Resonant Drag Instabilities in protoplanetary disks: the streaming instability and new, faster-growing instabilities

Abstract

We identify and study a number of new, rapidly growing instabilities of dust grains in protoplanetary disks, which may be important for planetesimal formation. The study is based on the recognition that dust-gas mixtures are generically unstable to a Resonant Drag Instability (RDI), whenever there is relative streaming motion between the two phases and the gas (absent dust) supports undamped linear modes. Due to the drag forces between dust and gas, each gas mode is associated with a set of instabilities (RDIs) which generically act to clump the dust. We show that the "streaming instability" is an RDI associated with epicyclic oscillations; this provides simple interpretations for its mechanisms and accurate analytic expressions for its growth rates and fastest-growing wavelengths. We extend this to identify a variety of additional RDIs associated with different waves -- e.g., buoyancy, acoustic, and magnetohydrodynamic oscillations -- each of which leads to new instabilities. Most importantly, we identify the disk "settling instability" (or vertical-epicyclic RDI), which occurs as dust settles vertically into the midplane of a rotating disk. For small grains, this instability grows many orders of magnitude faster than the standard streaming instability: in fact, its growth rate is independent of grain size. Growth timescales for realistic dust-to-gas ratios are comparable to the disk orbital period, and the characteristic wavelengths are more than an order of magnitude larger than the streaming instability (allowing the instability to concentrate larger masses). This suggests in the process of settling, dust will band into rings (then filaments or clumps), potentially seeding dust traps, high-metallicity regions (which in turn seed the streaming instability), or overdensities that could coagulate or directly collapse to planetesimals.