Modal analysis of gravitational instabilities in nearly Keplerian, counter-rotating collisionless discs

TL;DR

This paper analyzes the stability of counter-rotating, self-gravitating stellar discs around a central mass using a modified WKB approach, revealing that all modes are unstable with growth rates influenced by disc mass ratios and azimuthal wave numbers.

Contribution

It introduces a modal analysis of gravitational instabilities in counter-rotating stellar discs using a novel WKB formulation, highlighting the instability characteristics for different azimuthal modes.

Findings

All eigenmodes are unstable with complex eigenspectra.

Eigenfunctions are more compact for m=1 than m=2.

Pattern speeds are always prograde relative to the more massive disc.

Abstract

We present a modal analysis of instabilities of counter-rotating, self-gravitating collisionless stellar discs, using the recently introduced modified WKB formulation of spiral density waves for collisionless systems (Gulati \& Saini). The discs are assumed to be axisymmetric and in coplanar orbits around a massive object at the common center of the discs. The mass in both discs is assumed to be much smaller than the mass of the central object. For each disc, the disc particles are assumed to be in near circular orbits. The two discs are coupled to each other gravitationally. The perturbed dynamics of the discs evolves on the order of the precession time scale of the discs, which is much longer than the Keplerian time scale. We present results for the azimuthal wave number $m=1$ and $m=2$, for the full range of disc mass ratio between the prograde and retrograde discs. The eigenspectra are in general complex, therefore all eigenmodes are unstable. Eigenfunctions are radially more compact for $m = 1$ as compared to $m = 2$. Pattern speed of eigenmodes is always prograde with respect to the more massive disc. The growth rate of unstable modes increases with increasing mass fraction in the retrograde disc, and decreases with $m$; therefore $m=1$ instability is likely to play the dominant role in the dynamics of such systems.