21,960 research outputs found
Complete spatial characterization of an optical wavefront using a variable-separation pinhole pair
We present a technique for measuring the transverse spatial properties of an
optical wavefront. Intensity and phase profiles are recovered by analysis of a
series of interference patterns produced by the combination of a scanning
X-shaped slit and a static horizontal slit; the spatial coherence may be found
from the same data. We demonstrate the technique by characterizing high
harmonic radiation generated in a gas cell, however the method could be
extended to a wide variety of light sources.Comment: 4 pages, 3 figures, 1 tabl
Macroevolutionary Patterns In The Evolutionary Radiation Of Archosaurs (Tetrapoda: Diapsida)
The rise of archosaurs during the Triassic and Early Jurassic has been treated as a classic example of an evolutionary radiation in the fossil record. This paper reviews published studies and provides new data on archosaur lineage origination, diversity and lineage evolution, morphological disparity, rates of morphological character change, and faunal abundance during the TriassicâEarly Jurassic. The fundamental archosaur lineages originated early in the Triassic, in concert with the highest rates of character change. Disparity and diversity peaked later, during the Norian, but the most significant increase in disparity occurred before maximum diversity. Archosaurs were rare components of EarlyâMiddle Triassic faunas, but were more abundant in the Late Triassic and pre-eminent globally by the Early Jurassic. The archosaur radiation was a drawn-out event and major components such as diversity and abundance were discordant from each other. Crurotarsans (crocodile-line archosaurs) were more disparate, diverse, and abundant than avemetatarsalians (bird-line archosaurs, including dinosaurs) during the Late Triassic, but these roles were reversed in the Early Jurassic. There is no strong evidence that dinosaurs outcompeted or gradually eclipsed crurotarsans during the Late Triassic. Instead, crurotarsan diversity decreased precipitously by the end-Triassic extinction, which helped usher in the age of dinosaurian dominance
Charge trapping in polymer transistors probed by terahertz spectroscopy and scanning probe potentiometry
Terahertz time-domain spectroscopy and scanning probe potentiometry were used
to investigate charge trapping in polymer field-effect transistors fabricated
on a silicon gate. The hole density in the transistor channel was determined
from the reduction in the transmitted terahertz radiation under an applied gate
voltage. Prolonged device operation creates an exponential decay in the
differential terahertz transmission, compatible with an increase in the density
of trapped holes in the polymer channel. Taken in combination with scanning
probe potentionmetry measurements, these results indicate that device
degradation is largely a consequence of hole trapping, rather than of changes
to the mobility of free holes in the polymer.Comment: 4 pages, 3 figure
Characterization of Collective Gaussian Attacks and Security of Coherent-State Quantum Cryptography
We provide a simple description of the most general collective Gaussian
attack in continuous-variable quantum cryptography. In the scenario of such
general attacks, we analyze the asymptotic secret-key rates which are
achievable with coherent states, joint measurements of the quadratures and
one-way classical communication.Comment: 4 pages, 1 figure + 1 Table, REVteX. More descriptive titl
A study of the kinematics and binary-induced shaping of the planetary nebula HaTr 4
We present the first detailed spatio-kinematical analysis and modelling of
the planetary nebula HaTr 4, one of few known to contain a post-common-envelope
central star system. Common envelope evolution is believed to play an important
role in the shaping of planetary nebulae, but the exact nature of this role is
yet to be understood. High spatial- and spectral- resolution spectroscopy of
the [OIII]5007 nebular line obtained with VLT-UVES are presented alongside deep
narrowband Ha+[NII]6584 imagery obtained using EMMI-NTT, and together the two
are used to derive the three-dimensional morphology of HaTr 4. The nebula is
found to display an extended ovoid morphology with an enhanced equatorial
region consistent with a toroidal waist - a feature believed to be typical
amongst planetary nebulae with post-common-envelope central stars. The nebular
symmetry axis is found to lie perpendicular to the orbital plane of the central
binary, concordant with the idea that the formation and evolution of HaTr 4 has
been strongly influenced by its central binary.Comment: 9 pages, 5 figures, accepted for publication in MNRA
Ground-layer wavefront reconstruction from multiple natural guide stars
Observational tests of ground layer wavefront recovery have been made in open
loop using a constellation of four natural guide stars at the 1.55 m Kuiper
telescope in Arizona. Such tests explore the effectiveness of wide-field seeing
improvement by correction of low-lying atmospheric turbulence with ground-layer
adaptive optics (GLAO). The wavefronts from the four stars were measured
simultaneously on a Shack-Hartmann wavefront sensor (WFS). The WFS placed a 5 x
5 array of square subapertures across the pupil of the telescope, allowing for
wavefront reconstruction up to the fifth radial Zernike order. We find that the
wavefront aberration in each star can be roughly halved by subtracting the
average of the wavefronts from the other three stars. Wavefront correction on
this basis leads to a reduction in width of the seeing-limited stellar image by
up to a factor of 3, with image sharpening effective from the visible to near
infrared wavelengths over a field of at least 2 arc minutes. We conclude that
GLAO correction will be a valuable tool that can increase resolution and
spectrographic throughput across a broad range of seeing-limited observations.Comment: 25 pages, 8 figures, to be published in Astrophys.
