Dust dynamics during protoplanetary disc clearing

We consider the dynamics of dust and gas during the clearing of protoplanetary discs. We work within the context of a photoevaporation/viscous model for the evolution of the gas disc, and use a two-fluid model to study the dynamics of dust grains as the gas disc is cleared. Small (less than or similar to 10 mu m) grains remain well coupled to the gas, but larger (similar to 1 mm) grains are subject to inward migration from large radii (similar to 50 au), suggesting that the time-scale for grain growth in the outer disc is similar to 10(4)-10(5) yr. We describe in detail the observable appearance of discs during clearing, and find that pressure gradients in the gas disc result in a strong enhancement of the local dust-to-gas ratio in a ring near to the inner disc edge. Lastly, we consider a simple model of the disc-planet interaction, and suggest that observations of disc masses and accretion rates provide a straightforward means of discriminating between different models of disc clearing.