Implications of a High Population II B/Be Ratio

Abstract

The observed B/Be ratio in extreme Pop 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 Pop II halo star HD140283 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 $14 \la {\rm B/Be} \la 50$. The possibility of a high Pop 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 Pop 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 $7.6 \la$ B/Be $\la 14$. 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 $ν$-process in supernovae, either of which may be helpful in explaining the observed \bor11/\bor10 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.