The Close Binary Fraction of Solar-type Stars Is Strongly Anticorrelated with Metallicity

There is now strong evidence that the close binary fraction (P < 104 days; a < 10 au) of solar-type stars (M1 ≈ 0.6–1.5 ) decreases significantly with metallicity. Although early surveys showed that the observed spectroscopic binary (SB) fractions in the galactic disk and halo are similar (e.g., Carney–Latham sample), these studies did not correct for incompleteness. In this study, we examine five different surveys and thoroughly account for their underlying selection biases to measure the intrinsic occurrence rate of close solar-type binaries. We reanalyze (1) a volume-limited sample of solar-type stars, (2) the Carney-Latham SB survey of high proper motion stars, (3) various SB samples of metal-poor giants, (4) the APOGEE survey of radial velocity (RV) variables, and (5) eclipsing binaries (EBs) discovered by Kepler. The observed APOGEE RV variability fraction and Kepler EB fraction both decrease by a factor of ≈4 across −1.0 < [Fe/H] < 0.5 at the 22σ and 9σ confidence levels, respectively. After correcting for incompleteness, all five samples/methods exhibit a quantitatively consistent anticorrelation between the intrinsic close binary fraction (a < 10 au) and metallicity: Fclose = 53% ± 12%, 40% ± 6%, 24% ± 4%, and 10% ± 3% at [Fe/H] = −3.0, −1.0, −0.2 (mean field metallicity), and +0.5, respectively. We present simple fragmentation models that explain why the close binary fraction of solar-type stars strongly decreases with metallicity while the wide binary fraction, close binary fraction of OB stars, and initial mass function are all relatively constant across −1.5 ≲ [Fe/H] < 0.5. The majority of solar-type stars with [Fe/H] ≲ −1.0 will interact with a stellar companion, which has profound implications for binary evolution in old and metal-poor environments such as the galactic halo, bulge, thick disk, globular clusters, dwarf galaxies, and high-redshift universe.