Diffusive Instabilities in Dusty Disks: Linear Growth and Nonlinear Breakdown

TL;DR

This paper investigates the linear growth and nonlinear evolution of diffusive instabilities in dusty disks, revealing that without additional physics, the nonlinear phase leads to collapse rather than saturation, but a modified model can prevent blowup.

Contribution

The study extends previous models by including gas feedback and nonlinear simulations, showing that diffusive instabilities can cause collapse unless a saturation mechanism is introduced.

Findings

Linear instability criterion remains approximately valid with gas feedback.

Nonlinear evolution leads to finite-time collapse in standard models.

A modified closure can prevent blowup and control peak densities.

Abstract

We revisit the diffusive instability in dusty disks that arises when the dust mass diffusivity and/or viscosity decreases sufficiently steeply with increasing dust density. Our updated model includes an incompressible, viscous gas that responds azimuthally and couples to the dust through drag. We show that the basic criterion for diffusion-slope-driven instability remains approximately $\beta_\mathrm{diff}\lesssim -2$ for small dust stopping times, with gas feedback providing only modest quantitative changes for parameters motivated by streaming-instability turbulence. We perform nonlinear numerical calculations and confirm linear growth and mode selection toward the fastest-growing wavenumber. However, for power-law closures $D\propto\Sigma^{\beta_\mathrm{diff}}$ with $\beta_\mathrm{diff}<0$, the nonlinear evolution does not saturate. Instead, steepening gradients amplify the nonlinear dust-pressure term and drive finite-time collapse into increasingly sharp spikes. Motivated by the absence of multidimensional saturation channels in our 1D framework, we test a simple piecewise closure in which the negative diffusion slope operates only over a finite density interval. This modification eliminates blowup and produces peak densities controlled by the imposed saturation scale. Our results support diffusive instabilities as a linear organizing mechanism in dusty turbulence, while highlighting that realistic nonlinear saturation requires additional physics beyond the present closure.