7,339 research outputs found
Homogenization in magnetic-shape-memory polymer composites
Magnetic-shape-memory materials (e.g. specific NiMnGa alloys) react with a
large change of shape to the presence of an external magnetic field. As an
alternative for the difficult to manifacture single crystal of these alloys we
study composite materials in which small magnetic-shape-memory particles are
embedded in a polymer matrix. The macroscopic properties of the composite
depend strongly on the geometry of the microstructure and on the
characteristics of the particles and the polymer.
We present a variational model based on micromagnetism and elasticity, and
derive via homogenization an effective macroscopic model under the assumption
that the microstructure is periodic. We then study numerically the resulting
cell problem, and discuss the effect of the microstructure on the macroscopic
material behavior. Our results may be used to optimize the shape of the
particles and the microstructure.Comment: 17 pages, 4 figure
Field Theoretical Description of Quantum Hall Edge Reconstruction
We propose a generalization of the chiral Luttinger liquid theory to allow
for a unified description of quantum Hall edges with or without edge
reconstruction. Within this description edge reconstruction is found to be a
quantum phase transition in the universality class of one-dimensional dilute
Bose gas transition, whose critical behavior can be obtained exactly. At
principal filling factors , we show the additional edge modes due to
edge reconstruction modifies the point contact tunneling exponent in the low
energy limit, by a small and non-universal amount.Comment: 4 pages with 1 ps figure embedde
Quasi-Lie schemes and Emden--Fowler equations
The recently developed theory of quasi-Lie schemes is studied and applied to
investigate several equations of Emden type and a scheme to deal with them and
some of their generalisations is given. As a first result we obtain t-dependent
constants of the motion for particular instances of Emden equations by means of
some of their particular solutions. Previously known results are recovered from
this new perspective. Finally some t-dependent constants of the motion for
equations of Emden type satisfying certain conditions are recovered
Recommended from our members
Associated reading skills in children with a history of Specific Language Impairment (SLI)
A large cohort of 200 eleven-year-old children with Specific Language Impairment (SLI) were assessed on basic reading accuracy and on reading comprehension as well as language tasks. Reading skills were examined descriptively and in relation to early language and literacy factors. Using stepwise regression analyses in which age and nonverbal IQ were controlled for, it was found that a single word reading measure taken at 7 years was unsurprisingly a strong predictor of the two different types of reading ability. However, even with this measure included, a receptive syntax task (TROG) entered when reading accuracy score was the DV. Furthermore, a test of expressive syntax/narrative and a receptive syntax task completed at 7 years entered into the model for word reading accuracy. When early reading accuracy was excluded from the analyses, early phonological skills also entered as a predictor of both reading accuracy and comprehension at 11 years. The group of children with a history of SLI were then divided into those with no literacy difficulties at 11 and those with some persisting literacy impairment. Using stepwise logistic regression, and again controlling for IQ and age, 7 years receptive syntax score (but not tests of phonology, expressive vocabulary or expressive syntax/narrative) entered as a positive predictor of membership of the ‘no literacy problems’ group regardless of whether early reading accuracy was controlled for in step one. The findings are discussed in relation to the overlap of SLI and dyslexia and the long term sequelae of language impairment
Genetic consequences of Quaternary climatic oscillations in the Himalayas: Primula tibetica as a case study based on restriction site-associated DNA sequencing.
