43,186 research outputs found
Design, fabrication and evaluation of chalcogenide glass Luneburg lenses for LiNbO3 integrated optical devices
Optical waveguide Luneburg lenses of arsenic trisulfide glass are described. The lenses are formed by thermal evaporation of As2S3 through suitably placed masks onto the surface of LiNbO3:Ti indiffused waveguides. The lenses are designed for input apertures up to 1 cm and for speeds of f/5 or better. They are designed to focus the TM sub 0 guided mode of a beam of wavelength, external to the guide, of 633 nm. The refractive index of the As2S3 films and the changes induced in the refractive index by exposure to short wavelength light were measured. Some correlation between film thickness and optical properties was noted. The short wavelength photosensitivity was used to shorten the lens focal length from the as deposited value. Lenses of rectangular shape, as viewed from above the guide, as well as conventional circular Luneburg lenses, were made. Measurements made on the lenses include thickness profile, general optical quality, focal length, quality of focal spot, and effect of ultraviolet irradiation on optical properties
Cosmological Symmetry Breaking, Pseudo-scale invariance, Dark Energy and the Standard Model
The energy density of the universe today may be dominated by the vacuum
energy of a slowly rolling scalar field. Making a quantum expansion around such
a time dependent solution is found to break fundamental symmetries of quantum
field theory. We call this mechanism cosmological symmetry breaking and argue
that it is different from the standard phenomenon of spontaneous symmetry
breaking. We illustrate this with a toy scalar field theory, whose action
displays a U(1) symmetry. We identify a symmetry, called pseudo-scale
invariance, which sets the cosmological constant exactly equal to zero, both in
classical and quantum theory. This symmetry is also broken cosmologically and
leads to a nonzero vacuum or dark energy. The slow roll condition along with
the observed value of dark energy leads to a value of the background scalar
field of the order of Planck mass. We also consider a U(1) gauge symmetry
model. Cosmological symmetry breaking, in this case, leads to a non zero mass
for the vector field. We also show that a cosmologically broken pseudo-scale
invariance can generate a wide range of masses.Comment: 18 pages, no figure
The infall of the Virgo elliptical galaxy M60 toward M87 and the gaseous structures produced by Kelvin-Helmholtz instabilities
We present Chandra observations of hot gas structures, characteristic of gas stripping during infall, in the Virgo cluster elliptical galaxy M60 (NGC4649) located 1 Mpc east of M87. 0.5â2 keV Chandra X-ray images show a sharp leading edge in the surface brightness 12.4±0.1 kpc north and west of the galaxy center in the direction of M87 characteristic of a merger cold front due to M60's motion through the Virgo ICM. We measured a temperature of 1.00±0.02 keV for abundance 0.5Zâ inside the edge and 1.37+0.35â0.19 keV for abundance 0.1Zâ in the Virgo ICM free stream region. We find that the observed jump in surface brightness yields a density ratio of 6.44+1.04â0.67 between gas inside the edge and in the cluster free stream region. If the edge is a cold front due solely to the infall of M60 in the direction of M87, we find a pressure ratio of 4.7+1.7â1.4 and Mach number 1.7±0.3. For 1.37 keV Virgo gas we find a total infall velocity for M60 of 1030±180 kmsâ1. We calculate the motion in the plane of the sky to be 1012+183â192 kmâ1 implying an inclination angle Ο=11±3 degrees. Surface brightness profiles show the presence of a faint diffuse gaseous tail. We identify filamentary, gaseous wing structures caused by the galaxy's motion through the ICM. The structure and dimensions of these wings are consistent with simulations of Kelvin-Helmholtz instabilities as expected if the gas stripping is close to inviscid
An Economic analysis of the potential for precision farming in UK cereal production
The results from alternative spatial nitrogen application studies are analysed in economic terms and compared to the costs of precision farming hardware, software and other services for cereal crops in the UK. At current prices, the benefits of variable rate application of nitrogen exceed the returns from a uniform application by an average of ÂŁ22 haâ1 The cost of the precision farming systems range from ÂŁ5 to ÂŁ18 haâ1 depending upon the system chosen for an area of 250 ha. The benefits outweigh the associated costs for cereal farms in excess of 80 ha for the lowest price system to 200â300 ha for the more sophisticated systems. The scale of benefits obtained depends upon the magnitude of the response to the treatment and the proportion of the field that will respond. To be cost effective, a farmed area of 250 ha of cereals, where 30% of the area will respond to variable treatment, requires an increase in crop yield in the responsive areas of between 0·25 and 1.00 t haâ1 (at ÂŁ65 tâ1) for the basic and most expensive precision farming systems, respectively
