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We present two-dimensional hydrodynamic simulations of self-gravitatingprotostellar disks subject to axisymmetric infall from envelopes andirradiation from the central star, to explore disk fragmentation due togravitational instability (GI), and the fragmented clump evolution. We assumethat the disk is built gradually and smoothly by the infall, resulting in goodnumerical convergence. We confirm that for disks around solar-mass stars,infall at high rates at radii beyond ~50 AU leads to disk fragmentation. Atlower infall rates <1e-5 Msun/yr, however, irradiation suppressesfragmentation. We find that, once formed, the fragments or clumps migrateinward on typical type-I time scales of ~2e3 yr initially, but later migrationdeviates from the type-I time scale when the clump becomes more massive thanthe local disk mass, and/or when they starts to open gaps. As they migrate, theclumps accrete from the disk at a rate 1e-3 to 1e-1 MJupiter/yr, consistentwith analytic estimates that assume a 1-2 Hill radii cross section. Most clumpscan grow to their isolation masses >0.1 Msun quickly. The eventual fates ofthese clumps, however, diverges depending on the migration speed: 3 out of 13clumps become massive enough (brown dwarf mass) to open gaps in the disk andessentially stop migrating; 4 are tidally destroyed during inward migration; 6migrate across the inner simulated disk boundary. A simple analytic model forclump evolution is derived to explain these different fates. Overall, ourresults indicate that fast migration, accretion, and tidal destruction of theclumps pose challenges to the scenario of giant planet formation by GI in situ,but may provide a formation mechanism for close binary systems.

Challenges in Forming Planets by Gravitational Instability: Disk Irradiation and Clump Migration, Accretion & Tidal Destructio