On the theory of disc photoevaporation

We discuss a hydrodynamical model for the dispersal of protoplanetary discs around young, low-mass ( 100), the FUV constricts the X-ray flow and may dominate the mass loss. Simulations of low-mass discs with inner holes demonstrate a further stage of disc clearing, which we call ‘thermal sweeping’. This process occurs when the mid-plane pressure drops to sufficiently low values. At this stage, a bound, warm, X-ray heated region becomes sufficiently large and unstable, such that the remaining disc material is cleared on approximately dynamical time-scales. This process significantly reduces the time taken to clear the outer regions of the disc, resulting in an expected transition disc population that will be dominated by accreting objects, as indicated by recent observations.