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ACE News #74 - Aug 22, 2003

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Galactic Cosmic Ray Neon and Iron-Group Isotopic Abundances Agree with Wolf-Rayet Star Model

The 22Ne/20Ne abundance ratio has previously been shown by several experiments to be enhanced in the Galactic Cosmic Rays (GCR) relative to that in the solar wind. The Cosmic Ray Isotope Spectrometer (CRIS) on ACE has made the most precise measurement of this isotope ratio to date. We have derived the abundance ratio at the cosmic-ray source using the measured 21Ne/20Ne abundance as a "tracer" of secondary production of the Ne isotopes as cosmic rays propagate through the Galaxy (21Ne is very rare in the solar system and should also be rare in the cosmic-ray source). The 22Ne/20Ne abundance ratio obtained for the cosmic-ray source is a factor of 5.0 +/- 0.2 greater than in the solar wind. This ratio is also significantly greater than that found in anomalous cosmic rays (ACRs), solar energetic particles (SEPs), most meteoritic neon samples (Ne-A through Ne-E), and interplanetary dust particles (IDPs; left-hand figure).

It has been suggested previously that this overabundance might result from enrichment of the interstellar medium by high-velocity stellar winds from Wolf-Rayet (WR) stars. Wolf- Rayet stars are believed to be evolutionary products of very massive stars that have lost a large fraction of their mass because of such winds. We have compared our results with modeling calculations by Meynet et al. of abundance ratios for neon, iron, and nickel isotopes ejected from the surface of WR stars. Their model assumes that the GCRs are a mix of WR-wind material and material with solar system (SS) composition. In the right-hand figure above we show the predictions for a WR star with an initial mass of 60 solar masses and solar metallicity compared with the CRIS isotope source ratios (the metallicity of a star is the fraction of its mass in elements heavier than He). The mixing ratio of WR wind material and SS material that is required for agreement between the calculated and measured 22Ne/20Ne ratios is f = 0.052. Recent theoretical studies suggest that this ratio is physically reasonable. Given this normalization, the model predictions for GCR/SS ratios of heavier isotopes (right-hand figure) appear to be in good agreement with the measured abundances, though the measurement uncertainties for the rarest isotopes, e.g. 61Ni/58Ni, are rather large. We conclude that these data are consistent with an enrichment of Wolf-Rayet material in the pool of matter from which cosmic rays are accelerated.

Submitted by W. R. Binns of Washington University.

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