Photospheric nitrogen abundances and carbon 12C/13C ratios of red giant stars. (5th November 2019)

• Record Type: Journal Article
• Authors: Takeda, Yoichi; Omiya, Masashi; Harakawa, Hiroki; Sato, Bun'ei
• Is Part Of: Publications of the Astronomical Society of Japan. Volume 71:Number 6(2019:Dec.)
• Publication Date: 2019-11-05
• Subjects: stars: abundances -- stars: atmospheres -- stars: evolution -- stars: late-type; Astronomy -- Periodicals; 520.5
• DOI: 10.1093/pasj/psz104
• Languages: English
• ISSNs: 0004-6264

Abstract

Abstract: Nitrogen abundances and carbon isotope ratios ( 12 C$/$ 13 C) 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 12 C$/$ 13 C ratios for 239 late-G/early-K giant stars by applying the spectrum-fitting technique to the 12 CN and 13 CN lines in the ∼8002–8005 Å 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 12 C$/$ 13 C 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 of the photospheric 12 C$/$ 13 C ratios in red giants is still incomplete, requiring more observational studies.