The Close Binary Fraction of Solar-type Stars is Strongly
  Anti-correlated with Metallicity
  There is now strong evidence that the close binary fraction (P < 10$^4$ days;
a < 10 AU) of solar-type stars ($M_1$ = 0.6-1.5M$_{\odot}$) 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 re-analyze: (1) a volume-limited sample of
solar-type stars, (2) an 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) Kepler eclipsing binaries (EBs). The observed APOGEE RV
variability fraction and Kepler EB fraction both decrease by a factor of
$\approx$4 across $-$1.0 < [Fe/H] < 0.5 at the 22$\sigma$ and 9$\sigma$
confidence levels, respectively. After correcting for incompleteness, all five
samples exhibit a quantitatively consistent anti-correlation between the
intrinsic close binary fraction (a < 10 AU) and metallicity: $F_{\rm close}$ =
53%$\pm$12%, 40%$\pm$6%, 24%$\pm$4%, and 10%$\pm$3% at [Fe/H] = $-$3.0, $-$1.0,
$-$0.2 (mean field metallicity), and +0.5, respectively. We present
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 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.
