Chromospherically active stars: Lithium and CNO abundances in northern RS CVn stars

Vilnius University, Faculty of Physics, Institute of Theoretical Physics and Astronomy, Saulėtekio av. 3, 10257 Vilnius, Lithuania

^⋆ Corresponding author: barkha.bale@ff.vu.lt

Received: 31 July 2025
Accepted: 9 September 2025

Abstract

Aims. We carried out a detailed investigation of Lthium and CNO abundances, including carbon isotope ratios, in RS CVn stars to assess the role of magnetic activity in the mixing of stellar atmospheres.

Methods. We obtained high-resolution spectra at the Moletai Astronomical Observatory. Lithium abundances were determined by spectral synthesis of the 6707 Å line and the CNO abundances using the C₂ band heads at 5135 and 5635.5 Å, CN bands at 6470–6490 Å and 7980–8005 Å, and the [O I] line at 6300 Å. By fitting the ¹³CN band at 8004.7 Å, we determined the carbon isotope ratios.

Results. We determined the main atmospheric parameters and investigated the chemical composition of 32 RS CVn stars. Lithium abundances were determined for 13 additional stars using archival spectra. We report that *iot Gem and HD 179094 have carbon isotope ratios already affected by extra-mixing, even though they are in the evolutionary stage below the red giant branch luminosity bump. About half of the low-mass giants, for which the lithium abundance was determined, follow the first dredge-up predictions; however, other stars show reduced Lithium abundances, as predicted by thermohaline-induced mixing. The intermediate-mass stars show reduced Lithium abundances reduced, as predicted by rotation-induced mixing.

Conclusions. In low-mass, chromospherically active RS CVn stars, extra-mixing of lithium and carbon isotopes may begin earlier than in normal giants. The Li-rich RS CVn giant V*OP And has large C/N and carbon isotope ratios and raises questions about the origin of its lithium enhancement.