LiBeB production by nuclei and neutrinos

The production of LiBeB isotopes by nuclear and neutrino spallation are compared in the framework of Galactic evolutionary models. As motivated by γ-ray observations of Orion, different possible sources of low-energy C and O nuclei are considered, such as supernovae of various masses and WC stars. We confirm that the low-energy nuclei (LEN), injected in molecular clouds by stellar winds and Type II supernovae originating from the most massive progenitors, can very naturally reproduce the observed Be and B evolution in the early Galaxy (halo phase). Assuming the global importance of the LEN component, we compute upper and lower bounds to the neutrino process contribution corresponding to limiting cases of LEN particle spectra. A consistent solution is found with a spectrum of the kind proposed by Ramaty et al, e.g., flat up to E_c = 30 MeV nucleon^-1 and decreasing abruptly above. This solution fulfills the challenge of explaining, at the same time, the general Be and B evolution and their solar system abundances, without overproducing ⁷Li at very low metallicities, and the meteoritic ¹¹B/¹⁰B ratio. In this case, neutrino spallation is constrained to play a limited role in the genesis of the solar system ¹¹B. Galactic cosmic rays (GCRs) become operative late in the evolution of the disk ([Fe/H] > -1), but their contribution to the solar abundances of ⁹Be, ¹⁰B, and ¹¹B is not dominant (35%, 30%, and 20% respectively). Thus, with this LEN spectrum, GCRs are not the main source of ⁹Be and B in the Galaxy. The most favorable case for neutrinos (adopting the same kind of spectrum) has E_c = 20 MeV nucleon^-1. Even in this case, the neutrino yields of Woosley & Weaver must be reduced by a factor of 5 to avoid ¹¹B overproduction. Furthermore, this solution leads to a high B/Be ratio at [Fe/H] = -2, difficult to reconcile with the observations, unless specifically non-local thermodynamic equilibrium corrections to the boron abundance are large. On the other hand, if neutrino spallation does play an important role in the production of Galactic ¹¹B, then LEN processes are relegated to a more local phenomenon. However, in this case, unless neutrino spallation can also produce ⁹Be (and to some extent ¹⁰B and ⁶Li), a new source of primary ⁹Be must be found.