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arXiv · 2207.08905

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

Claudia Aguilera-Gómez·Matías I. Jones·Julio Chanamé
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Abstract

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 associated usually with thermohaline mixing acting after the RGB bump. Peculiar Li-enriched RGB stars might also be related to either enhanced extra-mixing or pollution from external sources. 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. Multiple-epoch observations needed for exoplanet detection are used to decrease the effects of telluric contamination in 12C/13C measurements. 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 extra-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 shows a correlation between 12C/13C and [Fe/H]. There is no evident impact of binarity either on Li or on 12C/13C. Our sample shows behavior consistent with additional mixing acting after the RGB bump. The 12C/13C adds new clues to describe extra-mixing, and could well be the best tool to study mixing in red giants. Additional measurements of 12C/13C in field stars would greatly improve our ability to compare with models and understand mixing mechanisms.

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Claudia Aguilera-Gómez, Matías I. Jones, Julio Chanamé. 2022-07-18. Evidence of extra-mixing in field giants as traced by lithium and carbon isotope ratio. https://doi.org/10.1051/0004-6361%2F202244518

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