Collapse and Fragmentation of Magnetic Molecular Cloud Cores with the
  Enzo AMR MHD Code. I. Uniform Density Sphere
  Magnetic fields are important contributers to the dynamics of collapsing
molecular cloud cores, and can have a major effect on whether collapse results
in a single protostar or fragmentation into a binary or multiple protostar
system. New models are presented of the collapse of magnetic cloud cores using
the adaptive mesh refinement (AMR) code Enzo2.0. The code was used to calculate
the ideal magnetohydrodynamics (MHD) of initially spherical, uniform density
and rotation clouds with density perturbations, i.e., the Boss and Bodenheimer
(1979) standard isothermal test case for three dimensional (3D) hydrodynamics
(HD) codes. After first verifying that Enzo reproduces the binary fragmentation
expected for the non-magnetic test case, a large set of models was computed
with varied initial magnetic field strengths and directions with respect to the
cloud core axis of rotation (parallel or perpendicular), density perturbation
amplitudes, and equations of state. Three significantly different outcomes
resulted: (1) contraction without sustained collapse, forming a denser cloud
core, (2) collapse to form a single protostar with significant spiral arms, and
(3) collapse and fragmentation into binary or multiple protostar systems, with
multiple spiral arms. Comparisons are also made with previous MHD calculations
of similar clouds with barotropic equations of state. These results for the
collapse of initially uniform density spheres illustrate the central importance
of both magnetic field direction and field strength for determining the outcome
of dynamic protostellar collapse.
