Streaming instabilities in stratified protoplanetary disks
Structure of linearly unstable modes found in stratified, dusty protoplanetary disks
The classic streaming instability of Youdin & Goodman (2005) is the leading mechanism for the formation of planetesimals from small grains or pebbles. However, the original linear theory of the instability neglects the vertical structure expected in real protoplanetary disks. We generalize the linear stability theory of dusty protoplanetary disks to account for stratification. We show that the dominant form of instability is one driven by the vertical gradient in the disk's rotation velocity, unlike Youdin & Goodman's streaming instability. We call these modes `vertically-shearing streaming instabilities' (VSSIs). Interestingly, VSSIs were discovered in numerical simulations earlier by Ishitsu et al. (2009).