56,488 research outputs found
Orbit classification in the meridional plane of a disk galaxy model with a spherical nucleus
We investigate the regular or chaotic nature of star orbits moving in the
meridional plane of an axially symmetric galactic model with a disk and a
spherical nucleus. We study the influence of some important parameters of the
dynamical system, such as the mass and the scale length of the nucleus, the
angular momentum or the energy, by computing in each case the percentage of
chaotic orbits, as well as the percentages of orbits of the main regular
resonant families. Some heuristic arguments to explain and justify the
numerically derived outcomes are also given. Furthermore, we present a new
method to find the threshold between chaos and regularity for both Lyapunov
Characteristic Numbers and SALI, by using them simultaneously.Comment: Published in Celestial Mechanics & Dynamical Astronomy (CMDA) journa
Feynman path-integral approach to the QED3 theory of the pseudogap
In this work the connection between vortex condensation in a d-wave
superconductor and the QED gauge theory of the pseudogap is elucidated. The
approach taken circumvents the use of the standard Franz-Tesanovic gauge
transformation, borrowing ideas from the path-integral analysis of the
Aharonov-Bohm problem. An essential feature of this approach is that
gauge-transformations which are prohibited on a particular multiply-connected
manifold (e.g. a superconductor with vortices) can be successfully performed on
the universal covering space associated with that manifold.Comment: 15 pages, 1 Figure. Int. J. Mod. Phys. B 17, 4509 (2003). Minor
changes from previous versio
The Singing Insects of Michigan
Excerpt: The so-called singing insects are all those that make loud, rhythmical noises. They include members of three groups of Orthoptera (Gryllidae, Tettigoniidae, and Acridoidea) and one family of Homoptera (Cicadidae). There are about 300 noisy species in these four groups in eastern North America, perhaps a thousand in all of North America, and 25-30 thousand in the entire world. Only about 1000 of the world species have been studied in any detail, mostly in North America, Europe, Japan, and Australia
Information technology and productivity: where are we now and where are we going?
Productivity growth in the U.S. economy jumped during the second half of the 1990s, a resurgence that many analysts linked to developments in information technology (IT). However, shortly after this consensus emerged, demand for IT products fell sharply, leading to a debate about the connection between IT and productivity and about the sustainability of the faster growth. ; This article contributes to this debate in two ways. First, the authors provide updated estimates of the proximate sources of growth using a growth accounting framework that focuses on information technology. Their results confirm that the acceleration in labor productivity after 1995 was driven by the greater use of IT capital goods and the more rapid efficiency gains in the production of these goods. Second, to assess whether the pickup in productivity growth is sustainable, the authors analyze the steady-state properties of a multisector growth model. This exercise generates a range for labor productivity growth of 2 percent to 2 3/4 percent per year, which suggests that much-and possibly all-of the resurgence is sustainable. ; The analysis also highlights that future increases in output will depend on the pace of technological advance in the semiconductor industry and on the extent to which products embodying these advances diffuse through the economy.Information technology ; Productivity ; Technology ; Economic development
Discrete Fourier Transform in Nanostructures using Scattering
In this paper we show that the discrete Fourier transform can be performed by
scattering a coherent particle or laser beam off a two-dimensional potential
that has the shape of rings or peaks. After encoding the initial vector into
the two-dimensional potential, the Fourier-transformed vector can be read out
by detectors surrounding the potential. The wavelength of the laser beam
determines the necessary accuracy of the 2D potential, which makes our method
very fault-tolerant.Comment: 6 pages, 5 EPS figures, REVTe
ROSAT HRI Detection of the 16 ms Pulsar PSR J0537-6910 Inside SNR N157B
Based on a deep ROSAT HRI observation, we have detected a pulsed signal in
the 0.1-2 keV band from PSR J0537-6910 --- the recently discovered pulsar
associated with the supernova remnant N157B in the Large Magellanic Cloud. The
measured pulse period 0.01611548182 ms (+- 0.02 ns), Epoch MJD 50540.5, gives a
revised linear spin-down rate of , slightly
greater than the previously derived value. The narrow pulse shape (FWHM = 10%
duty cycle) in the ROSAT band resembles those seen in both XTE and ASCA data (>
2 keV), but there is also marginal evidence for an interpulse. This ROSAT
detection enables us to locate the pulsar at R.A., Dec (J2000) =
. With its uncertainty , this
position coincides with the centroid of a compact X-ray source. But the pulsed
emission accounts for only about 10% of the source luminosity in the 0.1-2 keV band. These results support our previous
suggestions: (1) The pulsar is moving at a high velocity ();
(2) A bow shock, formed around the pulsar, is responsible for most of the X-ray
emission from the source; (3) A collimated outflow from the bow shock region
powers a pulsar wind nebula that accounts for an elongated non-thermal radio
and X-ray feature to the northwest of the pulsar.Comment: 6 pages including 3 figures. To be published in ApJ
Simulations of the Kelvin-Helmholtz instability driven by coronal mass ejections in the turbulent corona
Recent high resolution AIA/SDO images show evidence of the development of the
Kelvin-Helmholtz instability, as coronal mass ejections (CMEs) expand in the
ambient corona. A large-scale magnetic field mostly tangential to the interface
is inferred, both on the CME and on the background sides. However, the magnetic
field component along the shear flow is not strong enough to quench the
instability. There is also observational evidence that the ambient corona is in
a turbulent regime, and therefore the criteria for the development of the
instability are a-priori expected to differ from the laminar case. To study the
evolution of the Kelvin-Helmholtz instability with a turbulent background, we
perform three-dimensional simulations of the incompressible magnetohydrodynamic
equations. The instability is driven by a velocity profile tangential to the
CME-corona interface, which we simulate through a hyperbolic tangent profile.
The turbulent background is generated by the application of a stationary
stirring force. We compute the instability growth-rate for different values of
the turbulence intensity, and find that the role of turbulence is to attenuate
the growth. The fact that the Kelvin-Helmholtz instability is observed, sets an
upper limit to the correlation length of the coronal background turbulence
- …