The eccentricity distribution of wide binaries and their individual measurements

Eccentricity of wide binaries is difficult to measure due to their long orbital periods. With Gaia's high-precision astrometric measurements, eccentricity of a wide binary can be constrained by the angle between the separation vector and the relative velocity vector (the v-r angle). In this paper, by using the v-r angles of wide binaries in Gaia Early Data Release 3, we develop a Bayesian approach to measure the eccentricity distribution as a function of binary separations. Furthermore, we infer the eccentricities of individual wide binaries and make them publicly available. Our results show that the eccentricity distribution of wide binaries at 10² AU is close to uniform and becomes superthermal at >10³ AU, suggesting two formation mechanisms dominating at different separation regimes. The close binary formation, most likely disc fragmentation, results in a uniform eccentricity distribution at <10² AU. The wide binary formation that leads to highly eccentric wide binaries at >10³ AU may be turbulent fragmentation and/or the dynamical unfolding of compact triples. With Gaia, measuring eccentricities is now possible for a large number of wide binaries, opening a new window to understanding binary formation and evolution.