Global Simulations of Self-gravitating Magnetized Protoplanetary Disks

In the early stages of a protoplanetary disk, turbulence generated by gravitational instability (GI) should feature significantly in the disk's evolution. At the same time, the disk may be sufficiently ionized for magnetic fields to play some role in the dynamics. In this paper, we report on global three-dimensional magnetohydrodynamical simulations of a self-gravitating protoplanetary disk using the meshless finite mass 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 magnetorotational instability alone. We also determine the dynamo's nonlinear back reaction on the gravito-turbulent 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 magnetorotational instability. Our simulations use ideal MHD, an admittedly poor approximation in protoplanetary disks, and thus, future studies should explore the full gamut of nonideal MHD. In preparation for that, we exhibit a small number of ohmic runs that reveal that the dynamo, if anything, is stronger in a nonideal environment. This work confirms that magnetic fields are a potentially critical ingredient in gravito-turbulent young disks, possibly controlling their evolution, especially via their enhancement of (potentially episodic) accretion.