Chaotic dynamics around astrophysical objects with nonisotropic stresses

The existence of chaotic behavior for the geodesics of the test particles orbiting compact objects is a subject of much current research. Some years ago, Gu\'eron and Letelier [Phys. Rev. E \textbf{66}, 046611 (2002)] reported the existence of chaotic behavior for the geodesics of the test particles orbiting compact objects like black holes induced by specific values of the quadrupolar deformation of the source using as models the Erez--Rosen solution and the Kerr black hole deformed by an internal multipole term. In this work, we are interesting in the study of the dynamic behavior of geodesics around astrophysical objects with intrinsic quadrupolar deformation or nonisotropic stresses, which induces nonvanishing quadrupolar deformation for the nonrotating limit. For our purpose, we use the Tomimatsu-Sato spacetime [Phys. Rev. Lett. \textbf{29} 1344 (1972)] and its arbitrary deformed generalization obtained as the particular vacuum case of the five parametric solution of Manko et al [Phys. Rev. D 62, 044048 (2000)], characterizing the geodesic dynamics throughout the Poincar\'e sections method. In contrast to the results by Gu\'eron and Letelier we find chaotic motion for oblate deformations instead of prolate deformations. It opens the possibility that the particles forming the accretion disk around a large variety of different astrophysical bodies (nonprolate, e.g., neutron stars) could exhibit chaotic dynamics. We also conjecture that the existence of an arbitrary deformation parameter is necessary for the existence of chaotic dynamics.Comment: 7 pages, 5 figure

Attenuation Imaging with Pulse-Echo Ultrasound based on an Acoustic Reflector

Ultrasound attenuation is caused by absorption and scattering in tissue and is thus a function of tissue composition, hence its imaging offers great potential for screening and differential diagnosis. In this paper we propose a novel method that allows to reconstruct spatial attenuation distribution in tissue based on computed tomography, using reflections from a passive acoustic reflector. This requires a standard ultrasound transducer operating in pulse-echo mode, thus it can be implemented on conventional ultrasound systems with minor modifications. We use calibration with water measurements in order to normalize measurements for quantitative imaging of attenuation. In contrast to earlier techniques, we herein show that attenuation reconstructions are possible without any geometric prior on the inclusion location or shape. We present a quantitative evaluation of reconstructions based on simulations, gelatin phantoms, and ex-vivo bovine skeletal muscle tissue, achieving contrast-to-noise ratio of up to 2.3 for an inclusion in ex-vivo tissue.Comment: Accepted at MICCAI 2019 (International Conference on Medical Image Computing and Computer Assisted Intervention

Secuencia did?ctica basada en el modelo de desplazamiento como aproximaci?n al concepto de n?mero negativo, en los estudiantes de primer grado del colegio de San Sim?n, sede Restrepo

152 p. Recurso Electr?nicoEsta investigaci?n est? enfocada al campo de la educaci?n matem?tica, espec?ficamente hacia una de las dificultades m?s comunes que pueden presentar los estudiantes de secundaria, como es la interpretaci?n del signo en los n?meros enteros y su distinci?n con los signos de operaci?n. De acuerdo con Bruno, (2001); Whitacre, (2012) y Bofferding, (2014) es posible disminuir las dificultades que presentan los estudiantes, en el aprendizaje de los n?meros enteros, si se ense?a el concepto de n?mero negativo desde los primeros grados de escolaridad, y una de las propuestas para hacerlo, es utilizando el modelo de desplazamiento que plantea Cid, (2003). Es por esto que, el presente trabajo tiene como prop?sito aproximar a los estudiantes de primero de primaria al concepto de n?mero negativo, a partir de una secuencia did?ctica basada en el modelo de desplazamiento, que busca verificar si dicha secuencia aporta al aprendizaje de estos ni?os, e inducir si es posible utilizar la ense?anza de este tema en primaria, como alternativa de soluci?n a las dificultades de aprendizaje que presentan los estudiantes de secundaria. Por esta raz?n fue necesario llevar a cabo una investigaci?n-acci?n para que, seg?n Kemmis, (2010), se involucraran procesos de indagaci?n, autorreflexi?n, participaci?n y evaluaci?n constantes durante todo el proceso de investigaci?n, permitiendo as?, no solo la construcci?n de una secuencia did?ctica acorde a las necesidades y condiciones de su momento, sino tambi?n, la validaci?n de algunas propuestas planteadas por los autores antes mencionados.The present paper is focused on the mathematics teaching field, and more specifically on a situation that secondary school students face very commonly, such as the absolute numbers sign?s interpretation is, and the distinction between that and the operational signs. According to Bruni, (2001); Whitaker, (2012) and Bofferding, (2014) it is possible to diminish those difficulties by teaching the negative numbers concept to school students from the beginning of elementary school. One way to do that is by using the ?displacement model? proposed by Cid, (2003). For that reason, this paper is focused on the first grade elementary school students, whom we want to get acquainted whit the negative number concept throughout this research work, starting with a ?displacement model? related didactic sequence, attempting to verify if that sequence really helps children with that learning objective, or not, and by doing so, to learn if it is possible to use the teaching of negative numbers in elementary school as an alternative to solve the mathematics learning related issues observed in our secondary school students. To solve this question it was necessary to carry out an ?action ? related investigation? in which, according to Kemmis, (2010) inquiry, self-reflection, participation and evaluation processes where involved along the whole action to allow us not only to build a didactical sequence fitting the real needs and conditions of the educational environment, but to validate some of the above noted authors. Key words Model, Integers, Model displacement, Teaching sequence, First grad

