Characterizing fragmentation and sub-Jovian clump properties in magnetized young protoplanetary discs

Abstract

We study the initial development, structure, and evolution of protoplanetary clumps formed in three-dimensional resistive magnetohydrodynamic (MHD) simulations of self-gravitating discs. The magnetic field grows by means of the recently identified gravitational instability dynamo. Clumps are identified and their evolution is tracked finely both backward and forward in time. Their properties and evolutionary path is compared with clumps in companion simulations without magnetic fields. We find that magnetic and rotational energy are important in the clumps' outer regions, while in the cores, despite appreciable magnetic field amplification, thermal pressure is most important in counteracting gravity. Turbulent kinetic energy is of a smaller scale than magnetic energy in the clumps. Compared with non-magnetized clumps, rotation is less prominent, which results in lower angular momentum in much better agreement with observations. In order to understand the very low sub-Jovian masses of clumps forming in MHD simulations, we revisit the perturbation theory of magnetized sheets finding support for a previously proposed magnetic destabilization in low-shear regions. This can help explaining why fragmentation ensues on a scale more than an order of magnitude smaller than that of the Toomre mass. The smaller fragmentation scale and the high magnetic pressure in clumps' envelopes explain why clumps in magnetized discs are typically in the super-Earth to Neptune mass regime rather than super-Jupiter as in conventional disc instability. Our findings put forward a viable alternative to core accretion to explain widespread formation of intermediate-mass planets.