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

We investigate the evolution of initially metal-free, low-mass Red Giantstars through the He core flash at the tip of the Red Giant Branch. The lowentropy barrier between the helium- and hydrogen-rich layers enables apenetration of the helium flash driven convective zone into the inner tail ofthe extinguishing H-burning shell. As a consequence, protons are mixed intohigh-temperature regions triggering a H-burning runaway. The subsequentdredge-up of matter processed by He and H burning enriches the stellar surfacewith large amounts of helium, carbon and nitrogen. Extending previous resultsby Hollowell et al. (1990) and Fujimoto et al. (2000), who claimed that theH-burning runaway is an intrinsic property of extremely metal-poor low-massstars, we found that its occurrence depends on additional parameters like theinitial composition and the treatment of various physical processes. We perform some comparisons between predicted surface chemical abundances andobservational measurements for extremely metal-deficient stars. As in previousinvestigations, our results disclose that although the described scenarioprovides a good qualitative agreement with observations, considerablediscrepancies still remain. They may be due to a more complex evolutionary pathof `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 someinteresting differences between models whose Red Giant Branch progenitor hasexperienced the H-flash and canonical models. In particular, the AsymptoticGiant Branch evolution of extremely metal-deficient stars and the occurrence ofthermal pulses are strongly affected by the previous RGB evolution.Comment: 7 figures, AASTeX, submitted to Ap