The Nature of the Lithium Rich Giants. Mixing episodes on the RGB and early-AGB

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, eight 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. We suggest that the Li-rich stars are formed mainly as a precursor to the deeper mixing process which produces anomalously low carbon isotopic ratios. Low-mass RGB stars, which later undergo the helium flash, produce Li at the phase referred to as the bump in the luminosity function. In intermediate-mass stars, the Li-rich phase occurs on the early-AGB. 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.