Intermediate-mass Asymptotic Giant Branch Stars and Sources of 26Al, 60Fe, 107Pd, and 182Hf in the Solar System

Abstract

We explore the possibility that the short-lived radionuclides 26Al, 60Fe, 107Pd, and 182Hf inferred to be present in the proto-solar cloud originated from 3-8 M⊙ asymptotic giant branch (AGB) stars. Models of AGB stars with initial mass above 5 M⊙ are prolific producers of 26Al owing to hot bottom burning (HBB). In contrast, 60Fe, 107Pd, and 182Hf are produced by neutron captures: 107Pd and 182Hf in models ≲ 5 M⊙, and 60Fe in models with higher mass. We mix stellar yields from solar-metallicity AGB models into a cloud of solar mass and composition to investigate whether it is possible to explain the abundances of the four radioactive nuclides at the Sun's birth using one single value of the mixing ratio between the AGB yields and the initial cloud material. We find that AGB stars that experience efficient HBB (≥6 M⊙) cannot provide a solution because they produce too little 182Hf and 107Pd relative to 26Al and 60Fe. Lower-mass AGB stars cannot provide a solution because they produce too little 26Al relative to 107Pd and 182Hf. A self-consistent solution may be found for AGB stars with masses in between (4-5.5 M⊙), provided that HBB is stronger than in our models and the 13C(α, n)16O neutron source is mildly activated. If stars of M < 5.5 M⊙ are the source of the radioactive nuclides, then some basis for their existence in proto-solar clouds needs to be explored, given that the stellar lifetimes are longer than the molecular cloud lifetimes