A great advantage of the proposed solution is that it can account not only for the Li-production but also for the subsequent Li-depletion. Indeed, we find that after the mixing depth has reduced to less than , Li begins to be destroyed on a time-scale consistent with the results of De la Reza et al. (1996). It should be emphasized that it is even more difficult to deplete Li quickly after its production than to produce it, and our scenario deals with this naturally.
At the same time, however, it cannot be applied to the Li-rich () star V42 in M5 (Carney et al. 1998), which appears to be a (low-mass) post-AGB star. Due to the timescales, the Li-enrichment cannot have happened already during the first red giant phase. On the other side, since V42 is only as bright as the RGB-tip (), but hotter and thus smaller, capturing a companion would have happened already on the RGB. Thus, our scenario fails for this star, which otherwise appears to be typical M5 member, showing standard -enhancement and even the O-Na-anticorrelation (Carney et al. 1998). We can only speculate that its Li-overabundance happened (via the Cameron-Fowler mechanism) during the AGB, where additional deep mixing initiated hot bottom burning as is standard in intermediate-mass stars (Sackmann & Boothroyd 1992). Due to the thin envelope of this star very modest extra mixing might already be sufficient. We will investigate this possibility in forthcoming work.
Our scenario does neither provide a direct link to the dust shell formation. Siess & Livio (1999) have ascribed the shell detachment to an increased mass loss during the planet's engulfing but in our scenario this event is separated from the Li-enrichment episode by a time interval of years. The following two speculations towards a solution of this problem can be envisaged: (1) The mass of the radiative zone is negligible, and the angular velocity inside it scales as (Denissenkov & Tout 2000); hence, the ratio of the centrifugal acceleration to the gravity scales as ; as after engulfing the planet this ratio is expected to become close to unity near the BCE, then during the subsequent inward excursion of the step in the rotation profile it will surely exceed unity somewhere in the radiative zone, which may initiate dynamical processes of the angular momentum transfer outwards; the latter may be responsible for the increased mass loss. (2) The process of planet engulfing itself may be associated with various dynamical and thermodynamical processes, for instance, a deepening of the convective envelope (Siess & Livio 1999), which may redistribute the material with the high angular momentum throughout the radiative zone; in this case the fast mixing will be able to penetrate the zone of correct mixing depths from the very beginning. Whether such phenomena happen in a real red giant can be verified only by 3D hydrodynamical simulations.
© European Southern Observatory (ESO) 2000
Online publication: June 8, 2000