The boron-to-beryllium ratio in halo stars: A signature of cosmic-ray nucleosynthesis in the early galaxy

We discuss Galactic cosmic-ray (GCR) spallation production of Li, Be, and B in the early Galaxy with particular attention to the uncertainties in the predictions of this model. The observed correlation between the Be abundance and the metallicity in metal-poor Population II stars requires that Be was synthesized in the early Galaxy. We show that the observations and such Population II GCR synthesis of Be are quantitatively consistent with the big bang nucleosynthesis production of ⁷Li. We find that there is a nearly model independent lower bound to B/Be of ∼7 for GCR synthesis. Recent measurements of B/Be ∼ 10 in HD 140283 are in excellent agreement with the predictions of Population II GCR nucleosynthesis. Measurements of the boron abundance in additional metal-poor halo stars is a key diagnostic of the GCR spallation mechanism. We also show that Population II GCR synthesis can produce amounts of ⁶Li which may be observed in the hottest halo stars.