6,748 research outputs found
From ergodic to non-ergodic chaos in Rosenzweig-Porter model
The Rosenzweig-Porter model is a one-parameter family of random matrices with
three different phases: ergodic, extended non-ergodic and localized. We
characterize numerically each of these phases and the transitions between them.
We focus on several quantities that exhibit non-analytical behaviour and show
that they obey the scaling hypothesis. Based on this, we argue that non-ergodic
chaotic and ergodic regimes are separated by a continuous phase transition,
similarly to the transition between non-ergodic chaotic and localized phases.Comment: 12 page
Studying a dual-species BEC with tunable interactions
We report on the observation of controllable spatial separation in a
dual-species Bose-Einstein condensate (BEC) with Rb and Rb.
Interparticle interactions between the different components can change the
miscibility of the two quantum fluids. In our experiments, we clearly observe
the immiscible nature of the two simultaneously Bose-condensed species via
their spatial separation. Furthermore the Rb Feshbach resonance near 155
G is used to change them between miscible and immiscible by tuning the
Rb scattering length. Our apparatus is also able to create Rb
condensates with up to atoms which represents a significant
improvement over previous work
3-D Simulations of Protostellar Jets in Stratified Ambient Media
We present fully three-dimensional hydrodynamical simulations of radiative
cooling jets propagating into stratified isothermal ambient media with
power-law density and pressure distributions. The parameters used are mainly
suitable for protostellar jets but results applicable to extragalactic jets are
also presented. Comparisons are made with previous simulations of jets through
homogeneous media. We find that for radiative cooling jets propagating into
regions where the ambient medium has an increasing density (and pressure)
gradient, the ambient gas tends to compress the cold, low-pressure cocoon of
shocked material that surrounds the beam and destroy the bow shock-like
structure at the head. The compressing medium collimates the jet and promotes
the development of Kelvin-Helmholtz instabilities which cause beam focusing,
wiggling and the formation of internal traveling shocks,
, via pinching along the beam. This remarkably resembles the structure of
some observed systems (e.g. Haro 6-5B northern and HH 24G jets). These effects
are larger for jets with smaller density ratio between jet and environment
(tested for =1, 3, and 10) and larger Mach number
(tested for 12 and 24, where is the jet velocity and the
ambient sound speed). In an ambient medium of decreasing density (and
pressure), the beam is poorly collimated and relaxes, becoming faint. This
could explain ''invisible'' jet sections, like the gap between the parent
source and collimated beam (e.g., in HH30 jet). Although, on average, jets
propagating into an increasing (decreasing) density environment are decelerated
(accelerated) by the increasing (decreasing) ram pressure of the ambient
medium, we find that their propagation velocities have an oscillating pattern.Comment: 33 pp, LaTeX file, 13 figures upon request. To appear in the
Astrophys. J., vol 471, nov. 10t
Magnetic Field Effects on the Head Structure of Protostellar Jets
We present the results of 3-D SPMHD numerical simulations of
supermagnetosonic, overdense, radiatively cooling jets. Two initial magnetic
configurations are considered: (i) a helical and (ii) a longitudinal field. We
find that magnetic fields have important effects on the dynamics and structure
of radiative cooling jets, especially at the head. The presence of a helical
field suppresses the formation of the clumpy structure which is found to
develop at the head of purely hydrodynamical jets. On the other hand, a cooling
jet embedded in a longitudinal magnetic field retains clumpy morphology at its
head. This fragmented structure resembles the knotty pattern commonly observed
in HH objects behind the bow shocks of HH jets. This suggests that a strong
(equipartition) helical magnetic field configuration is ruled out at the jet
head. Therefore, if strong magnetic fields are present, they are probably
predominantly longitudinal in those regions. In both magnetic configurations,
we find that the confining pressure of the cocoon is able to excite
short-wavelength MHD K-H pinch modes that drive low-amplitude internal shocks
along the beam. These shocks are not strong however, and it likely that they
could only play a secondary role in the formation of the bright knots observed
in HH jets.Comment: 14 pages, 2 Gif figures, uses aasms4.sty. Also available on the web
page http://www.iagusp.usp.br/preprints/preprint.html. To appear in The
Astrophysical Journal Letter
Modeling the spectrum of gravitational waves in the primordial Universe
Recent observations from type Ia Supernovae and from cosmic microwave
background (CMB) anisotropies have revealed that most of the matter of the
Universe interacts in a repulsive manner, composing the so-called dark energy
constituent of the Universe. The analysis of cosmic gravitational waves (GW)
represents, besides the CMB temperature and polarization anisotropies, an
additional approach in the determination of parameters that may constrain the
dark energy models and their consistence. In recent work, a generalized
