The NLTE Analyses of Carbon Emission Lines in the Atmospheres of O- and B-type Stars

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 and 18,535 to appear at effective temperature Teff ≥ 17,500 K, those of C ii 6151 and 6461 to appear at Teff > 25,000 K, and those of C iii 5695, 6728-44 and 9701-17 to appear at Teff ≥ 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 , the upper levels are mainly populated from the C iii reservoir through the Rydberg states. For C iii 5695 and 6728-44 , the lower levels are depopulated due to photon losses in UV transitions at 885, 1308, and 1426-28 , 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 ≤Teff ≤ 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 ϵC = 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.