Nuclear and Gamma-ray Production by Supernova Ejecta

The fastest ejecta of supernova explosions propagate as a precursor to the main supernova shock wave, and can be quite energetic. The spectrum of such fast ejecta is estimated based on recent analytic and numerical supernova models, and found to be a power law having a cutoff at an energy of order 10 MeV/nucl, the precise value of which depends on the supernova mass and energetics. With cutoffs in this range there can be significant flux with energies above the thresholds for $\gamma$-ray and Li, Be, and B production. These nuclear interactions should inevitably accompany the passage of prompt supernova ejecta through the surrounding medium. The fast particle composition is that of the outermost layers of the progenitor; if the progenitor experienced significant loss of its envelope to a companion or by mass loss, then the composition is nonsolar, and in particular, metal-rich. Such a composition and spectrum of fast particles is required to explain the recent COMPTEL observations of $\gamma$-ray emission in Orion. Supernova ejecta from one progenitor having lost a large amount of mass are shown to quantitatively account for the Orion $\gamma$ rays, and to imply the presence of other, weaker lines and of a supernova explosion in Orion in the last $\la 10^4$ years. Implications for this mechanism as a mode of nucleosynthesis, and as a potential supernova diagnostic, are discussed. Model dependences and uncertainties are noted, and the need is shown for accurate measurements of $\gamma$-ray and spallation production cross sections at energies near threshold.