Carbon-enhanced metal-poor stars: a window on AGB nucleosynthesis and binary evolution. II. Statistical analysis of a sample of 67 CEMP-ss stars

Many observed CEMP stars are found in binary systems and show enhanced abundances of $s$-elements. The origin of the chemical abundances of these CEMP-$s$ stars is believed to be accretion in the past of enriched material from a primary star in the AGB phase. We investigate the mechanism of mass transfer and the process of nucleosynthesis in low-metallicity AGB stars by modelling the binary systems in which the observed CEMP-$s$ stars were formed. For this purpose we compare a sample of $67$ CEMP-$s$ stars with a grid of binary stars generated by our binary evolution and nucleosynthesis model. We classify our sample CEMP-$s$ stars in three groups based on the observed abundance of europium. In CEMP$-s/r$ stars the europium-to-iron ratio is more than ten times higher than in the Sun, whereas it is lower than this threshold in CEMP$-s/nr$ stars. No measurement of europium is currently available for CEMP-$s/ur$ stars. On average our models reproduce well the abundances observed in CEMP-$s/nr$ stars, whereas in CEMP-$s/r$ stars and CEMP-$s/ur$ stars the abundances of the light-$s$ elements are systematically overpredicted by our models and in CEMP-$s/r$ stars the abundances of the heavy-$s$ elements are underestimated. In all stars our modelled abundances of sodium overestimate the observations. This discrepancy is reduced only in models that underestimate the abundances of most of the $s$-elements. Furthermore, the abundance of lead is underpredicted in most of our model stars. These results point to the limitations of our AGB nucleosynthesis model, particularly in the predictions of the element-to-element ratios. Finally, in our models CEMP-$s$ stars are typically formed in wide systems with periods above 10000 days, while most of the observed CEMP-$s$ stars are found in relatively close orbits with periods below 5000 days.Comment: 23 pages, 8 figures, accepted for publication on Astronomy & Astrophysic

Carbon-enhanced metal-poor stars: a window on AGB nucleosynthesis and binary evolution. I. Detailed analysis of 15 binary stars with known orbital periods

AGB stars are responsible for producing a variety of elements, including carbon, nitrogen, and the heavy elements produced in the slow neutron-capture process ($s$-elements). There are many uncertainties involved in modelling the evolution and nucleosynthesis of AGB stars, and this is especially the case at low metallicity, where most of the stars with high enough masses to enter the AGB have evolved to become white dwarfs and can no longer be observed. The stellar population in the Galactic halo is of low mass ($\lesssim 0.85M_{\odot}$) and only a few observed stars have evolved beyond the first giant branch. However, we have evidence that low-metallicity AGB stars in binary systems have interacted with their low-mass secondary companions in the past. The aim of this work is to investigate AGB nucleosynthesis at low metallicity by studying the surface abundances of chemically peculiar very metal-poor stars of the halo observed in binary systems. To this end we select a sample of 15 carbon- and $s$-element-enhanced metal-poor (CEMP-$s$) halo stars that are found in binary systems with measured orbital periods. With our model of binary evolution and AGB nucleosynthesis, we determine the binary configuration that best reproduces, at the same time, the observed orbital period and surface abundances of each star of the sample. The observed periods provide tight constraints on our model of wind mass transfer in binary stars, while the comparison with the observed abundances tests our model of AGB nucleosynthesis.Comment: 18 pages, 20 figures, accepted for publication on A&

Simulation of Shaking Table Tests to Study Soil-Structure Interaction by Means of Two Different Constitutive Models

The paper presents the main results of a FEM 3-D model reproducing a physical model subjected to shaking table tests. The tests, performed at the EERC laboratory of Bristol University, have been simulated by means of a new numerical model based on a recent constitutive model characterized by isotropic and kinematic hardening and devoted to granular soil. The shaking table tests have been performed using: a six-degree of freedom shaking table; a shear-stack; a scaled one-storey steel frame; the Leigthon Buzzard Sand. The tests have been characterized by 11 shaking runs. As regards the 3-D numerical modeling, the linear elastic material has been considered for the structure, instead the soil has been modeled both with a cap-hardening Drucker-Prager model, often implemented in commercial codes, and with the above mentioned new constitutive model, implemented in the utilized FEM code by the Research Group of Catania University. Thanks to the great quantity of experimental data, the power of the proposed numerical model in simulation/prediction of dynamic soil-structure interaction can be verified and compared with the capability of other numerical models based on simpler constitutive models

