Creation of7Li and Destruction of3He,9Be,10B, and11B in Low‐Mass Red Giants, Due to Deep Circulation

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Abstract
It has been demonstrated that 7Li can be created in low-mass red giant stars via the Cameron-Fowler mechanism, due to extra deep mixing and the associated "cool bottom processing." Under certain conditions, this 7Li creation can take the place of the 7Li destruction normally expected. Note that such extra mixing on the red giant branch (RGB) has previously been invoked to explain the observed 13C enhancement. This new 7Li production can account for the recent discovery of surprisingly high lithium abundances in some low-mass red giants (a few of which are superrich lithium stars, with abundances higher than that in the interstellar medium). The amount of 7Li produced can exceed log ε(7Li)~4, but depends critically on the details of the extra mixing mechanism (mixing speeds, geometry, episodicity). If the deep circulation is a relatively long-lived, continuous process, lithium-rich RGB stars should be completely devoid of beryllium and boron. Cool bottom processing leads to 3He destruction in low-mass stars; in contrast to the 7Li creation, the extent of 3He depletion is largely independent of the details of the extra mixing mechanism. The overall contribution from solar-metallicity stars (from 1 to 40 M☉) is expected to be a net destruction of 3He, with an overall 3He survival fraction g3≈0.9±0.2 (weighted average over all stellar masses); this is in contrast to the conclusion from standard dredge-up theory, which would predict that stars are net producers of 3He (with g~2.4±0.5). Population II stars experience even more severe 3He depletion, with 0.3g30.7. Destruction of 3He in low-mass stars is consistent with the requirements of Galactic chemical evolution models; it would also result in some relaxation of the upper bound on the primordial (D+3He)/H abundance, thus relaxing the lower bound on the cosmic baryon density Ωb from big bang nucleosynthesis calculations. For reference, we also present the effects of standard first and second dredge-up on the helium, lithium, beryllium, and boron isotopes.