Innermost stable circular orbits around magnetized rotating massive stars

In 1998, Shibata and Sasaki [Phys. Rev. D 58, 104011 (1998)] presented an approximate analytical formula for the radius of the innermost stable circular orbit (ISCO) of a neutral test particle around a massive, rotating and deformed source. In the present paper, we generalize their expression by including the magnetic dipole moment. We show that our approximate analytical formulas are accurate enough by comparing them with the six-parametric exact solution calculated by Pach\'on et. al. [Phys. Rev. D 73, 104038 (2006)] along with the numerical data presented by Berti and Stergioulas [MNRAS 350, 1416 (2004)] for realistic neutron stars. As a main result, we find that in general, the radius at ISCO exhibits a decreasing behavior with increasing magnetic field. However, for magnetic fields below 100GT the variation of the radius at ISCO is negligible and hence the non-magnetized approximate expression can be used. In addition, we derive approximate analytical formulas for angular velocity, energy and angular momentum of the test particle at ISCO.Comment: 8 pages, 3 figure

Realistic Exact Solution for the Exterior Field of a Rotating Neutron Star

A new six-parametric, axisymmetric and asymptotically flat exact solution of Einstein-Maxwell field equations having reflection symmetry is presented. It has arbitrary physical parameters of mass, angular momentum, mass--quadrupole moment, current octupole moment, electric charge and magnetic dipole, so it can represent the exterior field of a rotating, deformed, magnetized and charged object; some properties of the closed-form analytic solution such as its multipolar structure, electromagnetic fields and singularities are also presented. In the vacuum case, this analytic solution is matched to some numerical interior solutions representing neutron stars, calculated by Berti & Stergioulas (Mon. Not. Roy. Astron. Soc. 350, 1416 (2004)), imposing that the multipole moments be the same. As an independent test of accuracy of the solution to describe exterior fields of neutron stars, we present an extensive comparison of the radii of innermost stable circular orbits (ISCOs) obtained from Berti & Stergioulas numerical solutions, Kerr solution (Phys. Rev. Lett. 11, 237 (1963)), Hartle & Thorne solution (Ap. J. 153, 807, (1968)), an analytic series expansion derived by Shibata & Sasaki (Phys. Rev. D. 58 104011 (1998)) and, our exact solution. We found that radii of ISCOs from our solution fits better than others with realistic numerical interior solutions.Comment: 13 pages, 13 figures, LaTeX documen

Photofission of heavy nuclei at energies up to 4 GeV

Total photofission cross sections for 238U, 235U, 233U, 237Np, 232Th, and natPb have been measured simultaneously, using tagged photons in the energy range Egamma=0.17-3.84 GeV. This was the first experiment performed using the Photon Tagging Facility in Hall B at Jefferson Lab. Our results show that the photofission cross section for 238U relative to that for 237Np is about 80%, implying the presence of important processes that compete with fission. We also observe that the relative photofission cross sections do not depend strongly on the incident photon energy over this entire energy range. If we assume that for 237Np the photofission probability is equal to unity, we observe a significant shadowing effect starting below 1.5 GeV.Comment: 4 pages of RevTex, 6 postscript figures, Submitted to Phys. Rev. Let

Exploiting the potential of bioreactors for creating spatial organization in the soil microbiome: A strategy for increasing sustainable agricultural practices

