Time filter

Source Type

Boa Vista, Brazil

Rodrigues H.,Federal University of Rio de Janeiro | Duarte S.B.,Brazilian Center for Research in Physics (CBPF) | De Oliveira J.C.T.,Federal University of Roraima
Astrophysical Journal | Year: 2011

High-mass compact stars have been reported recently in the literature, providing strong constraints on the properties of the ultra dense matter beyond the saturation nuclear density. In view of these results, the calculations of quark star or hybrid star equilibrium structure must be compatible with the provided observational data. But since the equations of state used in describing quark matter are in general too soft in comparison with the equation of states used to describe the hadronic or nuclear matter, the calculated quark star models presented in the literature are in general not suitable to explain the stability of highly-compact massive objects. In this work, we present the calculations of a spherically symmetric quark star structure by using an equation of state that takes into account the superconducting color-flavor locked phase of the strange quark matter. In addition, some fundamental aspects of QCD (asymptotic freedom and confinement) are considered by means of a phenomenological description of the deconfined quark phase, the density-dependent quark mass model. The quark matter behavior introduced by this model stiffens the corresponding equation of state. We thus investigate the influence of this model on the mass-radius diagram of quark stars. We obtain massive quark stars due to the stiffness of the equation of state, when a reasonable parameterization of the color superconducting gap is used. Models of quark stars enveloped by a nucleonic crust composed of a nuclear lattice embedded in an electron gas, with nuclei close to neutron drip line, are also discussed. © 2011. The American Astronomical Society. All rights reserved. Source

Campos M.,University of Sao Paulo | Campos M.,Federal University of Roraima | Lima J.A.S.,University of Sao Paulo
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

We discuss the gravitational collapse of a spherically symmetric massive core of a star in which the fluid component is interacting with a growing vacuum energy density. The influence of the variable vacuum in the collapsing core is quantified by a phenomenological β parameter as predicted by dimensional arguments and the renormalization group approach. For all reasonable values of this free parameter, we find that the vacuum energy density increases the collapsing time, but it cannot prevent the formation of a singular point. However, the nature of the singularity depends on the value of β. In the radiation case, a trapped surface is formed for β≤1/2, whereas for β1/2, a naked singularity is developed. In general, the critical value is β=1-2/3(1+ω), where ω is the parameter describing the equation of state of the fluid component. © 2012 American Physical Society. Source

Naka L.N.,Louisiana State University | Naka L.N.,Federal University of Roraima | Bechtoldt C.L.,National Institute of Amazonian Research | Magalli Pinto Henriques L.,National Institute of Amazonian Research | Brumfield R.T.,Louisiana State University
American Naturalist | Year: 2012

Suture zones represent natural forums in which to examine the role of geography and ecology in the speciation process. Here, we conduct a comparative analysis designed to investigate the location of avian phylogeographic breaks and contact zones in the Guiana Shield, northern Amazonia.We use distributional and genetic data from 78 pairs of avian taxa to address whether phylogeographic breaks and contact zones are associated with contemporary landscape features. Using spatially explicit statistical models, we found that phylogeographic breaks and contact zones are not randomly distributed throughout the landscape. In general, geographic breaks cluster along physical barriers (rivers, nonforested habitats, and small mountain ranges), whereas contact zones aggregate where these barriers either break down or are easier to overcome, such as around rivers' headwaters. Our results indicate that although major Amazonian rivers are often key determinants of taxon boundaries, the "riverine barrier effect" is a synergistic consequence of the wide lower reaches of some rivers, coupled with nonriverine landscape features at the headwaters. Our data suggest that ancestral refugia are not necessary to explain current distribution patterns and that pairs of codistributed taxa do not seem to be the result of simultaneous diversification processes. © 2012 by The University of Chicago. Source

Flores A.S.,Fundacao Estadual do Meio Ambiente | Rodrigues R.S.,Federal University of Roraima
Acta Botanica Brasilica | Year: 2010

(Leguminosae diversity in a savanna area of Roraima, Brazil). Leguminosae is the most diverse family in the savannas of Roraima, which are the largest savanna areas in the Brazilian Amazon. This paper presents a floristic survey and an analysis of the geographic distribution patterns of Leguminosae taxa in a savanna area at Boa Vista, Roraima (02°52′07″N; 60°43′03″W). The survey identified 80 taxa belonging to 38 genera. Six taxa are new records for the flora of Roraima. The best-represented genera were Aeschynomene and Chamaecrista, both with seven species each. Leguminosae- Papilionoideae was the most diverse subfamily based on the number of genera (24) and infrageneric taxa (53). The phytogeographical analysis revealed predominance of taxa with a Neotropical distribution. The savanna site analyzed represents a legume diverse area, because it concentrates 87% of legume diversity referred for the savannas of Roraima. Source

De Campos M.,Federal University of Roraima
International Journal of Geometric Methods in Modern Physics | Year: 2016

In this work, we study a particle creation mechanism at the expense of the energy stored in an extra dimension with the respective process of compactification, and hence the emergence of 4D space. The spacetime resulting can be the Friedmann-Robertson-Walker (FRW) type, but also can be the cosmological term plus cold dark matter (CDM) type, depending on the model's parameters. Concurrent with the compactification process we have also the homogenization process, since initially the spacetime is anisotropic. © 2016 World Scientific Publishing Company. Source

Discover hidden collaborations