The Source of Cosmic Rays: 3. Supernova Grain Composition

We show that the expected composition of cosmic rays accelerated from the erosion products of fast supernova grains in the hot interstellar medium (HISM) of ejecta enriched superbubbles, where the bulk ( 85%) of Galactic supernovae occur, is consistent with the currently available cosmic ray data. In particular, we show that if the cosmic ray metals come from the sputtering of fast refractory grains that condense in expanding supernova ejecta and populate the superbubbles, then: (i) the abundances of the refractory cosmic ray metals, including C and O, are consistent with the required cosmic ray source abundances; (ii) the problematic cosmic ray C-to-O ratio is explained by refractory oxide and graphite grain abundances; (iii) the scattering of ambient gas by the fast grains can account for the relative abundance of cosmic ray volatiles; (iv) the mean time delay for cosmic ray acceleration is consistent with the ACE measurements; and (v) the resulting cosmic ray metallicity is consistent with the Galactic evolution of the Be abundance. In addition, we point out that acceleration in the HISM is energetically much more favorable than acceleration in the denser phases of the interstellar medium because the bulk of the supernovae occur in the HISM where the shocks suffer much lower radiative losses than those of supernovae in the denser average interstellar medium.