The energetics, evolution, and stellar depletion of Li6 in the early   Galaxy
  Motivated by the recent report of a high Li6 ``plateau'' extending to low metallicities in Galactic halo stars, we study the energetics of an early production of Li6 through the interaction of energetic particles with the interstellar medium. We then explore the potential of various candidate sources of pre-galactic energetic particles and show that, in general, they fail to satisfy the observational and theoretical requirements (especially if the Li6 plateau is at a considerably higher level than observed). Succesful candidates appear to be: supernova explosions with abnormally low metal yield; the massive black hole in the Galactic center (provided that it was formed early on and that it was then radiating much more efficiently than today); and, perhaps, an early accretion phase of supermassive black holes in galaxies. Assuming that Li6 is indeed pre-galactic, we study the galactic evolution of the light isotopes Li7, Li6, and Be in a self-consistent way. We find that the existence of a Li6 plateau is hard to justify, unless a fine-tuned and metallicity-dependent depletion mechanism of Li6 in stellar envelopes is invoked. The depletion of Li6 should be different, both in magnitude and in its metallicity dependence, than the depletion required to explain current observations of Li (mostly Li7) in halo stars. If the recently reported Li6 ``plateau'' is confirmed, our analysis suggests important implications for our understanding of the production, evolution, and stellar depletion of the Li isotopes. 
Nikos Prantzos (Institut d'Astrophysique de Paris)