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

We explore the possibility that the short-lived radionuclides 26_Al, 60_Fe, 107_Pd, and 182_Hf 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 26_Al owing to hot bottom burning (HBB). In contrast, 60_Fe, 107_Pd, and 182_Hf are produced by neutron captures: 107_Pd and 182_Hf in models ≲ 5 M_⊙, and 60_Fe 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 182_Hf and 107_Pd relative to 26_Al and 60_Fe. Lower-mass AGB stars cannot provide a solution because they produce too little 26_Al relative to 107_Pd and 182_Hf. 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 13_C(α, n)16_O 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.