8,138 research outputs found
PopIII signatures in the spectra of PopII/I GRBs
We investigate signatures of population III (PopIII) stars in the
metal-enriched environment of GRBs originating from population II-I (PopII/I)
stars by using abundance ratios derived from numerical simulations that follow
stellar evolution and chemical enrichment. We find that at more than
of PopII/I GRBs explode in a medium previously enriched by PopIII stars
(we refer to them as GRBIIIII). Although the formation of
GRBIIIII is more frequent than that of pristine PopIII GRBs
(GRBIIIs), we find that the expected GRBIIIII observed rate is
comparable to that of GRBIIIs, due to the usually larger luminosities of these
latter. GRBIIIII events take place preferentially in small
proto-galaxies with stellar masses , star formation rates
and metallicities . On the other hand,
galaxies with are dominated by metal enrichment
from PopIII stars and should preferentially host GRBIIIII. Hence,
measured GRB metal content below this limit could represent a strong evidence
of enrichment by pristine stellar populations. We discuss how to discriminate
PopIII metal enrichment on the basis of various abundance ratios observable in
the spectra of GRBs' afterglows. By employing such analysis, we conclude that
the currently known candidates at redshift -- i.e. GRB 050904
\cite[][]{2006Natur.440..184K} and GRB 130606A \cite[][]{2013arXiv1312.5631C}
-- are likely not originated in environments pre-enriched by PopIII stars.Comment: 9 pages, 7 figures; MNRAS accepte
Constraining the PopIII IMF with high-z GRBs
We study the possibility to detect and distinguish signatures of enrichment
from PopIII stars in observations of PopII GRBs (GRBIIs) at high redshift by
using numerical N-body/hydrodynamical simulations including atomic and
molecular cooling, star formation and metal spreading from stellar populations
with different initial mass functions (IMFs), yields and lifetimes. PopIII and
PopII star formation regimes are followed simultaneously and both a top-heavy
and a Salpeter-like IMF for pristine PopIII star formation are adopted. We find
that the fraction of GRBIIs hosted in a medium previously enriched by PopIII
stars (PopIII-dominated) is model independent. Typical abundance ratios, such
as [Si/O] vs [C/O] and [Fe/C] vs [Si/C], can help to disentangle enrichment
from massive and intermediate PopIII stars, while low-mass first stars are
degenerate with regular PopII generations. The properties of galaxies hosting
PopIII-dominated GRBIIs are not very sensitive to the particular assumption on
the mass of the first stars.Comment: 9 pages, 4 figure
A cellular automaton for the factor of safety field in landslides modeling
Landslide inventories show that the statistical distribution of the area of
recorded events is well described by a power law over a range of decades. To
understand these distributions, we consider a cellular automaton to model a
time and position dependent factor of safety. The model is able to reproduce
the complex structure of landslide distribution, as experimentally reported. In
particular, we investigate the role of the rate of change of the system
dynamical variables, induced by an external drive, on landslide modeling and
its implications on hazard assessment. As the rate is increased, the model has
a crossover from a critical regime with power-laws to non power-law behaviors.
We suggest that the detection of patterns of correlated domains in monitored
regions can be crucial to identify the response of the system to perturbations,
i.e., for hazard assessment.Comment: 4 pages, 3 figure
Changing, priming, and acting on values: Effects via motivational relations in a circular model
Circular models of values and goals suggest that some motivational aims are consistent with each other, some oppose each other, and others are orthogonal to each other. The present experiments tested this idea explicitly by examining how value confrontation and priming methods influence values and value-consistent behaviors throughout the entire value system. Experiment 1 revealed that change in 1 set of social values causes motivationally compatible values to increase in importance, whereas motivationally incompatible values decrease in importance and orthogonal values remain the same. Experiment 2 found that priming security values reduced the better-than-average effect, but priming stimulation values increased it. Similarly, Experiments 3 and 4 found that priming security values increased cleanliness and decreased curiosity behaviors, whereas priming self-direction values decreased cleanliness and increased curiosity behaviors. Experiment 5 found that priming achievement values increased success at puzzle completion and decreased helpfulness to an experimenter, whereas priming with benevolence values decreased success and increased helpfulness. These results highlight the importance of circular models describing motivational interconnections between values and personal goals
Population III stars and the Long Gamma Ray Burst rate
Because massive, low-metallicity population III (PopIII) stars may produce
very powerful long gamma-ray bursts (LGRBs), high-redshift GRB observations
could probe the properties of the first stars. We analyze the correlation
between early PopIII stars and LGRBs by using cosmological
N-body/hydrodynamical simulations, which include detailed chemical evolution,
cooling, star formation, feedback effects and the transition between PopIII and
more standard population I/II (PopII/I) stars. From the Swift observed rate of
LGRBs, we estimate the fraction of black holes that will produce a GRB from
PopII/I stars to be in the range 0.028<f_{GRB}<0.140, depending on the assumed
upper metallicity of the progenitor. Assuming that as of today no GRB event has
been associated to a PopIII star, we estimate the upper limit for the fraction
of LGRBs produced by PopIII stars to be in the range 0.006<f_{GRB}<0.022. When
we apply a detection threshold compatible with the BAT instrument, we find that
the expected fraction of PopIII GRBs (GRB3) is ~10% of the full LGRB population
at z>6, becoming as high has 40% at z>10. Finally, we study the properties of
