Post-merger evolution of a neutron star-black hole binary with neutrino transport

We present a first simulation of the post-merger evolution of a black hole-neutron star binary in full general relativity using an energy-integrated general-relativistic truncated moment formalism for neutrino transport. We describe our implementation of the moment formalism and important tests of our code, before studying the formation phase of an accretion disk after a black hole-neutron star merger. We use as initial data an existing general-relativistic simulation of the merger of a neutron star of mass $1.4{M}_{\ensuremath{\bigodot}}$ with a black hole of mass $7{M}_{\ensuremath{\bigodot}}$ and dimensionless spin ${\ensuremath{\chi}}_{\mathrm{BH}}=0.8$. Comparing with a simpler leakage scheme for the treatment of the neutrinos, we find noticeable differences in the neutron-to-proton ratio in and around the disk, and in the neutrino luminosity. We find that the electron neutrino luminosity is much lower in the transport simulations, and that both the disk and the disk outflows are less neutron rich. The spatial distribution of the neutrinos is significantly affected by relativistic effects, due to large velocities and curvature in the regions of strongest emission. Over the short time scale evolved, we do not observe purely neutrino-driven outflows. However, a small amount of material ($3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}{M}_{\ensuremath{\bigodot}}$) is ejected in the polar region during the circularization of the disk. Most of that material is ejected early in the formation of the disk, and is fairly neutron rich (electron fraction ${Y}_{e}\ensuremath{\sim}0.15--0.25$). Through r-process nucleosynthesis, that material should produce high-opacity lanthanides in the polar region, and could thus affect the light curve of radioactively powered electromagnetic transients. We also show that by the end of the simulation, while the bulk of the disk remains neutron rich (${Y}_{e}\ensuremath{\sim}0.15--0.2$ and decreasing), its outer layers have a higher electron fraction: 10% of the remaining mass has ${Y}_{e}>0.3$. As that material would be the first to be unbound by disk outflows on longer time scales, and as composition evolution is slower at later times, the changes in ${Y}_{e}$ experienced during the formation phase of the disk could have an impact on nucleosynthesis outputs from neutrino-driven and viscously driven outflows. Finally, we find that the effective viscosity due to momentum transport by neutrinos is unlikely to have a strong effect on the growth of the magnetorotational instability in the post-merger accretion disk.

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