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Previous studies have shown that, for the three main stages of the development and evolution of asymptotic giant branch (AGB) star s-process models, the neutron exposure distribution (DNE) in the nucleosynthesis region can always be considered as an exponential function, i.e., ρ AGB ( τ ) = C/τ 0 exp( −τ/τ 0 ) in an effective range of the neutron exposure values. However, the specific expressions of the proportion factor C and the mean neutron exposure τ 0 in the exponential distribution function for different models are not completely determined in the related literature. Through dissecting the basic method to obtain the exponential DNE, and systematically analyzing the solution procedures of neutron exposure distribution functions in different stellar models, the general formulae, as well as their auxiliary equations, for calculating C and τ 0 are derived. Given the discrete neutron exposure distribution P k , the relationships of C and τ 0 with the model parameters can be determined. The result of this study has effectively solved the problem to analytically calculate the DNE in the current low-mass AGB star s-process nucleosynthesis model of 13 C-pocket radiative burning. [ABSTRACT FROM AUTHOR]

Abstract: A study on the distribution of neutron exposures in a low-mass asymptotic giant branch (AGB) star is presented, according to the s-process nucleosynthesis model with the 12C(α, n)16O reaction occurred under radiative conditions in the interpulse phases. The model parameters, such as the over- lap factor r of two successive convective thermal pulses, the mass ratio q of the 13C shell with respect to the He intershell, and the effective mass of 13C in the 13C shell, vary with the pulse number. Considering these factors, a calculating method for the distribution of neutron exposures in the He intershell has been presented. This method has the features of simplicity and universality. Using this method, the exposure distribution for the stellar model of a star with the mass of 3 M ʘ and the solar metallicity has been calculated. The results suggest that under the reasonable assumption that the number density of neutrons is uniform in the 13C shell, the final exposure distribution approaches to an exponential distribution. For a stellar model with the definite initial mass and metallicity, there is a definite relation between the mean neutron exposure τ0 and the neutron exposure Δτ of each pulse, namely τ0 = 0.434λ(q1, q2, …, qm max +1, …, r1, r2, …, rm max +1)Δτ, where m max is the total number of thermal pulses with the third dredge-up episode, and the proportional coeffcient λ(q 1 , q 2 , …, qm max +1, …, r 1 , r 2 , …, rm max +1) can be determined by an exponential curve fitting to the final exposure distribution. This new formula quantitatively unifies the classical model with the s-process nu- cleosynthesis model by means of neutron exposure distribution, and makes the classical model continue to offer guidance and constraints to the s-process nu- merical calculations of stellar models. [Copyright &y& Elsevier]

Taking account of the metallicity dependence of the s-process nucleosynthesis in the AGB stars, we adopted the wind accretion model with the condition of total angular momentum conservation and used the Monte-Carlo method to study the variations and the distributions of the orbital elements of the mild and strong Ba stars. The calculated results show that the level of heavy-element overabundance in a Ba star depends on the orbital period. Since there is a strong dependence of s-process yields on the initial stellar metallicity of the AGB star and a strong increase of the s-process yields in AGB stars with decreasing metallicity, the calculated results strongly suggest that the initial metallicity of the Ba star systems is another important parameter for the level of heavy-element overabundance in a Ba star. The strong Ba stars generally have lower metallicities than mild Ba stars. The masses of AGB progenitor and Ba star are other two parameters which also have some impact on the heavy-element overabundance in the Ba star. [ABSTRACT FROM AUTHOR]