Secular outflows from 3D MHD hypermassive neutron star accretion disc systems

Journal Article · 03 September 2023 · Monthly Notices of the Royal Astronomical Society

DOI: https://doi.org/10.1093/mnras/stad2653 · OSTI ID:2424705

Fahlman, Steven  ^[1]; Fernández, Rodrigo  ^[1]; Morsink, Sharon  ^[1]

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1. Univ. of Alberta, Edmonton, AB (Canada)

Magnetized hypermassive neutron stars (HMNSs) have been proposed as a way for neutron star 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, in this study, 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 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 BH–torus 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 two-component modelling of the GW170817 kilonova, self-consistent multidimensional detailed kilonova models are required to determine whether our outflows can power the blue kilonova.

• Research Organization: US Department of Energy (USDOE), Washington, DC (United States). Office of Science, Advanced Scientific Computing Research (ASCR)
• Sponsoring Organization: Natural Sciences and Engineering Research Council of Canada (NSERC); USDOE National Nuclear Security Administration (NNSA), Office of Defense Programs (DP)
• OSTI ID: 2424705
• Journal Information: Monthly Notices of the Royal Astronomical Society, Journal Name: Monthly Notices of the Royal Astronomical Society Journal Issue: 1 Vol. 526; ISSN 0035-8711; ISSN 1365-2966
• Publisher: Oxford University PressCopyright Statement
• Country of Publication: United States
• Language: English