Cosmic ray acceleration in superbubbles and the composition of cosmic rays

Abstract

We review the evidence for cosmic ray acceleration in the superbubble/hot phase of the interstellar medium, and discuss the implications for the composition of cosmic rays and the structure and evolution of the interstellar medium (ISM). We show that the bulk of the galactic supernovae, their expanding remnants, together with their metal-rich grain and gas ejecta, and their cosmic ray accelerating shocks, are all confined within the interiors of hot, low-density superbubbles, generated by the multiple supernova explosions of massive stars formed in giant OB associations. This superbubble/hot phase of the ISM provides throughout the age of the Galaxy a cosmic ray source of essentially constant metallicity for acceleration by the shocks of many supernovae over time scales of a few Myr, consistent with both Be/Fe evolution and ACE observations of 59Ni/59Co. We show that the expected metallicity (>2 times Solar) and filling factor (>50%) of the superbubble/hot phase is high enough that the composition of cosmic rays accelerated from fast, supernova grains in these superbubbles is quite consistent with the both Be/Fe and cosmic ray data, while their acceleration from grains in the well-mixed cooler phases of the ISM is not consistent with observations. We also show that if the refractory cosmic ray metals come from the sputtering of fast refractory grains then the accompanying scattering of ambient gas by these fast grains can also account for the relative abundance of cosmic ray volatiles.