Abstract

Fomalhaut harbours a cold, moderately eccentric dust belt with a planet candidate (Fom b) imaged near its inner edge. Based on available astrometric data, we perform a new orbital determination of Fom b using MCMC. We show that the orbit is highly eccentric (e~0.9), nearly apsidally aligned with the belt and subject to cross it. We study the secular interaction between the planet and the dust ring. We show that unless Fom b is a small mass object (Super-Earth sized at most), close encounters rapidly destroy the dust belt. Conversely, if Fom b is very small (sub-Earth regime), the belt's self-gravity prevents the planet from dynamically affecting it. In the intermediate mass regime, Fom b actually perturbs the belt without destroying it. But it inevitably drives it to high eccentricity. Moreover the belt turns out not to be apsidally aligned with Fom b. We show that this behaviour is due to the planet's high eccentricity. This contradicts both observations and orbital determination. We conclude that Fom b cannot account itself for the dust ring sculpting, and that its present orbital configuration may be recent. Consequently, another, more massive and less eccentric planet (Fom c) is required to dynamically control the belt. We present various scenarios involving two planets. We show that Fom b is likely to have been formerly trapped in mean-motion resonance with Fom c and that subsequent eccentricity increase caused it to cross Fom c's orbit and to jump on its present day orbit via a scattering event.

Presentation

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