Distribution of Neutron Exposures in the S-process Region of an AGB Star

There are two main approaches to study the s-process:the first is to use the classical models and the second is through calculating the nucleosynthesis in AGB(Asymptotic Giant Branch)stars with analytical models.For the old s-process models, the scenario of s-process in the convective zone can naturally lead to an exponential distribution of neutron exposures (hereafter "DNE" for the "distribution of neutron exposures").The classical-model parameter τ₀( mean neutron exposure)is related to the analytical-model parameters Δτ(the neutron exposure per pulse)and r ( overlap factor)by a simple equation:τ₀=-Δτ/lnr.This suggests the agreement between the two approaches.However, for a present model following Gallino et al.(1998), the DNE becomes very complex and can hardly be described analytically.Recent results show that the new analytical model still remains consistent with a classical model on the DNE, but the relation between τ₀ and Δτ takes a different form.In this paper, we review the history and the current status of studies on the DNE values in the s-process regions in AGB stars.Particularly, the neutron exposures (previously received by the matter)in the solar system as predicted by different AGB star s-process models are compared to the observed data(-constrained by the abundances of heavy elements).The latest study results are subsequently analyzed and discussed for their reliabilities.