Striations, integrals, hourglasses and collapse -- thermal instability
  driven magnetic simulations of molecular clouds
  The MHD version of the adaptive mesh refinement (AMR) code, MG, has been
employed to study the interaction of thermal instability, magnetic fields and
gravity through 3D simulations of the formation of collapsing cold clumps on
the scale of a few parsecs, inside a larger molecular cloud. The diffuse atomic
initial condition consists of a stationary, thermally unstable, spherical cloud
in pressure equilibrium with lower density surroundings and threaded by a
uniform magnetic field. This cloud was seeded with 10% density perturbations at
the finest initial grid level around n=1.1 cm^{-3} and evolved with
self-gravity included from the outset. Several cloud diameters were considered
(100 pc, 200 pc and 400 pc) equating to several cloud masses (17,000 Msun,
136,000 Msun and 1.1x10^6 Msun). Low-density magnetic-field-aligned striations
were observed as the clouds collapse along the field lines into disc-like
structures. The induced flow along field lines leads to oscillations of the
sheet about the gravitational minimum and an integral-shaped appearance. When
magnetically supercritical, the clouds then collapse and generate hourglass
magnetic field configurations with strongly intensified magnetic fields,
reproducing observational behaviour. Resimulation of a region of the highest
mass cloud at higher resolution forms gravitationally-bound collapsing clumps
within the sheet that contain clump-frame supersonic (M~5) and super-Alfvenic
(M_A~4) velocities. Observationally realistic density and velocity power
spectra of the cloud and densest clump are obtained. Future work will use these
realistic initial conditions to study individual star and cluster feedback.
