The envelope of thermally pulsing asymptotic giant branch (TP-AGB) stars undergoing periodic third dredge-up (TDU) episodes is enriched in both light and heavy elements, the ashes of a complex internal nucleosynthesis involving p, α, and n captures over hundreds of stable and unstable isotopes. In this paper, new models of lowmass AGB stars (2 M ), with metallicity ranging between Z = 0.0138 (the solar one) and Z = 0.0001, are presented. Main features are (1) a full nuclear network (from H to Bi) coupled to the stellar evolution code, (2) a mass loss-period-luminosity relation, based on available data for long-period variables, and (3) molecular and atomic opacities for C-and/or N-enhanced mixtures, appropriate for the chemical modifications of the envelope caused by the TDU. For each model, a detailed description of the physical and chemical evolutions is presented; moreover, we present a uniform set of yields, comprehensive of all chemical species (from hydrogen to bismuth). The main nucleosynthesis site is the thin 13 C pocket, which forms in the core-envelope transition region after each TDU episode. The formation of this 13 C pocket is the principal by-product of the introduction of a new algorithm, which shapes the velocity profile of convective elements at the inner border of the convective envelope: both the physical grounds and the calibration of the algorithm are discussed in detail. We find that the pockets shrink (in mass) as the star climbs the AGB, so that the first pockets, the largest ones, leave the major imprint on the overall nucleosynthesis. Neutrons are released by the 13 C(α, n) 16 O reaction during the interpulse phase in radiative conditions, when temperatures within the pockets attain T ∼ 1.0 × 10 8 K, with typical densities of (10 6 -10 7 ) neutrons cm −3 . Exceptions are found, as in the case of the first pocket of the metal-rich models (Z = 0.0138, Z = 0.006 and Z = 0.003), where the 13 C is only partially burned during the interpulse: the surviving part is ingested in the convective zone generated by the subsequent thermal pulse (TP) and then burned at T ∼ 1.5 × 10 8 K, thus producing larger neutron densities (up to 10 11 neutrons cm −3 ). An additional neutron exposure, caused by the 22 Ne(α, n) 25 Mg during the TPs, is marginally activated at large Z, but becomes an important nucleosynthesis source at low Z, when most of the 22 Ne is primary. The final surface compositions of the various models reflect the differences in the initial iron-seed content and in the physical structure of AGB stars belonging to different stellar populations. Thus, at large metallicities the nucleosynthesis of light s-elements (Sr, Y, Zr) is favored, whilst, decreasing the iron content, the overproduction of heavy s-elements (Ba, La, Ce, Nd, Sm) and lead becomes progressively more important. At low metallicities (Z = 0.0001) the main product is lead. The agreement with the observed [hs/ls] index observed in intrinsic C stars at different [Fe/H] is generally good. For the solar metallicity model, w...

