We present the results from an analysis of the \lambda 8727 forbidden [ C i ] line in high-resolution Gemini-S/bHROS spectra of three Carbon-Enhanced Metal-Poor ( CEMP ) stars .
Previous derivations of C abundances in CEMP stars primarily have used the blue bands of CH and the C _ { 2 } Swan system , features which are suspected to be sensitive to photospheric temperature inhomogeneities ( the so-called 3D effects ) .
We find the [ C/Fe ] ratios based on the [ C i ] abundances of the two most Fe-rich stars in our sample ( HE 0507-1653 : \mathrm { [ Fe / H ] } = -1.42 and HE 0054-2542 : \mathrm { [ Fe / H ] } = -2.66 ) to be in good agreement with previously determined values .
For the most Fe-deficient star in our sample ( HE 1005-1439 : \mathrm { [ Fe / H ] } = -3.08 ) , however , the [ C/Fe ] ratio is found to be 0.34 dex lower than the published molecular-based value .
We have carried out 3D local thermodynamic equilibrium ( LTE ) calculations for [ C i ] , and the resulting corrections are found to be modest for all three stars , suggesting that the discrepancy between the [ C i ] and molecular-based C abundances of HE 1005-1439Â is due to more severe 3D effects on the molecular lines .
Carbon abundances are also derived from C i high-excitation lines and are found to be 0.45 – 0.64 dex higher than the [ C i ] -based abundances .
Previously published non-LTE ( NLTE ) C i abundance corrections bring the [ C i ] and C i abundances into better agreement ; however , targeted NLTE calculations for CEMP stars are clearly needed .
We have also derived the abundances of nitrogen , potassium , and iron for each star .
The Fe abundances agree well with previously derived values , and the K abundances are similar to those of C-normal metal-poor stars .
Nitrogen abundances have been derived from resolved lines of the CN red system assuming the C abundances derived from the [ C i ] feature .
The abundances are found to be approximately 0.44 dex larger than literature values , which have been derived from CN blue bands near 3880 and 4215 Ã .
We discuss evidence that suggests that analyses of the CN blue system bands underestimate the N abundances of metal-poor giants .