5,909 research outputs found
Impact of Rotation-Driven Particle Repopulation on the Thermal Evolution of Pulsars
Driven by the loss of energy, isolated rotating neutron stars (pulsars) are
gradually slowing down to lower frequencies, which increases the tremendous
compression of the matter inside of them. This increase in compression changes
both the global properties of rotating neutron stars as well as their hadronic
core compositions. Both effects may register themselves observationally in the
thermal evolution of such stars, as demonstrated in this Letter. The
rotation-driven particle process which we consider here is the direct Urca (DU)
process, which is known to become operative in neutron stars if the number of
protons in the stellar core exceeds a critical limit of around 11% to 15%. We
find that neutron stars spinning down from moderately high rotation rates of a
few hundred Hertz may be creating just the right conditions where the DU
process becomes operative, leading to an observable effect (enhanced cooling)
in the temperature evolution of such neutron stars. As it turns out, the
rotation-driven DU process could explain the unusual temperature evolution
observed for the neutron star in Cas A, provided the mass of this neutron star
lies in the range of 1.5 to 1.9 \msun and its rotational frequency at birth was
between 40 (400 Hz) and 70% (800 Hz) of the Kepler (mass shedding) frequency,
respectively.Comment: Revised version, 7 pages 4 figures. To appear in Physics Letters
Discretization-dependent model for weakly connected excitable media
Pattern formation has been widely observed in extended chemical and biological processes. Although the biochemical systems are highly heterogeneous, homogenized continuum approaches formed by partial differential equations have been employed frequently. Such approaches are usually justified by the difference of scales between the heterogeneities and the characteristic spatial size of the patterns. Under different conditions, for example, under weak coupling, discrete models are more adequate. However, discrete models may be less manageable, for instance, in terms of numerical implementation and mesh generation, than the associated continuum models. Here we study a model to approach discreteness which permits the computer implementation on general unstructured meshes. The model is cast as a partial differential equation but with a parameter that depends not only on heterogeneities sizes, as in the case of quasicontinuum models, but also on the discretization mesh. Therefore, we refer to it as a discretization-dependent model. We validate the approach in a generic excitable media that simulates three different phenomena: the propagation of action membrane potential in cardiac tissue, in myelinated axons of neurons, and concentration waves in chemical microemulsions.We acknowledge the support from CAPES, grant
88881.065002/2014-01 of the Brazilian program Science
without borders, FAPEMIG, CNPq, UFJF, and from
MINECO of Spain under the Ramon y Cajal program,
grant number RYC-2012-11265Peer ReviewedPostprint (published version
Electrically Charged Strange Quark Stars
The possible existence of compact stars made of absolutely stable strange
quark matter--referred to as strange stars--was pointed out by E. Witten almost
a quarter of a century ago. One of the most amazing features of such objects
concerns the possible existence of ultra-strong electric fields on their
surfaces, which, for ordinary strange matter, is around V/cm. If
strange matter forms a color superconductor, as expected for such matter, the
strength of the electric field may increase to values that exceed
V/cm. The energy density associated with such huge electric fields is on the
same order of magnitude as the energy density of strange matter itself, which,
as shown in this paper, alters the masses and radii of strange quark stars at
the 15% and 5% level, respectively. Such mass increases facilitate the
interpretation of massive compact stars, with masses of around , as
strange quark stars.Comment: Revised version, references added, 6 pages, 4 figures, accepted for
publication in Physical Review
Transforming 2D Radar Remote Sensor Information from a UAV into a 3D World-View
This research work was supported by the "European Regional Development Fund" (EFRE) in the context of the aim of "Investment in Growth and Employment" (IWB) in Rhineland-Palatinate, Germany.Since unmanned aerial vehicles (UAVs) have been established in geoscience as a key and
accessible tool, a wide range of applications are currently being developed. However, not only the
design of UAVs themselves is vital to carry out an accurate investigation, but also the sensors and the
data processing are key parts to be considered. Several publications including accurate sensors are
taking part in pioneer research programs, but less is explained about how they were designed. Besides
the commonly used sensors such as a camera, one of the most popular ones is radar. The advantages
of a radar sensor to perform research in geosciences are the robustness, the ability to consider large
distances and velocity measurements. Unfortunately, these sensors are often expensive and there is a
lack of methodological papers that explain how to reduce these costs. To fill this gap, this article aims
to show how: (i) we used a radar sensor from the automotive field; and (ii) it is possible to reconstruct
a three-dimensional scenario with a UAV and a radar sensor. Our methodological approach proposes
a total of eleven stages to process the radar data. To verify and validate the process, a real-world
scenario reconstruction is presented with a system resolution reaching from two to three times the
radar resolution. We conclude that this research will help the scientific community to include the use
of radars in their research projects and programs, reducing costs and increasing accuracy.European Regional Development Fund" (EFRE) in the context of the aim of "Investment in Growth and Employment" (IWB) in Rhineland-Palatinate, German
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