Understand
Employing a simplified version of the Israel-Stewart formalism for general-relativistic shear-viscous hydrodynamics, we perform axisymmetric general-relativistic simulations for a rotating neutron star surrounded by a massive torus, which can be formed from differentially rotating stars.
- We show that with our choice of a shear-viscous hydrodynamics formalism, the simulations can be stably performed for a long time scale.
- We also demonstrate that with a possibly high shear-viscous coefficient, not only viscous angular momentum transport works but also an outflow could be driven from a hot envelope around the neutron star for a time scale $\gtrsim 100\,$ms with the ejecta mass $\gtrsim 10^{-2}M_\odot$ which is comparable to the typical mass for dynamical ejecta of binary neutron star mergers.
- This suggests that massive neutron stars surrounded by a massive torus, which are typical outcomes formed after the merger of binary neutron stars, could be the dominant source for providing neutron-rich ejecta, if the effective shear viscosity is sufficiently high, i.e., if the viscous $\alpha$ parameter is $\gtrsim 10^{-2}$.