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 (10^6 ≤ τ ≤ 2 x 10^7 yr) isotopes in the 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 τ_0 were obtained, and AGB stars were shown to produce several radioactive nuclei (especially ^(107)Pd, ^(205)Pb, ^(60)Fe, ^(93)Zr, ^(99)Tc, ^(135)Cs, and ^(182)Hf) in different 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 ^(107)Pd/^(108)Pd and depends on the value of τ_0. It is found that small masses M_(He). of He-shell material (10^(-4)-10^(-7) M_☉) enriched ins-process nuclei are sufficient to contaminate 1 M_☉ of the ISM to produce the ^(107)Pd 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 (τ_0 = 0.03 mbarn^(-1)) which contaminated the cloud with a dilution factor of M_(He)/M_☉ ~ 1.5 x 10^(-4). This will also contribute newly synthesized stable s-process nuclei in the amount of ~ 10^(-4) of their abundances already present in the protosolar cloud. Variations in the degree of homogenization (~ 30%) of the injected material may account for some of the small general isotopic anomalies found in meteorites. It is also found that ^(60)Fe is produced in small but significant quantities that may be sufficient to explain the observations if the time elapsed (Δ) from the contamination of the ISM to the formation of protoplanetary bodies is not higher than Δ = 5 x 10^6 yr. If Δ is longer, up to 10 x 10^6 yr, this would require the single AGB star to experience enhanced neutron densities (n_n ~ 3 x 10^9n cm^(-3)) in the s-processing zone in order to compensate for the branching at ^(59)Fe. 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 ^(26)Al production from the H-shell and find that the ^(26)Al abundance in the early solar system may be readily explained in a self-consistent manner. Moreover, ^(26)Al from AGB stars may contribute substantially to the galactic ^(26)Al y-source, while no significant y-flux from ^(60)Co (deriving from ^(60)Fe decay) is to be expected.

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