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This paper provides a detailed analysis of the main component of the slow neutron capture process (the s-process), which accounts for the solar abundances of half of the nuclei with 90 ≥ A ≥ 208. We examine the impact of the uncertainties of the two neutron sources operating in low-mass asymptotic giant branch (AGB) stars: the 13C(α, n)16O reaction, which releases neutrons radiatively during interpulse periods (kT ~ 8 keV), and the 22Ne(α, n)25Mg reaction, partially activated during the convective thermal pulses (TPs). We focus our attention on the branching points that mainly influence the abundance of s-only isotopes. In our AGB models, the 13C is fully consumed radiatively during interpulse. In this case, we find that the present uncertainty associated with the 13C(α, n)16O reaction has marginal effects on s-only nuclei. On the other hand, a reduction of this rate may increase the amount of residual (or unburned) 13C at the end of the interpulse: in this condition, the residual 13C is burned at higher temperature in the convective zone powered by the following TP. The neutron burst produced by the 22Ne(α, n)25Mg reaction has major effects on the branches along the s-path. The contributions of s-only isotopes with 90 ≥ A ≤ 204 are reproduced within solar and nuclear uncertainties, even if the 22Ne(α, n)25Mg rate is varied by a factor of 2. Improved β-decay and neutron capture rates of a few key radioactive nuclides would help to attain a comprehensive understanding of the solar main component. [ABSTRACT FROM AUTHOR]

Introduction: Mainstream (MS) grains - the dominant population (>90%) of presolar SiC - are believed to have originated from low-mass asymptotic giant branch (AGB) stars with close-to-solar metallicities. In comparison, presolar Y and Z grains are two minor populations (~1-5% each) of presolar SiC. The main difference between Y/Z and MS grains is that the former are generally more depleted in 29Si/28Si and enriched in 30Si/28Si than the latter. In addition, while MS and Z grains generally have lower-than-solar 12C/13C ratios, Y grains are defined as grains with 12C/13C = 100. In some Y and Z grains, large 30Si/28Si and 50Ti/48Ti excesses were observed, suggesting that these two minor types came from AGB stars with 1/3-1/2 Z8 [1]. However, the low-metallicity AGB stellar origins of Y and Z grains are challenged by the indistinguishable Mo isotopic compositions of MS, Y, and Z grains, in contrast to the varying Mo isotopic patterns predicted by AGB models for low-mass AGB stars of different metallicities [2]. Thus, the stellar origins of Y and Z grains remain enigmatic. Here, we provide the first piece of evidence that Y and Z grains show heavy-element isotopic compositions different than MS grains, namely higher 88Sr/87Sr and 138Ba/136Ba ratios. Results and Discussion: Light- and heavy-element isotopic data were collected with the Carnegie NanoSIMS 50L ion microprobe and the University of Chicago's CHILI instrument, respectively. Preliminary Sr and Ba data were reported in [3]. We obtained sufficiently precise Sr and Ba isotopic data for 11 Y and seven Z grains. We found that the Y and Z grains show higher 88Sr/87Sr (Fig. 1) and 138Ba/136Ba ratios when compared to MS grains data from the literature. The Si, Sr, and Ba isotopic compositions of our Y and Z grains can be consistently explained if the amount of 13C - the major neutron source for the s-process - is reduced by a factor of 1.0-7.8 (D6 -D1.5 cases) in FRANEC Torino AGB models for 0.15-0.50 Z8 AGB stars with respect to those (D1.5-U1.3 cases) required by MS grains for a 1.0 Z8 AGB star (Fig. 1). This scenario is in line with the previous finding based on Ti isotopes [2], but it fails to explain the indistinguishable Mo isotopic compositions of MS, Y, and Z grains. Also, our inferred amounts of 13C (D6-D1.5 cases) for the parent low-metallicity AGB stars of Y and Z grains, lie within those inferred for low-metallicity AGB stars based on stellar observations [4] but are, on average, relatively low. The unrepresentativeness of the parent stars of Y and Z grains may stem from the fact that the grains originated from a limited number of ancient low-metallicity stars, given that the abundance of Y and Z grains is more than an order of magnitude lower than the abundance of MS grains in primitive meteorites. Conclusions: The higher 88Sr/87Sr and 138Ba/136Ba ratios of our Y and Z grains provide strong support to the suggestion that these two minor types originated from low-metallicity AGB stars, which, however, is challenged by the indistinguishable Mo isotopic compositions of MS, Y, and Z grains. Since the varying Mo isotopic patterns predicted by AGB models for stars with different metallicities result mainly from the energy dependences of 95,96,97,98Mo(n,?) cross-sections [2], new cross-section measurements are needed. [ABSTRACT FROM AUTHOR]

