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

Abstract

We present simulations of collapsing 100 M_\\sun mass cores in the context of\nmassive star formation. The effect of variable initial rotational and magnetic\nenergies on the formation of massive stars is studied in detail. We focus on\naccretion rates and on the question under which conditions massive Keplerian\ndisks can form in the very early evolutionary stage of massive protostars. For\nthis purpose, we perform 12 simulations with different initial conditions\nextending over a wide range in parameter space. The equations of\nmagnetohydrodynamics (MHD) are solved under the assumption of ideal MHD. We\nfind that the formation of Keplerian disks in the very early stages is\nsuppressed for a mass-to-flux ratio normalised to the critical value \\mu below\n10, in agreement with a series of low-mass star formation simulations. This is\ncaused by very efficient magnetic braking resulting in a nearly instantaneous\nremoval of angular momentum from the disk. For weak magnetic fields,\ncorresponding to \\mu > 10, large-scale, centrifugally supported disks build up\nwith radii exceeding 100 AU. A stability analysis reveals that the disks are\nsupported against gravitationally induced perturbations by the magnetic field\nand tend to form single stars rather than multiple objects. We find\nprotostellar accretion rates of the order of a few 10^-4 M_\\sun yr^-1 which,\nconsidering the large range covered by the initial conditions, vary only by a\nfactor of ~ 3 between the different simulations. We attribute this fact to two\ncompeting effects of magnetic fields. On the one hand, magnetic braking\nenhances accretion by removing angular momentum from the disk thus lowering the\ncentrifugal support against gravity. On the other hand, the combined effect of\nmagnetic pressure and magnetic tension counteracts gravity by exerting an\noutward directed force on the gas in the disk thus reducing the accretion onto\nthe protostars.\n