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 time-scale 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. 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 (∼105-yr) decline in accretion rate on to the central star and in all disc-related emission shortward of 10 μm. 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.