Total C+N+O Abundances in the Atmospheres of Red Giants of Different Metallicities

Abstract

The sum of C+N+O in red giants presents interest due to its steady nature during stellar evolution of the star, although the abundance of its components, especially C and N, undergoes significant evolutionary changes. It is shown that there is a pronounced correlation between the total log ε(C+N+O) abundances in red giants and their metallicity index [Fe/H]. The linear relationship between log ε (C + N + O) and [Fe/H] indicates an increase in log ε (C+N+O) from 7.0 with [Fe/H] = -2.5 to 9.2 with [Fe/H] = +0.3, i.e., by more than 2 dex. In contrast, the abundance of (C+N+O) relative to Fe decreases by 0.7 dex over the same [Fe/H] interval. It is shown that the ratio between [(C+N+O)/Fe] and [Fe/H] is almost completely determined by the ratio between [O/Fe] and [Fe/H]; hence, it is confirmed that oxygen is the main contributor to C+N+O. Since current models of Galactic evolution explain the observed dependence of [O/Fe] on [Fe/H] reasonably well, they thereby also explain the observed dependence of [(C+N+O)/Fe] on [Fe/H].