The evolution of 6Li in standard cosmic-ray nucleosynthesis

We review the Galactic chemical evolution of Li-6 and compare these results with recent observational determinations of the lithium isotopic ratio. In particular, we concentrate on so-called standard Galactic cosmic-ray nucleosynthesis in which Li, Be, and B are produced (predominantly) by the inelastic scattering of accelerated protons and alpha off of CNO nuclei in the ambient interstellar medium. If O/Fe is constant at low metallicities, then the Li-6 versus Fe/H evolution - as well as Be and B versus Fe/H - has difficulty in matching the observations. However, recent determinations of Population II oxygen abundances, as measured via OH lines, indicate that O/Fe increases at lower metallicity; if this trend is confirmed, then the Li-6 evolution in a standard model of cosmic-ray nucleosynthesis is consistent with the data. We also show that another key indicator of (LiBeB)-Li-6 origin is the Li-6/Be ratio which also fits the available data if O/Fe is not constant at low metallicity. Finally we note that Li-6 evolution in this scenario can strongly constrain the degree to which Li-6 and Li-7 are depleted in halo stars. (C) 1999 Elsevier Science B.V. All rights reserved.