出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：In AGB stars of low mass and very low metallicity, [Fe/H] 2, a large abundance of primary
12C is mixed with the envelope by each third dredge up episode. The subsequent activation
of the H shell converts almost all CNO nuclei into 14N. Thus the H burning ashes contain 14N
from the original CNO nuclei, plus an increasing amount of primary 14N. During the subsequent
convective thermal instability in the He shell, all the 14N nuclides are converted to 22Ne
by 14N(a,g )18F(b n)18O, followed by 18O(a,g )22Ne. At the peak temperature reached at the
base of the thermal pulse, the 22Ne(a,n)25Mg reaction is partly activated, giving rise to a small
neutron exposure. At the same time, although the neutron capture cross section of 22Ne is very
small (s (22Ne, 30keV)=0.059
0.0057 mbarn, Beer et al. 1991), the very large amount of primary
22Ne acts as a major poison against the s process. This poison effect is substantial also in
case of addition of a 13C-pocket. Some fraction of primary 16O is also made in the thermal pulse
by a-capture on 12C (with mass fraction X(16O)
0.003, while X(12C)
0.20). Besides 12C and
22Ne, a number of light isotopes are largely produced in a primary way, among which one finds
19F (from neutron capture on 18O), 23Na, 25Mg, 26Mg. An effort should be devoted to improve
the accuracy of the cross sections of these light isotopes, in order to better constrain the s-process
efficiency and their production in low metallicity AGB stars. At very low metallicity, iron is directly
made starting from neutron captures on 22Ne and then used as a bridge for the build-up of
the s elements.
