Carbon abundance and the N/C ratio in atmospheres of A-, F- and G-type supergiants and bright giants

Based on our prior accurate determination of fundamental parameters for 36 Galactic A-, F- and G-type supergiants and bright giants (luminosity classes I and II), we undertook a non-LTE analysis of the carbon abundance in their atmospheres. It is shown that the non-LTE corrections to the C abundances derived from C i lines are negative and increase with the effective temperature Teff; the corrections are especially significant for the infrared C i lines with wavelengths 9060–9660 Å. The carbon underabundance as a general property of the stars in question is confirmed; a majority of the stars studied has the carbon deficiency [C/Fe] between −0.1 and −0.5 dex, with a minimum at −0.7 dex. When comparing the derived C deficiency with the N excess found by us for the same stars earlier, we obtain a pronounced N versus C anticorrelation, which could be expected from predictions of the theory. We found that the ratio [N/C] spans mostly the range from 0.3 to 1.7 dex. Both these enhanced [N/C] values and the C and N anomalies themselves are an obvious evidence of the presence on a star's surface of mixed material from stellar interiors; so, a majority of programme stars passed through the deep mixing during the main sequence (MS) and/or the first dredge-up (FD) phase. Comparison with theoretical predictions including rotationally induced mixing shows that the stars are either post-MS objects with the initial rotational velocities V0 = 200–300 km s−1 or post-FD objects with V0 = 0–300 km s−1. The observed N versus C anticorrelation reflects a dependence of the C and N anomalies on the V0 value: on average the higher V0 the greater the anomalies. It is shown that an absence of detectable lithium in the atmospheres of the stars, which is accompanied with the observed N excess and C deficiency, is quite explainable.