required to determine whether our outflows can power the blue kilonova.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 November 2023
 DOI:
 10.1093/mnras/stad2653
 arXiv:
 arXiv:2308.16237
 Bibcode:
 2023MNRAS.526..952F
 Keywords:

 accretion;
 accretion discs;
 MHD;
 neutrinos;
 nuclear reactions;
 nucleosynthesis;
 abundances;
 stars: black holes;
 stars: neutron;
 Astrophysics  High Energy Astrophysical Phenomena;
 General Relativity and Quantum Cosmology;
 Nuclear Theory
 EPrint:
 15 pages, 10 figures. Accepted for publication in MNRAS