the galaxies hosting our sample of GRB3. We find that the average metallicity
of the galaxies hosting a GRB3 is typically higher than the critical
metallicity used to select the PopIII stars, due to the efficiency in polluting
the gas above such low values. We also find that the highest probability of
finding a GRB3 is within galaxies with a stellar mass <10^7 Msun, independently
from the redshift.Comment: 8 pages,3 figures. Submitted to MNRAS, revised version after
referee's comment
A green method to prepare nanosilica modified graphene oxide to inhibit nanoparticles re-aggregation during melt processing
A graphene oxide-silica nanohybrid (GOS), self-assembled into a lasagna-like structure, was prepared in water and used as a filler for the melt preparation of polyamide 6 (PA6)-based nanocomposites. For sake of comparison, PA6-based materials were prepared under the same processing conditions by adding GO only or a physical mixture of GO and silica (GO+S). All the materials were characterized from a morphological, spectroscopic, thermal, dynamic-mechanical (DMA) and mechanical point of view. For all the nanocomposites, the interphase was studied either by analyzing loss factor plots coming from DMA measurements and by implementing a novel approach, i.e. combining solvent extraction, SEM and EDX measurements. While GO and GO+S showed an almost negligible effect on the macroscopic features of PA6, due to lack of dispersion, PA6-GOS ternary nanocomposites displayed an outstanding enhancement of mechanical and thermo-mechanical performance. This feature is likely due to the formation of an extended and strong interphase. In fact, the silica layers intercalated between GO lamellae played as an exfoliating agent, thus proving to be particularly efficient in avoiding self-aggregation of GO sheets, while those covalently attached to basal planes of GO acted as a rivet, capable to interlock the surrounding polymer chains. Finally, with respect to other techniques commonly adopted for the fabrication of polymer-graphene nanocomposites, the green route herein proposed does not involve any toxic solvent nor time-consuming protocols, and allows achieving remarkable improvements in stiffening (up to +180%), strengthening (up to +210%) and toughening (up to +210%) at extremely low filler contents (0.25% or 0.5%)
Parametric thermal analysis for the optimization of Double Walled Tubes layout in the Water Cooled Lithium Lead inboard blanket of DEMO fusion reactor
Within the roadmap that will lead to the nuclear fusion exploitation for electric energy generation, the construction of a DEMOnstration (DEMO) reactor is, probably, the most important milestone to be reached since it will demonstrate the technological feasibility and economic competitiveness of an industrial-scale nuclear fusion reactor. In order to reach this goal, several European universities and research centres have joined their efforts in the EUROfusion action, funded by HORIZON 2020 UE programme. Within the framework of EUROfusion research activities, ENEA and University of Palermo are involved in the design of the Water-Cooled Lithium Lead Breeding Blanket (WCLL BB), that is one of the two BB concepts under consideration to be adopted in the DEMO reactor. It is mainly characterized by a liquid lithium-lead eutectic alloy acting as breeder (lithium) and neutron multiplier (lead), as well as by subcooled pressurized water as coolant. Two separate circuits, both characterized by a pressure of 15.5 MPa and inlet/outlet temperatures of 295 °C/328 °C, are deputed to cool down the First Wall (FW) and the Breeder Zone (BZ). The former consists in a system of radial-toroidal-radial C-shaped squared channels where countercurrent water flow occurs while the latter relies in the use of bundles of poloidal-radial Double Walled Tubes (DWTs) housed within the breeder. A parametric thermal study has been carried out in order to assess the best DWTs' layout assuring that the structural material maximum temperature does not overcome the allowable limit of 550 °C and that the overall coolant thermal rise fulfils the design target value of 33 °C. The study has been performed following a theoretical-numerical approach based on the Finite Element Method (FEM) and adopting the quoted Abaqus FEM code. Main assumptions and models together with results obtained are herewith reported and critically discussed
Additive effect of non-alcoholic fatty liver disease on metabolic syndrome-related endothelial dysfunction in hypertensive patients
Metabolic syndrome (MS) is characterized by an increased risk of incident diabetes and cardiovascular (CV) events, identifying insulin resistance (IR) and endothelial dysfunction as key elements. Moreover, non-alcoholic fatty liver disease (NAFLD) is bidirectionally linked with MS as a consequence of metabolic and inflammatory abnormalities. We addressed the question if the evolution in NAFLD might worsen endothelium-dependent vasodilating response in MS hypertensives. We recruited 272 Caucasian newly-diagnosed never-treated hypertensive outpatients divided into three groups according to the presence/absence of MS alone or in combination with NAFLD. MS and NAFLD were defined according to the National Cholesterol Education Program-Adult Treatment Panel III (NCEP-ATPIII) and non-invasive fatty liver index, respectively. We determined IR by using the homeostasis model assessment (HOMA) index. Vascular function, as forearm blood flow (FBF), was determined through strain-gauge plethysmography after intra-arterial infusion of acetylcholine (ACh) and sodium nitroprusside. MS+NAFLD+ group showed worse metabolic, inflammatory and vascular profiles compared with MS-NAFLD- and MS+NAFLD-. HOMA resulted in being the strongest predictor of FBF both in the MS+NAFLD- and in the MS+NAFLD+ groups, accounting for 20.5% and 33.2% of its variation, respectively. In conclusion, we demonstrated that MS+NAFLD+ hypertensives show a worse endothelium-dependent vasodilation compared with MS+NAFLD-, allowing for consideration of NAFLD as an early marker of endothelial dysfunction in hypertensives
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