The neutron-capture and Maxwellian-averaged cross sections of 181Ta (n , γ) reaction have been computed making use of the TALYS code. Two main inputs of the TALYS calculation, nuclear level density (NLD) and radiative strength function (RSF), are calculated within a recent microscopic model that takes into account the exact thermal pairing plus independent-particle model and collective excitations within the phonon-damping model. A good agreement between the calculated and experimental NLD and RSF data of 182Ta was found, confirming the goodness of the microscopic nuclear model used. As the result, the deduced neutron-capture and Maxwellian-averaged cross sections of 181Ta (n , γ) reaction also agree reasonably well with the experimental and recommended data across the measured energy range. The impact of the predicted cross sections on the tantalum production in the slow neutron-capture process (s -process) is then studied by employing a recent version of FRUITY stellar evolutionary models in which the effect of magnetic fields is newly implemented. By calculating the isotopic composition ratios within the FUNS code, we obtain a slight depletion of tantalum as compared to the recommended data. The variation between the calculated and reference tungsten isotopic ratios in presolar stardust silicon carbide (SiC) grains is found to be negligible, as expected. However, the capability of our theoretical model to reproduce experimental nuclear data will allow us to investigate in detail the s -process branching points at 181W and 185W, for which experimental data from direct measurements are still unavailable. [ABSTRACT FROM AUTHOR]

Context. The origin of fluorine is a longstanding problem in nuclear astrophysics. It is widely recognized that asymptotic giant branch (AGB) stars are among the most important contributors to the Galactic fluorine production. Aims. In general, extant nucleosynthesis models overestimate the fluorine production by AGB stars with respect to observations. Although those differences are rather small at solar metallicity, low metallicity AGB stellar models predict fluorine surface abundances up to one order of magnitude larger than the observed ones. Methods. As part of a project devoted to reducing the uncertainties in the nuclear physics that affect the nucleosynthesis in AGB stellar models, we review the relevant nuclear reaction rates involved in the fluorine production or destruction. We perform this analysis on a model with initial mass M = 2 M☉ and Z = 0.001. Results. We found that the major uncertainties are due to the 13C(α,n)16O, the 19F(α,p)22Ne, and the 14N(p,γ)15O reactions. A change in the corresponding reaction rates within the present experimental uncertainties implies surface 19F variations at the AGB tip lower than 10%, thus much smaller than observational uncertainties. For some α capture reactions, however, cross sections at astrophysically relevant energies are determined on the basis of nuclear models, in which some low-energy resonance parameters are very poorly known. Thus, larger variations in the rates of those processes cannot be excluded. That being so, we explore the effects of the variation in some α capture rates well beyond the current published uncertainties. The largest 19F variations are obtained by varying the 15N(α,γ)19F and the 19F(α,p)22Ne reactions. Conclusions. The currently estimated uncertainties of the nuclear reaction rates involved in the production and destruction of fluorine produce minor 19F variations in the ejecta of AGB stars. Analysis of some α capture processes that assume a wider uncertainty range determines 19F abundances in better agreement with recent spectroscopic fluorine measurements at low metallicity. In the framework of this scenario, the 15N(α,γ)19F and the 19F(α,p)22Ne reactions show the strongest effects on fluorine nucleosynthesis. The presence of poorly known low-energy resonances make such a scenario possible, even if it is unlikely. We plan to measure these resonances directly. [ABSTRACT FROM AUTHOR]

Asymptotic giant branch (AGB) stars are thought to be among the most important sources of fluorine in our Galaxy. While observations and theory agree at close-to-solar metallicity, stellar models overestimate fluorine production in comparison to heavy elements at lower metallicities. We present predictions for 19F abundance for a set of AGB models with various masses and metallicities, in which magnetic buoyancy induces the formation of the 13C neutron source (the so-called 13C pocket). In our new models, fluorine is mostly created as a consequence of secondary 14N nucleosynthesis during convective thermal pulses, with a minor contribution from the 14N existing in the 13C pocket zone. As a result, AGB stellar models with magnetic-buoyancyinduced mixing show low 19F surface abundances which agree with fluorine spectroscopic observations at both low and near-solar metallicity.

