The Acoustic Resonant Drag Instability with a Spectrum of Grain Sizes

TL;DR

This paper investigates the linear growth and nonlinear behavior of the acoustic Resonant Drag Instability in dusty winds with a spectrum of grain sizes, revealing different regimes based on grain size relative to radiation wavelength.

Contribution

It introduces a detailed analysis of RDI with a continuous grain size spectrum, identifying distinct regimes and their implications for grain dynamics and growth.

Findings

Large grains exhibit strong correlations and turbulence in the constant-drift regime.

Smaller grains grow more slowly and show less RDI behavior in the non-constant-drift regime.

Constant-drift regime may facilitate grain growth through collisions.

Abstract

We study the linear growth and nonlinear saturation of the "acoustic Resonant Drag Instability" (RDI) when the dust grains, which drive the instability, have a wide, continuous spectrum of different sizes. This physics is generally applicable to dusty winds driven by radiation pressure, such as occurs around red-giant stars, star-forming regions, or active galactic nuclei. Depending on the physical size of the grains compared to the wavelength of the radiation field that drives the wind, two qualitatively different regimes emerge. In the case of grains that are larger than the radiation's wavelength -- termed the constant-drift regime -- the grain's equilibrium drift velocity through the gas is approximately independent of grain size, leading to strong correlations between differently sized grains that persist well into the saturated nonlinear turbulence. For grains that are smaller than the radiation's wavelength -- termed the non-constant-drift regime -- the linear instability grows more slowly than the single-grain-size RDI and only the larger grains exhibit RDI-like behavior in the saturated state. A detailed study of grain clumping and grain-grain collisions shows that outflows in the constant-drift regime may be effective sites for grain growth through collisions, with large collision rates but low collision velocities.