Industrial production of synthetic nitrogen fertilizers and their crop application have caused considerable environmental impacts. Some eco-friendly alternatives try to solve them but raise some restrictions. We tested a novel method to produce a nitrogen bioinoculant by enriching a soil microbial community in bioreactors supplying N2 by air pumping. The biomass enriched with diazotrophic bacteria was diluted and applied to N-depleted and sterilized soil of tomato plants. We estimated microbial composition and diversity by 16S rRNA metabarcoding from soil and bioreactors at different run times and during plant uprooting. Bioreactors promoted the N-fixing microbial community and revealed a hided diversity. One hundred twenty-four (124) operational taxonomic units (OTUs) were assigned to bacteria with a greater Shannon diversity during the reactor’s steady state. A total of 753 OTUs were found in the rhizospheres with higher biodiversity when the lowest concentration of bacteria was applied. The apparent bacterial abundance in the batch and continuous bioreactors suggested a more specific functional ecological organization. We demonstrate the usefulness of bioreactors to evidence hidden diversity in the soil when it passes through bioreactors. By obtaining the same growth of inoculated plants and the control with chemical synthesis fertilizers, we evidence the potential of the methodology that we have called directed bioprospecting to grow a complex nitrogen-fixing microbial community. The simplicity of the reactor’s operation makes its application promising for developing countries with low technological progress

Characterization and distribution of sedimentary facies of Cartagena Bay, Colombia.

En el análisis sedimentológico de 253 muestras de sedimentos de fondo y borde de playa de la Bahía de Cartagena, se identificaron 26 facies sedimentarias. Las muestras fueron tomadas con draga tipo Van Veen para el fondo, multimuestreador de sedimento tipo Beeker y apiques para los bordes de playa. Las muestras fueron tamizadas y se obtuvieron 15 categorías de acuerdo al tamaño de grano, posteriormente se realizó el análisis de carbonato de calcio (CaCO3) usando el calcímetro de Bernard. Los sedimentos fueron clasificados de acuerdo con el tamaño medio de grano y el contenido de CaCO3, para definir las facies sedimentarias existentes, donde la facies lodo litoclástico es la más representativa y se distribuye en la bahía interna y externa; al norte y sur de la Isla de Tierra Bomba. Se encontró que durante los últimos 30 años la distribución de las facies sedimentarias ha experimentado variaciones significativas: en la bahía interna ha disminuido el contenido de CaCO3 y al suroeste ha disminuido el tamaño de grano y el contenido de CaCO3; como resultado del creciente aporte de sedimentos del Canal de Dique y las condiciones oceanográficas presentes. Estas condiciones hidrodinámicas favorecen la meteorización y erosión de las formaciones coralinas presentes en el Caribe Colombiano.In the sedimentological analysis of 253 sediment samples from the edges and the bottom of the Cartagena bay, we identified 26 sedimentary facies. Samples were taken with Van Veen dredge for the bottom sediment Beeker multisampler and test pits for shorelines. Using the Bernard calcimeter we carried out the calcium carbonate (CaCO3) analysis in every sieved sample. Sediments were classified according to the average grain size and content of CaCO3, to define the existing sedimentary facies, where mud litoclast facies was the most representative and distributed to the north and south of the Tierra Bomba in the internal and external bay. We found that during the past 30 years the distribution of sedimentary facies has experienced significant changes: in the inner bay the content of CaCO3 has decreased and in the southwest the grain size and content of CaCO3 have decreased, as a result of the sediments contribution from Canal del Dique and present oceanographic conditions. These hydrodynamic conditions contribute to the weathering and erosion of coral formations present in the Colombian CaribbeanFil: Franco Arias, Diana Aillen. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto Geológico del Sur; Argentina. Universidad Industrial de Santander; ColombiaFil: Restrepo López, J. C.. Universidad del Norte; ColombiaFil: Sanabria Ruiz, N. Y.. Universidad Industrial de Santander; ColombiaFil: Gutierrez, Julio C.. Centro de Investigaciones Oceanográficas e Hidrográficas; Colombi

The double-Kerr equilibrium configurations involving one extreme object

We demonstrate the existence of equilibrium states in the limiting cases of the double-Kerr solution when one of the constituents is an extreme object. In the `extreme-subextreme' case the negative mass of one of the constituents is required for the balance, whereas in the `extreme-superextreme' equilibrium configurations both Kerr particles may have positive masses. We also show that the well-known relation |J|=M^2 between the mass and angular momentum in the extreme single Kerr solution ceases to be a characteristic property of the extreme Kerr particle in a binary system.Comment: 12 pages, 3 figures, submitted to Class. Quantum Gra