Type Ia supernovae are used as distance indicators to measure the expansion rate of the Universe and to constrain the nature of dark energy. Current and upcoming surveys will allow to extend supernova Hubble diagrams to higher redshifts and to improve further their statistics. It is accepted that Type Ia supernovae are thermonuclear explosions of carbon-oxygen white dwarfs in binary systems. However, the identification of their progenitors, the evolutionary path leading to the explosion and the explosion mechanism itself have not been identified yet. This is critical, as we need to understand the potential evolution of their luminosity with cosmic time and, thus, with their stellar progenitors. We will review the current situation, considering observational hints. We will focus on our recent models, that follow the evolution of carbon-oxygen white dwarfs accreting mass up to thermonuclear runaway, and on their dependence with the initial metallicity of the white dwarf progenitors.

In this paper, we analyze the effects induced by rotation on low-mass asymptotic giant branch stars. We compute two sets of models, M = 2.0M☉ at [Fe/H] = 0 and M = 1.5M☉ at [Fe/H] = -1.7, by adopting main-sequence rotation velocities in the range 0-120 km s-1. 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 13Cpocket (the major neutron source) with 14N (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 lesser extent, those of light-s elements (Sr, Y, Zr) decrease with increasing rotation velocities up to 60 km s-1. However, for larger initial rotation velocities, the production of light-s and, to a lesser 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 the Goldreich-Schubert-Fricke instability and meridional circulations determines an increase of light-s and, to a lesser extent, heavy-s elements, while lead is strongly reduced. For both metallicities, the rotation-induced instabilities active during the interpulse phase reduce the neutron-to-seed 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. [ABSTRACT FROM AUTHOR]

ABSTRACT We provide an individual analysis of 94 carbon-enhanced metal-poor stars showing an s-process 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 13C-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. [ABSTRACT FROM AUTHOR]

Thermally pulsing Asymptotic Giant Branch stars are responsible for the nucleosynthesis of the main component of the cosmic s-elements. The most important neutron source is the 13C(α, n)16O reaction. Owing to the presence of a subthreshold resonance, the low energy extrapolation is a rather complex task. The rate quoted in the literature differ up to a factor of 4 at typical stellar energies. The latest improvements in computer power allows us to calculate the evolution of TP-AGB stars coupled with a full nuclear network, extending from hydrogen to lead. Here we discuss the effects of the variation of the 13C(α, n)16O rate on the predicted neutron capture nucleosynthesis. [Copyright &y& Elsevier]

Context. The details of the mass-loss process in the late stages of low- and intermediate-mass stellar evolution are not well understood, in particular its dependence on stellar parameters. Mira variables are highly suitable targets for studying this mass-loss process. Aims. Here, we follow up on our earlier finding that a near-to-mid-infrared (NIR-MIR) colour versus pulsation period diagram shows two sequences of Miras that can be distinguished by the third dredge-up (3DUP) indicator technetium in those stars. While IR colours are good indicators of the dust mass-loss rate (MLR) from Miras, no corresponding sequences have been found using the gas MLR. However, investigations of the gas MLR have been hampered by data limitations. We aim to alleviate these limitations with new observational data. Methods. We present new optical spectra of a well-selected sample of Miras. We searched these spectra for absorption lines of Tc and other 3DUP indicators, and combine our findings with gas MLRs and expansion velocities from the literature. Furthermore, we extend the analysis of the MIR emission to WISE data and compare the broadband spectral energy distributions (SEDs) of Miras with and without Tc. Results. We find no systematic difference in gas MLRs between Miras with and without Tc. However, the gas envelopes of Tc-poor Miras appear to have a higher terminal expansion velocity than those of Miras with Tc. Furthermore, our analysis of the IR photometry strongly corroborates the earlier finding that Tc-poor Miras have a higher MIR emission than Tc-rich ones, by as much as a factor of two. We model the IR colours with DARWIN and stationary wind models and conclude that Miras with and without Tc have different dust content or dust properties. Conclusions. We discuss several hypotheses and interpretations of the observations and conclude that the reduction of free oxygen by 3DUP of carbon and iron-depleted dust grains in Tc-rich stars are the most convincing explanations for our observations. [ABSTRACT FROM AUTHOR]

