Blue metal-poor stars ( BMPs ) are main sequence stars that appear bluer and more luminous than normal turnoff stars .
They were originally singled out by using B - V and U - B colour cuts .
Early studies found that a larger fraction of field BMP stars were binaries compared to normal halo stars .
Thus , BMP stars are ideal field blue straggler candidates for investigating internal stellar evolution processes and binary interaction .
In particular , the presence or depletion in lithium in their spectra is a powerful indicator as to their origin .
They are either old , halo blue stragglers experiencing internal mixing processes or mass transfer ( Li-depletion ) , or intermediate-age , single stars of possibly extragalactic origin ( 2.2 dex halo plateau Li ) .
However , we note that internal mixing processes can lead to an increased level of Li .
Hence , this study combines photometry and spectroscopy to unveil the origin of various BMP stars .
We first show how to separate binaries from young blue stars using photometry , metallicity , and lithium .
Using a sample of 80 BMP stars ( T > 6300 K ) , we find that 97 % of the BMP binaries have V - Ks _ { 0 } < 1.08 \pm 0.03 , while BMP stars that are not binaries lie above this cut in 2/3 of the cases .
This cut can help classify stars which lack radial velocities from follow-up observations .
We then trace the origin of two BMP stars from the photometric sample by conducting a full chemical analysis using new high-resolution and high signal-to-noise spectra .
Based on their radial velocities , Li , \alpha , and s- and r-process abundances we show that BPS CS22874-042 is a single star ( A ( Li ) = 2.38 \pm 0.10 dex ) while with A ( Li ) = 2.23 \pm 0.07 dex CD-48 2445 is a binary , contrary to earlier findings .
Our analysis emphasises that field blue stragglers can be segregated from single metal-poor stars , using ( V - Ks ) colours with a fraction of single stars polluting the binary sample , but not vice versa .
These two groups can only be properly separated by using information from stellar spectra , illustrating the need for accurate and precise stellar parameters and high resolution , high S/N spectra in order to fully understand and classify this intriguing class of stars .
Our high-resolution spectrum analysis confirms the findings from the colour cuts and shows that CS 22874 - 042 is single , while CD - 48 2445 most likely is a binary .
Moreover , the stellar abundances show that both stars formed in situ ; CS 22874 - 042 carry traces of massive star enrichment and CD - 48 2445 shows indications of AGB mass transfer mixed with gases ejected possibly from neutron star mergers .