Bulk viscosities and r-mode of massive neutron stars

The r-modes are oscillatory modes that exist in neutron stars. They are characterized by Coriolis force acting as the restoring force and are particularly important because they have been shown to drive gravitational radiation (GR) unstable for all rotational frequencies of perfect fluid stars. Dissipative phenomena such as shear and bulk viscosities suppress the growth of GR driven instabilities. We report that there are two separate instability windows for the model considered. It is seen that between temperatures of 1.8 × 10⁸K and 2.67 × 10⁹K, the instability is completely suppressed due to hyperon bulk viscosity. The minima of the two windows occur at ∼ 0.15Ω_K. We also find that the instability can reduce the angular velocity up to 0.3Ω_K as it crosses the first window, and it doesn't lose any angular momentum as it crosses the second window