Creation of Li-7 and Destruction of He-3, Be-9, B-10, and B-11 in Low Mass Red Giants, Due to Deep Circulation

Abstract

It has been demonstrated that Li-7 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" (CBP). Under certain conditions, this Li-7 creation can take the place of the Li-7 destruction normally expected. Note that such extra mixing on the red giant branch (RGB) has previously been invoked to explain the observed C-13 enhancements. This new Li-7 production can account for the recent discovery of surprisingly high lithium abundances in some low mass red giants (a few of which are super-rich lithium stars, with abundances higher than that in the interstellar medium). The amount of Li-7 produced can exceed log epsilon(Li-7) = 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. CBP also leads to He-3 destruction in low mass stars; in contrast to Li-7 creation, the extent of He-3 depletion is largely independent of the details of the extra mixing mechanism. The overall contribution from solar-metallicity stars (1 - 40 Msun) is expected to be net destruction of He-3, with overall He-3 survival fraction g_3 = 0.9 +/- 0.2 (weighted average) --- in contrast to standard dredge-up, which would predict stars are net producers of He-3 (g_3:dr = 2.4 +/- 0.5). Pop II stars experience even more severe He-3 depletion, with 0.3 < g_3 < 0.7. Destruction of He-3 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 + He-3)/H abundance.