The wide binary fraction of solar-type stars: emergence of metallicity dependence at a < 200 AU

TL;DR

This study investigates how the fraction of wide binary stars among solar-type stars depends on metallicity, revealing a significant anti-correlation at smaller separations, which informs models of binary formation mechanisms.

Contribution

It provides the first comprehensive analysis of metallicity dependence of wide binary fractions across a broad separation range using Gaia data and spectroscopic surveys.

Findings

Wide binary fraction is constant at large separations regardless of metallicity.

At smaller separations, the binary fraction decreases with increasing metallicity.

The results suggest different formation mechanisms dominate at different binary separations.

Abstract

We combine a catalog of wide binaries constructed from Gaia DR2 with [Fe/H] abundances from wide-field spectroscopic surveys to quantify how the binary fraction varies with metallicity over separations $50 \lesssim s/{\rm AU} \lesssim 50,000$. At a given distance, the completeness of the catalog is independent of metallicity, making it straightforward to constrain intrinsic variation with [Fe/H]. The wide binary fraction is basically constant with [Fe/H] at large separations ($s \gtrsim 250$\,AU) but becomes quite rapidly anti-correlated with [Fe/H] at smaller separations: for $50 < s/{\rm AU} < 100$, the binary fraction at $\rm [Fe/H] = -1$ exceeds that at $\rm [Fe/H] = 0.5$ by a factor of 3, an anti-correlation almost as strong as that found for close binaries with $a < 10$ AU. Interpreted in terms of models where disk fragmentation is more efficient at low [Fe/H], our results suggest that $100 < a/{\rm AU} < 200$ is the separation below which a significant fraction of binaries formed via fragmentation of individual gravitationally unstable disks rather than through turbulent core fragmentation. We provide a public catalog of 8,407 binaries within 200 pc with spectroscopically-determined [Fe/H] for at least one component.