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A few percent of red giants are enriched in Lithium with $A(\mathrm{Li}) >1.5$. The evolutionary phase of the Li-rich red giants has remained uncertainbecause they could be placed both on the red-giant branch (RGB) near the bumpluminosity and in the red clump (RC) region. However, thanks toasteroseismology, it has been found that most of them are actually RC stars.Starting at the bump luminosity, RGB progenitors of the RC stars experienceextra mixing in the radiative zone separating the H-burning shell from theconvective envelope followed by a series of convective He-shell flashes at theRGB tip, known as the He-core flash. Therefore, the He-core flash was proposedto cause fast extra mixing in the stars at the RGB tip that is needed for theCameron-Fowler mechanism to produce Li. Alternatively, we propose that the RGBstars are getting enriched in Li by the same extra mixing that starts at thebump luminosity and initially leads to a decrease of the surface Li abundancebut that is getting enhanced and begins to produce Li when the stars areapproaching the RGB tip. We discuss five mechanisms of the RGB extra mixing,namely, the joint operation of rotation-driven meridional circulation andturbulent diffusion, the Azimuthal Magneto-Rotational Instability (AMRI),thermohaline convection, buoyancy of magnetic flux tubes, and internal gravitywaves, and, based on results of (magneto-) hydrodynamics simulations, come tothe conclusion that it is the mechanism of the AMRI that is most likely tosupport our hypothesis.

Enhanced extra mixing in low-mass stars approaching the RGB tip and the problem of Li-rich red-clump stars