Dynamics of stellar systems with collisions: eigenvalues and eigenfunctions in nearly collisionless limit

TL;DR

This paper analyzes how weak collisions affect the decay of perturbations in a self-gravitating stellar system, revealing the transition from Landau quasi-modes to true eigenmodes as collision frequency approaches zero.

Contribution

It provides analytic expressions and numerical verification for the eigenfunctions and damping rates of the least-damped mode in nearly collisionless stellar systems.

Findings

Weak collisions suppress van Kampen modes

Landau quasi-modes become true eigenmodes with decreasing collision frequency

Analytic and numerical results agree on eigenfunction behavior

Abstract

We examine the decay of perturbations in an infinite homogeneous self-gravitating model with a Maxwellian distribution function (DF) when weak collisions are present. In collisionless systems within the stable parameter range, the eigenvalue spectrum consists of a continuous set of real frequencies associated with van Kampen modes, which are singular eigenfunctions of the stellar DF. An initial perturbation in the stellar density and gravitational potential decays exponentially through a superposition of these modes, a phenomenon known as Landau damping. However, the perturbation in the stellar DF does not decay self-similarly; it becomes increasingly oscillatory in velocity space over time, indicating the absence of eigenfunctions corresponding to the Landau damping eigenfrequencies. Consequently, we refer to perturbations undergoing Landau damping as quasi-modes rather than true eigenmodes. Even rare collisions suppress the formation of steep DF gradients in velocity space. C. S. Ng & A. Bhattacharjee demonstrated that introducing collisions eliminates van Kampen modes and transforms Landau quasi-modes into true eigenmodes forming a complete set. As the collision frequency approaches zero, their eigenfrequencies converge to those of the collisionless Landau quasi-modes. In this study, we investigate the behavior of the eigenfunction of the least-damped aperiodic mode as the collision frequency approaches zero. We derive analytic expressions for the eigenfunction in the resonance region and for the damping rate as a function of collision frequency. Additionally, we employ the standard matrix eigenvalue problem approach to numerically verify our analytical results.