By using updated stellar low mass stars models, we can systematically investigate the nucleosynthesis processes occurring in AGB stars, when these objects experience recurrent thermal pulses and third dredge-up episodes. In this paper we present the database dedicated to the nucleosynthesis of AGB stars: the FRUITY (FRANEC Repository of Updated Isotopic Tables & Yields) database.An interactive web-based interface allows users to freely download the full (from H to Bi) isotopic composition, as it changes after each third dredge-up episode and the stellar yields the models produce. A first set of AGB models, having masses in the range 1.5 ≤ M/M ⊙ ≤ 3.0 and metallicities 1 × 10 −3 ≤ Z ≤ 2 × 10 −2 , is discussed here. For each model, a detailed description of the physical and the chemical evolution is provided. In particular, we illustrate the details of the s-process and we evaluate the theoretical uncertainties due to the parametrization adopted to model convection and mass loss. The resulting nucleosynthesis scenario is checked by comparing the theoretical [hs/ls] and [Pb/hs] ratios to those obtained from the available abundance analysis of s-enhanced stars. On the average, the variation with the metallicity of these spectroscopic indexes is well reproduced by theoretical models, although the predicted spread at a given metallicity is substantially smaller than the observed one. Possible explanations for such a difference are briefly discussed. An independent check of the third dredge-up efficiency is provided by the C-stars luminosity function. Consequently, theoretical C-stars luminosity functions for the Galactic disk and the Magellanic Clouds have been derived. We generally find a good agreement with observations. Tables & Yields), which is available on the web pages of the Teramo Observatory (INAF) 1 .This database has been organized under a relational model through the MySQL Database Management System. This software links input data to logical indexes, optimizing their arrangement and speeding up the response time to the user query. Its web interface has been developed through a set of Perl 2 scripts, which allow to submit the query strings resulting from filling out appropriate fields to the managing system. It contains our predictions for the surface composition of AGB stars undergoing TDU episodes and the stellar yields they produce. Tables for AGB models having initial masses 1.5 ≤ M/M ⊙ ≤ 3.0 and 1 × 10 −3 ≤ Z ≤ 2 × 10 −2 are available. FRUITY will be expanded soon by including AGB models with larger initial mass and/or lower Z.In Sections 2 and 3 of the present paper we describe the stellar models and the related nucleosynthesis results. In Section 4 we address the main uncertainties affecting our models while comparisons with available photometric and spectroscopic data are discussed in Section 6. Conclusions are drawn in Section 7. The FRANEC codeThe stellar models of the FRUITY database have been obtained by means of the FRANEC code (Frascati RAphson-Newton Evolutionary Code -Chieffi et al. 1998). T...

We derive a strong bound on the axion-photon coupling g(aγ) 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(aγ). We show that the estimated value of g(aγ) 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 O/H in the same range of the Galactic Globular Clusters, we obtain an upper bound g(aγ)<0.66×10(-10)  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.

We present the first detailed and homogeneous analysis of the s-element content in Galactic carbon stars of N-type. Abundances of Sr,Y, Zr (low-mass selements, or ls) and of Ba, La, Nd, Sm and Ce (high-mass s-elements, hs) are derived using the spectral synthesis technique from high-resolution spectra. The N-stars analyzed are of nearly solar metallicity and show moderate s-element enhancements, similar to those found in S stars, but smaller than those found in the only previous similar study (Utsumi 1985), and also smaller than those found in supergiant post-AGB stars. This is in agreement with the present understanding of the envelope s-element enrichment in giant stars, which is increasing along the spectral sequence M→MS→S→SC→C during the AGB phase. We compare the observational data with recent s-process nucleosynthesis models for different metallicities and stellar masses. Good agreement is obtained between low mass AGB star models (M 3M ⊙ ) and s-elements observations. In low mass AGB stars, the 13 C(α, n) 16 O reaction is the main source of neutrons for the s-process; a moderate spread, however, must exist in the abundance of 13 C that is burnt in different stars. By combining information deriving from the detection of Tc, the infrared colours and the theoretical relations between stellar mass, metallicity and the final C/O ratio, we conclude that most (or maybe all) of the N-stars studied in this work are intrinsic, thermally-pulsing AGB stars; their abundances are the consequence of the operation of third dredge-up and are not to be ascribed to mass transfer in binary systems.

We present new spectroscopic observations for a sample of C(N)-type red giants. These objects belong to the class of Asymptotic Giant Branch stars, experiencing thermal instabilities in the He-burning shell (thermal pulses). Mixing episodes called third dredge-up enrich the photosphere with newly synthesized 12 C in the He-rich zone, and this is the source of the high observed ratio between carbon and oxygen (C/O ≥ 1 by number). Our spectroscopic abundance estimates confirm that, in agreement with the general understanding of the late evolutionary stages of low and intermediate mass stars, carbon enrichment is accompanied by the appearance of s-process elements in the photosphere. We discuss the details of the observations and of the derived abundances, focusing in particular on rubidium, a neutron-density sensitive element, and on the s-elements Sr, Y and Zr belonging to the first s-peak. The critical reaction branching at 85 Kr, which determines the relative enrichment of the studied species, is discussed.Subsequently, we compare our data with recent models for s-processing in Thermally Pulsing Asymptotic Giant Branch stars, at metallicities relevant for our sample. A remarkable agreement between model predictions and observations is found. Thanks to the different neutron density prevailing in low and intermediate mass stars, comparison with the models allows us to conclude that most C(N) stars are of low mass (M 3 M ⊙ ). We also analyze the 12 C/ 13 C ratios measured, showing that most of them cannot be explained by canonical stellar models. We discuss how this fact would require the operation of an ad hoc additional mixing, currently called Cool Bottom Process, operating only in low mass stars during the first ascent of the red giant branch and, perhaps, also during the asymptotic giant branch.-3 -

A new set of stellar models in the mass range 1.2 to 9 M ⊙ is presented. The adopted chemical compositions cover the typical galactic values, namely 0.0001 ≤ Z ≤ 0.02 and 0.23 ≤ Y ≤ 0.28. A comparison among the most recent compilations of similar stellar models is also discussed. The main conclusion is that the differencies among the various evolutionary results are still rather large.For example, we found that the H-burning evolutionary time may differ up to 20%. An even larger disagreement is found for the He-burning phase (up to 40-50 %). Since the connection between the various input physics and the numerical algorithms could amplify or counterbalance the effect of a single ingredient on the resulting stellar model, the origin of this discrepancies is not evident. However most of these discrepancies, which are clearly found in the evolutionary tracks, are reduced on the isochrones. By means of our updated models we show that the ages inferred by the theory of stellar evolution is in excellent agreement with those obtained by using other independent methods applied to the nearby Open Clusters. Finally, the theoretical initial/final mass relation is revised.