Eccentricity of wide binary stars

Eccentricity is one of the fundamental orbital parameters in orbital dynamics. It not only provides critical constraints on binary formation mechanisms, but also plays a key role in the formation of hot Jupiters and close binaries via the secular three-body interaction. However, eccentricity is challenging to measure for wide binaries due to their extremely long orbital periods. In this talk, I will show that we can infer the eccentricity distribution of wide binaries even when we only have a snapshot of their orbits. In particular, with Gaia data, we use the “v-r angle”, the angle between the separation vector and the relative velocity vector in a wide binary, to infer binary eccentricities. I will present that the eccentricity distribution strongly depends on binary separations, and it becomes a mysterious super-thermal distribution at >1000 AU. I will further discuss how eccentricities change among subsamples of wide binaries, especially equal-mass wide binaries. This v-r angle method provides a new technique for measuring eccentricities for long-period systems, critical for understanding binary formation, evolution, and stellar orbits in the Galactic center and globular clusters.