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...

Abstract. High dispersion spectra (R ∼ > 40 000) for a quite large number of stars at the main sequence turn-off and at the base of the giant branch in NGC 6397 and NGC 6752 were obtained with the UVES on Kueyen (VLT UT2). The [Fe/H] values we found are −2.03 ± 0.02 ± 0.04 and −1.42 ± 0.02 ± 0.04 for NGC 6397 and NGC 6752 respectively, where the first error bars refer to internal and the second ones to systematic errors (within the abundance scale defined by our analysis of 25 subdwarfs with good Hipparcos parallaxes). In both clusters the [Fe/H]'s obtained for TO-stars agree perfectly (within a few percent) with that obtained for stars at the base of the RGB. The [O/Fe] = 0.21 ± 0.05 value we obtain for NGC 6397 is quite low, but it agrees with previous results obtained for giants in this cluster. Moreover, the star-to-star scatter in both O and Fe is very small, indicating that this small mass cluster is chemically very homogenous. On the other hand, our results show clearly and for the first time that the O-Na anticorrelation (up to now seen only for stars on the red giant branches of globular clusters) is present among unevolved stars in the globular cluster NGC 6752, a more massive cluster than NGC 6397. A similar anticorrelation is present also for Mg and Al, and C and N. It is very difficult to explain the observed Na-O, and Mg-Al anticorrelation in NGC 6752 stars by a deep mixing scenario; we think it requires some non internal mechanism.

We present the 26 Al and 60 Fe yields produced by a generation of solar metallicity stars ranging in mass between 11 and 120 M . We discuss the production sites of these -ray emitters and quantify the relative contributions of the various components. We provide the contributions of the wind, the C convective shell, and the explosive Ne/C burning to the total 26 Al yield together with the contributions of the He convective shell, the C convective shell, and the explosive Ne/C burning to the 60 Fe yield. We conclude that, at variance with current beliefs, 26 Al is mainly produced by the explosive C/Ne burning over most of the mass interval analyzed here, while 60 Fe is mainly produced by the C convective shell and the He convective shell. By means of these yields we try to reproduce two quite strong observational constraints related to the abundances of these nuclei in the interstellar medium, i.e., the number of 1.8 photons per Lyman continuum photon, R GxL , and the 60 Fe/ 26 Al -ray line flux ratio. R GxL is found to be roughly constant along the Galactic plane (and of the order of 1:25 ; 10 À11 ), while the 60 Fe/ 26 Al ratio has been recently measured by both RHESSI (0:17 AE 0:05) and SPI (INTEGRAL) (0:11 AE 0:03). We can quite successfully fit simultaneously both ratios for a quite large range of exponents of the power-law initial mass function. We also address the fit to 2 Velorum, and we find that a quite large range of initial masses, at least from 40 to 60 M , do eject an amount of 26 Al (through the wind) compatible with the current upper limit quoted for this W-R star: such a result removes a long-standing discrepancy between the models and the observational data.