When Is Moving Dust Unstable?

What’s one thing the interstellar medium, protoplanetary disks, stellar interiors, and the environments around black holes all have in common? They all contain dust grains moving within a fluid — and two scientists from the California Institute of Technology say we’ve been missing an important part of their behavior.

Pairing of Fluids and Dust

Crab Nebula

Hubble view of the Crab Nebula. Supernova ejecta are another instance of a coupled system of dust grains and fluid. [NASA/ESA/J. Hester and A. Loll (Arizona State University)]

Fluids — which can refer to liquids, gases, or plasmas — rarely exist in isolation in astrophysics. More often than not, fluids come laden with dust particles; examples of dusty fluids include the environments near star-forming regions, in planetary atmospheres, in the disks surrounding young stars, or even around active galactic nuclei. Since these fluid/dust systems are abundant across the universe and are fundamental to many key astrophysical processes, it’s important that we understand how they behave.

Caltech scientists Jonathan Squire and Philip Hopkins ask one particular question: what happens when dust particles move at a different speed than the fluid surrounding them?

Relative Motion

Relative motion of dust through fluid can arise naturally through many mechanisms. Radiation pressure, for instance, preferentially accelerates dust grains relative to gas in environments around active galactic nuclei or in the envelopes of stars, causing the dust to stream through the surrounding fluid. Or the fluid of a planetary atmosphere might be supported against gravity by thermal pressure, causing the heavier dust grains to settle downward through the fluid.

In a new study, Squire and Hopkins suggest that this relative streaming motion between dust grains and fluid can easily create instabilities — and this can have profound implications for our understanding of many fields of astrophysics.

Instabilities Found Everywhere

The authors used analytic calculations to show that coupled fluid/dust systems can develop “resonant drag instabilities” whenever the dust grains stream faster than any wave in the fluid.

planetary atmosphere

In a planetary atmosphere like the one shown in this artist’s impression, a fluid might be supported against gravity, whereas dust grains are not. This would create relative motion of the dust particles as they settle. [Max Planck Society]

These instabilities, it turns out, are quite easy to trigger, because astrophysical fluids host a variety of waves, any of which can form the basis for a resonant drag instability. Examples include sound waves, magnetosonic waves, Brunt–Väisälä waves, epicyclic oscillations, and others. The instabilities triggered by the streaming dust in the presence of these waves grow over time, causing spatial clumping of the dust and eventually seeding turbulence if they’re strong enough.

Squire and Hopkins present a way of calculating the growth rates and properties of these resonant drag instabilities in different fluids, and they demonstrate the behavior of the instabilities in three example fluid systems: hydrodynamic, magnetohydrodynamic, and stratified fluids.

The authors argue that the consequences of the resonant drag instability affect regions and processes like planetesimal formation, cool-star winds, active galactic nuclei torii and winds, starburst regions, H II regions, supernovae ejecta, and the circumgalactic medium. Their work toward understanding this instability is therefore broadly applicable across astronomical fields, providing critical insight into processes in our universe.


J. Squire and P. F. Hopkins 2018 ApJL 856 L15. doi:10.3847/2041-8213/aab54d

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