  Massive stars disproportionately influence their surroundings. How they form
has only started to become clear recently through radiation gas dynamical
simulations. However, until now, no simulation has simultaneously included both
magnetic fields and ionizing radiation. Here we present the results from the
first radiation-magnetohydrodynamical (RMHD) simulation including ionization
feedback, comparing an RMHD model of a 1000 M_sol rotating cloud to earlier
radiation gas dynamical models with the same initial density and velocity
distributions. We find that despite starting with a strongly supercritical mass
to flux ratio, the magnetic field has three effects. First, the field offers
locally support against gravitational collapse in the accretion flow,
substantially reducing the amount of secondary fragmentation in comparison to
the gas dynamical case. Second, the field drains angular momentum from the
collapsing gas, further increasing the amount of material available for
accretion by the central, massive, protostar, and thus increasing its final
mass by about 50% from the purely gas dynamical case. Third, the field is wound
up by the rotation of the flow, driving a tower flow. However, this flow never
achieves the strength seen in low-mass star formation simulations for two
reasons: gravitational fragmentation disrupts the circular flow in the central
regions where the protostars form, and the expanding H II regions tend to
further disrupt the field geometry. Therefore, outflows driven by ionization
heating look likely to be more dynamically important in regions of massive star
formation.
