Can ^(59)Ni Synthesized in OB Associations Decay to ^(59)Co Before Being Accelerated to Cosmic-ray Energies?

Abstract

Observations from the Cosmic Ray Isotope Spectrometer (CRIS) aboard NASA's Advanced Composition Explorer (ACE) have shown that all relevant galactic cosmic-ray isotopic ratios measured are consistent with an OB-association origin of galactic cosmic rays (GCRs). Additionally CRIS measurements of the isotopic abundances of ^(59)Ni and ^(59)Co have shown that the 59Ni has completely decayed into ^(59)Co, indicating a delay of >10^5 years between nucleosynthesis and acceleration. However, it has been suggested that shocks generated from high-velocity Wolf-Rayet winds in the OB-association environment must accelerate nuclei synthesized in nearby core-collapse supernovae on a time scale short compared to the ^(59)Ni half-life of 7.6x10^4 years. If this were the case, it would imply that OB associations could not be the source of most galactic cosmic rays. In this paper, we describe the OB-association history and environment and show that the time scales for acceleration are such that most ^(59)Ni should be expected to decay naturally in that setting, strengthening the argument that OB associations are the likely source of a substantial fraction of galactic cosmic rays.

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