There is now strong evidence that the close binary fraction ( P < 10 ^ { 4 } days ; a < 10 AU ) of solar-type stars ( M _ { 1 } \approx 0.6 - 1.5 M _ { \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 ( Raghavan et al .
2010 ) , ( 2 ) the SB survey of high-proper-motion stars ( Latham et al .
2002 ) , ( 3 ) various SB samples of metal-poor giants ( Carney et al .
2003 ; Hansen et al .
2015 , 2016 ) , ( 4 ) the APOGEE survey of radial velocity ( RV ) variables ( Badenes et al .
2018 ) , and ( 5 ) eclipsing binaries ( EBs ) discovered by Kepler ( Kirk et al .
2016 ) .
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 / methods exhibit a quantitatively consistent anti-correlation between the intrinsic close binary fraction ( a < 10 AU ) and metallicity : F _ { 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 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 \lesssim [ Fe/H ] < 0.5 .
The majority of solar-type stars with [ Fe/H ] \lesssim - 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 .