OB Associations, Supernova‐generated Superbubbles, and the Source of Cosmic Rays

Abstract

We have considered the effects of both the spatial and temporal clustering of OB stars and their subsequent core-collapse supernovae on their generation of superbubbles and their resultant role as the primary source of cosmic rays. Employing a wide range of astronomical and astrophysical observations, we determine quantitatively the fraction of Galactic core-collapse supernovae that occur in superbubbles. We show that the fraction of core-collapse supernovae occurring in superbubbles is high, ranging from ~80% (solely temporal correlations) to ~90% (only spatial correlations). In addition, we find that the singleton end of our stellar cluster distribution is sufficient to reproduce the observed relative number of OB field stars. Core-collapse supernovae (Types II and Ib/c) constitute 85% of Galactic supernovae; only a small fraction of the remaining class of supernovae, Type Ia, occur in superbubbles. Thus, ~75% of all Galactic supernovae are expected to occur within superbubbles. The occurrence of the great majority of Galactic supernovae in superbubbles has major implications for cosmic-ray acceleration. Acceleration of cosmic-ray nuclei heavier than He in enriched (ZSB ≈ 3 Z☉) superbubble interiors can consistently explain the anomalous cosmic-ray 22Ne/20Ne ratio, the cosmic-ray actinde/Pt group and UPuCm/Th ratios, and the constant LiBeB/(C+O) ratio observed in very old, metal-poor stars. Finally, although only ~75% of supernovae occur in superbubbles, ~88% of the cosmic-ray heavy particles are accelerated there because of the factor of ~3 enhanced superbubble core metallicity.