Carbon-enhanced metal-poor stars in sdss/segue. I. carbon abundance estimation and frequency of cemp stars

Abstract

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] < 5 kpc), the frequency of the CEMP giants does not increase at low metallicity ([Fe/H]