3,118 research outputs found
Classification of image distortions in terms of Petrov types
An observer surrounded by sufficiently small spherical light sources at a
fixed distance will see a pattern of elliptical images distributed over the
sky, owing to the distortion effect (shearing effect) of the spacetime geometry
upon light bundles. In lowest non-trivial order with respect to the distance,
this pattern is completely determined by the conformal curvature tensor (Weyl
tensor) at the observation event. In this paper we derive formulas that allow
to calculate these distortion patterns in terms of the Newman-Penrose
formalism. Then we represent the distortion patterns graphically for all Petrov
types, and we discuss their dependence on the velocity of the observer.Comment: 22 pages, 8 eps-figures; revised version, parts of Introduction and
Conclusions rewritte
The Size Distribution of Trans-Neptunian Bodies
[Condensed] We search 0.02 deg^2 for trans-Neptunian objects (TNOs) with
m<=29.2 (diameter ~15 km) using the ACS on HST. Three new objects are
discovered, roughly 25 times fewer than expected from extrapolation of the
differential sky density Sigma(m) of brighter objects. The ACS and other recent
TNO surveys show departures from a power law size distribution. Division of the
TNO sample into ``classical Kuiper belt'' (CKB) and ``Excited'' samples reveals
that Sigma(m) differs for the two populations at 96% confidence. A double power
law adequately fits all data. Implications include: The total mass of the CKB
is ~0.010 M_Earth, only a few times Pluto's mass, and is predominately in the
form of ~100 km bodies. The mass of Excited objects is perhaps a few times
larger. The Excited class has a shallower bright-end size distribution; the
largest objects, including Pluto, comprise tens of percent of the total mass
whereas the largest CKBOs are only ~2% of its mass. The predicted mass of the
largest Excited body is close to the Pluto mass; the largest CKBO is ~60 times
less massive. The deficit of small TNOs occurs for sizes subject to disruption
by present-day collisions, suggesting extensive depletion by collisions. Both
accretion and erosion appearing to have proceeded to more advanced stages in
the Excited class than the CKB. The absence of distant TNOs implies that any
distant (60 AU) population must have less than the CKB mass in the form of
objects 40 km or larger. The CKB population is sparser than theoretical
estimates of the required precursor population for short period comets, but the
Excited population could be a viable precursor population.Comment: Revised version accepted to the Astronomical Journal. Numerical
results are very slightly revised. Implications for the origins of
short-period comets are substantially revised, and tedious material on
statistical tests has been collected into a new Appendi
Guiding chemical pulses through geometry: Y-junctions
We study computationally and experimentally the propagation of chemical
pulses in complex geometries.The reaction of interest, CO oxidation, takes
place on single crystal Pt(110) surfaces that are microlithographically
patterned; they are also addressable through a focused laser beam, manipulated
through galvanometer mirrors, capable of locally altering the crystal
temperature and thus affecting pulse propagation. We focus on sudden changes in
the domain shape (corners in a Y-junction geometry) that can affect the pulse
dynamics; we also show how brief, localized temperature perturbations can be
used to control reactive pulse propagation.The computational results are
corroborated through experimental studies in which the pulses are visualized
using Reflection Anisotropy Microscopy.Comment: submitted to Phys. Rev.
Flight Mechanics and Control of Escape Manoeuvres in Hummingbirds. I. Flight Kinematics
Hummingbirds are nature’s masters of aerobatic manoeuvres. Previous research shows that hummingbirds and insects converged evolutionarily upon similar aerodynamic mechanisms and kinematics in hovering. Herein, we use three-dimensional kinematic data to begin to test for similar convergence of kinematics used for escape flight and to explore the effects of body size upon manoeuvring. We studied four hummingbird species in North America including two large species (magnificent hummingbird, Eugenes fulgens, 7.8 g, and blue-throated hummingbird, Lampornis clemenciae, 8.0 g) and two smaller species (broad-billed hummingbird, Cynanthus latirostris, 3.4 g, and black-chinned hummingbirds Archilochus alexandri, 3.1 g). Starting from a steady hover, hummingbirds consistently manoeuvred away from perceived threats using a drastic escape response that featured body pitch and roll rotations coupled with a large linear acceleration. Hummingbirds changed their flapping frequency and wing trajectory in all three degrees of freedom on a stroke-by-stroke basis, likely causing rapid and significant alteration of the magnitude and direction of aerodynamic forces. Thus it appears that the flight control of hummingbirds does not obey the ‘helicopter model’ that is valid for similar escape manoeuvres in fruit flies. Except for broad-billed hummingbirds, the hummingbirds had faster reaction times than those reported for visual feedback control in insects. The two larger hummingbird species performed pitch rotations and global-yaw turns with considerably larger magnitude than the smaller species, but roll rates and cumulative roll angles were similar among the four species
Optical Turbulence Measurements and Models for Mount John University Observatory
Site measurements were collected at Mount John University Observatory in 2005
and 2007 using a purpose-built scintillation detection and ranging system.
