Orbital Migration of Protoplanets in a Marginally Gravitationally Unstable Disk. II. Migration, Merging, and Ejection

TL;DR

This study uses advanced 3D hydrodynamics simulations to explore how protoplanets in gravitationally unstable disks migrate, merge, or get ejected, providing insights into the origins of free-floating planets.

Contribution

It demonstrates the use of the Enzo AMR code to simulate protoplanet evolution in MGU disks, confirming previous findings and highlighting frequent ejections as a formation pathway for free-floating planets.

Findings

Protoplanets undergo chaotic orbital evolution without monotonic migration.

Mergers among protoplanets significantly reduce their numbers over time.

Frequent ejections support the hypothesis that free-floating planets originate from such disks.

Abstract

Protoplanets formed in a marginally gravitationally unstable (MGU) disk by either core accretion or disk instability will be subject to dynamical interactions with massive spiral arms, possibly resulting in inward or outward orbital migration, mergers with each other, or even outright ejection from the protoplanetary system. The latter process has been hypothesized as a possible formation scenario for the unexpectedly high frequency of unbound gas giant exoplanets (free floating planets, FFP). Previous calculations with the EDTONS fixed grid three dimensional (3D) hydrodynamics code found that protoplanets with masses from 0.01 $M_\oplus$ to 3 $M_{Jup}$ could undergo chaotic orbital evolutions in MGU disks for $\sim$ 1000 yrs without undergoing monotonic inward or outward migration. Here the Enzo 2.5 adaptive mesh refinement (AMR) 3D hydrodynamics code is used to follow the formation and orbital evolution of protoplanets in MGU disks for up to 2000 yrs. The Enzo results confirm the basic disk fragmentation results of the EDTONS code, as well as the absence of monotonic inward or outward orbital migration. In addition, Enzo allows protoplanet mergers to occur, unlike EDTONS, resulting in a significant decrease in the number of protoplanets that survive for 1000 to 2000 yrs in the Enzo models. These models also imply that gas giants should be ejected frequently in MGU disks that fragment into large numbers of protoplanets, supporting ejection as a possible source mechanism for the observed FFPs.