Global simulations of self-gravitating magnetized protoplanetary disks
  In the early stages of a protoplanetary disk, when its mass is a significant
fraction of its star's, turbulence generated by gravitational instability (GI)
should feature significantly in the disk's evolution. At the same time, the
disk may be sufficiently ionised for magnetic fields to play some role in the
dynamics. Though usually neglected, the impact of magnetism on the GI may be
critical, with consequences for several processes: the efficiency of accretion,
spiral structure formation, fragmentation, and the dynamics of solids. In this
paper, we report on global three-dimensional magnetohydrodynamical simulations
of a self-gravitating protoplanetary disk using the meshless finite mass (MFM)
Lagrangian technique. We confirm that GI spiral waves trigger a dynamo that
amplifies an initial magnetic field to nearly thermal amplitudes (plasma beta <
10), an order of magnitude greater than that generated by the
magneto-rotational instability alone. We also determine the dynamo's nonlinear
back reaction on the gravitoturbulent flow: the saturated state is
substantially hotter, with an associated larger Toomre parameter and weaker,
more 'flocculent' spirals. But perhaps of greater import is the dynamo's
boosting of accretion via a significant Maxwell stress; mass accretion is
enhanced by factors of several relative to either pure GI or pure MRI. Our
simulations use ideal MHD, an admittedly poor approximation in protoplanetary
disks, and thus future studies should explore the full gamut of non-ideal MHD.
In preparation for that, we exhibit a small number of Ohmic runs that reveal
that the dynamo, if anything, is stronger in a non-ideal environment. This work
confirms that magnetic fields are a potentially critical ingredient in
gravitoturbulent young disks, possibly controlling their evolution, especially
via their enhancement of (potentially episodic) accretion.
