Thinkshop 2020

Snow-lines are regions of protoplanetary discs where volatiles transition from the solid-phase to the gas phase. They play an important role in the chemical evolution of protoplanetary discs and perhaps planet/planetesimal formation. The majority of work treats these transitions as passive, uncoupled from the dynamics. I will argue that snow-lines in the outer regions of protoplanetary discs (where the dis cooling is optically thin), are thermally unstable. Namely, condensation leads to an increase in the solid abundance which leads to increased cooling and more condensation (or vica-versa). I will demonstrate a dynamical simulation that actively couples the condensation physics, to the radiative transfer and hence the disc's temperature, dust dynamics and growth. I will use this simulation to show that snow-lines are not static, but dynamically evolve in otherwise stationary discs and drive the dynamics in the outer regions of protoplanetary discs. We find the CO snow-line can move 10s AU on timescales of a few 1e5 years, creating further structures and rings in the disc and even multiple snow-lines. This thermal instability at snow-lines is likely important for the chemical, thermal and dynamical evolution of protoplanetary discs. It perhaps even plays a role in explaining ringed ALMA discs and planet/planetesimal formation.