On the Indeterministic Nature of Star Formation on the Cloud Scale
  Molecular clouds are turbulent structures whose star formation efficiency
(SFE) is strongly affected by internal stellar feedback processes. In this
paper we determine how sensitive the SFE of molecular clouds is to randomised
inputs in the star formation feedback loop, and to what extent relationships
between emergent cloud properties and the SFE can be recovered. We introduce
the yule suite of 26 radiative magnetohydrodynamic (RMHD) simulations of a
10,000 solar mass cloud similar to those in the solar neighbourhood. We use the
same initial global properties in every simulation but vary the initial mass
function (IMF) sampling and initial cloud velocity structure. The final SFE
lies between 6 and 23 percent when either of these parameters are changed. We
use Bayesian mixed-effects models to uncover trends in the SFE. The number of
photons emitted early in the cluster's life and the length of the cloud provide
are the strongest predictors of the SFE. The HII regions evolve following an
analytic model of expansion into a roughly isothermal density field. The more
efficient feedback is at evaporating the cloud, the less the star cluster is
dispersed. We argue that this is because if the gas is evaporated slowly, the
stars are dragged outwards towards surviving gas clumps due to the
gravitational attraction between the stars and gas. While star formation and
feedback efficiencies are dependent on nonlinear processes, statistical models
describing cloud-scale processes can be constructed.
