tokens: with the update of Goriely (1999) . The reactions and -decays are also introduced when relevant, in particular for the Bi and Po isotopes. 3. Nucleosynthesis process during core He-burning Fig. 2 Upper panel : Kippenhahn diagram of our 2 during the first dredge-up and core He burning phases; the green and blue dotted lines delineate the region of maximum nuclear energy production by H- and He-burning, respectively. Middle panel : evolution of the central neutron density , central C mass fraction () and neutron exposure . Lower panel : evolution of the central temperature and density . When the star leaves the main sequence and the central temperature reaches K (for a density ), the burning of the CNO by-product C via produces a first burst of neutrons (Fig. 2 ). This occurs during the first dredge-up of our 2 model star and leads to a maximum neutron density of and a time-integrated neutron exposure of , where is the most probable relative neutron-nucleus velocity at the temperature . The main neutron emission lasts until is depleted at the center and then proceeds at a much lower rate in the contracting shells surrounding the -free core. Eventually helium ignites off-center and after a series of shell flashes, convection reaches the center at an age yr. The temperature in the core reaches K and can trigger the production of a small amount of neutrons by the activation of the reaction. The neutron exposure increases up to but remains too small to generate an efficient -process nucleosynthesis. Instead, it is responsible for the re-arrangements of the abundances of specific heavy nuclei. As shown in Fig. 3 , elements such as K, Cd, Xe, Gd, Dy, Yb or Hg can be significantly produced with abundances increased by more than 0.4 dex with respect to their initial value and others partially destroyed, like Ag, Sm or Eu. At the temperatures encountered in the stellar center during core He burning, we notice that Te, Lu, Ta and Os become -unstable and fully decay ( Takahashi Yokoi 1987 ). When helium is exhausted in the core, the chemical profiles in the central 0.4–0.5 are frozen and the abundances of these heavy nuclei will be part of the future white dwarf. We note that the distributions of overabundances of elements heavier than typically show very small differences between our 2 and 3 stellar models, almost independently of the metallicity. Fig. 3 Final overproduction factors [X/Fe] of all stable nuclei with (full squares) at the center of our 2 model star at the end of core helium burning. The s-only nuclei are shown by red diamonds connected by a solid line. The vertical line depicts the total decay of the s-only nucleus Te. 4. <italic>S</italic>-process nucleosynthesis during the AGB phase of a 2 <italic>M</italic><inline-formula specific-use="simple-math"><sub>⊙</sub></inline-formula> [Fe/H] = –0.5 model star Mixing of the nuclear species beyond the convective boundary in the LM and DM models essentially depends on three parameters: , and .
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