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The âSchool Foodshedâ: schools and fast-food outlets in a London borough
Purpose â The purpose of this paper is to explore the location of fast-food outlets around secondary schools and the influence of fast-food availability on the food choices of school children in an inner-London borough.
Design/methodology/approach â A number of methods including: mapping of outlets relative to schools; sampling food; gathering data on secondary school food policies; observing food behaviour in fast food outlets and focus groups with young people. Findings were fed back to a committee consisting of representatives from nutrition, public health, planning services and local community groups.
Findings â There are concentrations of fast-food outlets near schools and students reported use of these, including âstoriesâ of skipping lunch in order to save money and eat after school at these outlets. Food from fast-food outlets was high in fat, saturated fat and salt, but these are not the only source of high such foods, with many of the students reporting buying from shops near the school or on the way to or from school. At lunchtime food outlets were less likely to be used by school students in areas near schools that have a âclosed gateâ policy.
Research limitations/implications â The âsnapshotâ nature of the research limited what can be said about the food behaviours of the children outside school hours.
Practical implications â The local policy context requires action to improve both the food offered in schools and the immediate environment around the school in order to tackle fast-food and other competitive foods on offer outside the school.
Originality/value â This is one of the first studies in the UK to systematically map fast food outlets around schools and explore what might be done. This research shows how it is possible to link the findings of local research and develop local responses from both public health and local authority planning perspectives. The research moves away from a mere documenting of problems to devising integrated public health solutions. The findings show how public health and planning services can work together to the mutual benefit of each other
Keplerian discs around post-AGB stars: a common phenomenon?
Aims: We aim at showing that the broad-band SED characteristics of our sample
of post-AGB stars are best interpreted, assuming the circumstellar dust is
stored in Keplerian rotating passive discs.
Methods: We present a homogeneous and systematic study of the Spectral Energy
Distributions (SEDs) of a sample of 51 post-AGB objects. The selection criteria
to define the whole sample were tuned to cover the broad-band characteristics
of known binary post-AGB stars. The whole sample includes 20 dusty RV Tauri
stars from the General Catalogue of Variable Stars (GCVS). We supplemented our
own Geneva optical photometry with literature data to cover a broad range of
fluxes from the UV to the far-IR.
Results: All the SEDs display very similar characteristics: a large IR excess
with a dust excess starting near the sublimation temperature, irrespective of
the effective temperature of the central star. Moreover, when available, the
long wavelength fluxes show a black-body slope indicative of the presence of a
component of large mm sized grains.
Conclusions: We argue that in all systems, gravitationally bound dusty discs
are present. The discs must be puffed-up to cover a large opening angle for the
central star and we argue that the discs have some similarity with the passive
discs detected around young stellar objects. We interpret the presence of a
disc to be a signature for binarity of the central object, but this will need
confirmation by long-term monitoring of the radial velocities. We argue that
dusty RV Tauri stars are those binaries which happen to be in the Population II
instability strip.Comment: 29 pages, 5 figures, accepted for publication in A&
The Birmingham-CfA cluster scaling project - I: gas fraction and the M-T relation
We have assembled a large sample of virialized systems, comprising 66 galaxy
clusters, groups and elliptical galaxies with high quality X-ray data. To each
system we have fitted analytical profiles describing the gas density and
temperature variation with radius, corrected for the effects of central gas
cooling. We present an analysis of the scaling properties of these systems and
focus in this paper on the gas distribution and M-T relation. In addition to
clusters and groups, our sample includes two early-type galaxies, carefully
selected to avoid contamination from group or cluster X-ray emission. We
compare the properties of these objects with those of more massive systems and
find evidence for a systematic difference between galaxy-sized haloes and
groups of a similar temperature. We derive a mean logarithmic slope of the M-T
relation within R_200 of 1.84+/-0.06, although there is some evidence of a
gradual steepening in the M-T relation, with decreasing mass. We recover a
similar slope using two additional methods of calculating the mean temperature.
Repeating the analysis with the assumption of isothermality, we find the slope
changes only slightly, to 1.89+/-0.04, but the normalization is increased by
30%. Correspondingly, the mean gas fraction within R_200 changes from
(0.13+/-0.01)h70^-1.5 to (0.11+/-0.01)h70^-1.5, for the isothermal case, with
the smaller fractional change reflecting different behaviour between hot and
cool systems. There is a strong correlation between the gas fraction within
0.3*R_200 and temperature. This reflects the strong (5.8 sigma) trend between
the gas density slope parameter, beta, and temperature, which has been found in
previous work. (abridged)Comment: 27 pages, accepted for publication in MNRAS; uses longtable.sty &
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