Exploring the nucleosynthesis region of metal-poor Stars

Abstract

The chemical abundances of the very metal poor double-enhanced stars are excellent information to set new constraints on models of neutron-capture processes at low metallicity. There have been many theoretical studies of s-process nucleosynthesis in low-mass AGB stars. Using the parametric approach based on the radiative s-process nucleosynthesis model, we calculate the following five parameters for a series of metal-poor stars. They are: the mass fraction of ¹³C pocket q, the overlap factor r, the neutron exposure per interpulse Δτ, and the component coefficients that correspond to relative contribution from the s-process and the r-process. We find that the mass fraction of ¹³C pocket q deduced for the Pb stars is comparable to the overlap factor r, which is about 10 times larger than normal AGB model; q ~ 0.05; and the neutron exposure per interpulse Δτ for all Pb stars are about 10 times smaller than the ST case (Δτ ~ 7.0mb⁻¹). Although the two fundamental parameters Δτ and q obtained for the Pb stars are very different from the AGB stellar model, the results of the larger value of q and the smaller value of Δτ can also explain the abundance distribution of the Pb stars. This suggest that the q change to larger than that of normal AGB model. Then, this factor will result in the descent of the density of ¹³C in the nuclear synthesis region directly. So, the neutron exposure Δτ will also decrease to the same extent. Although the neutron number density in the larger initial mass AGB stars (m > 3M_⊙) is high, the neutron irradiation time is shorter, obviously the neutron exposure per interpulse in the AGB stars should be smaller. It is noteworthy that the total amount of ¹³C in metal poor condition is close to the ST case, which is consistent with the primary nature of the neutron source.