DWT-based Method for Partial Discharge Pattern Recognition

The new proposed method of pattern recognition is based on the application of Multiresolution Signal Decomposition (MSD) technique of wavelet transform. This technique has shown interesting properties in capturing the embedded horizontal, vertical and diagonal variations within an image obtained from the PD pattern in a separable form. This feature has been exploited to identify in the PD patterns MSD, relative at various family of PD sources, some detail images typical of a single discharge phenomenon. The classification of a generic PD phenomenon is feasible through a comparison between its detail images and the detail images typical of a single discharge phenomenon. Tests have been performed on specimens having single defects. The obtained results prove that the proposed improved classification methods is quite efficient and accurate. [DOI: 10.1685 / CSC06133] About DO

Modelling the observed properties of carbon-enhanced metal-poor stars using binary population synthesis

The stellar population in the Galactic halo is characterised by a large fraction of CEMP stars. Most CEMP stars are enriched in $s$-elements (CEMP-$s$ stars), and some of these are also enriched in $r$-elements (CEMP-$s/r$ stars). One formation scenario proposed for CEMP stars invokes wind mass transfer in the past from a TP-AGB primary star to a less massive companion star which is presently observed. We generate low-metallicity populations of binary stars to reproduce the observed CEMP-star fraction. In addition, we aim to constrain our wind mass-transfer model and investigate under which conditions our synthetic populations reproduce observed abundance distributions. We compare the CEMP fractions and the abundance distributions determined from our synthetic populations with observations. Several physical parameters of the binary stellar population of the halo are uncertain, e.g. the initial mass function, the mass-ratio and orbital-period distributions, and the binary fraction. We vary the assumptions in our model about these parameters, as well as the wind mass-transfer process, and study the consequent variations of our synthetic CEMP population. The CEMP fractions calculated in our synthetic populations vary between 7% and 17%, a range consistent with the CEMP fractions among very metal-poor stars recently derived from the SDSS/SEGUE data sample. The results of our comparison between the modelled and observed abundance distributions are different for CEMP-$s/r$ stars and for CEMP-$s$ stars. For the latter, our simulations qualitatively reproduce the observed distributions of C, Na, Sr, Ba, Eu, and Pb. Contrarily, for CEMP-$s/r$ stars our model cannot reproduce the large abundances of neutron-rich elements such as Ba, Eu, and Pb. This result is consistent with previous studies, and suggests that CEMP-$s/r$ stars experienced a different nucleosynthesis history to CEMP-$s$ stars.Comment: 17 pages, 11 figures, accepted for publication on Astronomy and Astrophysic

Sicilian Jurassic Phisiography and Geologic Realms

Two tectono-sedimentary domains, which were deformed during the Neogene and evolved into two large structural sectors, characterize the Sicilian Jurassic: the Maghrebides and Peloritani. Africa margin sediments, passing downward to Triassic successions and perhaps originally to Paleozoic deposits, characterize the former. The latter belongs to the European "Calabrian Arc", where the Jurassic transgressively rests on a continental substrate (i.e. the crystalline Variscan basement). These domains are characterized by four sedimentary facies: shallow platform-derived limestones; condensed seamount-type red limestones; nodular limestones with ammonites; deep radiolarites and shales. These facies are illustrated in a dozen of stratigraphic logs. The drowning of most Triassic-Liassic carbonate platforms or ramps and the deepening of adjacent basins came with inferred Jurassic strike-slip tectonics, connected to the relative movement of Africa (Gondwanan part) vs Europe (Laurasian part); the same strike-slip tectonics may have caused scattered intraplate volcanic seamounts found in Maghrebides. During the Jurassic the Maghrebide realm was characterized by the interfingering of basins and carbonate platforms. During the Early and Middle Liassic, carbonate platforms and ramps were dominant. Since Toarcian either radiolarites in some basins or Ammonite-bearing calcareous muds developed with intervening basaltic flows, and were accompanied by condensed pelagic carbonates on the ensialic seamount-type highs. The Peloritani realm displays similar characteristics, but with later transgression on the basement, several strike-slip basins and without any volcanoes

Grazing Impacts on Rangeland Condition in Semi-Arid South-Western Africa

The savannah biome, consisting of a dense herbaceous layer and a relatively open woody layer in competitive balance, constitutes 64% of the land surface of Namibia, an arid country in south-western Africa, and is used mainly for extensive cattle and sheep ranching. About half of the savannah area is affected by dense to moderately dense bush-thickening, resulting in a ten-fold decrease in the rangeland’s grass-based carrying capacity and a concomitant loss in meat production of about US$115 million per year (De Klerk, 2004). Bushencroached areas typically have densities \u3e 2 000 bushes/ha with \u3e 90% belonging to a single species. High grazing pressure by specialist grazers, such as domestic cattle, is often blamed for rangeland degradation. There is an urgent need to understand the dynamics of bush encroachment and devise grazing strategies to contain it