Unbound Star-forming Molecular Clouds
  We explore whether observed molecular clouds could include a substantial
population of unbound clouds. Using simulations which include only turbulence
and gravity, we are able to match observed relations and naturally reproduce
the observed scatter in the cloud size-linewidth coefficient, at fixed surface
density. We identify the source of this scatter as a spread in the intrinsic
virial parameter. Thus these observational trends do not require that clouds
exist in a state of dynamical equilibrium. We demonstrate that cloud virial
parameters can be accurately determined observationally with an appropriate
size estimator. All our simulated clouds eventually form collapsing cores,
regardless of whether the cloud is bound overall. This supports the idea that
molecular clouds do not have to be bound to form stars or to have observed
properties like those of nearby low-mass clouds.
