Magnetic fields during the early stages of massive star formation - I. Accretion and disc evolution

We present simulations of collapsing 100 M_⊙ 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 discs 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 discs in the very early stages is suppressed for a mass-to-flux ratio normalized to the critical value μ 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 disc. For weak magnetic fields, corresponding to μ≳ 10, large-scale, centrifugally supported discs build up with radii exceeding 100 au. A stability analysis reveals that the discs 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_⊙ 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 disc 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 disc thus reducing the accretion on to the protostars.