Cosmic-Ray Acceleration from Supernova Ejecta in Superbubbles

Abstract

We suggest that the cosmic rays are accelerated primarily out of the supernova ejecta-enriched matter in the interiors of superbubbles. These hot, low-density superbubbles, which reach dimensions of several hundred parsecs, are generated by the winds and ejecta of supernova explosions of massive stars formed in giant molecular cloud OB associations that last for tens of megayears. Since these bubbles expand with shell velocities that are much faster than the dispersion velocities of the O and B star progenitors of the supernovae that power the bubbles, the bulk of the supernovae occur in their cores. The expanding remnants of each of these supernovae fill only less than 1% of this core before they have slowed to sonic velocities. Thus, the bulk of these supernovae remnants, together with their metal-rich grain and gas ejecta and their cosmic-ray-accelerating shocks, are well confined within the cores of superbubbles. These cores can thus provide a source of cosmic-ray matter of essentially constant metallicity throughout the age of the Galaxy, which is required to account for the constancy of cosmic-ray-produced Be relative to supernova-produced Fe observed in halo stars formed in the early Galaxy. The interactions of the grains and gas in metal-rich superbubbles, with recurrent supernova shocks every ~3×105 yr, also reconcile the requirement of a supernova ejecta source of cosmic rays with the recent observations that require a greater than 105 yr delay between nucleosynthesis and acceleration for the cosmic-ray metals. Supernova-enriched bubble metallicity may also explain the X-ray emission from the interiors of superbubbles in the Large Magellanic Cloud.