Abundance of 26Al and 60Fe in evolving Giant Molecular Clouds
  The nucleosynthesis and ejection of radioactive $^{26}$Al (t$_{1/2} \sim$
0.72\,Myr) and $^{60}$Fe, (t$_{1/2} \sim$ 2.5\,Myr) into the interstellar
medium is dominated by the stellar winds of massive stars and supernova type II
explosions. Studies of meteorites and their components indicate that the
initial abundances of these short-lived radionuclides in the solar
protoplanetary disk were higher than the background levels of the galaxy
inferred from $\gamma$--ray astronomy and models of the galactic chemical
evolution. This observation has been used to argue for a late-stage addition of
stellar debris to the Solar System's parental molecular cloud or,
alternatively, the solar protoplanetary disk, thereby requiring a special
scenario for the formation of our Solar System. Here, we use supercomputers to
model---from first principles---the production, transport and admixing of
freshly synthesized $^{26}$Al and $^{60}$Fe in star-forming regions within
giant molecular clouds. Under typical star-formation conditions, the levels of
$^{26}$Al in most star-forming regions are comparable to that deduced from
meteorites, suggesting that the presence of short-lived radionuclides in the
early Solar System is a generic feature of the chemical evolution of giant
molecular clouds. The $^{60}$Fe/$^{26}$Al yield ratio of $\approx 0.2$
calculated from our simulations is consistent with the galactic value of 0.15
$\pm$ 0.06 inferred from $\gamma$--ray astronomy but is significantly higher
than most current Solar System measurements indicate. We suggest that estimates
based on differentiated meteorites and some chondritic components may not be
representative of the initial $^{60}$Fe abundance of the bulk Solar System.
