Rapid destruction of protoplanetary discs due to
  externalphotoevaporation in star-forming regions
  We analyse N-body simulations of star-forming regions to investigate the
effects of external far and extreme ultra-violet photoevaporation from massive
stars on protoplanetary discs. By varying the initial conditions of simulated
star-forming regions, such as the spatial distribution, net bulk motion (virial
ratio), and density, we investigate which parameters most affect the rate at
which discs are dispersed due to external photoevaporation. We find that disc
dispersal due to external photoevaporation is faster in highly substructured
star-forming regions than in smooth and centrally concentrated regions.
Sub-virial star-forming regions undergoing collapse also show higher rates of
disc dispersal than regions that are in virial equilibrium or are expanding. In
moderately dense ($\sim$100 M$_{\odot}$ pc$^{-3}$) regions, half of all
protoplanetary discs with radii $\geq$ 100 AU are photoevaporated within 1 Myr,
three times faster than is currently suggested by observational studies. Discs
in lower-density star-forming regions ($\sim$10 M$_{\odot}$ pc$^{-3}$) survive
for longer, but half are still dispersed on short timescales ($\sim$2 Myr).
This demonstrates that the initial conditions of the star forming regions will
greatly impact the evolution and lifetime of protoplanetary discs. These
results also imply that either gas giant planet formation is extremely rapid
and occurs before the gas component of discs is evaporated, or gas giants only
form in low-density star-forming regions where no massive stars are present to
photoevaporate gas from protoplanetary discs.
