For a sample of metal-poor stars ( -3.3 \leq [ Fe/H ] \leq - 2.2 ) that have high-resolution spectroscopic abundance determinations , we have measured equivalent widths ( EW ) of the Ca ii K , Mg i b and near-infrared ( NIR ) Ca ii triplet lines using low-resolution spectra of the Sloan Digital Sky Survey ( SDSS ) , calculated effective temperatures from ( g - z ) _ { 0 } color , deduced stellar surface gravities by fitting stellar isochrones , and determined metallicities based on the aforementioned quantities .
Metallicities thus derived from the Ca ii K line are in much better agreement with the results determined from high-resolution spectra than the values given in the SDSS Data Release 7 ( DR7 ) .
The metallicities derived from the Mg i b lines have a large dispersion owing to the large measurement errors , whereas those deduced from the Ca ii triplet lines are too high due to both non-local thermodynamical equilibrium ( NLTE ) effects and measurement errors .
Abundances after corrected for the NLTE effect for the Mg i b lines and Ca ii triplet lines are also presented .
Following this method , we have identified six candidates of ultra-metal-poor stars with [ Fe/H ] \sim - 4.0 from a sample of 166 metal-poor star candidates .
One of them , SDSS J102915+172927 , was recently confirmed to be an ultra-metal-poor ( [ Fe/H ] < -4.0 ) star with the lowest metallicity ever measured .
Follow-up high-resolution spectroscopy for the other five ultra-metal-poor stars in our sample will therefore be of great interest .