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Description
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 the first global three-dimensional magnetohydrodynamical simulations of a self-gravitating protoplanetary 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 essential ingredient in gravitoturbulent young disks, possibly controlling their evolution, especially
via their enhancement of (potentially episodic) accretion.
