Wolf-Rayet stars, OB associations, and the origin of galactic cosmic rays

Abstract

We have measured the isotopic abundances of neon and several refractory species in the galactic cosmic rays (GCRs) using the cosmic ray isotope spectrometer (CRIS) aboard the ACE spacecraft. The ^(22)Ne/^(20)Ne ratio at the cosmic-ray source that we have obtained is 0.387 ± 0.007 (stat.) ± 0.022 (syst.), which corresponds to enhancement by a factor of 5.3 ± 0.3 over that in the solar wind. Our ACE-CRIS data, and data from other experiments, are compared to recent results from two-component Wolf-Rayet (WR) models. The three largest deviations of galactic cosmic ray isotope ratios from solar-system ratios predicted by these models, ^(12)C/^(16)O,^(22)Ne/^(20)Ne, and ^(58)Fe/^(56)Fe, are, in fact, very close to those observed. Furthermore, all of the isotope ratios that we have measured are consistent with a GCR source consisting of ~20% of WR material mixed with ~80% material with solar-system composition. Since WR stars are evolutionary products of OB stars, and most OB stars exist in OB associations that form superbubbles, the good agreement of our data with WR models suggests that OB associations within superbubbles are the likely source of at least a substantial fraction of GCRs.