Nucleosynthesis in the s process takes place in the He burning layers of low mass AGB stars and during the He and C burning phases of massive stars. The s process contributes about half of the element abundances between Cu and Bi in solar system material. Depending on stellar mass and metallicity the resulting s-abundance patterns exhibit characteristic features, which provide comprehensive information for our understanding of the stellar life cycle and for the chemical evolution of galaxies. The rapidly growing body of detailed abundance observations, in particular for AGB and post-AGB stars, for objects in binary systems, and for the very faint metal-poor population represents exciting challenges and constraints for stellar model calculations. Based on updated and improved nuclear physics data for the s-process reaction network, current models are aiming at ab initio solution for the stellar physics related to convection and mixing processes. Progress in the intimately related areas of observations, nuclear and atomic physics, and stellar modeling is reviewed and the corresponding interplay is illustrated by the general abundance patterns of the elements beyond iron and by the effect of sensitive branching points along the sprocess path. The strong variations of the s-process efficiency with metallicity bear also interesting consequences for Galactic chemical evolution.

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 study the s-process abundances (A 90) at the epoch of the solar system formation. Asymptotic giant branch yields are computed with an updated neutron capture network and updated initial solar abundances. We confirm our previous results obtained with a Galactic chemical evolution (GCE) model: (1) as suggested by the s-process spread observed in disk stars and in presolar meteoritic SiC grains, a weighted average of s-process strengths is needed to reproduce the solar s distribution of isotopes with A > 130; and (2) an additional contribution (of about 25%) is required in order to represent the solar s-process abundances of isotopes from A = 90 to 130. Furthermore, we investigate the effect of different internal structures of the 13 C pocket, which may affect the efficiency of the 13 C(α, n)16 O reaction, the major neutron source of the s process. First, keeping the same 13 C profile adopted so far, we modify by a factor of two the mass involved in the pocket; second, we assume a flat 13 C profile in the pocket, and we test again the effects of the variation of the mass of the pocket. We find that GCE s predictions at the epoch of the solar system formation marginally depend on the size and shape of the 13 C pocket once a different weighted range of 13 C-pocket strengths is assumed. We obtain that, independently of the internal structure of the 13 C pocket, the missing solar system s-process contribution in the range from A = 90 to 130 remains essentially the same.

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Employing high-resolution spectra obtained with the near-UV-sensitive detector on the Keck I HIRES, supplemented by data obtained with the McDonald Observatory 2d-coudé, we have performed a comprehensive chemical composition analysis of the bright r-process-rich metal-poor red giant star HD 221170. Analysis of 57 individual neutral and ionized species yielded abundances for a total of 46 elements and significant upper limits for an additional five. Model stellar atmosphere parameters were derived with the aid of $200 Fe peak transitions. From more than 350 transitions of 35 neutron-capture (Z > 30) species, abundances for 30 neutron-capture elements and upper limits for three others were derived. Utilizing 36 transitions of La, 16 of Eu, and seven of Th, we derive ratios of log (Th / La) ¼ À0:73 (¼ 0:06) and log (Th /Eu) ¼ À0:60 (¼ 0:05), values in excellent agreement with those previously derived for other r-process-rich metal-poor stars such as CS 22892À052, BD +17 3248, and HD 115444. Based on the Th/Eu chronometer, the inferred age is 11:7 AE 2:8 Gyr. The abundance distribution of the heavier neutron-capture elements (Z ! 56) is fitted well by the predicted scaled solar system r-process abundances, as also seen in other r-process-rich stars. Unlike other r-process-rich stars, however, we find that the abundances of the lighter neutron-capture elements (37 < Z < 56) in HD 221170 are also in agreement with the abundances predicted for the scaled solar r-process pattern.

We provide an individual analysis of 94 carbon-enhanced metal-poor stars showing an sprocess enrichment (CEMP-s) collected from the literature. The s-process enhancement observed in these stars is ascribed to mass transfer by stellar winds in a binary system from a more massive companion evolving faster towards the asymptotic giant branch (AGB) phase. The theoretical AGB nucleosynthesis models have been presented in Bisterzo et al. (Paper I of this series). Several CEMP-s show an enhancement in both s-and r-process elements (CEMP-s/r). In order to explain the peculiar abundances observed in CEMP-s/r, we assume that the molecular cloud from which CEMP-s formed was previously enriched in r-elements by supernova pollution.A general discussion and the method adopted in order to interpret the observations have been provided in Bisterzo et al. (Paper II of this series). We present in this paper a detailed study of spectroscopic observations of individual stars. We consider all elements from carbon to bismuth, with particular attention to the three s-process peaks, ls (Y, Zr), hs (La, Nd, Sm) and Pb, and their ratios [hs/ls] and [Pb/hs]. The presence of an initial r-process contribution may be typically evaluated by [La/Eu]. We found possible agreements between theoretical predictions and spectroscopic data. In general, the observed [Na/Fe] (and [Mg/Fe]) provides information on the AGB initial mass, while [hs/ls] and [Pb/hs] are mainly indicators of the s-process efficiency. A range of 13 C-pocket strengths are required to interpret the observations. However, major discrepancies between models and observations exist. We highlight star by star the agreements and the main problems encountered and, when possible, we suggest potential indications for further studies. These discrepancies provide starting points of debate for unsolved problems in which spectroscopic and theoretical studies may intervene.

A large sample of carbon‐enhanced metal‐poor stars enriched in s‐process elements (CEMP‐s) have been observed in the Galactic halo. These stars of low mass (M∼ 0.9 M⊙) are located on the main‐sequence or the red‐giant phase, and do not undergo third dredge‐up (TDU) episodes. The s‐process enhancement is most plausibly due to accretion in a binary system from a more massive companion when on the asymptotic giant branch (AGB) phase (now a white dwarf). In order to interpret the spectroscopic observations, updated AGB models are needed to follow in detail the s‐process nucleosynthesis. We present nucleosynthesis calculations based on AGB stellar models obtained with Frascati Raphson‐Newton Evolutionary Code (franec) for low initial stellar masses and low metallicities. For a given metallicity, a wide spread in the abundance of the s‐process elements is obtained by varying the amount of 13C and its profile in the pocket, where the 13C(α, n)16O reaction is the major neutron source, releasing neutrons in radiative conditions during the interpulse phase. We also account for the second neutron source 22Ne(α, n)25Mg, partially activated during convective thermal pulses. We discuss the surface abundance of elements from carbon to bismuth, for AGB models of initial masses M= 1.3–2 M⊙, low metallicities ([Fe/H] from −1 down to −3.6) and for different 13C‐pocket efficiencies. In particular, we analyse the relative behaviour of the three s‐process peaks: light‐s (ls at magic neutron number N= 50), heavy‐s (hs at N= 82) and lead (N= 126). Two s‐process indicators, [hs/ls] and [Pb/hs], are needed in order to characterize the s‐process distribution. In the on‐line material, we provide a set of data tables with surface predictions. Our final objective is to provide a full set of theoretical models of low‐mass low‐metallicity s‐process‐enhanced stars. In a forthcoming paper, we will test our results through a comparison with observations of CEMP‐s stars.

Abstract. We provide an updated discussion of the sample of CEMP-s and CEMP-s/r stars collected from the literature. Observations are compared with the theoretical nucleosynthesis models of asymptotic giant branch (AGB) stars presented by Bisterzo et al. (2010Bisterzo et al. ( , 2011Bisterzo et al. ( , 2012, in the light of the most recent spectroscopic results.