Light-Element Evolution and Cosmic-Ray Energetics

Using cosmic-ray energetics as a discriminator, we investigate the viability of evolutionary models for the light elements Li, Be, and B (LiBeB). We find that models in which the cosmic rays are accelerated mainly out of the average interstellar medium which is increasingly metal-poor at early times significantly underpredict the measured Be abundance of the early Galaxy, the possible increase in [O/Fe] with decreasing [Fe/H] indicated by some recent data notwithstanding. On the other hand, if the cosmic-ray metals are accelerated primarily out of supernova ejecta-enriched superbubbles, such that the cosmic-ray source composition as a function of [Fe/H] remains similar to that of the current epoch, the measured Be abundances are consistent with a cosmic-ray acceleration efficiency that is in very good agreement with the current epoch data. This model requires the incorporation of neutrino-produced ¹¹B. We show that, even though the production histories of the cosmic-ray-produced B and Be and the neutrino-produced ¹¹B are different, B/Be can remain essentially constant as a function of [Fe/H]. We also find that neither the above cosmic-ray origin models nor a model employing low-energy cosmic rays originating from the supernovae of only very massive progenitors can account for the ⁶Li data at values of [Fe/H] below -2.