Gravity Versus Magnetic Fields in Forming Molecular Clouds
  Magnetic fields are dynamically important in the diffuse interstellar medium.
Understanding how gravitationally bound, star-forming clouds form requires
modeling of the fields in a self-consistent, supernova-driven, turbulent,
magnetized, stratified disk. We employ the FLASH magnetohydrodynamics code to
follow the formation and early evolution of clouds with final masses of 3-8
$\times 10^3 M_{\odot}$ within such a simulation. We use the code's adaptive
mesh refinement capabilities to concentrate numerical resolution in zoom-in
regions covering single clouds, allowing us to investigate the detailed
dynamics and field structure of individual self-gravitating clouds in a
consistent background medium. Our goal is to test the hypothesis that dense
clouds are dynamically evolving objects far from magnetohydrostatic
equilibrium. We find that the cloud envelopes are magnetically supported with
field lines parallel to density gradients and flow velocity, as indicated by
the histogram of relative orientations and other statistical measures. In
contrast, the dense cores of the clouds are gravitationally dominated, with
gravitational energy exceeding internal, kinetic, or magnetic energy and
accelerations due to gravity exceeding those due to magnetic or thermal
pressure gradients. In these regions field directions vary strongly, with a
slight preference towards being perpendicular to density gradients, as shown by
three-dimensional histograms of relative orientation.
