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تسجيل مستخدم جديدNLTE carbon abundance determination in selected A- and B-type stars and the interpretation of Cione emission lines
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We constructed a comprehensive model atom for Cione -- Cii using the most up-to-date atomic data available and evaluated the non-local thermodynamic equilibrium (NLTE) line formation for Cione and Cii 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 Cione 8335, 9405,AA singlet lines and at Teff~$>$~15,000~K (log~$g$ = 4) in the Cione 9061 -- 9111,AA,, 9603 -- 9658,AA, triplet lines. A prerequisite of the emission phenomenon is the overionization-recombination mechanism resulting in a depopulation of the lower levels of Cione 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 Cione 2479, 1930, and 1657,AA. We analysed the lines of Cione and Cii in Vega, HD~73666, Sirius, 21~Peg, $pi$~Cet, HD~22136, and $iota$ Her taking advantage of their observed high-resolution spectra. The Cione 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, Cione and Cii, including the Cione emission lines, were found to be consistent. We show that the predicted Cione emission phenomenon depends strongly on whether accurate or approximate electron-impact excitation rates are applied.
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