thesis - IRS, The Infrared Spectrograph

5.4. Nucleosynthesis in low- and intermediate-mass stars 795.4 Nucleosynthesis in low- and intermediate-mass starsWe will recall the main nucleosynthetic and convective mixing events that may possibly alterthe surface chemical composition of a low-/intermediate-mass star in the course of its evolution.According to the classical scenario 1 four processes are important (see e.g. Forestini &Charbonnel 1997 for a recent extended analysis), namely:The first dredge-up. At the base of the RGB the outer convective envelope reaches regionsof partial hydrogen burning (CN cycle). As a consequence, the surface abundance of 4 He isincreased (and that of H depleted), 14 N and 13 C are enhanced at the expense of 12 C, while16 O remains almost unchanged.The second dredge-up. This occurs in stars initially more massive than 3-5 M ⊙ (dependingon composition) during the early AGB phase. The convective envelope penetrates intothe helium core (the H-burning shell is extinguished) so that the surface abundances of 4 Heand 14 N increase, while those of 12 C, 13 C and 16 O decrease.The third dredge-up. This takes place during the TP-AGB evolution in stars more massivethan ≈ 1.5 M ⊙ for solar composition, starting at lower masses for lower metallicities (seeMarigo et al. 1999). It actually consists of several mixing episodes occurring at thermalpulses during which significant amounts of 4 He and 12 C, and smaller quantities of othernewly-synthesized products (e.g. 16 O, 22 Ne, 25 Mg, s-process elements) are convected to thesurface.Hot bottom burning. This occurs in the most massive and luminous AGB stars (with initialmasses M 4 − 4.5M ⊙ , depending on metallicity). The convective envelope penetratesdeeply into the hydrogen-burning shell, and the CN-cycle nucleosynthesis actually occursin the deepest envelope layers of the star. As a consequence, besides the synthesis of newhelium, 12 C is first converted into 13 C and then into 14 N. In the case of high temperaturesand after a sufficiently long time, the ON cycle can also be activated, so that 16 O is burnedinto 14 N.It should be remarked that the third dredge-up and hot-bottom burning are the processesthat are expected to produce the most significant changes in CNO and He surface abundances,being affected at the same time by the largest uncertainties in the theory of stellar evolution.This latter point motivates the adoption of free parameters (e.g. the dredge-up efficiency) todescribe these processes in synthetic TP-AGB models, that are discussed next.5.5 Synthetic TP-AGB calculationsIn order to interpret the abundance data reported in Sect. 5.3.1, synthetic evolutionary calculationsof the TP-AGB phase have been carried out with the aid of the model developed byMarigo et al. (1996, 1998, 1999), Marigo (1998, 2001, 2002), to whom we refer for all details.1 We do not consider here any additional extra-mixing process, such as the one invoked to explain the observedabundance anomalies in low-mass RGB stars (see e.g. Charbonnel 1995).