Wide Twin Binaries are Extremely Eccentric: Evidence of Twin Binary Formation in Circumbinary Disks

The Gaia mission recently revealed an excess population of equal-mass "twin" wide binaries, with mass ratio q ≳ 0.95, extending to separations of at least 1000 au. The origin of this population is an enigma: Twin binaries are thought to form via correlated accretion in circumbinary disks, but the typical observed protostellar disks have radii of ∼100 au, far smaller than the separations of the widest twins. Here, we infer the eccentricity distribution of wide twins from the distribution of their v–r angles, i.e., the angle between the components' separation and relative velocity vectors. We find that wide twins must be on extremely eccentric orbits. For the excess-twin population at 400–1000 au, we infer a near-delta-function excess of high-eccentricity systems, with eccentricity 0.95 ≲ e ≤ 1. These high eccentricities for wide twins imply pericenter distances of order 10 au and suggest that their orbits were scattered via dynamical interactions in their birth environments, consistent with a scenario in which twins are born in circumbinary disks and subsequently widened. These results further establish twin wide binaries as a distinct population and imply that wide twins can be used as a probe of the dynamical history of stellar populations.