Production of lithium, beryllium, and boron by hypernovae and the possible hypernova-gamma-ray burst connection

We investigate a possible nucleosynthetic signature of highly energetic explosions of C-O cores (" hypernovae " [HNe]) that might be associated with gamma-ray bursts (GRBs). We note that the direct impact of C- and O-enriched hypernova ejecta on the ambient hydrogen and helium leads to spallation reactions that can produce large amounts of the light nuclides lithium, beryllium, and boron (LiBeB). Using analytic velocity spectra of the hypernova ejecta, we calculate the LiBeB yields of different exploding C-O cores associated with observed hypernovae. The deduced yields are much higher than those produced by similar (direct) means in normal Type II supernovae (SNe) and are higher than the commonly used ones arising from shock wave acceleration induced by Type II supernova explosions. To avoid overproduction of these elements in our Galaxy, hypernovae should be rare events, with ≲3 × 10^-2 hypernovae per supernova, assuming a constant HN/SN ratio over time; this result also implies that the HN production of Be is only a fraction of other sources, e.g., superbubbles. Our limit to the HN/SN ratio is in good agreement with that of long-duration GRBs if we assume that the gamma-ray emission is focussed in a solid angle Ω so that 〈(Ω/4π)^-1〉 ≲ 3 × 10⁴. This encouraging result supports the possible HN-GRB association. Thus, Galactic LiBeB abundance measurements offer a promising way to probe the HN rate history and the possible HN-GRB correlation. On the other hand, if hypernovae are associated with massive pregalactic stars (Population III), they would produce an LiBeB pre-enrichment in protogalactic gas, which could show up as a plateau in the lowest metallicities of the Be-Fe relation in halo stars.