2,470 research outputs found
Scandium: A key element for understanding Am stars
{\rm Context.} Atomic diffusion is believed to cause the abundance anomalies
observed in AmFm stars. However, the detailed process has still not been
well-established. For instance, two possible scenarios for the diffusion theory
are presently envisaged. They differ mainly by the depth from which the
abundance anomalies emanate. The first scenario predicts that the abundances
are modified in the superficial regions of the star, just below the hydrogen
convection zone. The second scenario predicts that a much deeper extension of
the mixing zone exists due to the convection caused by Fe accumulation in
regions below the hydrogen convection zone. {\rm Aims.} We calculate much more
accurate radiative accelerations of Sc than previously, to better understand
the observed abundance anomalies of this element. We believe that it is a key
element to use as a diagnostic tool for understanding AmFm stars.
{\rmMethods.} The method employed to obtain these radiative accelerations is
based on an interpolation from the parameters of the so-called SVP parametric
method.
{\rm Results.} The radiative accelerations, shown here in a typical Am
stellar model, are discussed in light of the observed anomalies of Ca and Sc.
Our results suggest that the deeper mixing scenario is not entirely
satisfactory: the mixing zone should be deeper than what is predicted by recent
models to account for observed Sc underabundances. Our results seem more
compatible with the scenario where the abundances anomalies are created in the
superficial regions. However, only detailed evolutionary modelling with mass
loss and diffusion of all important species, including Ca and Sc, with accurate
radiative accelerations, will be able to give more insight into where the
source of these anomalies occur in AmFm stars.Comment: 6 pages, 3 figures, accepted for publication in A&
Stratification of the elements in the atmospheres of blue horizontal-branch stars
Blue horizontal-branch (BHB) stars with approximately larger
than 11500 K show several observational anomalies. In globular clusters, they
exhibit low rotational velocities, abundance anomalies (as compared to cluster
abundances), photometric jumps and gaps and spectroscopic gravities lower than
predicted by canonical models. It is commonly believed that the low rotational
velocities of these stars permit atomic diffusion to be efficient in their
atmosphere thereby causing the observed anomalies. Recent detections of
vertical stratification of iron (and some other chemical elements) in several
BHB stars concur with this framework. In this paper, improved model atmospheres
that include the vertical stratification of the elements are applied to BHB
stars to verify if they can explain their observational anomalies. The results
from theoretical model atmospheres are consistent with the photometric jumps
and gaps observed for BHB stars in globular clusters. It is found that iron
stratification in the theoretical models and that obtained from observations
have similar tendancies. Our results also show that the spectroscopic gravities
obtained while using chemically homogeneous model atmospheres to fit
observations are underestimated. These results significantly strengthen the
belief that atomic diffusion is responsible for these BHB-star anomalies.Comment: 6 pages, 6 figure
A mechanistic investigation of Pickering emulsion polymerization
Pickering emulsion polymerization offers a versatile way of synthetising hybrid core–shell latexes where a polymer core is surrounded by an armour of inorganic nanoparticles. A mechanistic understanding of the polymerization process is limited which restricts the use of the technique in the fabrication of more complex, multilayered colloids. In this paper clarity is provided through an in-depth investigation into the Pickering emulsion polymerization of methyl methacrylate (MMA) in the presence of nano-sized colloidal silica (Ludox TM-40). Mechanistic insights are discussed by studying both the adsorption of the stabiliser to the surface of the latex particles and polymerization kinetics. The adhesion of the Pickering nanoparticles was found not to be spontaneous, as confirmed by cryo-TEM analysis of MMA droplets in water and monomer-swollen PMMA latexes. This supports the theory that the inorganic particles are driven towards the interface as a result of a heterocoagulation event in the water phase with a growing oligoradical. The emulsion polymerizations were monitored by reaction calorimetry in order to establish accurate values for monomer conversion and the overall rate of polymerizations (Rp). Rp increased for higher initial silica concentrations and the polymerizations were found to follow pseudo-bulk kinetics
