The evolution of dust in discs influenced by external photoevaporation
  Protoplanetary discs form and evolve in a wide variety of stellar
environments and are accordingly exposed to a wide range of ambient far
ultraviolet (FUV) field strengths. Strong FUV fields are known to drive
vigorous gaseous flows from the outer disc. In this paper we conduct the first
systematic exploration of the evolution of the solid component of discs subject
to external photoevaporation. We find that the main effect of photoevaporation
is to reduce the reservoir of dust at large radii and this leads to more
efficient subsequent depletion of the disc dust due to radial drift. Efficient
radial drift means that photoevaporation causes no significant increase of the
dust to gas ratio in the disc. We show that the disc lifetime in both dust and
gas is strongly dependent on the level of the FUV background and that the
relationship between these two lifetimes just depends on the Shakura-Sunyaev
$\alpha$ parameter, with the similar lifetimes observed for gas and dust in
discs pointing to higher $\alpha$ values ($\sim 10^{-2}$). On the other hand
the distribution of observed discs in the plane of disc size versus flux at
$850~\mu$m is better reproduced by lower $\alpha$ ($\sim 10^{-3}$). We find
that photoevaporation does not assist rocky planet formation but need not
inhibit mechanisms (such as pebble accretion at the water snow line) which can
be effective sufficiently early in the disc's lifetime (i.e. well within a
Myr).
