Magnetic fields during the early stages of massive star formation - I.
  Accretion and disk evolution
  We present simulations of collapsing 100 M_\sun mass cores in the context of
massive star formation. The effect of variable initial rotational and magnetic
energies on the formation of massive stars is studied in detail. We focus on
accretion rates and on the question under which conditions massive Keplerian
disks can form in the very early evolutionary stage of massive protostars. For
this purpose, we perform 12 simulations with different initial conditions
extending over a wide range in parameter space. The equations of
magnetohydrodynamics (MHD) are solved under the assumption of ideal MHD. We
find that the formation of Keplerian disks in the very early stages is
suppressed for a mass-to-flux ratio normalised to the critical value \mu below
10, in agreement with a series of low-mass star formation simulations. This is
caused by very efficient magnetic braking resulting in a nearly instantaneous
removal of angular momentum from the disk. For weak magnetic fields,
corresponding to \mu > 10, large-scale, centrifugally supported disks build up
with radii exceeding 100 AU. A stability analysis reveals that the disks are
supported against gravitationally induced perturbations by the magnetic field
and tend to form single stars rather than multiple objects. We find
protostellar accretion rates of the order of a few 10^-4 M_\sun yr^-1 which,
considering the large range covered by the initial conditions, vary only by a
factor of ~ 3 between the different simulations. We attribute this fact to two
competing effects of magnetic fields. On the one hand, magnetic braking
enhances accretion by removing angular momentum from the disk thus lowering the
centrifugal support against gravity. On the other hand, the combined effect of
magnetic pressure and magnetic tension counteracts gravity by exerting an
outward directed force on the gas in the disk thus reducing the accretion onto
the protostars.
