The observed boron/beryllium ratio in extreme Population II stars has been interpreted as evidence of Be and B synthesis by early Galactic cosmic rays. However, a recent reanalysis of the boron abundance in the Population II halo star HD 140283 suggests that B/H may be larger than previously reported, by as much as a factor of 4. This would yield a B/Be ratio lying in the range 14 ≲ B/Be ≲ 50. The possibility of a high Population II B/Be ratio stresses the importance of the upper limit to the B/Be ratio arising from cosmic-ray production. It is found that the limit to cosmic-ray-produced B/Be depends upon the assumed cosmic-ray spectrum. For any Population II comic-ray spectrum that is a single power law in either total energy per nucleon or in momentum, the B/Be ratio is constrained to lie in the range 7.6 ≲ B/Be ≲ 14. Thus, if the new B/Be ratio is correct, it requires either a bimodal cosmic-ray flux with a large low-energy component, or, for another B source, possibly the proposed v-process in supernovae, either of which may be helpful in explaining the observed 11B/10B ratio. Finally, it is noted that the boron reanalysis highlights the uncertainty in our knowledge of the B/Be ratio, and the need for additional data on Be and B abundances.
- Cosmic rays
- Nuclear reactions, nucleosynthesis, abundances
- Stars: Population II