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Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE,General Relativity and Quantum Cosmology, gr-qc
Abstract:
We study the merger of black hole-neutron star binaries by fully
general-relativistic neutrino-radiation-hydrodynamics simulations throughout
the coalescence, particularly focusing on the role of neutrino irradiation in
dynamical mass ejection. Neutrino transport is incorporated by an approximate
transfer scheme based on the truncated moment formalism. While we fix the mass
ratio of the black hole to the neutron star to be 4 and the dimensionless spin
parameter of the black hole to be 0.75, the equations of state for
finite-temperature neutron-star matter are varied. The hot accretion disk
formed after tidal disruption of the neutron star emits a copious amount of
neutrinos with the peak total luminosity ~1--3x10^53 erg s^(-1) via thermal
pair production and subsequent electron/positron captures on free nucleons.
Nevertheless, the neutrino irradiation does not modify significantly the
electron fraction of the dynamical ejecta from the neutrinoless
beta-equilibrium value at zero temperature of initial neutron stars. The mass
of the wind component driven by neutrinos from the remnant disk is negligible
compared to the very neutron-rich dynamical component, throughout our
simulations performed until a few tens milliseconds after the onset of merger,
for the models considered in this study. These facts suggest that the ejecta
from black hole-neutron star binaries are very neutron rich and are expected to
accommodate strong r-process nucleosynthesis, unless magnetic or viscous
processes contribute substantially to the mass ejection from the disk. We also
find that the peak neutrino luminosity does not necessarily increase as the
disk mass increases, because tidal disruption of a compact neutron star can
result in a remnant disk with a small mass but high temperature.
