Can the Core Helium Flash Synthesize Lithium and Dredge it Up?
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2025
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Abstract
The existence of lithium-rich (Li-rich) red giant stars challenges standard stellar evolution models. Although several mechanisms have been proposed, the origin of Li enrichment remains a mystery. Recent asteroseismic and spectroscopic evidence suggests that most Li-rich giants are low-mass red clump (RC) stars that have undergone the core helium (He) flash, indicating a possible connection with this event. Motivated by multi-dimensional hydrodynamical simulations, in this work I investigate whether the core He-flash in low-mass stars can trigger a proton ingestion episode (PIE) that produces lithium. Using the MESA stellar evolution code I compare standard evolution with models where a PIE is induced by implementing enhanced overshooting, in a 1.2 Msun star with subsolar metallicity ([Fe/H] = -0.3). These models show that PIEs can occur at near-solar metallicity, producing a secondary hydrogen flash followed by a dredge-up event. Although lithium and beryllium are synthesized during the PIE, which if they were dredged up they could enhance the surface Li-abundance up to a value of A(Li)~4.8, being able to explain even the most Li-rich giants, both Li and Be are destroyed before the dredge-up event, resulting in no surface enrichment. In the PIE-induced models, the post-PIE evolution diverges from standard models, with a second RGB ascent and secondary He-flash occurring before stable core He-burning. The PIE also produces substantial s-process elements via the 13C(alpha, n)16O reaction, which are dredged up to the surface, enhancing their abundance by 2–3 dex. The non-standard evolution and strong s-process enhancement contradict current observations of Li-rich red clump giants. While PIEs remain a theoretically promising mechanism for internal Li enrichment, current results do not support them as the origin of Li-rich giants. However, open questions remain before the scenario can be definitively ruled out.
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Tesis (Master’s degree in Astrophysics)--Pontificia Universidad Católica de Chile, 2025