Early in this decade our theoretical work demonstrated that all AGB stars in the mass range ˜ 4 to ˜ 7 M⊙ pass through a stage when a tremendous amount of lithium [up to log ε(7Li) ˜ 4.5] is created and transported to the surface. These lithium-rich AGB stars are predicted to occupy a narrow luminosity range between Mbol = −6 and −7, in excellent agreement with the observations of Smith & Lambert (1989), and might be useful as approximate standard candles. Recently, we found that even low mass stars (˜ 1 to ˜ 2 M⊙) on the RGB could create a tremendous amount of surface lithium. In both the AGB and RGB cases, it is the Cameron-Fowler mechanism that is responsible for the lithium creation.
In the AGB stars, it is hot bottom burning (nuclear burning at the base of the convective envelope) that produces the lithium. In the RGB stars, it is “cool bottom processing” that can lead to either lithium production or destruction. Cool bottom processing results when extra mixing (presumably rotation-induced) transfers material from the cool convective envelope down to the outer wing of the hydrogen-burning shell (where nuclear reactions can take place) and back out to the envelope. If the extra mixing is slow, 7Li is destroyed; if it is fast enough, then 7Li is created - for sufficiently fast and deep extra mixing, log ε(7Li) ˜ 4 is possible.
Unlike 7Li, the 3He abundance is almost independent of the mixing speed, and is constrained by observations of 12C/13C or [C/Fe] on the RGB. Cool bottom processing causes low mass stars of sub-solar metallicity to be net destroyers of 3He, rather than net producers. This is in contrast to previous theoretical predictions, and has a far-reaching effect on our understanding of galactic chemical evolution of 3He.