Bulk viscosity in hyperonic star and r-mode instability

Abstract: We consider a rotating neutron star with the presence of hyperons in its core, using an equation of state in an effective chiral model within the relativistic mean field approximation. We calculate the hyperonic bulk viscosity coefficient due to nonleptonic weak interactions. By estimating the damping timescales of the dissipative processes, we investigate its role in the suppression of gravitationally driven instabilities in the $r$-mode. We observe that $r$-mode instability remains very much significant for hyperon core temperature of around $10^8 $K, resulting in a comparatively larger instability window. We find that such instability can reduce the angular velocity of the rapidly rotating star considerably upto $\sim0.04 \Omega_K$, with $\Omega_K$ as the Keplerian angular velocity.