Vitellogenin receptors during vitellogenesis in the rainbow trout Oncorhynchus mykiss
Rainbow trout vitellogenin receptors have been characterized by ligand blotting and Scatchard analysis. Their evolution has been studied over a reproductive cycle in a broodstock of 2-year-old females. The receptors were prepared from ovarian membrane homogenates and were solubilized using n-octyl-beta-D-glucopyranoside. The visualization of the receptor by ligand blotting using 125iodine-vitellogenin after sodium dodecyl sulfate electrophoresis revealed the existence of one major binding component corresponding to a protein of 113 kDa. The Scatchard transformation of the binding data revealed a single class of binding sites. The variations of the binding characteristics (Kd and maximum binding) were investigated during vitellogenesis. This study revealed that the Kd was not affected by oocyte growth during vitellogenesis, but was highly decreased in ovulated eggs. The receptor number increased during the same period from 35 to 860 fM per oocyte, while the receptor number per mm2 of oocyte membrane surface was doubled during the same period. (Résumé d'auteur
Independent responsive behaviour and communication in hydrogel objects
In this work, we show the fabrication of soft hydrogel alginate-based objects, namely fibres and beads, that have an individually programmed time delay in their response to a shared environmental stimulus. We utilize the enzyme urease to programme a self-regulated change in pH, which in turn activates the designed response of gel fibre disintegration or a change in gel bead colour. This design allows for independent response behaviour of a collection of bodies in a single closed system, as well as inter-material communication on shorter length scales. The incorporation of responsive time control directly into soft matter objects demonstrates an advance in the field of autonomous materials
A Rapid and Computationally Inexpensive Method to Virtually Implant Current and Next-Generation Stents into Subject-Specific Computational Fluid Dynamics Models
Computational modeling is often used to quantify hemodynamic alterations induced by stenting, but frequently uses simplified device or vascular representations. Based on a series of Boolean operations, we developed an efficient and robust method for assessing the influence of current and next-generation stents on local hemodynamics and vascular biomechanics quantified by computational fluid dynamics. Stent designs were parameterized to allow easy control over design features including the number, width and circumferential or longitudinal spacing of struts, as well as the implantation diameter and overall length. The approach allowed stents to be automatically regenerated for rapid analysis of the contribution of design features to resulting hemodynamic alterations. The applicability of the method was demonstrated with patient-specific models of a stented coronary artery bifurcation and basilar trunk aneurysm constructed from medical imaging data. In the coronary bifurcation, we analyzed the hemodynamic difference between closed-cell and open-cell stent geometries. We investigated the impact of decreased strut size in stents with a constant porosity for increasing flow stasis within the stented basilar aneurysm model. These examples demonstrate the current method can be used to investigate differences in stent performance in complex vascular beds for a variety of stenting procedures and clinical scenarios
THE VISIBILITY AND INVISIBILITY OF HERDERS' KRAALS IN SOUTHERN AFRICA, WITH REFERENCE TO A POSSIBLE EARLY CONTACT PERIOD KHOEKHOE KRAAL AT KFS 5, WESTERN CAPE
The Europeans who landed on the shores of the South African Cape from the late 15th century onwards encountered local herders whom they later referred to as the Hottentots (now known as the Khoekhoe). There are written references to the settlements and livestock of these pastoralists, but archaeologists have not had much success in discovering any such sites. This absence of archaeological evidence for recent Khoekhoe kraals has been interpreted by some scholars as an indication for a general archaeological invisibility of nomadic pastoralist sites. This article reports on the archaeology of an extensive, low density surface spread of artefacts, KFS 5 (Western Cape), which possibly represents a Khoekhoe kraal dating to the time of the first contact with Europeans. Data are compared to other archaeological evidence of cattle pens in southern Africa and the issues of the visibility of prehistoric and historic kraals are re-addresse
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