Bulk viscosity in hyperonic star and r-mode instability

TL;DR

This paper investigates how hyperonic bulk viscosity affects r-mode instability in rotating neutron stars, showing that hyperons can significantly dampen gravitational wave-driven instabilities at certain temperatures.

Contribution

It introduces a calculation of hyperonic bulk viscosity within an effective chiral model and assesses its impact on r-mode damping in hyperonic neutron stars.

Findings

Hyperons increase bulk viscosity, affecting r-mode damping.

R-mode instability remains significant at temperatures around 10^8 K.

Instability can reduce star’s angular velocity up to about 4% of Keplerian velocity.

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.