A physical picture for the acoustic resonant drag instability

TL;DR

This paper provides a clear physical explanation of the acoustic resonant drag instability in gas-dust mixtures, revealing how coupled mechanisms lead to instability and offering insights for simulations and astrophysical phenomena.

Contribution

It introduces a transparent physical picture of the acoustic RDI, supported by mathematical analysis, clarifying its mechanisms and conditions for occurrence.

Findings

Acoustic RDI arises from converging sound waves and drifting dust interactions.

The instability remains strong even far from resonance.

Simulations of RDIs do not require fine-tuning of domain dimensions.

Abstract

Mixtures of gas and dust are pervasive in the universe, from AGN and molecular clouds to proto-planetary discs. When the two species drift relative to each other, a large class of instabilities can arise, called resonant drag instabilities (RDIs). The most famous RDI is the streaming instability, which plays an important role in planet formation. On the other hand, acoustic RDIs, the simplest kind, feature in the winds of cool stars, AGN, or starburst regions. Unfortunately, owing to the complicated dynamics of two coupled fluids (gas and dust), the underlying physics of most RDIs is mysterious. In this paper, we develop a clear physical picture of how the acoustic RDI arises and support this explanation with transparent mathematics. We find that the acoustic RDI is built on two coupled mechanisms. In the first, the converging flows of a sound wave concentrate dust. In the second, a drifting dust clump excites sound waves. These processes feed into each other at resonance, thereby closing an unstable feedback loop. This physical picture helps decide where and when RDIs are most likely to happen, and what can suppress them. Additionally, we find that the acoustic RDI remains strong far from resonance. This second result suggests that one can simulate RDIs without having to fine-tune the dimensions of the numerical domain.