Mass Fraction of ¹³C-Pocket in Metal-Poor AGB Stars and the Primary Nature of Neutron Source

Chemical abundances of very metal-poor s-rich stars contain excellent information to set new constraints on models of neutron-capture processes at low metallicity. Using the parametric approach based on the radiative s-process nucleosynthesis model, we obtain the mass fraction q of ¹³C-pocket, the overlap factor r, the neutron exposure per interpulse Δτ, and the component coefficients of the s-process and the r-process for 25 s-rich stars, respectively. We find that q deduced for the lead stars is comparable to the overlap factor r, which is larger than the standard case (hereafter ST case) of the AGB model (q~0.05) about 10 times, and Δτ are about 10 times smaller than the ST case (Δτ=7.0mbarn^-1). Although the two parameters obtained for the lead stars are very different from the ST case of the AGB stellar model, it is worth noting that the total amounts of ¹³C in metal-poor condition are close to the ST case. The above relation is a significant evidence for the primary nature of the neutron source and the lead stars could be polluted by low-mass AGB stars. Because interpulse period declines with increasing stellar mass, for high-mass AGB star, the neutron irradiation may be terminated due to their shorter interpulse period. Thus the neutron exposure per interpulse of the larger AGB stars should be about 10 times smaller than the ST case. In this case, the primary nature of the neutron source also exists.