White dwarfs with a surface electrical charge distribution: Equilibrium and stability

The equilibrium configuration and the radial stability of white dwarfs composed of charged perfect fluid are investigated. These cases are analyzed through the results obtained from the solution of the hydrostatic equilibrium equation. We regard that the fluid pressure and the fluid energy density follow the relation of a fully degenerate electron gas. For the electric charge distribution in the object, we consider that it is centralized only close to the white dwarfs' surfaces. We obtain larger and more massive white dwarfs when the total electric charge is increased. To appreciate the effects of the electric charge in the structure of the star, we found that it must be in the order of $10^{20}\,[{\rm C}]$ with which the electric field is about $10^{16}\,[{\rm V/cm}]$. For white dwarfs with electric fields close to the Schwinger limit, we obtain masses around $2\,M_{\odot}$. We also found that in a system constituted by charged static equilibrium configurations, the maximum mass point found on it marks the onset of the instability. This indicates that the necessary and sufficient conditions to recognize regions constituted by stable and unstable equilibrium configurations against small radial perturbations are respectively $dM/d\rho_c>0$ and $dM/d\rho_c<0$.Comment: This is a preprint. The original paper will be published in EPJ

Stellar equilibrium configurations of white dwarfs in the f(R,T)f(R,T) gravity

In this work we investigate the equilibrium configurations of white dwarfs in a modified gravity theory, na\-mely, $f(R,T)$ gravity, for which $R$ and $T$ stand for the Ricci scalar and trace of the energy-momentum tensor, respectively. Considering the functional form $f(R,T)=R+2\lambda T$, with $\lambda$ being a constant, we obtain the hydrostatic equilibrium equation for the theory. Some physical properties of white dwarfs, such as: mass, radius, pressure and energy density, as well as their dependence on the parameter $\lambda$ are derived. More massive and larger white dwarfs are found for negative values of $\lambda$ when it decreases. The equilibrium configurations predict a maximum mass limit for white dwarfs slightly above the Chandrasekhar limit, with larger radii and lower central densities when compared to standard gravity outcomes. The most important effect of $f(R,T)$ theory for massive white dwarfs is the increase of the radius in comparison with GR and also $f(R)$ results. By comparing our results with some observational data of massive white dwarfs we also find a lower limit for $\lambda$, namely, $\lambda >- 3\times 10^{-4}$.Comment: To be published in EPJ

Visualization of electric field lines in an engineering education context

The electromagnetic theory presents a unifying explanation of electric and magnetic phenomena underlying our technological society. It is a fundamental physical theory taught in engineering schools at university level. In this theory the electromagnetic field is a vector field permeating space. An important aspect relating to students difficulties and misconceptions is the difficulty in visualizing vector fields. With the goal of enhancing student understanding and studying student engagement we have developed high quality 3D visualizations of electromagnetic situations. These make use of accurate computation of the field lines, together with realistic rendering using the open source software Blender. We present examples of electrostatic situations with both an assessment of the student understanding and an evaluation of the students' perceptions of the importance of the visualizations. Complex interplay between visualization specific issues and the abstract notion of the field is identified in the students' conceptions. It is found that the visualizations are not used as substitutes of other learning resources. They are perceived as allowing a quick access to content and prompting motivation. The adequacy of the visualization to the subject content as well as the capacity to use it as self-assessment is valued by the students

An analysis of helium primordial nucleosynthesis with a variable cosmological coupling

The synthesis of helium in the early Universe depends on many input parameters, including the value of the gravitational coupling during the period when the nucleosynthesis takes place. We compute the primordial abundance of helium as function of the gravitational coupling, using a semi-analytical method, in order to track the influence of $G$ in the primordial nucleosynthesis. To be specific, we construct a cosmological model with varying $G$, using the Brans-Dicke theory. The greater the value of $G$ at nucleosynthesis period, the greater the abundance of helium predicted. Using the observational data for the abundance of the primordial helium, constraints for the time variation of $G$ are established

Organização do trabalho em instituição de P&D e construção do conhecimento no agronegócio.

