Gamma-ray sources, supernova remnants, OB associations, and the origin of cosmic rays

Although supernova explosions are widely thought to give rise to cosmic rays (nucleons), there is, as yet, no direct evidence from individual objects to support this view. A possible tool in this respect is the detection of γ-rays emitted by supernova remnants (SNRs) via π0 decay. However, the accumulating γ-ray data show that SNRs as a class are not γ-ray sources, but rather that γ-ray sources are, in general, closely linked with young objects. We examine, among other possibilities, whether a restricted class of SNRs are actually γ-ray sources; we restrict the class to those SNRs physically linked with extreme Population I objects. Spatial coincidences between SNRs and OB associations or H II regions (SNOBs) are sought by various methods, and this yields a list of about 30 objects (which is certainly incomplete). From the Cos B data, one finds that five (perhaps six) out of 11 as yet unidentified γ-ray sources (above 100 MeV) are associated with SNOBs, and there is a hint that as much as three-fourths of the best identified SNOBs are seen in γ-rays. The associated probabilities of chance coincidence are approx.10-4. Angular and other statistical considerations also support this association. Pending confirmation, if a substantial proportion of the observed γ-rays does come from π0 decay, SNOBs appear to be a major source of galactic cosmic rays, in which cosmic-ray (> or =2 GeV) energy densities in the range approx.10--100 times the solar neighborhood value are found. A phenomenological scenario is suggested. Cosmic rays are produced by a two-step process, in which low-energy (MeV range) particles are injected by young stars pertaining to an OB association, and are subsequently accelerated by the shock wave of a neighboring supernova explosion. Isolated SNRs, possible links with light-element production, and the links between SNOBS and other astronomical objects are discussed