Viscous damping of r-mode oscillations in compact stars with quark matter

TL;DR

This paper investigates how different phases of quark matter inside compact stars affect the damping of r-mode oscillations, revealing conditions under which stars can rotate rapidly without instability, and predicting a population of fast-spinning binaries.

Contribution

It provides detailed calculations of viscous damping timescales for r-modes in stars with various quark matter phases, including CFL and ungapped phases, and explores their impact on stellar stability.

Findings

Ungapped quark matter supports damping at 10^8 K to 5x10^9 K.

Pure CFL stars are not damped at 10^10 K to 10^11 K.

Hybrid stars can have large spin frequencies outside instability windows.

Abstract

We determine characteristic timescales for the viscous damping of r-mode oscillations in rapidly rotating compact stars that contain quark matter. We present results for the color-flavor-locked (CFL) phase of dense quark matter, in which the up, down and strange quarks are gapped, as well as the normal (ungapped) quark phase. While the ungapped quark phase supports a temperature window between 10^8 K and 5x10^9 K where the r-mode is damped even for rapid rotation, the r-mode in a rapidly rotating pure CFL star is not damped in the temperature range 10^10 K - 10^11 K. Rotating hybrid stars with quark matter cores display an instability window whose width is determined by the amount of quark matter present, and they can have large spin frequencies outside this window. Except at high temperatures T > 10^10 K, the presence of a quark phase allows for larger critical frequencies and smaller spin-periods compared to rotating neutron stars. If low-mass X-ray binaries contain a large amount of ungapped or CFL quark matter, then our estimates of the r-mode instability suggest that there should be a population of rapidly rotating binaries at frequencies greater than 1000 Hz which have not yet been observed.