Carbon abundances of reference late-type stars from 1D analysis of atomic C i and molecular CH lines

A comprehensive model atom was constructed for C i using the most up-to-date atomic data. We evaluated non-local thermodynamical equilibrium (NLTE) line formation for neutral carbon in classical one-dimensional (1D) models representing atmospheres of late-type stars, where carbon abundance varies from the solar value down to [C/H] = −3. NLTE leads to stronger C i lines compared with their local thermodynamical equilibrium (LTE) strength and negative NLTE abundance corrections, Δ<inf>NLTE</inf>. The deviations from LTE are large for the strong lines in the infrared (IR), with Δ<inf>NLTE</inf> = −0.10 to −0.45 dex, depending on stellar parameters, and minor for the weak lines in the visible spectral range, with |Δ<inf>NLTE</inf>| ≤ 0.03 dex. The NLTE abundance corrections were found to be dependent on the carbon abundance in the model. As the first application of the treated model atom, carbon NLTE abundances were determined for the Sun and eight late-type stars with well-determined stellar parameters that cover the −2.56 ≤ [Fe/H] ≤ −1.02 metallicity range. Consistent abundances from the visible and IR lines were found for the Sun and the most metal-rich star of our sample, when applying a scaling factor of <it>S</it><inf>H</inf> = 0.3 to the Drawinian rates of C+H collisions. Carbon abundances were also derived from the molecular CH lines and agree with those from the atomic C i lines for each star. We present the NLTE abundance corrections for lines of C i in the grid of model atmospheres applicable to carbon-enhanced (CEMP) stars.