22,138 research outputs found
Dissipation of dark matter
Fluids often display dissipative properties. We explore dissipation in the
form of bulk viscosity in the cold dark matter fluid. We constrain this model
using current data from supernovae, baryon acoustic oscillations and the cosmic
microwave background. Considering the isotropic and homogeneous background
only, viscous dark matter is allowed to have a bulk viscosity
Pas, also consistent with the expected integrated Sachs-Wolfe effect
(which plagues some models with bulk viscosity). We further investigate the
small-scale formation of viscous dark matter halos, which turns out to place
significantly stronger constraints on the dark matter viscosity. The existence
of dwarf galaxies is guaranteed only for much smaller values of the dark matter
viscosity, Pas.Comment: 10 pages, 3 figures, published in PR
Experimental Generation and Observation of Intrinsic Localized Spin Wave Modes in an Antiferromagnet
By driving with a microwave pulse the lowest frequency antiferromagnetic
resonance of the quasi 1-D biaxial antiferromagnet (C_2 H_5 NH_3)_2 CuCl_4 into
an unstable region intrinsic localized spin waves have been generated and
detected in the spin wave gap. These findings are consistent with the
prediction that nonlinearity plus lattice discreteness can lead to localized
excitations with dimensions comparable to the lattice constant.Comment: 10 pages, 4 figures, accepted for publication in Physical Review
Letter
Kinematic and morphological modeling of the bipolar nebula Sa2-237
We present [OIII]500.7nm and Halpha+[NII] images and long-slit, high
resolution echelle spectra in the same spectral regions of Sa2--237, a possible
bipolar planetary nebula. The image shows a bipolar nebula of about 34" extent,
with a narrow waist, and showing strong point symmetry about the central
object, indicating it's likely binary nature. The long slit spectra were taken
over the long axis of the nebula, and show a distinct ``eight'' shaped pattern
in the velocity--space plot, and a maximum projected outflow velocity of
V=106km/s, both typical of expanding bipolar planetary nebulae. By model
fitting the shape and spectrum of the nebula simultaneously, we derive the
inclination of the long axis to be 70 degrees, and the maximum space velocity
of expansion to be 308 km/s. Due to asymmetries in the velocities we adopt a
new value for the system's heliocentric radial velocity of -30km/s. We use the
IRAS and 21cm radio fluxes, the energy distribution, and the projected size of
Sa2-237 to estimate it's distance to be 2.1+-0.37kpc. At this distance Sa2-237
has a luminosity of 340 Lsun, a size of 0.37pc, and -- assuming constant
expansion velocity -- a nebular age of 624 years. The above radial velocity and
distance place Sa2--237 in the disk of the Galaxy at z=255pc, albeit with
somewhat peculiar kinematics.Comment: 10pp, 4 fig
Long beating wavelength in the Schwarz-Hora effect
Thirty years ago, H.Schwarz has attempted to modulate an electron beam with
optical frequency. When a 50-keV electron beam crossed a thin crystalline
dielectric film illuminated with laser light, electrons produced the
electron-diffraction pattern not only at a fluorescent target but also at a
nonfluorescent target. In the latter case the pattern was of the same color as
the laser light (the Schwarz-Hora effect). This effect was discussed
extensively in the early 1970s. However, since 1972 no reports on the results
of further attempts to repeat those experiments in other groups have appeared,
while the failures of the initial such attempts have been explained by Schwarz.
The analysis of the literature shows there are several unresolved up to now
contradictions between the theory and the Schwarz experiments. In this work we
consider the interpretation of the long-wavelength spatial beating of the
Schwarz-Hora radiation. A more accurate expression for the spatial period has
been obtained, taking into account the mode structure of the laser field within
the dielectric film. It is shown that the discrepancy of more than 10% between
the experimental and theoretical results for the spatial period cannot be
reduced by using the existing quantum models that consider a collimated
electron beam.Comment: 3 pages, RevTe
3-D Photoionization Structure and Distances of Planetary Nebulae II. Menzel 1
We present the results of a spatio-kinematic study of the planetary nebula
Menzel 1 using spectro-photometric mapping and a 3-D photoionization code. We
create several 2-D emission line images from our long-slit spectra, and use
these to derive the line fluxes for 15 lines, the Halpha/Hbeta extinction map,
and the [SII] line ratio density map of the nebula. We use our photoionization
code constrained by these data to derive the three-dimensional nebular
structure and ionizing star parameters of Menzel 1 by simultaneously fitting
the integrated line intensities, the density map, and the observed morphologies
in several lines, as well as the velocity structure. Using theoretical
evolutionary tracks of intermediate and low mass stars, we derive a mass for
the central star of 0.63+-0.05 Msolar. We also derive a distance of 1050+_150
pc to Menzel 1.Comment: To be published in ApJ of 10th February 2005. 12 figure
Level statistics for quantum -core percolation
Quantum -core percolation is the study of quantum transport on -core
percolation clusters where each occupied bond must have at least occupied
neighboring bonds. As the bond occupation probability, , is increased from
zero to unity, the system undergoes a transition from an insulating phase to a
metallic phase. When the lengthscale for the disorder, , is much greater
than the coherence length, , earlier analytical calculations of quantum
conduction on the Bethe lattice demonstrate that for the metal-insulator
transition (MIT) is discontinuous, suggesting a new universality class of
disorder-driven quantum MITs. Here, we numerically compute the level spacing
distribution as a function of bond occupation probability and system size
on a Bethe-like lattice. The level spacing analysis suggests that for ,
, the quantum percolation critical probability, is greater than , the
geometrical percolation critical probability, and the transition is continuous.
In contrast, for , and the transition is discontinuous such that
these numerical findings are consistent with our previous work to reiterate a
new universality class of disorder-driven quantum MITs.Comment: 8 pages, 11 figure
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