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 present a comprehensive study of the abundance evolution of the elements from H to U in the Milky Way halo and local disk. We use a consistent chemical evolution model, metallicity dependent isotopic yields from low and intermediate mass stars and yields from massive stars which include, for the first time, the combined effect of metallicity, mass loss and rotation for a large grid of stellar masses and for all stages of stellar evolution. The yields of massive stars are weighted by a metallicity dependent function of the rotational velocities, constrained by observations as to obtain a primary-like 14 N behavior at low metallicity and to avoid overproduction of s-elements at intermediate metallicities. We show that the solar system isotopic composition can be reproduced to better than a factor of two for isotopes up to the Fe-peak, and at the 10% level for most pure s-isotopes, both light ones (resulting from the weak s-process in rotating massive stars) and the heavy ones (resulting from the main s-process in low and intermediate mass stars). We conclude that the light element primary process (LEPP), invoked to explain the apparent abundance deficiency of the s-elements with A< 100, is not necessary. We also reproduce the evolution of the heavy to light s-elements abundance ratio ([hs/ls]) -recently observed in unevolved thin disk stars -as a result of the contribution of rotating massive stars at sub-solar metallicities. We find that those stars produce primary F and dominate its solar abundance and we confirm their role in the observed primary behavior of N. In contrast, we show that their action is insufficient to explain the small observed values of 12 C/ 13 C in halo red giants, which is rather due to internal processes in those stars.

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 -

The decomposition of the Solar system abundances of heavy isotopes into their sand r-components plays a key role in our understanding of the corresponding nuclear processes and the physics and evolution of their astrophysical sites. We present a new method for determining the s-and r-components of the Solar system abundances, fully consistent with our current understanding of stellar nucleosynthesis and galactic chemical evolution. The method is based on a study of the evolution of the solar neighborhood with a state-of-the-art 1-zone model, using recent yields of low and intermediate mass stars as well as of massive rotating stars. We compare our results with previous studies and we provide tables with the isotopic and elemental contributions of the s-and r-processes to the Solar system composition.

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Abundances of lithium, heavy elements and carbon isotope ratios have been measured in 12 J-type galactic carbon stars. The abundance analysis shows that in these stars the abundances of s-process elements with respect to the metallicity are nearly normal. Tc is not present in most of them, although upper limits have been derived for WZ Cas and WX Cyg, perhaps two SC-type rather than J-type carbon stars. The Rb abundances, obtained from the resonance 7800Å Rb I line, are surprisingly low, probably due to strong non-LTE effects in the formation of this line in cool carbon-rich stars. Lithium and 13 C are found to be enhanced in all the stars. These results together with the nitrogen abundances and oxygen isotope ratios measured by Lambert et al. (1986) and Harris et al. (1987) are used to discuss the origin of J-stars. The luminosity and variability class of the stars studied would indicate that they are low mass (M 2 − 3 M ⊙ ), less evolved objects than the normal carbon stars, although the presence of some luminous (M bol < −5.5) J-stars in our galaxy (WZ Cas may be an example) and in other galaxies, suggests the existence of at least two types of J-stars, with different formation scenarios depending upon the initial mass of the parent star. Standard evolutionary AGB models are difficult to reconcile with all the observed chemical characteristics. In fact, they suggest the existence of an extra-mixing mechanism which transports material from the convective envelope down to hotter regions where some nuclear burning occurs. This mechanism would act preferably on the early-AGB phase in low-mass stars. Mixing at the He-core flash and the binary system hypothesis are also discussed as alternatives to the above scenario. Subject headings: stars: abundances -stars: carbon -stars: evolutionnucleosynthesis 12 C/ 13 C ratios can be obtained in current AGB star models of M≥ 4 M ⊙ if hot H-burning takes place at the bottom of the convective envelope (the so-called hot bottom burning, HBB)(Lattanzio 1999; Sackmann & Boothroyd 1992). However, the performance of the CN-cycle at the same time destroys 12 C and, in consequence, the C/O ratio in the envelope is reduced and the star again becomes O-rich. Thus, a fine-tuning of the parameters of the AGB models (mass, mixing-length, mass-loss rate, metallicity, etc.), that determine the chemistry of the envelope, seems to be required to obtain a J-star. Mixing at the He-core flash has also been proposed as an alternative scenario to form J-stars. In this event an -4injection of carbon-rich material from the core into the hydrogen-rich shell may occur. The introduction of core material ( 12 C and 4 He) into a proton-rich region yields enhanced 12 C and 13 C, with perhaps a small enhancement of 14 N (Deupree & Cole 1983). The presence of strong λ6708Å Li I lines is frequent in J-stars. In a Li survey of galactic C-stars, Boffin et al. (1993) found that among 30 Li-rich stars in a sample of 250 C-stars ∼ 50% are of J-type. This figure increases up to ∼ 70% if the Li-rich phenomenon is c...

Extant chemical evolution models underestimate the Galactic production of Sr, Y and Zr as well as the Solar System abundances of s-only isotopes with 904.0 M ⊙ ) are negligible; 4) the inclusion of rotation implies a downward shift of the whole distribution with an higher efficiency for the heavy s-only isotopes, leading to a flatter s-only distribution; 5) different prescriptions on convection or mass-loss produce nearly rigid shifts of the whole distribution.In summary, a variation of the standard paradigm of AGB nucleosynthesis would allow to reconcile models predictions with Solar System s-only abundances. Nonetheless, the LEPP cannot be definitely ruled out, because of the uncertainties still affecting stellar and Galactic chemical evolution models.