The Close Binary Fraction of Solar-type Stars is Strongly Anti-correlated with Metallicity

TL;DR

This study demonstrates that the intrinsic close binary fraction of solar-type stars decreases significantly with increasing metallicity, revealing a strong anti-correlation across multiple surveys after correcting for biases.

Contribution

It provides a comprehensive analysis correcting for selection biases across five surveys, establishing a consistent anti-correlation between binary fraction and metallicity, and offers theoretical models explaining this trend.

Findings

Close binary fraction decreases from 53% at [Fe/H] = -3.0 to 10% at [Fe/H] = +0.5.

APOGEE RV variability and Kepler EB fractions both decline by factors of about 4 across the metallicity range.

The trend is explained by fragmentation models, with implications for binary evolution in various environments.

Abstract

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.