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Based on our prior accurate determination of fundamental parameters for 36Galactic A-, F- and G-type supergiants and bright giants (luminosity classes Iand II), we undertook a non-LTE analysis of the carbon abundance in theiratmospheres. It is shown that the non-LTE corrections to the C abundancesderived from C I lines are negative and increase with the effective temperatureTeff; the corrections are especially significant for the infrared C I lineswith wavelengths 9060-9660 \AA. The carbon underabundance as a general propertyof the stars in question is confirmed; a majority of the stars studied has thecarbon 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 thesame stars earlier, we obtain a pronounced N vs. C anti-correlation, whichcould 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] valuesand the C and N anomalies themselves are an obvious evidence of the presence ona star's surface of mixed material from stellar interiors; so, a majority ofprogramme stars passed through the deep mixing during the main sequence (MS)and/or the first dredge-up (FD) phase. Comparison with theoretical predictionsincluding rotationally-induced mixing shows that the stars are either post-MSobjects with the initial rotational velocities V0 = 200-300 km/s or post-FDobjects with V0 = 0-300 km/s. The observed N vs. C anti-correlation reflects adependence of the C and N anomalies on the V0 value: on average the higher V0the greater the anomalies. It is shown that an absence of detectable lithium inthe atmospheres of the stars, which is accompanied with the observed N excessand C deficiency, is quite explainable.

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