Characterizing thermal sweeping: a rapid disc dispersal mechanism

Abstract

(Abridged) We consider the properties of protoplanetary discs that are\nundergoing inside-out clearing by photoevaporation. In particular, we aim to\ncharacterise the conditions under which a protoplanetary disc may undergo\n`thermal sweeping', a rapid (< 1e4 years) disc destruction mechanism proposed\nto occur when a clearing disc reaches sufficiently low surface density at its\ninner edge and where the disc is unstable to runaway penetration by the X-rays.\nWe use a large suite of 1D radiation-hydrodynamic simulations to probe the\nobservable parameter space, which is unfeasible in higher dimensions. These\nmodels allow us to determine the surface density at which thermal sweeping will\ntake over the disc's evolution and to evaluate this critical surface density as\na function of X-ray luminosity, stellar mass and inner hole radius. We find\nthat this critical surface density scales linearly with X-ray luminosity,\nincreases with inner hole radius and decreases with stellar mass and we develop\nan analytic model that reproduces these results.\n This surface density criterion is then used to determine the evolutionary\nstate of protoplanetary discs at the point that they become unstable to\ndestruction by thermal sweeping. We find that transition discs created by\nphotoevaporation will undergo thermal sweeping when their inner holes reach\n20-40 AU, implying that transition discs with large holes and no accretion\n(which were previously a predicted outcome of the later stages of all flavours\nof photoevaporation model) will not form.\n We emphasise that the surface density criteria that we have developed apply\nto all situations where the disc develops an inner hole that is optically thin\nto X-rays. It thus applies not only to the case of holes originally created by\nphotoevaporation but also to holes formed, for example, by the tidal influence\nof planets.\n