We review the Galactic chemical evolution of 6Li 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 α off of CNO nuclei in the ambient interstellar medium. If O/Fe is constant at low metallicities, then the 6Li 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 6Li evolution in a standard model of cosmic-ray nucleosynthesis is consistent with the data. We also show that another key indicator of 6LiBeB origin is the 6Li/Be ratio which also fits the available data if O/Fe is not constant at low metallicity. Finally we note that 6Li evolution in this scenario can strongly constrain the degree to which 6Li and 7Li are depleted in halo stars.
Bibliographical noteFunding Information:
We are grateful to Elisabeth Vangioni-Flam and Michel Cassé for many useful discussions and comments on an earlier version of this work. We would also like to dedicate this work in the memory of David Schramm, an advisor to us both. He brought to the area of cosmic-ray nucleosynthesis the same insight and unparalleled enthusiasm he showed for all of his many research interests. This work was supported in part by DoE grant DE-FG02-94ER-40823 at the University of Minnesota.
- Cosmic rays
- Galaxies: abundances
- Nuclear reactions, nucleosynthesis, abundances