Context. The chemical evolution of neutron capture elements in the Milky Way is still a matter of debate. Although more and more studies investigate their chemical behaviour, there is still a lack of a significant large sample of abundances of a key heavy element: lead. Aims. Lead is the final product of the s-process nucleosynthesis channel and is one of the most stable heavy elements. The goal of this article is to present the largest catalogue of homogeneous Pb abundances, in particular for metallicities higher than −1.0 dex, and then to study the lead content of the Milky Way. Methods. We analysed high-resolution spectra from the ESO UVES and FEROS archives. Atmospheric parameters were taken from the AMBRE parametrisation. We used the automated abundance method GAUGUIN to derive lead abundances in 653 slow-rotating FGK-type stars from the 368.34 nm Pb I line. Results. We present the largest catalogue of Local Thermodynamic Equilibrium (LTE) and non-LTE lead abundances ever published with metallicities ranging from −2.9 to 0.6 dex and [Pb/Fe] from −0.7 to 3.3 dex. Within this sample, no lead-enhanced Asymptotic Giant Branch (AGB) stars were found, but nine lead-enhanced metal-poor stars ([Pb/Fe] > 1.5) were detected. Most of them were already identified as carbon-enhanced metal-poor stars with enrichments in other s-process species. The lead abundance of 13 Gaia Benchmark Stars are also provided. We then investigated the Pb content of the Milky Way disc by computing vertical and radial gradients and found a slightly decreasing [Pb/Fe] radial trend with metallicity. This trend together with other related ratios ([Pb/Eu], [Pb/Ba], and [Pb/α]) are interpreted thanks to chemical evolution models. The two-infall model closely reproduces the observed trends with respect to the metallicity. It is also found that the AGB contribution to the Pb Galactic enrichment has to be strongly reduced. Moreover, the contribution of massive stars with rather high rotational velocities should be favoured in the low-metallicity regime. [ABSTRACT FROM AUTHOR]

Experiments performed on Storage Rings have shown that lifetimes of beta-radionuclides can change dramatically as a function of theionization state. PANDORA (Plasmas for Astrophysics, Nuclear Decay Observation and Radiation for Archaeometry) aims at measuring, for the first time, nuclear β-decay rates in stellar-like conditions, especially for radionuclides involved in nuclear-astrophysics processes (BBN, s- processing, CosmoChronometers, Early Solar System formation). Compact magnetic plasma traps, where plasmas reach density ne~10n-1014 cm-3, and temperature Te~0.1-30 keV, are suitable for such studies. The decay rates can be measured as a function of the charge state distribution of the inplasma ions. The collaboration is now designing the plasma trap able to reach the needed plasma densities, temperatures and charge states distributions. A first list of radioisotopes, including tens of physics cases of potential interest is now available. Possible physics cases include, among the others, 2°4Tl, 63Ni, 6°Co, 171Tm, 147Pm, 85Kr, 176Lu and the pairs 187Re-187Os and 87Sr-87Rb, which play a crucial role as cosmo-clock. Physics cases are now under evaluation in terms of lifetime measurements feasibility in a plasma trap.

Stars are marvellous caldrons where all the elements of the Universe (apartfrom hydrogen and helium) have been synthesized. The solar system chemical distri-butionis the result of many pollution episodes from already extinct stellar generations, occurred at different epochs before the Sun formation. Main nucleosynthesis channels re-sponsiblefor the formation of heavy elements are the rapid neutron capture process (ther-process) and the slow neutron capture process (the s-process). Hereafter, I will describethe theory of the s-process and the stellar sites where it is active.