Abstract

A major hurdle in planet formation theory is that we do not understand how small pebbles congregate into big planetesimals. A promising way to overcome this metre-scale barrier involves a fluid dynamics phenomenon called the streaming instability (SI). It concentrates the pebbles into clumps that are dense enough to collapse gravitationally, thereby forming planetesimals.

Unfortunately, the mechanism responsible for the onset of the instability remains mysterious. This makes it hard to evaluate the robustness of the instability, or to understand how it saturates. It has recently been shown that the SI is a Resonant Drag Instability (RDI) involving inertial waves. In the first part of this talk, I build on this insight to produce a clear physical picture of how the SI develops.

Another problem is that the SI can only devellop in regions containing a high density of similar-sized pebbles. Those conditions are met in large-scale vortices, but no one knows if the SI can feed on vorticial flows. Indeed, any instability can only devellop in specific flows, and a priori the SI is tailored to Keplerian disc flows, not vortex flows. I answer this question in the second part of the talk. To do so, I develop a simple pen-and-paper model of a dust-laden vortex in a protoplanetary disc. I find that if the vortex is weak and anticyclonic, dust drifts towards its centre. I then build a vortex analog of the shearing box to analyse the local linear stability of my dusty vortex. I find that the dust’s drift powers an instability which closely resembles the SI. This result strengthens the case for vortex-induced planetesimal formation.