The energetics, evolution, and stellar depletion of Li in the early Galaxy
Institut d'Astrophysique de Paris, 98bis Bd. Arago, 75104 Paris, France e-mail: email@example.com
Accepted: 7 October 2005
Motivated by the recent report of a high 6Li “plateau” extending to low metallicities in Galactic halo stars, we study the energetics of an early production of 6Li 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 6Li 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 6Li is indeed pre-galactic, we study the galactic evolution of the light isotopes 7Li, 6Li, and Be in a self-consistent way. We find that the existence of a 6Li plateau is hard to justify, unless a fine-tuned and metallicity-dependent depletion mechanism of 6Li in stellar envelopes is invoked. The depletion of 6Li should be different, both in magnitude and in its metallicity dependence, than the depletion required to explain current observations of Li (mostly 7Li) in halo stars. If the recently reported 6Li “plateau” is confirmed, our analysis suggests important implications for our understanding of the production, evolution, and stellar depletion of the Li isotopes.
Key words: stars: abundances / ISM: cosmic rays
© ESO, 2006