Title: 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 the gas,
absent dust, supports undamped linear modes. 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 analysis to
more general dust streaming motions and other waves, including buoyancy and
magnetohydrodynamic oscillations, finding various new instabilities. Most
importantly, we identify the disk "settling instability," 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, with a growth rate that 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 that in the process of settling, dust will band
into rings then filaments or clumps, potentially seeding dust traps,
high-metallicity regions that in turn seed the streaming instability, or even
overdensities that coagulate or directly collapse to planetesimals.