The Close Binary Fraction as a Function of Stellar Parameters in APOGEE: A Strong Anti-Correlation With $\alpha$ Abundances.

We use observations from the APOGEE survey to explore the relationship between stellar parameters and multiplicity. We combine high-resolution repeat spectroscopy for 41,363 dwarf and subgiant stars with abundance measurements from the APOGEE pipeline and distances and stellar parameters derived using Gaia DR2 parallaxes from Sanders & Das 2018 to identify and characterise stellar multiples with periods below 30 years, corresponding to $\Delta$RV$_{max}\gtrsim$ 3 km s$^{-1}$, where $\Delta$RV$_{max}$ is the maximum APOGEE-detected shift in the radial velocities. Chemical composition is responsible for most of the variation in the close binary fraction in our sample, with stellar parameters like mass and age playing a secondary role. In addition to the previously identified strong anti-correlation between the close binary fraction and [Fe/H], we find that high abundances of $\alpha$ elements also suppress multiplicity at most values of [Fe/H] sampled by APOGEE. The anti-correlation between $\alpha$ abundances and multiplicity is substantially steeper than that observed for Fe, suggesting C, O, and Si in the form of dust and ices dominate the opacity of primordial protostellar disks and their propensity for fragmentation via gravitational stability. Near [Fe/H] = 0, the bias-corrected close binary fraction ($a<10$ au) decreases from $\approx$ 100\% at [$\alpha$/H] = $-$0.2 to $\approx$ 15\% near [$\alpha$/H] = 0.08, with a suggestive turn-up to $\approx$20\% near [$\alpha$/H] = 0.2. We conclude that the relationship between stellar multiplicity and chemical composition for sun-like dwarf stars in the field of the Milky Way is complex, and that this complexity should be accounted for in future studies of interacting binaries.