Dancing with the Stars: Formation of the Fomalhaut triple system and its effect on the debris disks

摘要

Fomalhaut is a triple system, with all components widely separated (~1E5 au).Such widely separated binaries are thought to form during cluster dissolution,but that process is unlikely to form such a triple system. We explore analternative scenario, where A and C form as a tighter binary from a singlemolecular cloud core (with semimajor axis ~1E4 au), and B is captured duringcluster dispersal. We use N-body simulations augmented with the Galactic tidalforces to show that such a system naturally evolves into a Fomalhaut-likesystem in about half of cases, on a timescale compatible with the age ofFomalhaut. From initial non-interacting orbits, Galactic tides drive cycles inB's eccentricity that lead to a close encounter with C. After several closeencounters, typically lasting tens of millions of years, one of the stars isejected. The Fomalhaut-like case with both components at large separations isalmost invariably a precursor to the ejection of one component, most commonlyFomalhaut C. By including circumstellar debris in a subset of the simulations,we also show that such an evolution usually does not disrupt the coherentlyeccentric debris disk around Fomalhaut A, and in some cases can even producesuch a disk. We also find that the final eccentricity of the disk around A andthe disk around C are correlated, which may indicate that the dynamics of thethree stars stirred C's disk, explaining its unusual brightness.