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: prantzos@iap.fr

Received: 24 July 2005
Accepted: 7 October 2005

Abstract

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.