Radioactive Al26 and Fe60 in the Milky Way: implications of the RHESSI detection of Fe60

Abstract

The recent detection of gamma-ray lines from radioactive Al26 and Fe60 in the Milky Way by the RHESSI satellite calls for a reassessment of the production sites of those nuclides. The observed gamma-ray line flux ratio is in agreement with calculations of nucleosynthesis in massive stars, exploding as SNII (Woosley and Weaver 1995); in the light of those results, this observation would suggest then that SNII are the major sources of Al26 in the Milky Way, since no other conceivable source produces substantial amounts of Fe60. However, more recent theoretical studies find that SNII produce much higher Fe60/Al26 ratios than previously thought and, therefore, they cannot be the major Al26 sources in the Galaxy (otherwise Fe60 would be detected long ago, with a line flux similar to the one of Al26). Wolf-Rayet stars, ejecting Al26 (but not Fe60) in their stellar winds, appear then as a most natural candidate. We point out, however, that this scenario faces also an important difficulty. Forthcoming results of ESA's INTEGRAL satellite, as well as consistent calculations of nucleosynthesis in massive stars (including stars of initial masses as high as 100 M_sun and metallicities up to 3 Z_sun), are required to settle the issue.