Photospheric nitrogen abundances and carbon 12C/13C ratios of red giant stars

TL;DR

This study measures nitrogen abundances and carbon isotope ratios in red giant stars to understand envelope mixing processes, revealing complex patterns and highlighting gaps in current understanding.

Contribution

It provides a large dataset of [N/Fe] and 12C/13C ratios in red giants, analyzing their relation to stellar parameters and mixing indicators, and discusses the diversity of isotope ratios.

Findings

[N/Fe] is generally supersolar and anti-correlated with [C/Fe].

12C/13C ratios vary widely (~5-50) without clear dependence on mixing indicators.

Mixing appears enhanced with increased luminosity and rotational velocity.

Abstract

Nitrogen abundances and carbon isotope ratios (12C/13C) in the atmospheres of red giants are known to be influenced by dredge-up of H-burning products and serve as useful probes to study the nature of evolution-induced envelope mixing. We determined the [N/Fe] and 12C/13C ratios for 239 late-G/early-K giant stars by applying the spectrum-fitting technique to the 12CN and 13CN lines in the ~8002-8005A region, with an aim to investigate how these quantities are related to other similar mixing-affected indicators which were already reported in our previous work. It was confirmed that [N/Fe] values are generally supersolar (typically by several tenths dex though widely differ from star to star), anti-correlated with [C/Fe], and correlated with [Na/Fe], as expected from theory. As seen from their dependence upon stellar parameters, it appears that mixing tends to be enhanced with an increase of stellar luminosity (or mass) and rotational velocity, which is also reasonable from the theoretical viewpoint. In contrast, the resulting 12C/13C ratios turned out to be considerably diversified in the range of ~5-50 (with a peak around ~20), without showing any systematic dependence upon C or N abundance anomalies caused by the mixing of CN-cycled material. It thus appears that our understanding on the photospheric 12C/13C ratios in red giants is still incomplete, for which more observational studies would be required.