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 $50\lesssim s/\mathrm{AU}\lesssim 50,000$. 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 $50<s/\mathrm{AU}<100$, the binary fraction at $[Fe/H]=-1$ exceeds that at $[Fe/H]=0.5$ 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 $100<a/\mathrm{AU}<200$ 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.

5 pages, 3 figures, accepted to MNRAS Letters. Catalog available at https://sites.google.com/site/dr2binariesspectra/data

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