17,955 research outputs found
Hands-On Universe: A Global Program for Education and Public Outreach in Astronomy
Hands-On Universe (HOU) is an educational program that enables students to
investigate the Universe while applying tools and concepts from science, math,
and technology. Using the Internet, HOU participants around the world request
observations from an automated telescope, download images from a large image
archive, and analyze them with the aid of user-friendly image processing
software. This program is developing now in many countries, including the USA,
France, Germany, Sweden, Japan, Australia, and others. A network of telescopes
has been established among these countries, many of them remotely operated, as
shown in the accompanying demo. Using this feature, students in the classroom
are able to make night observations during the day, using a telescope placed in
another country. An archive of images taken on large telescopes is also
accessible, as well as resources for teachers. Students are also dealing with
real research projects, e.g. the search for asteroids, which resulted in the
discovery of a Kuiper Belt object by high-school students. Not only Hands-On
Universe gives the general public an access to professional astronomy, but it
is also a more general tool to demonstrate the use of a complex automated
system, the techniques of data processing and automation. Last but not least,
through the use of telescopes located in many countries over the globe, a form
of powerful and genuine cooperation between teachers and children from various
countries is promoted, with a clear educational goal.Comment: 4 pages, 1 figure, to appear in the proceedings of the ADASS X
conference, Boston, October 2000, ASP conf. pro
Start-up inertia as an origin for heterogeneous flow
For quite some time non-monotonic flow curve was thought to be a requirement
for shear banded flows in complex fluids. Thus, in simple yield stress fluids
shear banding was considered to be absent. Recent spatially resolved
rheological experiments have found simple yield stress fluids to exhibit shear
banded flow profiles. One proposed mechanism for the initiation of such
transient shear banding process has been a small stress heterogeneity rising
from the experimental device geometry. Here, using Computational Fluid Dynamics
methods, we show that transient shear banding can be initialized even under
homogeneous stress conditions by the fluid start-up inertia, and that such
mechanism indeed is present in realistic experimental conditions
On the symmetries of special holonomy sigma models
In addition to superconformal symmetry, (1,1) supersymmetric two-dimensional
sigma models on special holonomy manifolds have extra symmetries that are in
one-to-one correspondence with the covariantly constant forms on these
manifolds. The superconformal algebras extended by these symmetries close as
W-algebras, i.e. they have field-dependent structure functions. It is shown
that it is not possible to write down cohomological equations for potential
quantum anomalies when the structure functions are field-dependent. In order to
do this it is necessary to linearise the algebras by treating composite
currents as generators of additional symmetries. It is shown that all cases can
be linearised in a finite number of steps, except for G_2 and SU(3). Additional
problems in the quantisation procedure are briefly discussed.Comment: 16 pages. Abstract improved and a few typographical errors correcte
Further search for a neutral boson with a mass around 9 MeV/c2
Two dedicated experiments on internal pair conversion (IPC) of isoscalar M1
transitions were carried out in order to test a 9 MeV/c2 X-boson scenario. In
the 7Li(p,e+e-)8Be reaction at 1.1 MeV proton energy to the predominantly T=0
level at 18.15 MeV, a significant deviation from IPC was observed at large pair
correlation angles. In the 11B(d,n e+e-)12C reaction at 1.6 MeV, leading to the
12.71 MeV 1+ level with pure T=0 character, an anomaly was observed at 9
MeV/c2. The compatibility of the results with the scenario is discussed.Comment: 12 pages, 5 figures, 2 table
Duration of Regrowth of Ryegrass (\u3ci\u3eLolium perenne\u3c/i\u3e) Swards: Effects on Rumen Fermentation of Lactating Dairy Cows
The relative importance of duration of sward regrowth and rumen fill and fermentation on the control of grazing time and intake rate during the first grazing session of the day were studied. Four lactating dairy cows were allowed to graze ryegrass (Lolium perenne) swards, with five different regrowth periods after mowing (6, 9, 16, 22 and 30 d). The cows were allowed to graze until they stopped voluntarily. Samples of rumen liquid were taken at approximately 0, 30, 60, 120 and 240 min after the grazing session was finished. Concentration of volatile fatty acids (VFA) followed a significant quadratic trend with a maximum concentration observed at approximately 110 min after cessation of grazing. In this study, rumen fill, VFA (either total or major components) and ammonia concentration as individual variables were not correlated with grazing time or dry matter intake
Tuning the emission wavelength of Si nanocrystals in SiO2 by oxidation
Si nanocrystals (diameter 2–5 nm) were formed by 35 keV Si + implantation at a fluence of 6 × 1016 Si/cm2 into a 100 nm thick thermally grown SiO2 film on Si (100), followed by thermal annealing at 1100 °C for 10 min. The nanocrystals show a broad photoluminescence spectrum, peaking at 880 nm, attributed to the recombination of quantum confined excitons. Rutherford backscattering spectrometry and transmission electron microscopy show that annealing these samples in flowing O2 at 1000 °C for times up to 30 min results in oxidation of the Si nanocrystals, first close to the SiO2 film surface and later at greater depths. Upon oxidation for 30 min the photoluminescence peak wavelength blueshifts by more than 200 nm. This blueshift is attributed to a quantum size effect in which a reduction of the average nanocrystal size leads to emission at shorter wavelengths. The room temperature luminescence lifetime measured at 700 nm increases from 12 µs for the unoxidized film to 43 µs for the film that was oxidized for 29 min
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