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 new set of models for intermediate mass AGB stars (4.0, 5.0 and, 6.0 M ⊙ ) at different metallicities (-2.15≤[Fe/H]≤+0.15). This integrates the existing set of models for low mass AGB stars (1.3≤M/M ⊙ ≤3.0) already included in the FRUITY database. We describe the physical and chemical evolution of the computed models from the Main Sequence up to the end of the AGB phase. Due to less efficient third dredge up episodes, models with large core masses show modest surface enhancements. The latter is due to the fact that the interpulse phases are short and, then, Thermal Pulses are weak. Moreover, the high temperature at the base of the convective envelope prevents it to deeply penetrate the radiative underlying layers. Depending on the initial stellar mass, the heavy elements nucleosynthesis is dominated by different neutron sources. In particular, the s-process distributions of the more massive models are dominated by the 22 Ne(α,n) 25 Mg reaction, which is efficiently activated during Thermal Pulses. At low metallicities, our models undergo hot bottom burning and hot third dredge up. We compare our theoretical final core masses to available white dwarf observations. Moreover, we quantify the weight that intermediate mass models have on the carbon stars luminosity function. Finally, we present the upgrade of the FRUITY web interface, now also including the physical quantities of the TP-AGB phase of all the models included in the database (ph-FRUITY).Subject headings: Stars: AGB and post-AGB -Physical data and processes: Nuclear reactions, nucleosynthesis, abundances found by varying the metallicity and the initial stellar mass. In fact, the three s-process peaks 2 receive different contributions depending on the physical environmental conditions (radiative or convective burning) and on the neutron-to-seed ratio (which is related to the metallicity).In this paper we also illustrate a new web interface (ph-FRUITY), to access tables containing the evolution of the most relevant physical quantities of our models. This paper is structured as follows. In §2 we describe the main features of our stellar evolutionary code, focusing on the most recent upgrades. In §3 we highlight the evolutionary phases prior to the AGB, which is analyzed in §4. In §5 we show the potentiality of our new web ph-FRUITY interface. The nucleosynthesis of all FRUITY models is discussed in detail in §6. Finally, in §7we report the discussion and our conclusions. The modelsAs already outlined, models presented in this paper (4.0-5.0-6.0 M ⊙ ) integrate the already existing set available on the FRUITY database (Cristallo et al. 2011), currently hosting Low Mass Stars AGB models (hereafter LMS-AGB; 1.3-1.5-2.0-2.5-3.0) with different initial metallicities (-2.15≤[Fe/H]≤+0.15). We add a further metallicity (Z = 0.002, corresponding to [Fe/H]=-0.85)in order to better sample the peak in the lead production (see below). In Table 1 we report all the models included in the FRUITY database (in bold the models added with this work), by specifying...

We present a comprehensive theoretical investigation of the evolutionary properties of intermediate-mass stars. The evolutionary sequences were computed from the Zero Age Main Sequence up to the central He exhaustion and often up to the phases which precede the carbon ignition or to the reignition of the H-shell which marks the beginning of the thermal pulse phase. The evolutionary tracks were constructed by adopting a wide range of stellar masses (3 ≤M/M ⊙ ≤ 15) and chemical compositions. In order to account for current uncertainties on the He to heavy elements enrichment ratio (∆Y /∆Z ), the stellar models were computed by adopting at Z=0.02 two different He contents (Y=0.27, 0.289) and at Z=0.04 three different He contents (Y=0.29, 0.34, and 0.37). Moreover, to supply a homogeneous evolutionary scenario which accounts for young Magellanic stellar systems the calculations were also extended toward lower metallicities (Z=0.004, Z=0.01), by adopting different initial He abundances.We evaluated for both solar (Z=0.02) and super-metal-rich (SMR, Z=0.04) models the transition mass M up between the stellar structures igniting carbon and those which develop a full electron degeneracy inside the carbon-oxygen core. We found that M up is of the order of 7.7 ± 0.5M ⊙ for solar composition, while for SMR structures an increase in the He content causes a decrease in M up , and indeed it changes from 9.5 ± 0.5M ⊙ at Y=0.29, to 8.7 ± 0.2M ⊙ at Y=0.34, and to 7.7 ± 0.2M ⊙ at Y=0.37. We also show that M up presents a nonlinear behavior with metallicity, and indeed it decreases when moving from Z=0.04 to Z ≈ 0.001 and increases at lower metal contents. This finding confirms the predictions by Cassisi & Castellani (1993) and more recently by Umeda et al. (1999) and suggests that the rate of SNe type Ia depends on the 3 chemical composition of the parent stellar population.This approach allows us to investigate in detail the evolutionary properties of classical Cepheids. In particular, we find that the range of stellar masses which perform the blue loop during the central He-burning phase narrows when moving toward metal-rich and SMR structures. This evidence and the substantial decrease in the evolutionary time spent by these structures inside the instability strip bring out that the probability to detect long-period Cepheids in SMR stellar systems is substantially smaller than in more metal-poor systems.Moreover and even more importantly, we find that the time spent by Cepheids along the subsequent crossings of the instability strip also depends on the stellar mass. In fact, our models suggest that low-mass, metal-poor Cepheids spend a substantial portion of their lifetime along the blueward excursion of the blue loop, while at higher masses (M/M ⊙ ≥ 8) the time spent along the redward excursion becomes longer. Models at solar chemical composition present an opposite behavior i.e. the time spent along the redward excursion is longer than the blueward excursion among low-mass Cepheids and vice versa for high-massCepheids. Oddly ...

