A solution to the proplyd lifetime problem
  Protoplanetary discs (PPDs) in the Orion Nebula Cluster (ONC) are irradiated
by UV fields from the massive star $\theta^1$C. This drives thermal winds,
inducing mass loss rates of up to $\dot{M}_\mathrm{wind}\sim
10^{-7}\,M_\odot$/yr in the `proplyds' (ionised PPDs) close to the centre. For
the mean age of the ONC and reasonable initial PPD masses, such mass loss rates
imply that discs should have been dispersed. However, ~80% of stars still
exhibit a NIR excess, suggesting that significant circumstellar mass remains.
This `proplyd lifetime problem' has persisted since the discovery of
photoevaporating discs in the core of the ONC by O'Dell & Wen (1994). In this
work, we demonstrate how an extended period of star formation can solve this
problem. Coupling N-body calculations and a viscous disc evolution model, we
obtain high disc fractions at the present day. This is partly due to the
migration of older stars outwards, and younger stars inwards such that the most
strongly irradiated PPDs are also the youngest. We show how the disc mass
distribution can be used to test the recent claims in the literature for
multiple stellar populations in the ONC. Our model also explains the recent
finding that host mass and PPD mass are only weakly correlated, in contrast
with other regions of similar age. We conclude that the status of the ONC as
the archetype for understanding the influence of environment on planet
formation is undeserved; the complex star formation history (involving star
formation episodes within ~0.8 Myr of the present day) results in confusing
signatures in the PPD population.
