s-process nucleosynthesis in low-mass AGB stars by the ¹³C(α,n)¹⁶O neutron source

In this review we outline the temporal growth of our knowledge on slow neutron captures (the so-called s-process), concentrating on its main part occurring during the final stages of stellar evolution for low or intermediate-mass stars when they approach for the second time the Red Giant Branch and are therefore called Asymptotic Giant Branch , or AGB, stars. In particular, we focus our attention on how, in this field, studies passed from a first era of inquiries based on nuclear systematics (now often referred to as the the phenomenological approach ), to numerical nucleosynthesis computations performed in stellar codes. We then discuss how these last were forced, by observational constraints, to almost abandon, for the synthesis of nuclei between Sr and Pb (i.e. the main component ), the rather naturally activated ²² 22 Ne( α α ,n) ²⁵ 25 Mg neutron source (operating efficiently at T ≳ 3.5 · · 10 ⁸ 8 K, i.e. 30 keV, and producing a neutron density nₙ 5 · 10⁸ n n ≳ 5 · 10 8 cm⁻³ cm - 3 ). This implied considering the alternative reaction ¹³ 13 C( α α ,n) ¹⁶ 16 O, that can be activated locally after each of the recurring mixing episodes from the envelope (collectively referred to as the Third Dredge Up , or TDU ). The mentioned crucial reaction occurs at a relatively low temperature ( T 8-9 · 10⁷ T ≃ 8 - 9 · 10 7 K., i.e. less than 8 keV), in the time intervals separating two subsequent thermal instabilities of the He shell (also named Thermal Pulses , or TP ). The layers where ¹³ 13 </jats:alternativ