出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：Approximately half of the solar abundances of nuclei heavier than iron are created in the deep
layers of asymptotic giant branch (AGB) stars via slow neutron captures (the s process). Freshly
made heavy elements, such as Zr, Ba, and Pb, are carried to the stellar surface by recurrent mixing
episodes and shed into the interstellar medium via strong stellar winds, thus contributing to
the chemical evolution of galaxies. In the past few years several new modelling tools and observational
constraints have added to our understanding of how the s process operates in AGB
stars of different initial masses and metallicities. For AGB stars of low masses ( 4 M
), the
13C(a
n)16O reaction is the main neutron source. The exact mixing mechanism leading to the
formation of 13C is still unknown and multidimensional hydrodynamic models are needed to address
this point. Recent stellar population modelling including s-process nucleosynthesis indicate
that the spread in the efficiency of the 13C neutron source is limited to a factor of two of the value
obtained when reasonable basic assumptions are applied to the mixing mechanism. This is confirmed
by new refined measurements of the isotopic composition of heavy elements in meteoritic
silicon carbide grains. On the other hand, observations of the Rb and Zr abundances in massive
AGB stars (
4 M
) indicate that the main neutron source in these stars is the 22Ne(a
n)25Mg
reaction. The increasing number of observations becoming available for the abundances of heavy
elements in post-AGB stars and planetary nebulae, the progeny of AGB stars, can also be used to
test the ideas above. At low metallicity, the main constraints for the s process come from observations
of s-process-enriched carbon-enhanced metal-poor (CEMP) stars. In particular, the origin
of CEMP stars enriched in both slow and rapid neutron-capture elements represent a challenge
to our understanding of the origin of heavy elements.
