A few percent of red giants are enriched in Lithium with A(Li)>1.5. The evolutionary phase of the Li-rich red giants has remained uncertain
because they could be placed both on the red-giant branch (RGB) near the bump
luminosity and in the red clump (RC) region. However, thanks to
asteroseismology, it has been found that most of them are actually RC stars.
Starting at the bump luminosity, RGB progenitors of the RC stars experience
extra mixing in the radiative zone separating the H-burning shell from the
convective envelope followed by a series of convective He-shell flashes at the
RGB tip, known as the He-core flash. Therefore, the He-core flash was proposed
to cause fast extra mixing in the stars at the RGB tip that is needed for the
Cameron-Fowler mechanism to produce Li. Alternatively, we propose that the RGB
stars are getting enriched in Li by the same extra mixing that starts at the
bump luminosity and initially leads to a decrease of the surface Li abundance
but that is getting enhanced and begins to produce Li when the stars are
approaching the RGB tip. We discuss five mechanisms of the RGB extra mixing,
namely, the joint operation of rotation-driven meridional circulation and
turbulent diffusion, the Azimuthal Magneto-Rotational Instability (AMRI),
thermohaline convection, buoyancy of magnetic flux tubes, and internal gravity
waves, and, based on results of (magneto-) hydrodynamics simulations, come to
the conclusion that it is the mechanism of the AMRI that is most likely to
support our hypothesis.Comment: 14 pages, 7 figures, submitted to MNRA