Abstract

Fomalhaut is a triple system, with all components widely separated (~100000 au). Unusually, Fomalhaut A has a bright debris disk with coherent eccentricity ~0.1, and Fomalhaut C is also know to have a debris disk. Widely separated binaries are thought to form during cluster dissolution, but that process is unlikely to form a triple system. We explore an alternative scenario, where A and C form as a tighter binary from a single molecular cloud core, and B is captured during cluster dispersal. We use N-body simulations augmented with the Galactic tidal forces to show that such a system naturally evolves into a Fomalhaut-like system in about half of cases, on a timescale compatible with the age of Fomalhaut. From non-interacting orbits, Galactic tides drive cycles in B's eccentricity that lead to a close encounter with C. After several close encounters, typically lasting tens of millions of years, one of the stars is ejected. The Fomalhaut-like case with both components at large separations is almost invariably a precursor to the ejection of one component, most commonly Fomalhaut C. By including circumstellar debris in a subset of the simulations, we also show that such an evolution usually does not disrupt the coherently eccentric debris disk around Fomalhaut A, and in some cases can even produce a coherently disk. We also find that the final eccentricity of the disk around A and the disk around C are correlated, which may point to the origin of the unusual debris disk around C.

Note - if anyone would like copies of the movies from Andrew's talk, please email shannon@ast.cam.ac.uk

Presentation

Talk slides - PDF