Insights on the dynamical history of the Fomalhaut system - Investigating the Fom c hypothesis

Abstract

The eccentric shape of the debris disk observed around Fomalhaut was first attributed to Fom b, a companion detected near the belt inner-edge, but new constraints on its orbit revealed that it is belt-crossing, highly eccentric $(e\sim 0.6-0.9)$, and can hardly account for the shape of the belt. The best scenario to explain this paradox is that there is another massive body in this system, Fom c, which drives the debris disk shape. The resulting planetary system is highly unstable, which hints at a dynamical scenario involving a recent scattering of Fom b on its current orbit, potentially with the putative Fom c. Our goal is to give insights on the probability for Fom b to have been set on its highly eccentric orbit by a close-encounter with the putative Fom c. We aim to study in particular the part played by mean-motion resonances with Fom c, which could have brought Fom b sufficiently close to Fom c for it to be scattered on its current orbit, but also delay this scattering event. Using N-body simulations, we found that the generation of orbits similar to that of Fom b, either in term of dimensions or orientation, is a robust process involving a scattering event and a further secular evolution of inner material with an eccentric massive body such as the putative Fom c. We found in particular that mean-motion resonances can delay scattering events, and thus the production of Fom b-like orbits, on timescales comparable to the age of the system, thus explaining the witnessing of an unstable configuration. We conclude that Fom b probably originated from an inner resonance with Fom c, which is at least Neptune-Saturn size, and was set on its current orbit by a scattering event with Fom c. Since Fom b could not have formed from material in resonance, our scenario also hints at former migration processes in this planetary system.

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• Influence of an inner disc on the orbital evolution of massive planets migrating in resonance
• Planetesimal driven migration as an explanation for observations of high levels of warm, exozodiacal dust
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• Constraining the orbits of sub-stellar companions imaged over short orbital arcs
• Fomalhaut b could be massive and sculpting the narrow, eccentric debris disc, if in mean-motion resonance with it
• Biases in orbit fitting of directly-imaged exoplanets with small orbital coverage
• ALMA imaging of the M-dwarf Fomalhaut C's debris disc
• Constraints on the gas content of the Fomalhaut debris belt. Can gas-dust interactions explain the belt's morphology?
• 94 Ceti: a triple star with a planet and dust disc
• Interferometric Detection and Orbit Modeling of the Subcomponent in the Hot-dust System $\kappa$ Tuc A: A Low-mass Star on an Eccentric Orbit in a Hierarchical-quintuple System