1999ApJ...510..217S

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{epsiv}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^dr₃ ∼ 2.4±0.5). Population II stars experience even more severe ³He depletion, with 0.3≲g₃≲0.7. 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+³HeMH abundance, thus relaxing the lower bound on the cosmic baryon density Ω_b 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.