  Migration of dense gaseous clumps that form in young protostellar disks via
gravitational fragmentation is investigated to determine the likelihood of
giant planet formation. High-resolution numerical hydrodynamics simulations in
the thin-disk limit are employed to compute the formation and long-term
evolution of a gravitationally unstable protostellar disk around a solar-mass
star. We show that gaseous clumps that form in the outer regions of the disk
(>100 AU) through disk fragmentation are often perturbed by other clumps or
disk structures, such as spiral arms, and migrate toward the central star on
timescales from a few 10^3 to few 10^4 yr. The migration timescale is slowest
when stellar motion in response to the disk gravity is considered. When
approaching the star, the clumps first gain mass (up to several tens of M_Jup),
but then quickly lose most of their diffuse envelopes through tidal torques.
Part of the clump envelope can be accreted on the central star causing an
FU-Ori-type accretion and luminosity outburst. The tidal mass loss helps the
clumps to significantly slow down or even halt their inward migration at a
distance of a few tens of AU from the protostar. The resulting clumps are
heavily truncated both in mass and size compared to their wider-orbit
counterparts, keeping only a dense and hot nucleus. During the inward
migration, the temperature in the clump interiors may exceed the molecular
hydrogen dissociation limit (2000 K) and the central region of the clump can
collapse into a gas giant protoplanet. We argue that FU-Orionis-type luminosity
outbursts may be the end product of disk fragmentation and clump inward
migration, ushering the formation of giant protoplanets on tens-of-au orbits in
systems such as HR~8799.
