On the Helium Flash in Low‐Mass Population III Red Giant Stars

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Abstract
We investigate the evolution of initially metal-free, low-mass red giant stars through the He core flash at the tip of the red giant branch (RGB). The low entropy barrier between the helium- and hydrogen-rich layers enables a penetration of the helium-flash-driven convective zone into the inner tail of the extinguishing H-burning shell. As a consequence, protons are mixed into high-temperature regions triggering an H-burning runaway. The subsequent dredge-up of matter processed by He and H burning enriches the stellar surface with large amounts of helium, carbon, and nitrogen. Extending previous results by Hollowell et al. and Fujimoto, Ikeda, & Iben, who claimed that the H-burning runaway is an intrinsic property of extremely metal-poor low-mass stars, we found that its occurrence depends on additional parameters like the initial composition and the treatment of various physical processes. We perform some comparisons between predicted surface chemical abundances and observational measurements for extremely metal-deficient stars. As in previous investigations, our results disclose that although the described scenario provides a good qualitative agreement with observations, considerable discrepancies still remain. They may be due to a more complex evolutionary path of "real" stars and/or some shortcomings in current evolutionary models. In addition, we analyze the evolutionary properties after the He core flash, during both the central and shell He-burning phases, allowing us to deduce some interesting differences between models whose RGB progenitor has experienced the H flash and canonical models. In particular, the asymptotic giant branch evolution of extremely metal-deficient stars and the occurrence of thermal pulses are strongly affected by the previous RGB evolution.
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