Evidence of extra-mixing in field giants as traced by lithium and carbon isotope ratio

Abstract

Although not predicted by standard stellar evolution, it is known that the surface abundance of light elements changes during the red giant branch (RGB) as a result of extra-mixing. This is associated usually with thermohaline mixing acting after the RGB bump. Peculiar lithium-enriched RGB stars might also be related to either enhanced extra-mixing or pollution from external sources. We measure the lithium (Li) abundance and carbon isotopic ratio C12/C13 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. The Li abundance pattern is complicated to interpret, but the comparison between RGB and core-He burning giants shows the effects of extra-mixing consistent with thermohaline. The most Li-enriched giant in the sample was classified as a RGB star close to the luminosity function bump with low C12/C13. Given that the C12/C13 should not be affected by external mechanisms, contamination by an external source, such as a planet, does not seem to be the source of the high Li. The C12C13 presents new clues to describe the extra-mixing. There is a decreasing correlation between mass and C12/C13 in the RGB and an increasing correlation in the horizontal branch, which, once again, is consistent with thermohaline mixing. Our data also shows a correlation between C12/C13 and [Fe/H]. There is no evident impact of binarity either on Li or C12/C13. Our sample shows behavior that is consistent with additional mixing acting after the RGB bump. Li, which is heavily affected by rotational mixing and other processes, does not show a clear trend. Instead, the C12/C13 could be the best tool to study mixing in red giants. Additional measurements of C12/C13 in field stars would greatly improve our ability to compare with models and understand the mixing mechanisms.Comment: 15 pages, 12 figures, submitted to A&A, comments welcom