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 \geq 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 ] \geq + 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 ] \sim –2.4 , followed by a sudden increase , by about a factor of three , to 39 % from [ Fe/H ] \sim –2.4 to [ Fe/H ] \sim –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 ] \leq - 2.5 , 31 % for [ Fe/H ] \leq - 3.0 , and 33 % for [ Fe/H ] \leq - 3.5 ; a roughly constant value .
For the main-sequence turnoff stars , we obtain a lower cumulative CEMP frequency , around 10 % for [ Fe/H ] \leq - 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 ] \leq - 2.5 to about 75 % for [ Fe/H ] \leq - 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 ] \leq - 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 .