Galactic Cosmic Rays and the Evolution of Light Elements

Abstract

We reevaluate the contribution of Galactic cosmic ray (GCR) spallation to the abundances of the Li, Be, and B elements at various metallicities. We calculate absolute yields, as was formerly done by Reeves and collaborators in their pioneering studies. We discuss all the relevant parameters (spectrum shape and flux, escape length, cross sections, and spectral index) in the light of updated cosmic-ray data. We assume a dependence of the flux (t) on the supernova rate, (t) ∝ (dNSN/dt)x, and we introduce the calculated production rates of LiBeB in a standard model of chemical evolution of the Galaxy. We find that the light elements are overproduced by GCR spallation with respect to their solar abundances unless (1) the cosmic-ray injection spectrum strongly flattens out at low energies or (2) the dependence of the cosmic-ray flux on the supernova rate is weak, i.e., x ≤ 1. We argue that constraint (1) should be expected in nonlinear shock acceleration, and is also supported by the relatively weak ionization rate of the interstellar medium; constraint (2) could result from the fact that supernovae occur in associations in superbubbles, where cosmic-ray fluxes of different supernovae do not add linearly. We argue that even with substantial modifications, GCRs are not able to account for the BeB abundances at low metallicites. We then discuss the new mechanisms proposed to account for these abundances. When incorporating a new mechanism of spallation, the above constraints are more demanding. Finally, we propose a solution to explain all the absolute abundances of 6Li,9Be,10B, and 11B, in which Galactic cosmic ray spallation and spallation by fast C and O nuclei originating from stellar winds and/or massive star explosions each produce about half of the solar/meteoritic abundances; neutrino-driven spallation in Type II supernovae contributes to 15% of the 11B meteoritic abundance.