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

Abstract

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.