2023A&A...670A..73A

Context. Although not predicted by standard stellar evolution, the surface abundance of light elements, such as lithium (Li), carbon, and nitrogen, changes during the red giant branch (RGB) as a result of extra mixing. This is usually associated with thermohaline mixing acting after the RGB bump. Peculiar Li-enriched RGB stars might also be related to either enhanced mixing or pollution from external sources.
Aims. We measure the Li abundance and carbon isotopic ratio ¹²C/¹³C in a sample of 166 field red giants with -0.3 ≤ [Fe/H] ≤ 0.2, targeted by the EXPRESS radial velocity program to analyze the effects of extra mixing.
Methods. We measured the abundances with spectral synthesis using high-quality spectra. Multiple-epoch observations needed for exoplanet detection were used to decrease the effects of telluric contamination in ¹²C/¹³C measurements.
Results. Due to the prevalence of upper limits, the Li abundance pattern is complicated to interpret, but the comparison between RGB and core He-burning giants shows effects of mixing consistent with thermohaline. The most Li-enriched giant in the sample, classified as a RGB star close to the RGB bump, has low ¹²C/¹³C. Given that the ¹²C/¹³C should not be affected by planet engulfment, this does not seem to be the source of the high Li. There is a decreasing correlation between mass and ¹²C/¹³C in the RGB and an increasing correlation in the horizontal branch, which, once again, is consistent with thermohaline mixing. Our data also show a correlation between ¹²C/¹³C and [Fe/H]. There is no evident impact of binarity either on Li or on ¹²C/¹³C.
Conclusions. Our sample shows behavior consistent with additional mixing acting after the RGB bump. The ¹²C/¹³C adds new clues which can be used to describe extra mixing, and it could well be the best tool to study mixing in giants. Additional measurements of ¹²C/¹³C in field stars would greatly improve our ability to compare data with models and understand mixing mechanisms.