The recently improved information on the stellar (n,gamma) cross sections of neutron-magic nuclei at N = 82, and in particular of 142Nd, 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. 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 13C source.Comment: 49 pagg., 5 figg., accepted for publication by Ap

We derive a strong bound on the axion-photon coupling $g_{a\gamma }$ from the analysis of a sample of 39 Galactic Globular Clusters. As recognized long ago, the $R$ parameter, i.e., the number ratio of stars in horizontal over red giant branch of old stellar clusters, would be reduced by the axion production from photon conversions occurring in stellar cores. In this regard, we have compared the measured $R$ with state-of-the-art stellar models obtained under different assumptions for $g_{a\gamma }$. We show that the estimated value of $g_{a\gamma }$ substantially depends on the adopted He mass fraction $Y$, an effect often neglected in previous investigations. Taking as a benchmark for our study the most recent determinations of the He abundance in H ii regions with $\mathrm{O}/\mathrm{H}$ in the same range of the Galactic Globular Clusters, we obtain an upper bound $g_{a\gamma }<0.66\times {10}^{-10}{\mathrm{GeV}}^{-1}$ at 95% confidence level. This result significantly improves the constraints from previous analyses and is currently the strongest limit on the axion-photon coupling in a wide mass range.

High dispersion spectra () 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 and 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 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.