Effects of uncertainties of the 22Ne( α, n) 25Mg and 13C( α, n) 16O reaction rates in the s-process yields

Abstract

In the low mass star AGB phase the region between H shell and He shell (He-intershell) is affected by brief convective instabilities (Thermal Pulses), due to trigger of the He burning in the He shell. In this conditions the neutron source 22 Ne( α , n) 25 Mg is marginally activated (releasing a short neuton burst with a peak neutron density n n ≤ 10 10 n cm −3 ), and may change isotopic ratios near s-path branchings sensitive to the peak neutron density. For each element we analyse the isotopic ratios obtained in the AGB nucleosynthesis model using three different 22 Ne( α , n) 25 Mg rates: first,with the "standard rate", evaluated in [F. Käppeler et al., Astrophys. J. 437, 396 (1994)]; second, with the standard rate multiplied by a factor of two, and third, the standard rate divided by a factor of two. This factor is close to the present uncertainty factor of the 22 Ne( α , n) 25 Mg rate. The same analysis is presented for 13 C( α , n) 16 O reaction rate, the primary neutron source in the He-intershell, which works in radiative conditions in a thin region in the top layers of the He-intershell ( 13 C-pocket) during interpulse periods. In this case the standard rate is evaluated in [G.R. Caughlan and W.A. Fowler, Atomic Data Nucl. Data Tables 40, 283 (1988)].