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

Abstract 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.