The fraction of binary systems in various stellar populations of the Galaxy and the distribution of their orbital parameters are important but not well-determined factors in studies of star formation , stellar evolution , and Galactic chemical evolution .
While observational studies have been carried out for a large sample of nearby stars , including some metal-poor , Population II stars , almost no constraints on the binary nature for extremely metal-poor ( EMP ; [ Fe/H ] < -3.0 ) stars have yet been obtained .
Here we investigate the fraction of double-lined spectroscopic binaries and carbon-enhanced metal-poor ( CEMP ) stars , many of which could have formed as pairs of low-mass and intermediate-mass stars , to estimate the lower limit of the fraction of binary systems having short periods .
The estimate is based on a sample of very metal-poor stars selected from the Sloan Digital Sky Survey , and observed at high spectral resolution in a previous study by Aoki et al .
That survey reported three double-lined spectroscopic binaries and 11 CEMP stars , which we consider along with a sample of EMP stars from the literature compiled in the SAGA database .
We have conducted measurements of the velocity components for stacked absorption features of different spectral lines for each double-lined spectroscopic binary .
Our estimate indicates that the fraction of binary stars having orbital periods shorter than 1000 days is at least 10 % , and possibly as high as 20 % , if the majority of CEMP stars are formed in such short-period binaries .
This result suggests that the period distribution of EMP binary systems is biased toward short periods , unless the binary fraction of low-mass EMP stars is significantly higher than that of other nearby stars .