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

TL;DR

This study analyzes Lithium and CNO abundances, including carbon isotope ratios, in a sample of 32 northern RS CVn stars to test how magnetic activity influences mixing in evolved stellar envelopes. Using high-resolution spectroscopy from Moletai and archival data, the authors determine Li abundances (with NLTE corrections), CNO abundances, and $^{12}$C/$^{13}$C ratios via spectral synthesis of multiple molecular bands, alongside precise atmospheric parameters, masses, and kinematics. The findings show that extra-mixing begins earlier in some RS CVn stars than in normal giants, evidenced by lowered $^{12}$C/$^{13}$C and C/N ratios in stars below the RGB bump, and that Li abundances generally track the standard evolution with 1DUP and subsequent mixing, though Li-rich cases like OP And challenge simple enrichment scenarios. Overall, the work supports a link between chromospheric activity and enhanced internal mixing in RS CVn systems and provides a detailed, publicly available dataset for testing mixing models under magnetic effects.

Abstract

Aims: We carried out a detailed investigation of Lithium 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 A line and the CNO abundances using the C2 band heads at 5135 and 5635.5 A CN bands at 6470- 6490 A and 7980 to 8005 A, and the [O I] line at 6300 A. By fitting the 13CN band at 8004.7 A, 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.

Figures (11)

• Figure 1: Examples of synthetic spectrum fits to the Li i line, C$_2$ band heads at 5135 Å, and the CN bands at 7995 Å and 8003 Å. Black dots represent the observed spectra, while solid green lines depict the best-fit synthetic spectra. The shaded region around the fit indicates the $\pm0.10$ dex abundance range.
• Figure 2: Comparisof NLTE vs LTE lithium abundances.
• Figure 3: Sample stars colour-coded by metallicity on a Toomre diagram. Dashed lines indicate constant total space velocity. The star *eta Boo, attributed to the thick disc, is marked with a star symbol.
• Figure 4: Atmospheric parameters derived in this study ($T_{\rm eff}$, left; $\log g$, middle; and [Fe/H], right) compared with values from studies listed in Sect. \ref{['resultstellar']}.
• Figure 5: Log $g$ versus $T_{\rm eff}$ diagram. Locations of *iot Gem (1.37 $M_\odot$) and HD 179094 (1.03 $M_\odot$) below the RGB luminosity bump are denoted by red arrows, on the PARSEC evolutionary tracks for solar metallicity from Bressan2012.