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.
Bibliographical noteCopyright 2013 The American Astronomical Society. First published in The Astronomical Journal, 146(5), 132, 2013. The original publication is available at http://www.doi.org/10.1088/0004-6256/146/5/132, published by IOP Publishing. Version archived for private and non-commercial use with the permission of the author/s and according to publisher conditions. For further rights please contact the publisher.
- methods: data analysis
- stars: abundances
- stars: fundamental parameters
- techniques: imaging spectroscopy