External photoevaporation of protoplanetary discs in sparse stellar
  groups: the impact of dust growth
  We estimate the mass loss rates of photoevaporative winds launched from the
outer edge of protoplanetary discs impinged by an ambient radiation field. We
focus on mild/moderate environments (the number of stars in the group/cluster
is N ~ 50), and explore disc sizes ranging between 20 and 250 AU. We evaluate
the steady-state structures of the photoevaporative winds by coupling
temperature estimates obtained with a PDR code with 1D radial hydrodynamical
equations. We also consider the impact of dust dragging and grain growth on the
final mass loss rates. We find that these winds are much more significant than
have been appreciated hitherto when grain growth is included in the modelling:
in particular, mass loss rates > 1e-8 M_sun/yr are predicted even for modest
background field strengths ( ~ 30 G_0) in the case of discs that extend to R >
150 AU. Grain growth significantly affects the final mass loss rates by
reducing the average cross section at FUV wavelengths, and thus allowing a much
more vigorous flow. The radial profiles of observable quantities (in particular
surface density, temperature and velocity patterns) indicate that these winds
have characteristic features that are now potentially observable with ALMA. In
particular, such discs should have extended gaseous emission that is dust
depleted in the outer regions, characterised by a non-Keplerian rotation curve,
and with a radially increasing temperature gradient.
