Asymptotic Giant Branch stars as a source of short-lived radioactive nuclei in the solar nebula

Abstract

We carried out a theoretical evaluation of the contribution of Asymptotic Giant Branch (AGB) stars to some short-lived (10<SUP>6</SUP> less than or equal to Tau-bar less than or equal to 2 x 10<SUP>7</SUP> yr) isotopes in the Interstellar Medium (ISM) and in the early solar system using stellar model calculations for thermally pulsing evolutionary phases of low-mass stars. The yields of s-process nuclei in the convective He-shell for different neutron exposures tau<SUB>0</SUB> were obtained, and AGB stars were shown to produce several radioactive nuclei (especially Pd-107, Pb-205, Fe-60, Zr-93, Tc-99, Cs-135, and Hf-182) in diferent amounts. Assuming either contamination of the solar nebula from a single AGB star or models for continuous injection and mixing from many stars into the ISM, we calculate the ratios of radioactive to stable nuclei at the epoch of the Sun's formation. The dilution factor between the AGB ejecta and the early solar system matter is obtained by matching the observed Pd-107/Pd-108 and depends on the value of tau<SUB>0</SUB>. It is found that small masses M<SUB>He</SUB> of He-shell material (10<SUP>-4</SUP>-10<SUP>-7</SUP> solar mass) enriched in s-process nuclei are sufficient to contaminate 1 solar mass of the ISM to produce the Pd-107 found in the early solar system. Predictions are made for all of the other radioactive isotopes. The optimal model to explain several observed radioactive species at different states of the proto-solar nebula involves a single AGB star with a low neutron exposure (tau<SUB>0</SUB> = 0.03 mbarn<SUP>-1</SUP>) which contaminated the cloud with a dilution factor of M<SUB>He</SUB>/solar mass approximately 1.5 x 10<SUP>-4</SUP>. This will also contribute newly synthesized stable s-process nuclei in the amount of approximately 10<SUP>-4</SUP> of their abundances already present in the proto-solar cloud. Variations in the degree of homogenization (approximately 30%) of the injected material may account for some of the small general isotopic anomalies found in meteorites. It is also found that Fe-60 is produced in small but significant quantities that may be sufficient to explain the observations if the time elapsed delta from the contamination of the ISM to the formation of protoplanetary bodies is not higher than delta = 5 x 10<SUP>6</SUP> yr. If delta is longer, up to 10 x 10<SUP>6</SUP> yr, this would require the single AGB star to experience enhanced neutron densities (n<SUB>n</SUB> approximately 3 x 10<SUP>9</SUP>n/cu cm) in the s-processing zone in order to compensate for the branching at Fe-59. The alternative model of long-term continuous ejection of matter from many AGB stars does not appear to match the observations. We also estimate the Al-26 production from the H-shell and find that the Al-26 abundance in the early solar system may be readily explained in a self-consistent manner. Moreover, Al-26 from AGB stars may contribute substantially to the galactic Al-26 gamma-source, while no significant gamma-flux from Co-60 (deriving from Fe-60 decay) is to be expected.