4.7 Article

Long-term 3D MHD simulations of black hole accretion discs formed in neutron star mergers

Journal

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 513, Issue 2, Pages 2689-2707

Publisher

OXFORD UNIV PRESS
DOI: 10.1093/mnras/stac948

Keywords

accretion, accretion discs; MHD; Neutrinos; nuclear reactions, nucleosynthesis, abundances; stars: black holes; stars: neutron

Funding

  1. Natural Sciences and Engineering Research Council of Canada (NSERC) [RGPIN-2017-04286]
  2. DOE
  3. Advanced Simulation and Computing Program
  4. Scientific Discovery through Advanced Computing Program
  5. WestGrid
  6. Shared Hierarchical Academic Research Computing Network (SHARCNET)
  7. Calcul Quebec
  8. Compute Canada
  9. Canada Foundation for Innovation, the Government of Ontario (Ontario Research Fund -Research Excellence)
  10. University of Toronto
  11. Faculty of Science at the University of Alberta

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In this study, we modified the solver to simulate accretion discs and found that a significant amount of mass ejection can occur during the evolution of accretion discs around black hole remnants. The velocity distribution and electron fraction distribution of the ejected mass exhibit characteristic features.
We examine the long-term evolution of accretion tori around black hole (BH) remnants of compact object mergers involving at least one neutron star, to better understand their contribution to kilonovae and the synthesis of r-process elements. To this end, we modify the unsplit magnetohydrodynamic (MILD) solver in FLASH 4.5 to work in non-uniform three-dimensional spherical coordinates, enabling more efficient coverage of a large dynamic range in length scales while exploiting symmetries in the system. This modified code is used to perform BII accretion disc simulations that vary the initial magnetic field geometry and disc compactness, utilizing a physical equation of state, a neutrino leakage scheme for emission and absorption, and modelling the BII's gravity with a pseudo-Newtonian potential. Simulations run for long enough to achieve a radiatively inefficient state in the disc. We find robust mass ejection with both poloidal and toroidal initial field geometries, and suppressed outflow at high disc compactness. With the included physics, we obtain bimodal velocity distributions that trace back to mass ejection by magnetic stresses at early times, and to thermal processes in the radiatively inefficient state at late times. The electron fraction distribution of the disc outflow is broad in all models, and the ejecta geometry follows a characteristic hourglass shape. We test the effect of removing neutrino absorption or nuclear recombination with axisymmetric models, finding similar to 50 per cent less mass ejection and more neutron-rich composition without neutrino absorption, and a subdominant contribution from nuclear recombination. Tests of the MHD and neutrino leakage implementations are included.

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