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Efficient magnetic braking is a formidable obstacle to the formation ofrotationally supported disks (RSDs) around protostars in magnetized densecores. We have previously shown, through 2D (axisymmetric) non-ideal MHDsimulations, that removing very small grains (VSGs: ~10 \AA$~$to few 100 \AA)can greatly enhance ambipolar diffusion and enable the formation of RSDs. Herewe extend the simulations of disk formation enabled by VSG removal to 3D. Wefind that the key to this scenario of disk formation is that the drift velocityof the magnetic field almost cancels out the infall velocity of the neutrals inthe $10^2$-$10^3$ AU-scale "pseudo-disk" where the field lines are mostseverely pinched and most of protostellar envelope mass infall occurs. As aresult, the bulk neutral envelope matter can collapse without dragging muchmagnetic flux into the disk-forming region, which lowers the magnetic brakingefficiency. We find that the initial disks enabled by VSG removal tend to beToomre-unstable, which leads to the formation of prominent spiral structuresthat function as centrifugal barriers. The piling-up of infall material nearthe centrifugal barrier often produces dense fragments of tens of Jupitermasses, especially in cores that are not too strongly magnetized. Somefragments accrete onto the central stellar object, producing bursts in massaccretion rate. Others are longer lived, although whether they can survivelong-term to produce multiple systems remains to be ascertained. Our resultshighlight the importance of dust grain evolution in determining the formationand properties of protostellar disks and potentially multiple systems.

Decoupling of Magnetic Fields in Collapsing Protostellar Envelopes and Disk Formation and Fragmentation