  Due to the gas rich environments of early circumstellar disks, the
gravitational collapse of cool, dense regions of the disk form fragments
largely composed of gas. During formation, disk fragments may attain increased
metallicities as they interact with the surrounding disk material, whether
through particle migration to pressure maxima or through mutual gravitational
interaction. In this paper, we investigate the ability of fragments to collect
and retain a significant solid component through gas-particle interactions in
high-resolution 3D self-gravitating shearing box simulations. The formation of
axissymmetric perturbations associated with gravitational instabilities allows
particles of intermediate sizes to concentrate through aerodynamic drag forces.
By the onset of fragmentation, the mass of local particle concentrations within
the fragment are comparable to that of the gas component and the sebsequent
gravitational collapse results in the formation of a solid core. We find that
these cores can be up to several tens of Earth masses, depending on grain size,
before the fragment center reaches temperatures which would sublimate solids.
The solid fraction and total mass of the fragment also depend on the
metallicity of the young parent protoplanetary disk, with higher initial
metallicities resulting in larger fragments and larger solid cores.
Additionally, the extended atmospheres of these soon-to-be gas giants or brown
dwarfs are occasionally enriched above the initial metallicity, provided no
solid core forms in the center and are otherwise lacking in heavier elements
when a core does form.