Verificaram-se relações da equipe e da parceria como modos organizacionais, com a produção em P&D na Embrapa, eleitos pela similitude com o que propõem Gibbons et al. (1996) sobre "a nova produção do conhecimento". Coletaram-se dados de gestão da Empresa, de 2000 a 2003, contabilizados sob quatro subconjuntos de indicadores: produção técnico-científica; produção de publicações técnicas; transferência de tecnologia e promoção da imagem; desenvolvimento de tecnologias, produtos e processos. Esses e mais a produção total são as variáveis de produção em P&D nesta pesquisa. Procedeu-se à análise estatística e à discussão qualitativa, buscando entender a subjetividade do objeto de trabalho. A correlação entre índices de parceria e números de equipes e, por outro lado, produção em P&D, é baixa. Discute-se a possível ausência de influência entre as variáveis independentes e dependentes. Consideraram-se as diferenças entre as unidades da Embrapa, esperando-se que aquelas com pessoal mais numeroso e/ou atuação mais abrangente estabeleçam um maior número de equipes. Maiores índices de parceria são esperados daquelas em territórios mais concentrados em C&T. Tais resultados não significam que equipes e parcerias sejam desprezíveis para a construção do conhecimento. De um lado, os achados quantitativos são limitados e, de outro, o universo estudado é rico em diversidade. Daí admitir-se que tais formatos organizacionais não devem ser desprezados como estratégias favoráveis ao novo modo de construção do conhecimento

Data mining framework for fatty liver disease classification in ultrasound: a hybrid feature extraction paradigm

PURPOSE: Fatty liver disease (FLD) is an increasing prevalent disease that can be reversed if detected early. Ultrasound is the safest and ubiquitous method for identifying FLD. Since expert sonographers are required to accurately interpret the liver ultrasound images, lack of the same will result in interobserver variability. For more objective interpretation, high accuracy, and quick second opinions, computer aided diagnostic (CAD) techniques may be exploited. The purpose of this work is to develop one such CAD technique for accurate classification of normal livers and abnormal livers affected by FLD. METHODS: In this paper, the authors present a CAD technique (called Symtosis) that uses a novel combination of significant features based on the texture, wavelet transform, and higher order spectra of the liver ultrasound images in various supervised learning-based classifiers in order to determine parameters that classify normal and FLD-affected abnormal livers. RESULTS: On evaluating the proposed technique on a database of 58 abnormal and 42 normal liver ultrasound images, the authors were able to achieve a high classification accuracy of 93.3% using the decision tree classifier. CONCLUSIONS: This high accuracy added to the completely automated classification procedure makes the authors' proposed technique highly suitable for clinical deployment and usage

Respostas hematológicas de Arapaima gigas (Pisces: Arapaimidae) parasitados naturalmente por protozoários e metazoários.

O presente estudo comparou os parâmetros sanguíneos de Arapaima gigas Schinz, 1822 de cultivo, parasitados por uma espécie de protozoário e três espécies de helmintos. Em uma das pisciculturas os peixes foram parasitados somente por Ichthyophthirius multifiliis (Ciliophora), enquanto na outra foram parasitados por I. multifiliis, Dawestrema cycloancistrium e Dawestrema cycloancistrioides (Monogenoidea) e Polyacanthorhynchus macrorhynchus (Acanthocephala). Determinou-se a contagem total de eritrócitos , trombócitos e leucócitos (total e diferencial), hematócrito, concentração de hemoglobina, volume corpuscular média (VCM), hemoglobina corpuscular média (HCM), concentração da hemoglobina corpuscular média (CHCM), glicose e proteínas totais. Os peixes parasitados por I. multifiliis, D. cycloancistrium, D. cycloancistrioides e P. macrorhynchus mostraram menores valores de hematócrito, VCM e níveis de glicose, e maior concentração de hemoglobina, HCM, CHCM, número de eritrócitos , monócitos , neutrófilos e eosinófilos totais quando comparados aos peixes parasitados somente por I. multifiliis. Portanto, esses resultados indicam uma resposta desses parâmetr os sanguíneos ao parasitismo em A. gigas