The dispersal of circumstellar discs: the role of the ultraviolet switch

We explore ultraviolet switch models for the dispersal of circumstellar discs in T Tauri stars that involve both photoevaporation by the central star and viscous evolution. We show that in combination these processes generate the observed 'two-time-scale' behaviour for the dispersal of such discs, whereby the disc is rapidly dispersed at the end of its life on a timescale that is a small fraction of the disc lifetime. This switch is activated when the accretion rate through the disc declines to a low level (a few times 10(-10) M. yr(-1)) such that it roughly matches the rate of photoevaporative, mass loss from the disc outside 5-10 au. At this point, the inner disc is deprived of further replenishment from larger radii and empties on to the central star on its own short viscous time-scale. This causes the rapid (similar to 10(5)-yr) decline in accretion rate on to the central star and in all disc-related emission shortward of 10 mum. We discuss the implications of this model for the detection of millimetre emission around weak-line T Tauri stars, and also point out the consequences of such a sudden draining for planet formation in the inner regions of circumstellar discs.