Testing viscous disc theory using the balance between stellar accretion
  and external photoevaporation of protoplanetary discs
  The nature and rate of (viscous) angular momentum transport in protoplanetary
discs (PPDs) has important consequences for the formation process of planetary
systems. While accretion rates onto the central star yield constraints on such
transport in the inner regions of a PPD, empirical constraints on viscous
spreading in the outer regions remain challenging to obtain. Here we
demonstrate a novel method to probe the angular momentum transport at the outer
edge of the disc. This method applies to PPDs that have lost a significant
fraction of their mass due to thermal winds driven by UV irradiation from a
neighbouring OB star. We demonstrate that this external photoevaporation can
explain the observed depletion of discs in the 3-5 Myr old $\sigma$ Orionis
region, and use our model to make predictions motivating future empirical
investigations of disc winds. For populations of intermediate-age PPDs, in
viscous models we show that the mass flux outwards due to angular momentum
redistribution is balanced by the mass-loss in the photoevaporative wind. A
comparison between wind mass-loss and stellar accretion rates therefore offers
an independent constraint on viscous models in the outer regions of PPDs.
