2000A&A...359..563C

We present a critical analysis of the nature of the so-called Li-rich RGB stars. For a majority of the stars, we have used Hipparcos parallaxes to determine masses and evolutionary states by comparing their position on the Hertzsprung-Russell diagram with theoretical evolutionary tracks. Among the twenty Li-rich giants whose location on the HR diagram we were able to determine precisely, five appear to be Li-rich because they have not completed the standard first dredge-up dilution, and three have abundances compatible with the maximum allowed by standard dilution. Thus, these should be re-classified as Li-normal. For the remaining stars, the high Li abundance must be a result of fresh synthesis of this fragile element. We identify two distinct episodes of Li production which occur in advanced evolutionary phases depending upon the mass of the star. Low-mass RGB stars, which later undergo the helium flash, produce Li at the phase referred to as the bump in the luminosity function. At this evolutionary phase, the outwardly-moving hydrogen shell burns through the mean molecular weight discontinuity created by the first dredge-up. Any extra-mixing process can now easily connect the ³He-rich envelope material to the outer regions of the hydrogen-burning shell, enabling Li production by the Cameron & Fowler (1971ApJ...164..111C) process. While very high Li abundances are then reached, this Li-rich phase is extremely short lived because once the mixing extends deep enough to lower the carbon isotopic ratio below the standard dilution value, the freshly synthesized Li is quickly destroyed. In intermediate-mass stars, the mean molecular weight gradient due to the first dredge-up is not erased until after the star has begun to burn helium in its core. The Li-rich phase in these stars occurs when the convective envelope deepens at the base of the AGB, permitting extra-mixing to play an effective role. Li production ceases when a strong mean molecular weight gradient is built up between the deepening convective envelope and the shell of nuclear burning that surrounds the inert CO core. This episode is also very short lived. Low-mass stars may undergo additional mixing at this phase. The compiled data provide constraints on the time scales for extra mixing and some insight on processes suggested in the literature. However, our results do not suggest any specific trigger mechanism. Since the Li-rich phases are extremely short, enrichment of the Li content of the ISM as a result of these episodes is negligible.