On a role of corotation radius in the low $T/W$ dynamical instability of differentially rotating stars

TL;DR

This paper explores how the corotation radius influences low $T/W$ dynamical instability in differentially rotating stars, highlighting the role of wave over-reflection and trapping in the instability mechanism.

Contribution

It introduces a linear perturbation model focusing on corotation radius effects and demonstrates how wave over-reflection contributes to the low $T/W$ instability.

Findings

Corotation radius is crucial in the instability mechanism.

Over-reflection of sound waves explains eigenfrequency dependence.

Trapped sound waves between surface and corotation radius drive instability.

Abstract

We investigate the nature of so-called low $T/W$ dynamical instability in a differentially rotating star by focusing on the role played by the corotation radius of the unstable oscillation modes. An one dimensional model of linear perturbation, which neglects dependence of variables on the coordinate along the rotational axis of the star, is solved to obtain stable and unstable eigenmodes. A linear eigenmode having a corotation radius, at which azimuthal pattern speed of the mode coincides with the stellar angular velocity, is categorized to either a complex (growing or damping) mode or a purely real mode belonging to a continuous spectrum of frequency. We compute canonical angular momentum and its flux to study eigenmodes with corotation radius. In a dynamically unstable mode, sound wave transports its angular momentum in such a way that the absolute value of the angular momentum is increased on both sides of the corotation radius. We further evaluate growth of amplitude of reflected sound wave incident to a corotation point and find that the over-reflection of the wave and the trapping of it between the corotation radius and the surface of the star may qualitatively explain dependences of eigenfrequencies on the stellar differential rotation. The results suggest that the low $T/W$ instability may be caused by over-reflection of sound waves trapped mainly between the surface of the star and a corotation radius.