In this paper we analyze the effects induced by rotation on low mass Asymptotic Giant Branch stars. We compute two sets of models, M=2.0 M ⊙ at [Fe/H]=0 and M=1.5 M ⊙ at [Fe/H]=-1.7, respectively, by adopting Main Sequence rotation velocities in the range 0÷120 km/s. At high metallicity, we find that the Goldreich-Schubert-Fricke instability, active at the interface between the convective envelope and the rapid rotating core, contaminates the 13 C-pocket (the major neutron source) with 14 N (the major neutron poison), thus reducing the neutron flux available for the synthesis of heavy elements. As a consequence, the yields of heavy-s elements (Ba, La, Nd, Sm) and, to a less extent, those of light-s elements (Sr, Y, Zr) decrease with increasing rotation velocities up to 60 km/s. However, for larger initial rotation velocities, the production of light-s and, to a less extent, that of heavy-s begins again to increase, due to mixing induced by meridional circulations. At low metallicity, the effects of meridional circulations are important even at rather low rotation velocity. The combined effect of Goldreich-Schubert-Fricke instability and meridional circulations determines an increase of light-s and, to a less extent, heavy-s elements, while lead is strongly reduced. For both metallicities, the rotation-induced instabilities active during the interpulse phase reduce the neutrons-to-seeds ratio, so that the spectroscopic indexes [hs/ls] and [Pb/hs] decrease by increasing the initial rotation velocity. Our analysis suggests that rotation could explain the spread in the s-process indexes, as observed in s-process enriched stars at different metallicities.

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Pulsating white dwarfs provide constraints to the evolution of progenitor stars. We revise He-burning stellar models, with particular attention to core convection and to its connection with the nuclear reactions powering energy generation and chemical evolution. Theoretical results are compared to the available measurements for the variable white dwarf GD 358, which indicate a rather large abundance of central oxygen (Metcalfe and coworkers). We show that the attempt to constrain the relevant nuclear reaction rate by means of the white dwarf composition is faced with a large degree of uncertainty related to evaluating the efficiency of convection-induced mixing. By combining the uncertainty of the convection theory with the error on the relevant reaction rate, we derive that the present theoretical prediction for the central oxygen mass fraction in white dwarfs varies between 0.3 and 0.9. Unlike previous claims, we find that models taking into account semiconvection and a moderate C-12(alpha,gamma)O-16 reaction rate are able to account for a high central oxygen abundance. The rate of the C-12(alpha,gamma)O-16 used in these models agrees with the one recently obtained in laboratory experiments by Kunz and coworkers. On the other hand, when semiconvection is inhibited, as in the case of classical models (bare Schwarzschild criterion) or in models with mechanical overshoot, an extremely high rate of the C-12(alpha,gamma)O-16 reaction is needed to account for a large oxygen production. Finally, we show that the apparent discrepancy between our result and those reported in previous studies depends on the method used to avoid the convective runaways (the so-called breathing pulses) that are usually encountered in modeling late stage of core He-burning phase

In this paper we present the evolution of a low mass model (initial mass M =1.5 M⊙) with a very low metal content (Z = 5 × 10 −5 , equivalent to [Fe/H]= −2.44). We find that, at the beginning of the AGB phase, protons are ingested from the envelope in the underlying convective shell generated by the first fully developed thermal pulse. This peculiar phase is followed by a deep third dredge up episode, which carries to the surface the freshly synthesized 13 C, 14 N and 7 Li. A standard TP-AGB evolution, then, follows. During the proton ingestion phase, a very high neutron density is attained and the s-process is efficiently activated. We therefore adopt a nuclear network of about 700 isotopes, linked by more than 1200 reactions, and we couple it with the physical evolution of the model. We discuss in detail the evolution of the surface chemical composition, starting from the proton ingestion up to the end of the TP-AGB phase.