From Dust to Planetesimals: Criteria for Gravitational Instability of Small Particles in Gas

Dust particles sediment toward the midplanes of protoplanetary disks, forming dust-rich sublayers encased in gas. What densities must the particle sublayer attain before it can fragment by self-gravity? We describe various candidate threshold densities. One of these is the Roche density, which is that required for a strengthless satellite to resist tidal disruption by its primary. Another is the Toomre density, which is that required for de-stabilizing self-gravity to defeat the stabilizing influences of pressure and rotation. We show that for sublayers containing aerodynamically well-coupled dust, the Toomre density exceeds the Roche density by many (up to about four) orders of magnitude. We present three-dimensional shearing box simulations of self-gravitating, stratified, dust-gas mixtures to test which of the candidate thresholds is relevant for collapse. All of our simulations indicate that the larger Toomre density is required for collapse. This result is sensible because sublayers are readily stabilized by pressure. Sound-crossing times for thin layers are easily shorter than free-fall times, and the effective sound speed in dust-gas suspensions decreases only weakly with the dust-to-gas ratio (as the inverse square root). Our findings assume that particles are small enough that their stopping times in gas are shorter than all other timescales. Relaxing this assumption may lower the threshold for gravitational collapse back down to the Roche criterion. In particular, if the particle stopping time becomes longer than the sound-crossing time, then sublayers may lose pressure support and become gravitationally unstable.