Cosmic rays and gamma-rays from OB stars

The possible acceleration of cosmic rays at the terminal shocks of OB star winds is investigated. Interest is focused on the interaction region downstream of the shock, its acceleration, and its transmission properties. Particles to be accelerated can come from the star (stellar energetic particles), from the shock itself through injection out of the stellar wind plasma, from the interstellar medium, and possibly from the interface between shocked wind and ambient medium. It is shown that stellar energetic particles lose far too much energy in the expanding wind to be relevant. Estimates of the scattering mean free path indicate that postshock convection prevents access of downstream particles to the shock unless the postshock region collapses due to cooling in a low density ambient medium. Disregarding electrons, shock-injection of nucleons, as estimated from analogous results at the earth's bow shock in the solar wind, will yield nonrelativistic nucleons with a power law distribution in momentum of approximately p to the -4th if they can be accelerated at the shock. This is believed to be possible at least intermittently on time scales limited by the stellar rotation rate. Thus, stellar winds might be sources of very low energy (nuclear) cosmic rays. At high ambient gas densities this may lead to nuclear gamma-rays.

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