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Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE
Abstract:
New viscous neutrino-radiation hydrodynamics simulations are performed for
accretion disks surrounding a spinning black hole with low mass $3M_\odot$ and
dimensionless spin 0.8 or 0.6 in full general relativity, aiming at modeling
the evolution of a merger remnant of massive binary neutron stars or low-mass
black hole-neutron star binaries. We reconfirm the following results found by
previous studies of other groups: 15-30% of the disk mass is ejected from the
system with the average velocity of $\sim $5-10% of the speed of light for the
plausible profile of the disk as merger remnants. In addition, we find that for
the not extremely high viscous coefficient case, the neutron richness of the
ejecta does not become very high, because weak interaction processes enhance
the electron fraction during the viscous expansion of the disk before the onset
of the mass ejection, resulting in the suppression of the lanthanide synthesis.
For high-mass disks, the viscous expansion timescale is increased by a
longer-term neutrino emission, and hence, the electron fraction of the ejecta
becomes even higher. We also confirm that the mass distribution of the electron
fraction depends strongly on the magnitude of the given viscous coefficient.
This demonstrates that a first-principle magnetohydrodynamics simulation is
necessary for black hole-disk systems with sufficient grid resolution and with
sufficiently long timescale (longer than seconds) to clarify the
nucleosynthesis and electromagnetic signals from them.
