Supernova grains: The source of cosmic-ray metals

We show that the Be abundances in old halo stars formed in the early Galaxy strongly suggest that the refractory cosmic-ray metals, in both the early Galaxy and the present epoch, are accelerated from fresh supernova ejecta rather than from the interstellar medium, as proposed in most current models. To resolve this problem, we suggest that the high-velocity grains formed in supernova ejecta are the injection source for these cosmic rays, including C and O. We show how this cosmic-ray injection source can refute previous arguments against cosmic-ray acceleration from fresh supernova ejecta: the enrichment of the highly refractory elements relative to the volatiles and the similarity of the refractory cosmic-ray source and solar elemental and isotopic abundance ratios. We also show that the cosmic-ray source O abundance may be the direct consequence of the condensation of O as refractory oxides and that the problematic cosmic-ray C-to-O ratio may be understood if a large fraction of the C in the ejecta is in graphite grains. We further show that if the refractory metals are accelerated from fresh ejecta, the cosmic-ray source ⁵⁴Fe/⁵⁶Fe should be higher than the corresponding solar value. Such a difference is apparent in some recent cosmic-ray measurements, but forthcoming Advanced Composition Explorer results are needed to confirm this finding.