We develop and test a method for the estimation of metallicities ( [ Fe/H ] ) and carbon abundance ratios ( [ C/Fe ] ) for carbon-enhanced metal-poor ( CEMP ) stars , based on application of artificial neural networks , regressions , and synthesis models to medium-resolution ( 1 - 2 Ã Â ) spectra and J - K colors .
We calibrate this method by comparison with metallicities and carbon abundance determinations for 118 stars with available high-resolution analyses reported in the recent literature .
The neural network and regression approaches make use of a previously defined set of line-strength indices quantifying the strength of the CaII K line and the CH G-band , in conjuction with J - K colors from the 2MASS Point Source Catalog .
The use of near-IR colors , as opposed to broadband B - V colors , is required because of the potentially large affect of strong molecular carbon bands on bluer color indices .

We also explore the practicality of obtaining estimates of carbon abundances for metal-poor stars from the spectral information alone , i.e. , without the additional information provided by photometry , as many future samples of CEMP stars may lack such data .
We find that , although photometric information is required for the estimation of [ Fe/H ] , it provides little improvement in our derived estimates of [ C/Fe ] , hence estimates of carbon-to-iron ratios based solely on line indices appear sufficiently accurate for most purposes .
Although we find that the spectral synthesis approach yields the most accurate estimates of [ C/Fe ] , in particular for the stars with the strongest molecular bands , it is only marginally better than is obtained from the line index approaches .

Using these methods we are able to reproduce the previously-measured [ Fe/H ] and [ C/Fe ] determinations with an accuracy of \approx 0.25 dex for stars in the metallicity interval -5.5 \leq { [ Fe / H ] } \leq - 1.0 , and with 0.2 \leq ( J - K ) _ { 0 } \leq 0.8 .
At higher metallicity the CaII K line begins to saturate , especially for the cool stars in our program , hence this approach is not useful in some cases .
As a first application , we estimate the abundances of [ Fe/H ] and [ C/Fe ] for the 56 stars identified as possibly carbon-rich , relative to stars of similar metal abundance , in the sample of “ strong G-band ” stars discussed by Beers , Preston , & Shectman .