Abundance of 26Al and 60Fe in evolving Giant Molecular Clouds

TL;DR

This study uses supercomputing simulations to show that short-lived radionuclides like $^{26}$Al and $^{60}$Fe are naturally abundant in star-forming regions of giant molecular clouds, aligning with some meteoritic data but challenging previous Solar System estimates.

Contribution

First-principles modeling of radionuclide production and distribution in giant molecular clouds reveals their natural abundance in star-forming regions, providing insights into Solar System formation.

Findings

$^{26}$Al levels are common in star-forming regions.

The $^{60}$Fe/$^{26}$Al ratio from simulations matches galactic gamma-ray data.

Simulated $^{60}$Fe/$^{26}$Al ratio is higher than some Solar System measurements.

Abstract

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.