26Al and 60Fe From Supernova Explosions

Abstract

Using recently calculated yields for Type II supernovae, along with models for chemical evolution and the distribution of mass in the interstellar medium, the current abundances and spatial distributions of two key gamma-ray radioactivities, $^{26}$Al and $^{60}$Fe, are determined. The estimated steady state production rates are 2.0 $\pm$ 1.0 M\sun \ Myr$^{-1}$ for $^{26}$Al and 0.75 $\pm$ 0.4 M\sun \ Myr$^{-1}$ for $^{60}$Fe. This corresponds to 2.2 $\pm$ 1.1 M\sun \ of $^{26}$Al and 1.7 $\pm$ 0.9 M\sun \ of $^{60}$Fe in the present interstellar medium. Sources of uncertainty are discussed, one of the more important being the current rate of core collapse supernovae in the Galaxy. Our simple model gives three per century, but reasonable changes in the star formation rate could easily accommodate a core collapse rate one-half as large, and thus one-half the yields. When these stellar and chemical evolution results are mapped into a three dimensional model of the Galaxy, the calculated 1809 keV gamma-ray flux map is consistent with the {\it Compton Gamma Ray Observatory} observations of a steep decline in the flux outside a longitude of $\pm$ 50$^\circ$ from the Galactic center, and the slight flux enhancements observed in the vicinity of spiral arms. Other potential stellar sources of $^{26}$Al and $^{60}$Fe are mentioned, especially the possibility of $^{60}$Fe synthesis in Type Ia supernovae. Predictions for the $^{60}$Fe mass distribution, total mass, and flux map are given.