Short-lived radioactivity in the early solar system: The Super-AGB star hypothesis

The composition of the most primitive solar system condensates, such as calcium-aluminum-rich inclusions (CAIs) and micron-sized corundum grains, show that short-lived radionuclides (SLR), e.g., ²⁶Al, were present in the early solar system. Their abundances require a local or stellar origin, which, however, is far from being understood. We present for the first time the abundances of several SLR up to ⁶⁰Fe predicted from stars with initial mass in the range approximately 7-11M_⊙. These stars evolve through core H, He, and C burning. After core C burning they go through a "Super"-asymptotic giant branch (Super-AGB) phase, with the H and He shells activated alternately, episodic thermal pulses in the He shell, a very hot temperature at the base of the convective envelope (approximately 10⁸K), and strong stellar winds driving the H-rich envelope into the surrounding interstellar medium. The final remnants of the evolution of Super-AGB stars are mostly O-Ne white dwarfs. Our Super-AGB models produce ²⁶Al/²⁷Al yield ratios approximately 0.02-0.26. These models can account for the canonical value of the ²⁶Al/²⁷Al ratio using dilutions with the solar nebula of the order of 1 part of Super-AGB mass per several 10² to several 10³ of solar nebula mass, resulting in associated changes in the O-isotope composition in the range Δ¹⁷O from 3 to 20‰. This is in agreement with observations of the O isotopic ratios in primitive solar system condensates, which do not carry the signature of a stellar polluter. The radionuclides ⁴¹Ca and ⁶⁰Fe are produced by neutron captures in Super-AGB stars and their meteoritic abundances are also matched by some of our models, depending on the nuclear and stellar physics uncertainties as well as the meteoritic experimental data. We also expect and are currently investigating Super-AGB production of SLR heavier than iron, such as ¹⁰⁷Pd.