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

TL;DR

This study analyzes carbon and nitrogen abundances in 36 Galactic supergiants and bright giants, revealing a clear N/C anti-correlation and evidence of deep mixing processes, with results aligning with models of rotationally-induced stellar mixing.

Contribution

It provides the first non-LTE analysis of carbon in these stars, confirming C underabundance and linking N/C ratios to stellar rotational history and mixing processes.

Findings

Most stars show C deficiency between -0.1 and -0.5 dex.

A pronounced N vs. C anti-correlation is observed.

Higher initial rotational velocities correlate with greater anomalies.

Abstract

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 \AA. 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 vs. C anti-correlation, 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 or post-FD objects with V0 = 0-300 km/s. The observed N vs. C anti-correlation 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.