Cosmic Rays From Supernova Remnants: a Brief Description of the Shock Acceleration of gas and Dust
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We summarize our model of galactic cosmic-ray (GCR) origin and acceleration, wherein a mixture of interstellar and/or circumstellar gas and dust is accelerated by a supernova remnant (SNR) blast wave. A detailed analysis of observed GCR abundances (Meyer et al., 1997), combined with the knowledge that many refractory elements known to be locked in grains in the interstellar medium (ISM) are abundant in cosmic rays, has lead us to revive an old suggestion (Epstein, 1980) that charged dust grains can be shock accelerated. Here, we outline results (presented more completely in Ellison et al., 1997) from a nonlinear shock model which includes (i) the direct acceleration of interstellar gas-phase ions, (ii) a simplified model for the direct acceleration of weakly charged grains to ∼100 keV amu−1 energies, simultaneously with the acceleration of the gas ions, (iii) the energy losses of grains colliding with the ambient gas, (iv) the sputtering of grains, and (v) the simultaneous acceleration of the sputtered ions to TeV energies. We show that the model produces GCR source abundance enhancements of the volatile, gas-phase elements, which are an increasing function of mass, as well as a net, mass independent, enhancement of the refractory, grain elements over protons, consistent with cosmic-ray observations. The GCR 22Ne and C excesses may also be accounted for in terms of the acceleration of 22Ne-C- enriched pre-SN Wolf-Rayet star wind material surrounding the most massive supernovae. The O excess seen in cosmic rays probably cannot be interpreted in terms of W-R star nucleosynthesis, but is easily accounted for in our model since 15 to 20% of O is trapped in grain cores and this O will be preferentially accelerated. We have expanded the parameter range explored in Ellison et al. (1997) to lower shock speeds and higher maximum cosmic-ray energies and find similar fits to the H/He ratio and the cosmic-ray source spectra.
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