Feeding vs. Falling: The growth and collapse of molecular clouds in a
  turbulent interstellar medium
  In order to understand the origin of observed molecular cloud properties, it
is critical to understand how clouds interact with their environments during
their formation, growth, and collapse. It has been suggested that
accretion-driven turbulence can maintain clouds in a highly turbulent state,
preventing runaway collapse, and explaining the observed non-thermal velocity
dispersions. We present 3D, AMR, MHD simulations of a kiloparsec-scale,
stratified, supernova-driven, self-gravitating, interstellar medium, including
diffuse heating and radiative cooling. These simulations model the formation
and evolution of a molecular cloud population in the turbulent interstellar
medium. We use zoom-in techniques to focus on the dynamics of the mass
accretion and its history for individual molecular clouds. We find that mass
accretion onto molecular clouds proceeds as a combination of turbulent and near
free-fall accretion of a gravitationally bound envelope. Nearby supernova
explosions have a dual role, compressing the envelope, boosting accreted mass,
but also disrupting parts of the envelope and eroding mass from the cloud's
surface. It appears that the inflow rate of kinetic energy onto clouds from
supernova explosions is insufficient to explain the net rate of charge of the
cloud kinetic energy. In the absence of self-consistent star formation,
conversion of gravitational potential into kinetic energy during contraction
seems to be the main driver of non-thermal motions within clouds. We conclude
that although clouds interact strongly with their environments, bound clouds
are always in a state of gravitational contraction, close to runaway, and their
properties are a natural result of this collapse.
