Instabilities in dusty non-isothermal protoplanetary discs

Protoplanetary discs (PPDs) can host a number of instabilities that may partake directly or indirectly in the process of planetesimal formation. These include the vertical shear instability (VSI), convective overstability (COS), streaming instability (SI), and dust settling instability (DSI), to name a few. Notably, the VSI and COS have mostly been studied in purely gaseous discs, while the SI and DSI have only been analysed in isothermal discs. How these instabilities operate under more general conditions is therefore unclear. To this end, we devise a local model of a PPD describing a non-isothermal gas interacting with a single species of dust via drag forces. Using this, we find that dust drag sets minimum length-scales below which the VSI and COS are suppressed. Similarly, we find that the SI can be suppressed on sufficiently small scales by the gas' radial buoyancy if it cools on roughly a dynamical time-scale. We show that the DSI can be effectively stabilized by vertical buoyancy, except at special radial and vertical length-scales. We also find novel instabilities unique to a dusty, non-isothermal gas. These result in a dusty analogue of the COS that operates in slowly cooled discs, and a dusty version of the VSI that is strongly enhanced by dust settling. We briefly discuss the possible implications of our results on planetesimal formation.