12C/13C of Kepler giant stars: The missing piece of the mixing puzzle

Abstract

Context. Despite a rich observational background, few spectroscopic studies have dealt with the measurement of the carbon isotopic ratio in giant stars. However, it is a key element in understanding the mixing mechanisms that occur in the interiors of giant stars. Aims. We present the CNO and 12C/13C abundances derived for 71 giant field stars. Then, using this new catalogue and complementary data from the Kepler and Gaia satellites, we study the efficiency of mixing occurring in the giant branch as a function of the stellar properties of the stars (e.g. mass, age, metallicity). Methods. We determined the abundances of CNO and more specifically the carbon isotopic ratio using the high-resolution FIbre-fed Echelle Spectrograph on the Nordic Optical Telescope, for 71 giant field stars. In addition, asteroseismology from the Kepler satellite is available for all stars, providing the stellar masses, ages, and evolutionary states. Finally, astrometry from the Gaia data is also available for most of the sample. We compare these new determinations with stellar evolution models taking into account the effects of transport processes. To exploit the complete potential of our extensive catalogue, and considering both the Milky Way evolution and the impact of stellar evolution, we built mock catalogues using the Besançon Galaxy model in which stellar evolution models taking into account the effects of thermohaline instability are included. Results. We confirm that the carbon isotopic ratio at the surface of core He-burning stars is lower than that of first-ascent RGB stars. The carbon isotopic ratio measured at the surface of the core He-burning stars increases with [Fe/H] and stellar mass, while it decreases with stellar age. These trends are all nicely explained by the thermohaline mixing that occurs in red giants. We show that our models can explain the behaviour of 12C/13C versus N/O, although the observations seem to show a lower N/O than the models. We also note that more constraints on the thick disc core He-burning stars are needed to understand this difference. Conclusions. Overall, the current model including thermohaline mixing is able to reproduce very well the 12C/13C with the stellar metallicity and with the stellar mass and age.