Wolf Rayet Stars and the Origin of the ²²Ne Excess in Cosmic Rays

Abstract

First order Fermi acceleration at the boundary between supersonic stellar winds from OB and Wolf-Rayet stars and the surrounding interstellar medium could be influential in the bulk energization of the local cosmic radiation (Cassé and Paul, 1980). Since wind acceleration is not supposed to accelerate thermal particles, a continuous injection of low energy particles (E ~ 1 to 10 MeV/n) is required. We keep open the possibility that these particles may be injected from the interior of the stellar cavity, i.e. by the mass-losing star itself via a flare-like surface activity for instance. Observations of flare activity on hot and massive stars are mandatory to settle this idea. In this context, we expect that the CR reservoir is the surface of young and active stars and that the difference between the CR source (CRS) composition (i.e. corrected for propagation effects in the interstellar medium, ISM) and the surface composition of young stars (reflecting for most of them the present local ISM) is principally due to selective effects at injection depending on the atomic properties of the elements. This idea is supported by 3 arguments (Cassé and Goret, 1978, Meyer et al., 1979) i) the general resemblance between solar CR elemental abundances and elemental CRS abundances (see e.g. Mewalt, 1980) ii) the correlation between the (CRS/local galactic) abundance ratio and the first ionization potential and iii) the fact that dust grains must have been thoroughly destroyed in the medium from which CR are extracted. In the interstellar gas in which dust grains are present, Ni, Fe, Mg and especially Ca and Al are highly depleted (Salpeter, 1977) whereas they are normally abundant in CR.