Effect of hyperon bulk viscosity on neutron-star r-modes

Neutron stars are expected to contain a significant number of hyperons in addition to protons and neutrons in the highest density portions of their cores. Following the work of Jones, we calculate the coefficient of bulk viscosity due to nonleptonic weak interactions involving hyperons in neutron-star cores, including new relativistic and superfluid effects. We evaluate the influence of this new bulk viscosity on the gravitational radiation driven instability in the r-modes. We find that the instability is completely suppressed in stars with cores cooler than a few times ${10}^9\hspace{0.5em}\mathrm{K},$ but that stars rotating more rapidly than 10–30% of maximum are unstable for temperatures around ${10}^{10}\hspace{0.5em}\mathrm{K}.$ Since neutron-star cores are expected to cool to a few times ${10}^9\hspace{0.5em}\mathrm{K}$ within seconds (much shorter than the r-mode instability growth time) due to direct Urca processes, we conclude that the gravitational radiation instability will be suppressed in young neutron stars before it can significantly change the angular momentum of the star.