Magneto-rotational Instability in the Accreting Envelope of a Protostar and the Formation of the Large-Scale Magnetic Field

We investigate the role of the magnetic field in the collapse of a gas-dust cloud into a massive gravitating object. Observations of one such object (G31.41) indicate that the magnetic field has an hourglass shape oriented along the rotation axis of the matter, due to the freezing-in of the magnetic-field lines in the accreting matter. It is believed that accretion in stellar disks is associated with the transport of angular momentum from the center to the periphery, which could be initiated by large-scale vortex structures arising in the presence of unstable rotational flows of matter. The numerical simulations have established that the equilibrium configuration of a gas-dust disk rotating in a spherically symmetrical gravitational potential is subject to the development of strong instability in the presence of a weak magnetic field. It is shown that the development of instability leads to a transport of angular momentum to the disk periphery by large-scale vortex structures, together with the accretion of matter onto the gravitating object. The magnetic-field lines near the equator take on a chaotic character, but an hourglass configuration is observed near the rotation axis, in agreement with observations.