Astrophysical ⁷Li as a product of Big Bang nucleosynthesis and galactic cosmic-ray spallation

Abstract

RECENTLY measured abundances of beryllium^1–4 and boron⁵ in a number of hot population II halo stars are orders of magnitude above the predicted abundances of those elements from standard Big Bang nucleosynthesis⁶. Be and B do not, however, show a plateau of constant abundance over a wide range of low metallicities and high temperatures, as is the case for ⁷Li (refs 7–15). The implication is that the ⁷Li abundance is largely primordial, whereas the Be and B abundances are due to galactic cosmic ray (GCR) spallation reactions^16–22 on top of a much smaller Big Bang component²³. But GCR spallation should also produce ⁷Li. As a consistency check on the combination of Big Bang nucleosynthesis and GCR spallation, we use the Be and B data to subtract from the measured ⁷Li abundance an estimate of the amount generated by GCR spallation^21,22 for each star in the sample, and then add to this baseline an estimate of the metallicity-dependent augmentation of ⁷Li, due to spallation. The slightly reduced primordial ⁷Li abundance is still consistent with Big Bang nucleosynthesis, and a single GCR spallation model can fit the Be, B and corrected ⁷Li abundances for all the stars in the sample.