The recently improved information on the stellar (n, γ) cross sections of neutron-magic nuclei at N = 82, and in particular of 142 Nd, turned out to represent a sensitive test for models of s-process nucleosynthesis. While these data were found to be incompatible with the classical approach based on an exponential distribution of neutron exposures, they provide significantly better agreement between the solar abundance distribution of s nuclei and the predictions of models for low mass AGB stars. The origin of this phenomenon is identified as being due to the high neutron exposures at low neutron density obtained between thermal pulses when the 13 C burns radiatively in a narrow layer of a few 10 −4 M ⊙ . This effect is studied in some detail, and the influence of the presently available nuclear physics data is discussed with respect to specific further requests. In this context, particular attention is paid to a consistent description of s-process branchings in the region of the rare earth elements.It is shown that -in certain cases -the nuclear data are sufficiently accurate that the resulting abundance uncertainties can be completely attributed to stellar modelling. Thus, the s process becomes important for testing the role of different stellar masses and metallicities as well as for constraining the assumptions for describing the low neutron density provided by the 13 C source.

We present a new analysis of neutron capture occurring in low-mass asymptotic giant branch (AGB) stars su †ering recurrent thermal pulses. We use dedicated evolutionary models for stars of initial mass in the range 1 to 3 and metallicity from solar to half solar. Mass loss is taken into account with the M _ Reimers parameterization. The third dredge-up mechanism is self-consistently found to occur after a limited number of pulses, mixing with the envelope freshly synthesized 12C and s-processed material from the He intershell. During thermal pulses, the temperature at the base of the convective region barely reaches being the temperature in units of 108 K), leading to a marginal activation of T 8 D 3 (T 8 the 22Ne(a, n)25Mg neutron source. The alternative and much faster reaction 13C(a, n)16O must then play the major role. However, the 13C abundance left behind by the H shell is far too low to drive the synthesis of the s-elements. We assume instead that at any third dredge-up episode, hydrogen downÑows from the envelope penetrate into a tiny region placed at the top of the 12C-rich intershell, of the order of a few 10~4At H reignition, a 13C-rich (and 14N-rich) zone is formed. Neutrons by the major 13C M _ . source are then released in radiative conditions at during the interpulse period, giving rise to an T 8 D 0.9 efficient s-processing that depends on the 13C proÐle in the pocket. A second small neutron burst from the 22Ne source operates during convective pulses over previously s-processed material diluted with fresh Fe seeds and H-burning ashes. The main features of the Ðnal s-process abundance distribution in the material cumulatively mixed with the envelope through the various third dredge-up episodes are discussed. Contrary to current expectations, the distribution cannot be approximated by a simple exponential law of neutron irradiations. The s-process nucleosynthesis mostly occurs inside the 13C pocket ; the form of the distribution is built through the interplay of the s-processing occurring in the intershell zones and the geometrical overlap of di †erent pulses.The 13C pocket is of primary origin, resulting from proton captures on newly synthesized 12C. Consequently, the s-process nucleosynthesis also depends on Fe seeds, a lower metallicity favoring the production of the heaviest elements. This allows a wide range of s-element abundance distributions to be produced in AGB stars of di †erent metallicities, in agreement with spectroscopic evidence and with the Galactic enrichment of the heavy s-elements at the time of formation of the solar system. AGB stars of metallicity are the best candidates for the buildup of the main component, i.e., for the s-Z^1 2 Z _ distribution of the heavy elements from the Sr-Y-Zr peak up to the Pb peak, as deduced by meteoritic and solar spectroscopic analyses. A number of AGB stars may actually show in their envelopes an sprocess abundance distribution almost identical to that of the main component. Eventually, the astrophysical origin of mainstream circumstel...

New models of thermally pulsing asymptotic giant branch (TP-AGB) stars of low mass and solar chemical composition are presented, namely, Z \ 0.02, and Y \ 0.28. The inÑuence of 1 ¹ M/M _ ¹ 3, various parameters (such as the initial core mass, the envelope mass, the mass-loss rate, the opacity, and the mixing length) on the properties of the models is discussed in detail. Our main Ðndings are the following :1. The third dredge-up (TDU) operates self-consistently (using the Schwarzschild criterion for convection and without invoking any extra-mixing) for masses as low as 1.5The minimum core mass for M _ . which TDU is found is This value is attained after about 10 thermal pulses, almost M H D 0.61 M _ . independently of the initial mass.2. During the early TP-AGB evolution, the relation between the pulse strength (i.e., the luminosity peak of the 3a burning during the pulse) and the core mass is in good agreement with previous Ðndings. However, when TDU is settled on, the strength of the pulse increases more rapidly as the penetration of the convective envelope into the He intershell increases. No asymptotic limit is found.3. Furthermore, the 3a luminosity peak is independent of the previous history : the strength of the pulse in a model with mass loss is the same as in a model without mass loss but having the same core and envelope masses.4. Unless extreme mass-loss rates are assumed, carbon stars are obtained in all the sequences of models with initial mass M º 1.5 after about 24-26 thermal pulses and 15-17 TDU episodes. At M _ C-star formation, the core mass is less than 0.7 and the luminosity is of the order of 104 The M _ , L _ . dredged-up mass increases up to a maximum and then decreases as mass loss and/or the advancement of the H-burning shell consume the envelope. When the envelope mass is reduced below approximately 0.5 TDU eventually vanishes. M _ , 5. If some amount of protons is di †used below the base of the H-rich envelope during TDU, in the interpulse a 13C-pocket is formed and then burnt radiatively via the 13C(a, n)16O reaction, before the onset of a new pulse. Thus, s-process nucleosynthesis occurs in a radiative environment characterized by a fairly low neutron density. In advanced thermal pulses, when the temperature at the bottom of the convective shell approaches 3 ] 108 K, a secondary source of neutrons comes from the marginal activation of the 22Ne(a, n)25Mg reaction.

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