66,719 research outputs found
Change of the plane of oscillation of a Foucault pendulum from simple pictures
The change of the plane of oscillation of a Foucault pendulum is calculated
without using equations of motion, the Gauss-Bonnet theorem, parallel
transport, or assumptions that are difficult to explain.Comment: 5 pages, 4 figure
Eastern Range Extension of \u3ci\u3eLeptoglossus Occidentalis\u3c/i\u3e With a Key to Leptoglossus Species of America North of Mexico (Heteroptera: Coreidae)
Leptoglossus occidentalis is reported for the first time from Illinois and Michigan, and confirmed for Indiana. A key to the species of Leptoglossus occurring in America north of Mexico is presented
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Numerical Simulation of Baroclinic Jovian Vortices
We examine the evolution of baroclinic vortices in a time-dependent, nonlinear numerical model of a Jovian atmosphere. The model uses a normal-mode expansion in the vertical, using the barotropic and first two baroclinic modes. Results for the stability of baroclinic vortices on an f plane in the absence of a mean zonal flow are similar to results of Earth vortex models, although the presence of a fluid interior on the Jovian planets shifts the stability boundaries to smaller length scales. The presence of a barotropic mean zonal flow in the interior stabilizes vortices against instability and significantly modifies the finite amplitude form of baroclinic instabilities. The effect of a zonal flow on a form of barotropic instability produces periodic oscillations in the latitude and longitude of the vortex as observed at the level of the cloud tops. This instability may explain some, but not all, observations of longitudinal oscillations of vortices on the outer planets. Oscillations in aspect ratio and orientation of stable vortices in a zonal shear flow are observed in this baroclinic model, as in simpler twodimensional models. Such oscillations are also observed in the atmospheres of Jupiter and Neptune. The meridional propagation and decay of vortices on a β plane is inhibited by the presence of a mean zonal flow. The direction of propagation of a vortex relative to the mean zonal flow depends upon the sign of the meridional potential vorticity gradient; combined with observations of vortex drift rates, this may provide a constraint on model assumption for the flow in the deep interior of the Jovian planets
The Arecibo Galaxy Environment Survey VII : A Dense Filament With Extremely Long HI Streams
We present completed observations of the NGC 7448 galaxy group and background
volume as part of the blind neutral hydrogen Arecibo Galaxy Environment Survey
(AGES). Our observations cover a region spanning 5x4 degrees, over a redshift
range of approximately -2,000 < cz < 20,000 km/s. A total of 334 objects are
detected, mostly in three overdensities at cz 7,500, cz 9,600 and
cz 11,400 km/s. The galaxy density is extremely high (15 per square
degree) and many (24%) show signs of extended HI emission, including some
features as much as 800 kpc in projected length. We describe the overall
characteristics of this environment : kinematics, typical galaxy colours and
mass to light ratios, and substructure. To aid in the cataloguing of this data
set, we present a new FITS viewer (FRELLED : Fits Realtime Explorer of Low
Latency in Every Dimension). This incorporates interactive source cataloguing
tools which increase our source extraction speed by approximately a factor of
50.Comment: 15 pages, 13 figures, accepted for publication in MNRA
Attack of the Flying Snakes : Formation of Isolated HI Clouds By Fragmentation of Long Streams
The existence of long (> 100 kpc) HI streams and small (< 20 kpc)
free-floating HI clouds is well-known. While the formation of the streams has
been investigated extensively, and the isolated clouds are often purported to
be interaction debris, little research has been done on the formation of
optically dark HI clouds that are not part of a larger stream. One possibility
is that such features result from the fragmentation of more extended streams,
while another idea is that they are primordial, optically dark galaxies. We
test the validity of the fragmentation scenario (via harassment) using
numerical simulations. In order to compare our numerical models with
observations, we present catalogues of both the known long HI streams (42
objects) and free-floating HI clouds suggested as dark galaxy candidates (51
objects). In particular, we investigate whether it is possible to form compact
features with high velocity widths (> 100 km/s), similar to observed clouds
which are otherwise intriguing dark galaxy candidates. We find that producing
such features is possible but extremely unlikely, occurring no more than 0.2%
of the time in our simulations. In contrast, we find that genuine dark galaxies
could be extremely stable to harassment and remain detectable even after 5 Gyr
in the cluster environment (with the important caveat that our simulations only
explore harassment and do not yet include the intracluster medium, heating and
cooling, or star formation). We also discuss the possibility that such objects
could be the progenitors of recently discovered ultra diffuse galaxies.Comment: 46 pages, 27 figures, accepted for publication in MNRA
Development of a HgCdTe photomixer and impedance matched GaAs FET amplifier
A research program for the development of a 10.6 micron HgCdTe photodiode/GaAs field effect transistor amplifier package for use at cryogenic temperatures (77k). The photodiode/amplifier module achieved a noise equivalent power per unit bandwidth of 5.7 times 10 to the 20th power W/Hz at 2.0 GHz. The heterodyne sensitivity of the HgCdTe photodiode was improved by designing and building a low noise GaAs field effect transistor amplifier operating at 77K. The Johnson noise of the amplifier was reduced at 77K, and thus resulted in an increased photodiode heterodyne sensitivity
A Statistical Description of AGN Jet Evolution from the VLBA Imaging and Polarimetry Survey (VIPS)
A detailed analysis of the evolution of the properties of core-jet systems
within the VLBA Imaging and Polarimetry Survey (VIPS) is presented. We find a
power-law relationship between jet intensity and width that suggests for the
typical jet, little if any energy is lost as it moves away from its core. Using
VLA images at 1.5 GHz, we have found evidence that parsec-scale jets tend to be
aligned with the the direction of emission on kiloparsec scales. We also found
that this alignment improves as the jets move farther from their cores on
projected scales as small as ~50-100 pc. This suggests that realignment of jets
on these projected scales is relatively common. We typically find a modest
amount of bending (a change in jet position angle of ~5 deg.) on these scales,
suggesting that this realignment may typically occur relatively gradually.Comment: Accepted to ApJ, 20 pages, 8 figure
Energy benchmarks for water clusters and ice structures from an embedded many-body expansion
We show how an embedded many-body expansion (EMBE) can be used to calculate
accurate \emph{ab initio} energies of water clusters and ice structures using
wavefunction-based methods. We use the EMBE described recently by Bygrave
\emph{et al.} (J. Chem. Phys. \textbf{137}, 164102 (2012)), in which the terms
in the expansion are obtained from calculations on monomers, dimers, etc. acted
on by an approximate representation of the embedding field due to all other
molecules in the system, this field being a sum of Coulomb and
exchange-repulsion fields. Our strategy is to separate the total energy of the
system into Hartree-Fock and correlation parts, using the EMBE only for the
correlation energy, with the Hartree-Fock energy calculated using standard
molecular quantum chemistry for clusters and plane-wave methods for crystals.
Our tests on a range of different water clusters up to the 16-mer show that for
the second-order M\o{}ller-Plesset (MP2) method the EMBE truncated at 2-body
level reproduces to better than 0.1 m/monomer the correlation energy
from standard methods. The use of EMBE for computing coupled-cluster energies
of clusters is also discussed. For the ice structures Ih, II and VIII, we find
that MP2 energies near the complete basis-set limit reproduce very well the
experimental values of the absolute and relative binding energies, but that the
use of coupled-cluster methods for many-body correlation (non-additive
dispersion) is essential for a full description. Possible future applications
of the EMBE approach are suggested
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