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We describe a method for the determination of stellar [C/Fe] abundance ratiosusing low-resolution (R = 2000) stellar spectra from the SDSS and SEGUE. Bymeans of a star-by-star comparison with a set of SDSS/SEGUE spectra withavailable estimates of [C/Fe] based on published high-resolution analyses, wedemonstrate that we can measure [C/Fe] from SDSS/SEGUE spectra with S/N > 15 toa precision better than 0.35 dex. Using the measured carbon-to-iron abundanceratios obtained by this technique, we derive the frequency of carbon-enhancedstars ([C/Fe] > +0.7) as a function of [Fe/H], for both the SDSS/SEGUE starsand other samples from the literature. We find that the differential frequencyslowly rises from almost zero to about 14% at [Fe/H] ~ -2.4, followed by asudden increase, by about a factor of three, to 39% from [Fe/H] ~ -2.4 to[Fe/H] ~ -3.7. We also examine how the cumulative frequency of CEMP starsvaries across different luminosity classes. The giant sample exhibits acumulative CEMP frequency of 32% for [Fe/H] < -2.5, 31% for [Fe/H] < -3.0, and33% for [Fe/H] < -3.5. For the main-sequence turnoff stars, we obtain a lowercumulative CEMP frequency, around 10% for [Fe/H] < -2.5. The dwarf populationdisplays a large change in the cumulative frequency for CEMP stars below [Fe/H]= -2.5, jumping from 15% for [Fe/H] < -2.5 to about 75% for [Fe/H] < -3.0. Whenwe impose a restriction with respect to distance from the Galactic mid-plane(|Z| < 5 kpc), the frequency of the CEMP giants does not increase at lowmetallicity ([Fe/H] < -2.5), but rather, decreases, due to the dilution ofC-rich material in stars that have undergone mixing with CNO-processed materialfrom their interiors. The frequency of CEMP stars near the main-sequenceturnoff, which are not expected to have experienced mixing, increases for[Fe/H] < -3.0. [abridged]

Carbon-enhanced Metal-poor Stars in SDSS/SEGUE. I. Carbon Abundance Estimation and Frequency of CEMP Stars