Interpreting the Star Formation Efficiency of Molecular Clouds with
  Ionising Feedback
  We investigate the origin of observed local star formation relations using
radiative magnetohydrodynamic simulations with self-consistent star formation
and ionising radiation. We compare these clouds to the density distributions of
local star-forming clouds and find that the most diffuse simulated clouds match
the observed clouds relatively well. We then compute both
observationally-motivated and theoretically-motivated star formation
efficiencies (SFEs) for these simulated clouds. By including ionising
radiation, we can reproduce the observed SFEs in the clouds most similar to
nearby Milky Way clouds. For denser clouds, the SFE can approach unity. These
observed SFEs are typically 3 to 10 times larger than the "total" SFEs, i.e.
the fraction of the initial cloud mass converted to stars. Converting observed
to total SFEs is non-trivial. We suggest some techniques for doing so, though
estimate up to a factor of ten error in the conversion.
