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NLTE carbon abundance determination in selected A- and B-type stars and the interpretation of C\ione\ emission lines

We constructed a comprehensive model atom for C\ione\ -- C\ii\ using the most up-to-date atomic data available and evaluated the non-local thermodynamic equilibrium (NLTE) line formation for C\ione\ and C\ii\ in classical 1D models representing the atmospheres of A and late B-type stars. Our NLTE calculations predict the emission that appears at effective temperature of 9250 to 10\,500~K depending on log~$g$ in the C\ione\ 8335, 9405\,Å singlet lines and at \Teff~$>$~15\,000~K (log~$g$ = 4) in the C\ione\ 9061 -- 9111\,Å\,, 9603 -- 9658\,Å\, triplet lines. A prerequisite of the emission phenomenon is the overionization-recombination mechanism resulting in a depopulation of the lower levels of C\ione\ to a greater extent than the upper levels. Extra depopulation of the lower levels of the transitions corresponding to the near-infrared lines, is caused by photon loss in the UV lines C\ione\ 2479, 1930, and 1657\,Å. We analysed the lines of C\ione\ and C\ii\ in Vega, HD~73666, Sirius, 21~Peg, $π$~Cet, HD~22136, and $ι$ Her taking advantage of their observed high-resolution spectra. The C\ione\ emission lines were detected in the four hottest stars, and they were well reproduced in our NLTE calculations. For each star, the mean NLTE abundances from lines of the two ionization stages, C\ione\ and C\ii, including the C\ione\ emission lines, were found to be consistent. We show that the predicted C\ione\ emission phenomenon depends strongly on whether accurate or approximate electron-impact excitation rates are applied.