Early evolution of clumps formed via gravitational instability in protoplanetary disks; precursors of Hot Jupiters?

Abstract

Although it is fairly established that Gravitational Instability (GI) should occur in the early phases of the evolution of a protoplanetary disk, the fate of the clumps resulting from disk fragmentation and their role in planet formation is still unclear. In the present study we investigate semi-analytically their evolution following the contraction of a synthetic population of clumps with varied initial structure and orbits coupled with the surrounding disk and the central star. Our model is based on recently published state-of-the-art 3D collapse simulations of clumps with varied thermodynamics. Various evolutionary mechanisms are taken into account, and their effect is explored both individually and in combination with others: migration and tidal disruption, mass accretion, gap opening and disk viscosity. It is found that, in general, at least 50% of the initial clumps survive tides, leaving behind potential gas giant progenitors after ~10^5 yr of evolution in the disk. The rest might be either disrupted or produce super-Earths and other low mass planets provided that a solid core can be assembled on a sufficiently short timescale, a possibility that we do not address in this paper. Extrapolating to million year timescales, all our surviving protoplanets would lead to close-in gas giants. This outcome might in part reflect the limitations of the migration model adopted, and is reminiscent of the analogous result found in core-accretion models in absence of fine-tuning of the migration rate. Yet it suggests that a significant fraction of the clumps formed by gravitational instability could be the precursors of Hot Jupiters.