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Motivated by the recent report of a high Li6 ``plateau'' extending to lowmetallicities in Galactic halo stars, we study the energetics of an earlyproduction of Li6 through the interaction of energetic particles with theinterstellar medium. We then explore the potential of various candidate sourcesof pre-galactic energetic particles and show that, in general, they fail tosatisfy the observational and theoretical requirements (especially if the Li6plateau is at a considerably higher level than observed). Succesful candidatesappear to be: supernova explosions with abnormally low metal yield; the massiveblack hole in the Galactic center (provided that it was formed early on andthat it was then radiating much more efficiently than today); and, perhaps, anearly accretion phase of supermassive black holes in galaxies. Assuming thatLi6 is indeed pre-galactic, we study the galactic evolution of the lightisotopes Li7, Li6, and Be in a self-consistent way. We find that the existenceof a Li6 plateau is hard to justify, unless a fine-tuned andmetallicity-dependent depletion mechanism of Li6 in stellar envelopes isinvoked. The depletion of Li6 should be different, both in magnitude and in itsmetallicity dependence, than the depletion required to explain currentobservations of Li (mostly Li7) in halo stars. If the recently reported Li6``plateau'' is confirmed, our analysis suggests important implications for ourunderstanding of the production, evolution, and stellar depletion of the Liisotopes.Comment: 12 pages, 4 figures, final version after language editing for  Astronomy and Astrophysics; references update

The energetics, evolution, and stellar depletion of $\mathsf{^6}$Li in the early Galaxy