A model for the formation of stellar associations and clusters from
  giant molecular clouds
  We present a large suite of MHD simulations of turbulent, star-forming giant
molecular clouds(GMCs) with stellar feedback, extending previous work by
simulating 10 different random realizations for each point in the parameter
space of cloud mass and size. It is found that oncethe clouds disperse due to
stellar feedback, both self-gravitating star clusters and unbound stars
generally remain, which arise from the same underlying continuum of
substructured stellar density, ie. the hierarchical cluster formation scenario.
The fraction of stars that are born within gravitationally-bound star clusters
is related to the overall cloud star formation efficiency set by stellar
feedback, but has significant scatter due to stochastic variations in the
small-scale details of the star-forming gas flow. We use our numerical results
to calibrate a model for mapping the bulk properties (mass, size, and
metallicity) of self-gravitating GMCs onto the star cluster populations they
form, expressed statistically in terms of cloud-level distributions.
Synthesizing cluster catalogues from an observed GMC catalogue in M83, we find
that this model predicts initial star cluster masses and sizes that are in good
agreement with observations, using only standard IMF and stellar evolution
models as inputs for feedback. Within our model, the ratio of the strength of
gravity to stellar feedback is the key parameter setting the masses of star
clusters, and of the various feedback channels direct stellar radiation(photon
momentum and photoionization) is the most important on GMC scales.
