Characterizing thermal sweeping: a rapid disc dispersal mechanism

Abstract

We consider the properties of protoplanetary discs that are undergoing inside-out clearing by photoevaporation. In particular, we aim to characterize the conditions under which a protoplanetary disc may undergo ‘thermal sweeping’, a rapid (≲10^4 years) disc destruction mechanism proposed to occur when a clearing disc reaches sufficiently low surface density at its inner edge and where the disc is unstable to runaway penetration by the X-rays. We use a large suite of 1D radiation-hydrodynamic simulations to probe the observable parameter space, which is unfeasible in higher dimensions. These models allow us to determine the surface density at which thermal sweeping will take over the disc's evolution and to evaluate this critical surface density as a function of X-ray luminosity, stellar mass and inner hole radius. We find that this critical surface density scales linearly with X-ray luminosity, increases with inner hole radius and decreases with stellar mass, and we develop an analytic model that reproduces these results. This surface density criterion is then used to determine the evolutionary state of protoplanetary discs at the point that they become unstable to destruction by thermal sweeping. We find that transition discs created by photoevaporation will undergo thermal sweeping when their inner holes reach 20–40 au, implying that transition discs with large holes and no accretion (which were previously a predicted outcome of the later stages of all flavours of the photoevaporation model) will not form. Thermal sweeping thus avoids the production of large numbers of large, non-accreting holes (which are not observed) and implies that the majority of holes created by photoevaporation should still be accreting. We emphasize that the surface density criteria that we have developed apply to all situations where the disc develops an inner hole that is optically thin to X-rays. It thus applies not only to the case of holes originally created by photoevaporation but also to holes formed, for example, by the tidal influence of planets.