Lithium in red giants: the roles of the He-core flash and the luminosity bump

Abstract

Lithium abundances for red giants in the GALAH DR3 survey are studied. The rare examples of Li-enriched stars with abundances A(Li) $\ge 1.5$ are confirmed to be He-core burning stars belonging to or evolved from the red clump with similar masses and metallicity: $M\simeq 1.1\pm 0.2{\mathrm{M}}_{\odot }$ and [Fe/H] $\simeq -0.3\pm 0.3$. Li enrichment over the Li abundance present in a star's predecessor at the tip of the red giant branch likely occurs in all these red clump stars. Examination of the elemental abundances (C to Eu) in the GALAH catalogue shows no anomalous abundances in red clump giants and, in particular, no dependence on the Li abundance, which ranges over at least five dex. Lithium synthesis is attributed to the He-core flash occurring in stars at the tip of the red giant branch. Models from the Modules for Experiments in Stellar Astrophysics (MESA) match the observed evolution of these stars along the red giant branch and to the red clump but only at the low effective temperature end of the observed spread of red clump giants. Run of Li abundance on the red giant branch is fairly well reproduced by MESA models. A speculation is presented that the series of He-core flashes not only leads to ${}^7$Li synthesis from a star's internal reservoir of ${}^3$He but also may lead to internal restructuring leading to the observed effective temperature spread of red clump stars at about a constant luminosity. Giants exhibiting marked Li enrichments are not found at other evolutionary phases and, in particular, not directly associated with the luminosity bump on the red giant branch for which the Li abundance increase does not exceed 0.3 dex.

20 Pages, 16 figures, 2 tables. Accepted for publication in MNRAS. Data available at https://zenodo.org/record/5086894

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• The Active Chromospheres of Lithium-Rich Red Giant Stars
• Many Roads Lead to Lithium: Formation Pathways For Lithium-Rich Red Giants
• Evidence of extra-mixing in field giants as traced by lithium and carbon isotope ratio
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• The Gaia-ESO Survey: Preparing the ground for 4MOST & WEAVE galactic surveys. Chemical evolution of lithium with machine learning
• Li-rich Giants in LAMOST Survey. III. The statistical analysis of Li-rich giants
• Are Lithium-Rich Giants Binaries? A Radial Velocity Variability Analysis of 1,400 Giants
• Lithium in Kepler Red Giants: Defining Normal and Anomalous