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

We describe a method for the determination of stellar [C/Fe] abundance ratios using low-resolution (R = 2000) stellar spectra from the Sloan Digital Sky Survey (SDSS) and its Galactic sub-survey, the Sloan Extension for Galactic Understanding and Exploration (SEGUE). By means of a star-by-star comparison with a set of SDSS/SEGUE spectra with available estimates of [C/Fe] based on published high-resolution analyses, we demonstrate that we can measure [C/Fe] from SDSS/SEGUE spectra with S/N >= 15 Å^-1 to a precision better than 0.35 dex for stars with atmospheric parameters in the range T _eff = [4400, 6700] K, log g = [1.0, 5.0], [Fe/H] = [-4.0, +0.5], and [C/Fe] = [-0.25, +3.5]. Using the measured carbon-to-iron abundance ratios obtained by this technique, we derive the frequency of carbon-enhanced stars ([C/Fe] >= +0.7) as a function of [Fe/H], for both the SDSS/SEGUE stars and other samples from the literature. We find that the differential frequency slowly rises from almost zero to about 14% at [Fe/H] ~ -2.4, followed by a sudden increase, by about a factor of three, to 39% from [Fe/H] ~ -2.4 to [Fe/H] ~ -3.7. Although the number of stars known with [Fe/H] < -4.0 remains small, the frequency of carbon-enhanced metal-poor (CEMP) stars below this value is around 75%. We also examine how the cumulative frequency of CEMP stars varies across different luminosity classes. The giant sample exhibits a cumulative CEMP frequency of 32% for [Fe/H] <= -2.5, 31% for [Fe/H] <= -3.0, and 33% for [Fe/H] <= -3.5 a roughly constant value. For the main-sequence turnoff stars, we obtain a lower cumulative CEMP frequency, around 10% for [Fe/H] <= -2.5, presumably due to the difficulty of identifying CEMP stars among warmer turnoff stars with weak CH G-bands. The dwarf population displays 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. When we 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 low metallicity ([Fe/H] < -2.5), but rather decreases due to the dilution of C-rich material in stars that have undergone mixing with CNO-processed material from their interiors. The frequency of CEMP stars near the main-sequence turnoff, which are not expected to have experienced mixing, increases for [Fe/H] <= -3.0. The general rise in the global CEMP frequency at low metallicity is likely due to the transition from the inner-halo to the outer-halo stellar populations with declining metallicity and increasing distance from the plane.