458 research outputs found
Preliminary report on IUE spectra of the Crab Nebula
The Crab Nebula is marginally observable with the IUE. Observations of the optically brightest filamentary regions, made with IUE in August 1979, show the C IV lambda 1549, He II lambda 1640, and C III lambda 1909 emission lines. The intensities of these lines were compared with the visual wavelength data. It appears that carbon is not overabundant in the Crab; carbon/oxygen is approximately normal and oxygen is slightly scarcer than normal as a fraction of the total mass
Comparison of Cell Sediment and Surface Grown Test Plaque Using Scanning Electron Microscopy
Scanning electron microscopy was used to compare the morphology, integrity and distribution of bacterial cells in a test plaque grown on the surface of enamel with that of the cell sediment plaque routinely used in a short-term intraoral caries model. Cultures of S. mutans IB-1600 or S. sobrinus 6715-13 were grown in complex media supplemented with either 2.0% sucrose (glucan plaque) or 0.2 % glucose (non-glucan plaque). Cell sediment (CS) plaque was prepared by centrifuging the cultures after incubation, recovering the cell sediment, and spreading it on Metricel membrane filter paper. Surface grown (SG) plaque was prepared by suspending saliva-coated bovine enamel in the culture inedium, incubating, and recovering the enamel assembly with bacterial accumulations. Cell morphology and integrity, as well as the appearance of glucan-like material produced by the cells, was similar in both CS and SG test plaques. The cell distribution however, varied in the SG plaque from extremes of all cells to all glucan, whereas the cell sediment plaque was more uniform in cell distribution. A highly standardized test plaque minimizes variability in the intraoral caries model. These findings support the contention that the bacterial cells in a cell sediment plaque are similar in morphology, integrity and glucan production to surface grown plaque, and have the added advantage of uniform distribution, which makes the cell sediment plaque more appropriate for intraoral caries model studies
Timelike surfaces in Lorentz covariant loop gravity and spin foam models
We construct a canonical formulation of general relativity for the case of a
timelike foliation of spacetime. The formulation possesses explicit covariance
with respect to Lorentz transformations in the tangent space. Applying the loop
approach to quantize the theory we derive the spectrum of the area operator of
a two-dimensional surface. Its different branches are naturally associated to
spacelike and timelike surfaces. The results are compared with the predictions
of Lorentzian spin foam models. A restriction of the representations labeling
spin networks leads to perfect agreement between the states as well as the area
spectra in the two approaches.Comment: a wrong sign corrected in equation (65
Relating Spin Foams and Canonical Quantum Gravity: A Discrete Step Evolution Formulation of Spin Foams
This article has been replaced by gr-qc/0412011Comment: This article has been replaced by gr-qc/041201
Preliminary Report on IUE Spectra of Crab Nebula
The Crab Nebula is marginally observable with the IUE. Observations of the optically brightest filamentary regions, made with IUE in August 1979, show the C IV x1549, He II x1640, and C III] x1909 emission lines. The intensities of these lines have been compared with visual-wavelength data. It appears that carbon is not overabundant in the Crab; carbon/- oxygen is approximately normal oxygen is slightly scarcer than normal as a fraction of the total mass.https://digitalcommons.chapman.edu/scs_books/1005/thumbnail.jp
Exchange interaction effects in inter-Landau level Auger scattering in a two-dimensional electron gas
We consider the influence of spin effects on the inter-Landau level
electron-electron scattering rate in a two-dimensional electron gas. Due to the
exchange spin splitting, the Landau levels are not equidistant. This leads to
the suppresion of Auger processes and a nonlinear dependence of the lifetime on
the concentration of the excited electrons even at very low excitation levels.Comment: 10 pages, 3 figure
The Asymmetric Wind in M82
We have obtained detailed imaging Fabry-Perot observations of the nearby
galaxy M82, in order to understand the physical association between the
high-velocity outflow and the starburst nucleus. The observed velocities of the
emitting gas in M82 reveal a bipolar outflow of material, originating from the
bright starburst regions in the galaxy's inner disk, but misaligned with
respect to the galaxy spin axis. The deprojected outflow velocity increases
with radius from 525 to 655 km/s. Spectral lines show double components in the
centers of the outflowing lobes, with the H-alpha line split by ~300 km/s over
a region almost a kiloparsec in size. The filaments are not simple surfaces of
revolution, nor is the emission distributed evenly over the surfaces. We model
these lobes as a composite of cylindrical and conical structures, collimated in
the inner ~500 pc but expanding at a larger opening angle of ~25 degrees beyond
that radius. We compare our kinematic model with simulations of
starburst-driven winds in which disk material surrounding the source is
entrained by the wind. The data also reveal a remarkably low [NII]/H-alpha
ratio in the region of the outflow, indicating that photoionization by the
nuclear starburst may play a significant role in the excitation of the optical
filament gas, particularly near the nucleus.Comment: 42 pages AASTeX with 16 figures; accepted for publication in ApJ;
figures reformatted for better printin
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Ultraviolet Spectrum of the Crab Nebula
Data from 65 hours of observation of the Crab Nebula with the International Ultraviolet Explorer are reported, together with new ground-based spectrophotometry. We have measured the important C IV λ1549, He II λ1640, and C III] λ1908 emission line intensities and placed upper limits on other ultraviolet features for the brightest filamentary region in the nebula. We have also measured some ultraviolet continuum surface brightnesses at two places in the Crab. The emission lines imply an average ionic abundance ratio n(C^+2)/n(O^+2 ) in the range from 0.4 to 1.5 in the observed gaseous condensations. The elemental abundance ratio of carbon to oxygen is probably in the same range. This range may be narrowed through the use of ionization calculations, but even our simple analysis shows that there is no perceptible excess of carbon, due to presupernova nucleosynthesis, in the observed region. The large helium abundance, small carbon and oxygen abundances, and presence of a neutron star in the Crab Nebula suggest that the presupernova star had a mass close to 8 M0 when it was on the main sequence
Propagation of light in area metric backgrounds
The propagation of light in area metric spacetimes, which naturally emerge as
refined backgrounds in quantum electrodynamics and quantum gravity, is studied
from first principles. In the geometric-optical limit, light rays are found to
follow geodesics in a Finslerian geometry, with the Finsler norm being
determined by the area metric tensor. Based on this result, and an
understanding of the non-linear relation between ray vectors and wave covectors
in such refined backgrounds, we study light deflection in spherically symmetric
situations, and obtain experimental bounds on the non-metricity of spacetime in
the solar system.Comment: 18pp, no figures, Journal versio
Auger-like Relaxation of Inter-Landau-Level Magneto-Plasmon Excitations in the Quantised Hall Regime
Auger relaxation in 2D strongly correlated electron gas can be represented as
an Auger-like process for neutral magnetoplasmon excitations. The case of
"dielectric" state with lack of free electrons (i.e. at integer filling )
is considered. Really the Auger-like process is a coalescence of two
magnetoplasmons which are converted into a single one of a different plasmon
mode with zero 2D wave-vector. This event turns out to be energetically allowed
for magnetoplasmons near their roton minima where the spectrum has the infinite
density of states. As a result the additional possibility appears for indirect
observation of the magnetorotons by means of anti-Stokes Raman scattering. We
find the rate of this process employing the technique of Excitonic
Representation for the relevant matrix element calculation.Comment: 4 page
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