2019ApJ...884..150A

We present a model atom for C I-C II-C III-C IV using the most up-to-date atomic data and evaluated the non-local thermodynamic equilibrium line formation in classical 1D atmospheric models of O-B-type stars. Our models predict the emission lines of C II 9903 Å and 18,535 Å to appear at effective temperature T_eff >= 17,500 K, those of C II 6151 Å and 6461 Å to appear at T_eff > 25,000 K, and those of C III 5695, 6728-44 and 9701-17 Å to appear at T_eff >= 35,000 K (log g = 4.0). Emission occurs in the lines of minority species, where the photoionization-recombination mechanism provides a depopulation of the lower levels to a greater extent than the upper levels. For C II 9903 and 18535 Å, the upper levels are mainly populated from the C III reservoir through the Rydberg states. For C III 5695 and 6728-44 Å, the lower levels are depopulated due to photon losses in UV transitions at 885, 1308, and 1426-28 Å, which become optically thin in the photosphere. We analyzed the lines of C I, C II, C III, and C IV for twenty-two O-B-type stars with temperature range between 15,800 <=T_eff <= 38,000 K. Abundances from emission lines of C I, C II, and C III are in agreement with those from absorption ones for most of the stars. We obtained log ε_C = 8.36 ± 0.08 from twenty B-type stars, that is in line with the present-day Cosmic Abundance Standard. The obtained carbon abundances in 15 Mon and HD 42088 from emission and absorption lines are 8.27 ± 0.11 and 8.31 ± 0.11, respectively.