Nucleosynthesis and Mixing on the Asymptotic Giant Branch. III. Predicted and Observeds‐Process Abundances

Abstract

We present the results of s-process nucleosynthesis calculations for Asymptotic Giant Branch (AGB) stars of different metallicities and different initial stellar masses (1.5 and 3 M ⊙ ) and comparisons of them with observational constraints from high resolution spectroscopy of evolved stars over a wide metallicity range. The computations were based on previously published stellar evolutionary models that account for the third dredge up phenomenon occurring late on the AGB. Neutron production is driven by the 13 C(α,n) 16 O reaction during the interpulse periods in a tiny layer in radiative equilibrium at the top of the He-and C-rich shell. The neutron source 13 C is manufactured locally by proton captures on the abundant 12 C; a few protons are assumed to penetrate from the convective envelope into the radiative layer at any third dredge up episode, when a chemical discontinuity is established between the convective envelope and the He-and Crich zone. A weaker neutron release is also guaranteed by the marginal activation of the reaction 22 Ne(α,n) 25 Mg during the convective thermal pulses. Owing to the lack of a consistent model for 13 C formation, the abundance of 13 C burnt per cycle is allowed to vary as a free parameter over a wide interval (a factor of 50). The s-enriched material is subsequently mixed with the envelope by the third dredge up, and the envelope composition is computed after each thermal pulse. We follow the changes in the photospheric abundance of the Ba-peak elements (heavy s, or 'hs') and that of the Zr-peak ones (light s, or 'ls'), whose logarithmic ratio [hs/ls] has often been adopted as an indicator of the s-process efficiency (e.g. of the neutron exposure). Our model predictions for this parameter show a complex trend versus metallicity. Especially noteworthy is the prediction that the flow along the s path at low metallicities drains the Zr-peak and Ba-peak and builds an excess at the doubly-magic 208 Pb, at the termination of the s path. We then discuss the effects on the models of variations in the crucial parameters of the 13 C pocket, finding that they are not critical for interpreting the results.The theoretical predictions are compared with published abundances of s elements for AGB giants of classes MS, S, SC, post-AGB supergiants, and for various classes of binary stars, which supposedly derive their composition by mass transfer from an AGB companion. This is done for objects belonging both to the Galactic disk and to the halo. The observations in general confirm the complex dependence of neutron captures on metallicity. They suggest that a moderate spread exists in the abundance of 13 C that is burnt in different stars. Although additional observations are needed, it seems that a good understanding has been achieved of s-process operation in AGB stars. Finally, the detailed

Short-lived nuclei in the early Solar System: Possible AGB sources

“…Certainly the high abundance of AGB debris found in meteorites is evidence of AGB components, but does not prove they were the carriers of live nuclei. A demonstration of the presence of 205 Pb would strongly support the idea that this stellar source was an AGB star.…”

“…With regard to possible matches with ESS values, we note that a compromise exists between the two cases (1. Table 6) would favor the survival of 205 Pb in the critical interval after a pulse (a few hundred years) before it is transferred to the envelope by dredge up. Hence IMS models are probably the most promising site for 205 Pb formation.…”

“…From equation 7 and the quantities in Tables 3 and 4, we find that a very small contribution (f 107 Pd = 4 × 10 −5 ) from a 1.5 M ⊙ star will provide the 107 Pd inventory with ∆ 1 + ∆ 2 = 8.75 × 10 6 yr. It will also provide 205 Pb with essentially the same upper bound as given in Table 4. A similar result is found for the 3 M ⊙ case.…”

C-enhanced metal poor stars and AGB nucleosynthesis at low Z

“…Busso et al 2001) do expect that, as the number of available neutrons for seed nuclei increases (because of the decrease in metallicity), the s-process path shifts more toward the third peak increasing the [Pb/Ba] ratio. This effect is not visible in the data, which require variable amount of 13 C in the pocket to explain the different values of [Pb/Ba] at a given metallicity (Busso et al 2001).…”

Evolution and Nucleosynthesis of Low and Intermediate-Mass Stars

“…Stellar yields are an essential ingredient of chemical evolution models. Karakas & Lattanzio (2014) [23] recently reviewed the stellar yields available from AGB stars, focusing on tabulated yields available in the literature (which do not include the many studies of the s-process including [3,4,8,16,27,34]). The main deficiencies identified were stellar yields for very metal-poor AGB stars and stellar yields of heavy elements for all metallicities for a significant range of initial mass.…”