We present a method for the determination of [ \alpha /Fe ] ratios from low-resolution ( R = 2000 ) SDSS/SEGUE stellar spectra .
By means of a star-by-star comparison with degraded spectra from the ELODIE spectral library and with a set of moderately high-resolution ( R = 15 , 000 ) and medium-resolution ( R = 6000 ) spectra of SDSS/SEGUE stars , we demonstrate that we are able to measure [ \alpha /Fe ] from SDSS/SEGUE spectra ( with S/N > 20 / 1 ) to a precision of better than 0.1 dex , for stars with atmospheric parameters in the range T _ { eff } = [ 4500 , 7000 ] K , \log~ { } g = [ 1.5 , 5.0 ] , and [ Fe/H ] = [ - 1.4 , + 0.3 ] , over the range [ \alpha /Fe ] = [ - 0.1 , + 0.6 ] .
For stars with [ Fe/H ] < -1.4 , our method requires spectra with slightly higher signal-to-noise to achieve this precision ( S/N > 25 / 1 ) .
Over the full temperature range considered , the lowest metallicity star for which a confident estimate of [ \alpha /Fe ] can be obtained from our approach is [ Fe/H ] \sim - 2.5 ; preliminary tests indicate that a metallicity limit as low as [ Fe/H ] \sim - 3.0 may apply to cooler stars .
As a further validation of this approach , weighted averages of [ \alpha /Fe ] obtained for SEGUE spectra of likely member stars of Galactic globular clusters ( M15 , M13 , and M71 ) and open clusters ( NGC 2420 , M67 , and NGC 6791 ) exhibit good agreement with the values of [ \alpha /Fe ] from previous studies .
The results of the comparison with NGC 6791 imply that the metallicity range for the method may extend to \sim + 0.5 .