Photometry of SN 2002ic and Implications for the Progenitor Mass-Loss History
We present new pre-maximum and late-time optical photometry of the Type
Ia/IIn supernova 2002ic. These observations are combined with the published
V-band magnitudes of Hamuy et al. (2003) and the VLT spectrophotometry of Wang
et al. (2004) to construct the most extensive light curve to date of this
unusual supernova. The observed flux at late time is significantly higher
relative to the flux at maximum than that of any other observed Type Ia
supernova and continues to fade very slowly a year after explosion. Our
analysis of the light curve suggests that a non-Type Ia supernova component
becomes prominent days after explosion. Modeling of the non-Type Ia
supernova component as heating from the shock interaction of the supernova
ejecta with pre-existing circumstellar material suggests the presence of a
cm gap or trough between the progenitor system and the
surrounding circumstellar material. This gap could be due to significantly
lower mass-loss years prior to explosion or
evacuation of the circumstellar material by a low-density fast wind. The latter
is consistent with observed properties of proto-planetary nebulae and with
models of white-dwarf + asymptotic giant branch star progenitor systems with
the asymptotic giant branch star in the proto-planetary nebula phase.Comment: accepted for publication in Ap
The Three-Nucleon System Near the N-d Threshold
The three-nucleon system is studied at energies a few hundred keV above the
N-d threshold. Measurements of the tensor analyzing powers and
for p-d elastic scattering at keV are presented
together with the corresponding theoretical predictions. The calculations are
extended to very low energies since they are useful for extracting the p-d
scattering lengths from the experimental data. The interaction considered here
is the Argonne V18 potential plus the Urbana three-nucleon potential. The
calculation of the asymptotic D- to S-state ratio for H and He, for
which recent experimental results are available, is also presented.Comment: Latex, 11 pages, 2 figures, to be published in Phy.Lett.
Hydrodynamics of the stream-disk impact in interacting binaries
We use hydrodynamic simulations to provide quantitative estimates of the
effects of the impact of the accretion stream on disks in interacting binaries.
For low accretion rates, efficient radiative cooling of the hotspot region can
occur, and the primary consequence of the stream impact is stream overflow
toward smaller disk radii. The stream is well described by a ballistic
trajectory, but larger masses of gas are swept up and overflow at smaller, but
still highly supersonic, velocities. If cooling is inefficient, overflow still
occurs, but there is no coherent stream inward of the disk rim. Qualitatively,
the resulting structure appears as a bulge extending downstream along the disk
rim. We calculate the mass fraction and velocity of the overflowing component
as a function of the important system parameters, and discuss the implications
of the results for X-ray observations and doppler tomography of cataclysmic
variables, low-mass X-ray binaries and supersoft X-ray sources.Comment: 16 pages, including 8 figures. 1 color figure as a jpeg. ApJ, in
pres
Up and down the number line: modelling collaboration in contrasting school and home environments
This paper is concerned with user modelling issues such as adaptive educational environments, adaptive information retrieval, and support for collaboration. The HomeWork project is examining the use of learner modelling strategies within both school and home environments for young children aged 5 â 7 years. The learning experience within the home context can vary considerably from school especially for very young learners, and this project focuses on the use of modelling which can take into account the informality and potentially contrasting learning styles experienced within the home and school
Feasibility investigation of integrated optics Fourier transform devices
The possibility of producing an integrated optics data processing device based upon Fourier transformations or other parallel processing techniques, and the ways in which such techniques may be used to upgrade the performance of present and projected NASA systems were investigated. Activities toward this goal include; (1) production of near-diffraction-limited geodesic lenses in glass waveguides; (2) development of grinding and polishing techniques for the production of geodesic lenses in LiNbO3 waveguides; (3) development of a characterization technique for waveguide lenses; and (4) development of a theory for corrected aspheric geodesic lenses. A holographic subtraction system was devised which should be capable of rapid on-board preprocessing of a large number of parallel data channels. The principle involved is validated in three demonstrations
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