| Abstract
| - 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.
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