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

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.