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

Nitrogen abundances and carbon isotope ratios (C-12/C-13) 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 C-12/C-13 ratios for 239 late-G/early-K giant stars by applying the spectrum-fitting technique to the (CN)-C-12 and (CN)-C-13 lines in the similar to 8002-8005 angstrom region, with the aim of investigating 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 of a dex, though widely differing 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 C-12/C-13 ratios turned out to be considerably diversified in the range of similar to 5-50 (with a peak around similar to 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 of the photospheric C-12/C-13 ratios in red giants is still incomplete, requiring more observational studies.