Chaplygin gas model was considered in a flat universe and the corresponding
spectrum of gravitational waves was obtained. The present work adds a massless
gas component to that model and the new spectrum is compared to the previous
one. The Chaplygin gas is also used to simulate a -CDM model by means
of a particular combination of parameters so that the Chaplygin gas and the
-CDM models can be easily distinguished in the theoretical scenarios
here established. The lack of direct observational data is partialy solved when
the signature of the GW on the CMB spectra is determined.Comment: Proc. of the Conference on Magnetic Fields in the Universe: from
laboratories and stars to primordial structures, AIP(NY), eds. E. M. de
Gouveia Dal Pino, G. Lugones & A. Lazarian (2005), in press. (8 pages, 11
figures
Consumption of Analeptics and Energizing Drinks to Improve Academic Performance in Students From First to Third Level of the Career of Medicine
Central nervous system stimulant drugs are frequently used by college students as factors to alleviate fatigue, maintain wakefulness, and stimulate cognitive abilities, which has become a problem because the physical or emotional consequences that may occur are not considered. produce. At the time of marketing these products they do not keep any type of control, it is freely distributed to the general public. Objective: to analyze how the consumption of analeptics and energy drinks affects the academic performance of students. Methods: An explanatory, cross-sectional, quantitative-qualitative study was carried out, to obtain data, the survey was applied to first to third level students of the Medicine School of the Polytechnic School of Chimborazo (ESPOCH). Results: From the compilation of the information it can be determined that 65% of the students use stimulants to increase their performance, without considering the consequences such as palpitations, insomnia, headache, tremor, vomiting, gastrointestinal discomfort as well as affections to different organs and body systems. Bottom Line: The cognitive effects of stimulants commonly bring about changes in performance by accelerating the production of dopamine in the brain, which is a neurotransmitter that is related to attention, pleasure, memory and performance. Adverse effects can vary from mild to very serious, causing irritability, headache, arrhythmias, hypertension, dizziness, and lesser degrees such as difficulty in bowel movement, itching or skin rash, among others.
Keywords: analleptic, stimuli, effects, academic performance.
Resumen
Los estudiantes universitarios consumen con frecuencia los fármacos estimulantes del sistema nervioso central como factores para aliviar la fatiga, mantener la vigilia y estimular las capacidades cognitivas, lo cual se ha convertido en un problema, porque no se consideran las consecuencias físicas o emocionales que se pueden producir. Al momento de comercializar estos productos no guardan algún tipo de control, es de libre distribución al público general. Objetivo: analizar como el consumo de analépticos y bebidas energizantes incide en el rendimiento académico de los estudiantes. Métodos: Se realizó un estudio explicativo de corte transversal, de tipo cuanti- cualitativo, para la obtención de datos se aplicó la encuesta a los estudiantes de primer a tercer nivel de la Carrera de Medicina de la Escuela Superior Politécnica de Chimborazo (ESPOCH). Resultados: De la recopilación de la información se puede determinar que un 65% de los estudiantes utilizan los estimulantes para elevar su rendimiento, sin considerar las consecuencias como palpitaciones, insomnio, cefalea, temblor, vómito, molestias gastrointestinales así como afecciones a distintos órganos y sistemas del cuerpo. Conclusión: Los efectos cognitivos de los estimulantes, comúnmente aportan cambios en el rendimiento al acelerar la producción de dopamina en el cerebro, que es un neurotransmisor que se relaciona con la atención, el placer, la memoria y el rendimiento. Los efectos adversos pueden variar de leves a muy graves causando irritabilidad, dolor de cabeza, arritmias, hipertensión, mareos y los de menor grado como dificultad para la evacuación intestinal, picazón o erupción en la piel, entre otras.
Palabras clave: analépticos, estímulos, efectos, rendimiento académico
The Ljapunov-Schmidt reduction for some critical problems
This is a survey about the application of the Ljapunov-Schmidt reduction for
some critical problems
Global axisymmetric Magnetorotational Instability with density gradients
We examine global incompressible axisymmetric perturbations of a
differentially rotating MHD plasma with radial density gradients. It is shown
that the standard magnetorotational instability, (MRI) criterion drawn from the
local dispersion relation is often misleading. If the equilibrium magnetic
field is either purely axial or purely toroidal, the problem reduces to finding
the global radial eigenvalues of an effective potential. The standard Keplerian
profile including the origin is mathematically ill-posed, and thus any solution
will depend strongly on the inner boundary. We find a class of unstable modes
localized by the form of the rotation and density profiles, with reduced
dependence on boundary conditions.Comment: 22 pages, 5 figure
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