The role of turbulent magnetic reconnection on the formation of
  rotationally supported protostellar disks
  The formation of protostellar disks out of molecular cloud cores is still not
fully understood. Under ideal MHD conditions, the removal of angular momentum
from the disk progenitor by the typically embedded magnetic field may prevent
the formation of a rotationally supported disk during the main protostellar
accretion phase of low mass stars. This has been known as the magnetic braking
problem and the most investigated mechanism to alleviate this problem and help
removing the excess of magnetic flux during the star formation process, the so
called ambipolar diffusion (AD), has been shown to be not sufficient to weaken
the magnetic braking at least at this stage of the disk formation. In this
work, motivated by recent progress in the understanding of magnetic
reconnection in turbulent environments, we appeal to the diffusion of magnetic
field mediated by magnetic reconnection as an alternative mechanism for
removing magnetic flux. We investigate numerically this mechanism during the
later phases of the protostellar disk formation and show its high efficiency.
By means 3D MHD simulations, we show that this mechanism is able to transport
magnetic flux to the outskirts of the disk progenitor at time scales compatible
with the collapse, allowing the formation of a rotationally supported disk
around the protostar of dimensions ~100 AU. Since MHD turbulence is expected to
be present in protostellar disks, this is a natural mechanism for removing
magnetic flux excess and allowing the formation of these disks. This mechanism
dismiss the necessity of postulating a hypothetical increase of the Ohmic
resistivity as discussed in the literature. Together with our earlier work
which showed that magnetic flux removal from molecular cloud cores is very
efficient, this work calls for reconsidering the relative role of AD for the
processes of star and planet formation.