The effects of Quaternary climatic oscillations on the demography of organisms vary across regions and continents. In taxa distributed in Europe and North America, several paradigms regarding the distribution of refugia have been identified. By contrast, less is known about the processes that shaped the species' spatial genetic structure in areas such as the Himalayas, which is considered a biodiversity hotspot. Here, we investigated the phylogeographic structure and population dynamics of Primula tibetica by combining genomic phylogeography and species distribution models (SDMs). Genomic data were obtained for 293 samples of P. tibetica using restriction site-associated DNA sequencing (RADseq). Ensemble SDMs were carried out to predict potential present and past distribution ranges. Four distinct lineages were identified. Approximate Bayesian computation analyses showed that each of them have experienced both expansions and bottlenecks since their divergence, which occurred during or across the Quaternary glacial cycles. The two lineages at both edges of the distribution were found to be more vulnerable and responded in different ways to past climatic changes. These results illustrate how past climatic changes affected the demographic history of Himalayan organisms. Our findings highlight the significance of combining genomic approaches with environmental data when evaluating the effects of past climatic changes
The Ionized Stellar Wind in Vela X-1 During Eclipse
We present a first analysis of a high resolution X-ray spectrum of the
ionized stellar wind of Vela X-1 during eclipse. The data were obtained with
the High Energy Transmission Grating Spectrometer onboard the Chandra X-ray
Observatory. The spectrum is resolved into emission lines with fluxes between
0.02 and 1.04x10^4 ph/cm^2/s. We identify lines from a variety of charge
states, including fluorescence lines from cold material, a warm photoionized
wind. We can exclude signatures from collisionally ionized plasmas. For the
first time we identify fluorescence lines from L-shell ions from lower Z
elements. We also detect radiative recombination continua from a kT = 10 eV
(1.2 x 10^5 K) photoionized optically thin gas. The fluorescence line fluxes
infer the existence of optically thick and clumped matter within or outside the
warm photoionized plasma.Comment: 4 pages, 2 figures, accepted by ApJ letter
Spectroscopic and physical parameters of Galactic O-type stars. I. Effects of rotation and spectral resolving power in the spectral classification of dwarfs and giants
The modern-era spectral classification of O-stars relies on either the
Walborn or the Conti-Mathys scheme. Since both of these approaches have been
developed using low-quality photographic data, their application to
high-quality digital data might not be straightforward and be hampered by
problems and complications that have not yet been appreciated. Using
high-resolution spectra obtained with the ESO/MPG 2.2\,m telescope in La Silla
and following the premises of the Walborn and Conti classification schemes, we
determined the spectral types and luminosity classes of 19 Galactic O-type
stars and compared them to those attributed by Walborn and Mathys based on
low-quality data. Our analysis reveals that the morphological spectral types
assigned using high-resolution data are systematically later (by up to 1.5
subtypes) then those attributed by Walborn. By means of line-profile
simulations, we show that part of this discrepancy is more likely caused by the
combined effect of stellar rotation and high spectral resolution on the depth
of helium lines used as spectral type indicators. In addition, we demonstrate
that at least for narrow-lined stars the "rotational effect" does not disappear
when the high-resolution spectra are degraded to the resolution of the Walborn
standards. We also find evidence of a systematic difference between our
high-resolution quantitative spectral types and those assigned by Mathys.
Rotation and spectral resolution are important third parameters in the spectral
classification of O-type stars. To obtain reliable spectral classes within the
Walborn approach, the unknown and the standard spectra must be compared at the
same resolution and \vsini. Owing to resolution effects, the Conti approach
might also need to be updated.Comment: paper accepted for publication in A&
Integrative metabolomics to identify molecular signatures of responses to vaccines and infections
Approaches to the identification of metabolites have progressed from early biochemical pathway evaluation to modern high-dimensional metabolomics, a powerful tool to identify and characterize biomarkers of health and disease. In addition to its relevance to classic metabolic diseases, metabolomics has been key to the emergence of immunometabolism, an important area of study, as leukocytes generate and are impacted by key metabolites important to innate and adaptive immunity. Herein, we discuss the metabolomic signatures and pathways perturbed by the activation of the human immune system during infection and vaccination. For example, infection induces changes in lipid (e.g., free fatty acids, sphingolipids, and lysophosphatidylcholines) and amino acid pathways (e.g., tryptophan, serine, and threonine), while vaccination can trigger changes in carbohydrate and bile acid pathways. Amino acid, carbohydrate, lipid, and nucleotide metabolism is relevant to immunity and is perturbed by both infections and vaccinations. Metabolomics holds substantial promise to provide fresh insight into the molecular mechanisms underlying the host immune response. Its integration with other systems biology platforms will enhance studies of human health and disease
A Search for Wolf-Rayet Stars in the Small Magellanic Cloud
We conducted an extensive search for Wolf-Rayet stars (W-Rs) in the SMC,
using the same interference filter imaging techniques that have proved
successful in finding W-Rs in more distant members of the Local Group.
Photometry of some 1.6 million stellar images resulted in some 20 good
candidates, which we then examined spectroscopically. Two of these indeed
proved to be newly found W-Rs, bringing the total known in the SMC from 9 to
11. Other finds included previously unknown Of-type stars (one as early as
O5f?p)),the recovery of the Luminous Blue Variable S18, and the discovery of a
previously unknown SMC symbiotic star. More important, however, is the fact
that there does not exist a significant number of W-Rs waiting to be discovered
in the SMC. The number of W-Rs in the SMC is a factor of 3 lower than in the
LMC (per unit luminosity), and we argue this is the result of the SMC's low
metallicity on the evolution of the most massive stars.Comment: Accepted by Astrophysical Journal. Postscript version available via
ftp.lowell.edu/pub/massey/smcwr.ps.gz Revised version contains slightly
revised spectral types for the Of stars but is otherwise unchange
- …