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

TL;DR

This study uses hydrodynamic simulations to investigate how gravitational instability in protostellar disks leads to fragmentation, clump migration, and potential destruction, impacting planet and binary star formation theories.

Contribution

The paper provides new insights into disk fragmentation, clump evolution, and the effects of irradiation and infall rates on planet formation via gravitational instability.

Findings

High infall rates cause disk fragmentation beyond ~50 AU.

Irradiation suppresses fragmentation at lower infall rates.

Clump migration and accretion dynamics influence their ultimate fate.

Abstract

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