出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：Observations of planetary nebulae by Sterling, Dinerstein and Bowers have revealed enhancements
in the element Germanium (Ge) by factors of 3 to 10 compared to solar. Germanium
can be produced by slow-neutron capture (s-process) in asymptotic giant branch (AGB) stars. We
compute the detailed nucleosynthesis of a series of AGB models of variousmasses (1.9 to 6.5M⊙)
and metallicities from the zero-aged main sequence to near the end of the thermally-pulsing AGB
phase. We include a partial mixing zone of constant mass at the deepest extent of each dredgeup
episode. During the interpulse period this mixing zone will produce a 13C pocket in the top
∼1/10th of the He-rich intershell, releasing free neutrons that can be captured by iron-seed nuclei.
Using a post-processing code that includes 156 species, 1260 reaction rates and time-dependent
convective mixing, we follow the nucleosynthesis of all stable isotopes up to 75As, including the
five stable Ge isotopes. The surface Ge abundance at the end of the evolution was not enhanced
in models with no partial mixing i.e. [Ge/Fe] ≈ 0, except the 6.5M⊙ models which had [Ge/Fe]
≈ 0.2. Models with partial mixing produced surface enhancements of at most a factor of 3.6,
i.e. [Ge/Fe] . 0.5 except the 2.5M⊙, Z = 0.004 model with [Ge/Fe] = 0.77. Given the difficulties
in matching the compositions of the most Ge-enriched PNe we suggest a late thermal pulse
mixed pure He-intershellmaterial, enriched in Ge, with the small amount of remaining convective
envelope.
