$r$-mode stabilization in rotating hyperon-rich neutron stars and its implications for GW190814

TL;DR

This paper explores how hyperon-induced bulk viscosity can stabilize rapidly rotating neutron stars with masses exceeding 2.5 solar masses, offering an alternative explanation for the GW190814 secondary as a hyperon-rich neutron star.

Contribution

It introduces a comprehensive model combining rotation, thermal effects, and hyperon bulk viscosity to analyze the stability of massive neutron stars, extending previous static or simplified approaches.

Findings

Hyperon bulk viscosity can effectively damp $r$-mode instabilities.

Rapid rotation combined with hyperon effects can stabilize neutron stars above 2.5 solar masses.

A hyperon-rich neutron star could explain GW190814's secondary object.

Abstract

The GW190814 event, involving a black hole of mass $22.2$--$24.3 M_{\odot}$ and a compact object of mass $2.50$--$2.67 M_{\odot}$, challenges our understanding of the mass gap between the heaviest neutron stars and the lightest black holes. If the secondary is a neutron star exceeding $2.5 M_{\odot}$, hyperons are likely to appear in its core, softening the equation of state. Rapid rotation can offset some of this softening, enabling higher maximum masses, but it may simultaneously excite the Chandrasekhar--Friedman--Schutz $r$-mode instability. Bulk viscosity arising from nonleptonic weak interactions in hyperonic matter provides an efficient damping mechanism that can stabilize such configurations. In this work, we investigate the combined effects of rotation, thermal evolution, and hyperon-induced bulk viscosity on the stability of massive neutron stars. We demonstrate a direct connection between the suppression of $r$-mode instabilities and the long-term dynamical stability of hyperon-rich stars, offering a plausible interpretation of the GW190814 secondary as a rapidly rotating, hyperon-rich neutron star rather than a low-mass black hole. Our unified framework extends beyond previous studies restricted to static equations of state or extreme viscous damping assumptions, providing new insights into the stability of massive, exotic neutron star configurations.