The eccentricity distribution of wide binaries and their individual measurements

TL;DR

This paper uses Gaia data to develop a Bayesian method for measuring the eccentricity distribution of wide binaries, revealing different formation mechanisms at various separations and enabling individual eccentricity estimates.

Contribution

It introduces a novel Bayesian approach leveraging Gaia's $v$-$r$ angles to infer wide binary eccentricity distributions and individual eccentricities, advancing understanding of binary formation.

Findings

Eccentricity distribution is near uniform at 100 AU.

Distribution becomes superthermal beyond 1000 AU.

Different formation mechanisms dominate at different separations.

Abstract

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 $10^2$ AU is close to uniform and becomes superthermal at $>10^{3}$ AU, suggesting two formation mechanisms dominating at different separation regimes. The close binary formation, most likely disk fragmentation, results in a uniform eccentricity distribution at $<10^{2}$ AU. The wide binary formation that leads to highly eccentric wide binaries at $>10^{3}$ 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.