profiling indicates a weak layer located at 12 - 14 km above sea
level and strong low altitude turbulence extending up to 5 km. During calm
weather conditions, an additional layer was detected at 6 - 8 km above sea
level. profiling suggests that tropopause layer velocities are nominally
12 - 30 m/s, and near-ground velocities range between 2 -- 20 m/s, dependent on
weather. Little seasonal variation was detected in either and
profiles. The average coherence length, , was found to be cm for
the full profile at a wavelength of 589 nm. The average isoplanatic angle,
, was arcsec. The mean turbulence altitude,
, was found to be km above sea level. No average in the
Greenwood frequency, , could be established due to the gaps present in the
\vw\s profiles obtained. A modified Hufnagel-Valley model was developed to
describe the profiles at Mount John, which estimates at 6 cm
and at 0.9 arcsec. A series of models were developed, based
on the Greenwood wind model with an additional peak located at low altitudes.
Using the model and the suggested model for moderate ground
wind speeds, is estimated at 79 Hz.Comment: 14 pages; accepted for publication in PAS
Flight Mechanics and Control of Escape Manoeuvres in Hummingbirds. II. Aerodynamic Force Production, Flight Control and Performance Limitations
The superior manoeuvrability of hummingbirds emerges from complex interactions of specialized neural and physiological processes with the unique flight dynamics of flapping wings. Escape manoeuvring is an ecologically relevant, natural behaviour of hummingbirds, from which we can gain understanding into the functional limits of vertebrate locomotor capacity. Here, we extend our kinematic analysis of escape manoeuvres from a companion paper to assess two potential limiting factors of the manoeuvring performance of hummingbirds: (1) muscle mechanical power output and (2) delays in the neural sensing and control system. We focused on the magnificent hummingbird (Eugenes fulgens, 7.8 g) and the black-chinned hummingbird (Archilochus alexandri, 3.1 g), which represent large and small species, respectively. We first estimated the aerodynamic forces, moments and the mechanical power of escape manoeuvres using measured wing kinematics. Comparing active-manoeuvring and passive-damping aerodynamic moments, we found that pitch dynamics were lightly damped and dominated by the effect of inertia, while roll dynamics were highly damped. To achieve observed closed-loop performance, pitch manoeuvres required faster sensorimotor transduction, as hummingbirds can only tolerate half the delay allowed in roll manoeuvres. Accordingly, our results suggested that pitch control may require a more sophisticated control strategy, such as those based on prediction. For the magnificent hummingbird, we estimated that escape manoeuvres required muscle mass-specific power 4.5 times that during hovering. Therefore, in addition to the limitation imposed by sensorimotor delays, muscle power could also limit the performance of escape manoeuvres
cDNA Sequence, Expression, and Transcript Stability of a Cold Acclimation-Specific Gene, cas18, of Alfalfa (Medicago falcata) Cells
Searching For Integrated Sachs-Wolfe Effect Beyond Temperature Anisotropies: CMB E-mode Polarization-Galaxy Cross Correlation
The cross-correlation between cosmic microwave background (CMB) temperature
anisotropies and the large scale structure (LSS) traced by the galaxy
distribution, or sources at different wavelengths, is now well known. This
correlation results from the integrated Sachs-Wolfe (ISW) effect in CMB
anisotropies generated at late times due to the dark energy component of the
Universe. In a reionized universe, the ISW quadrupole rescatters and
contributes to the large-scale polarization signal. Thus, in principle, the
large-scale polarization bump in the E-mode should also be correlated with the
galaxy distribution. Unlike CMB temperature-LSS correlation that peaks for
tracers at low redshifts this correlation peaks mostly at redshifts between 1
and 3. Under certain conditions, mostly involving a low optical depth to
reionization, if the Universe reionized at a redshift around 6, the cross
polarization-source signal is marginally detectable, though challenging as it
requires all-sky maps of the large scale structure at redshifts between 1 and
3. If the Universe reionized at a redshift higher than 10, it is unlikely that
this correlation will be detectable even with no instrumental noise all-sky
maps. While our estimates do not guarantee a detection unknown physics related
to the dark energy as well as still uncertain issues related to the large
angular scale CMB and polarization anisotropies may motivate attempts to
measure this correlation using upcoming CMB polarization E-mode maps.Comment: 13 pages; 3 figure panels, JCAP submitte
BrainGlobe Atlas API: a common interface for neuroanatomical atlases
Summary: Neuroscientists routinely perform experiments aimed at recording or manipulating neural activity,
uncovering physiological processes underlying brain function or elucidating aspects of brain
anatomy. Understanding how the brain generates behaviour ultimately depends on merging
the results of these experiments into a unified picture of brain anatomy and function. Brain
atlases are crucial in this endeavour: by outlining the organization of brain regions they provide
a reference upon which our understanding of brain function can be anchored. More recently,
digital high-resolution 3d atlases have been produced for several model organisms providing
an invaluable resource for the research community. Effective use of these atlases depends
on the availability of an application programming interface (API) that enables researchers to
develop software to access and query atlas data. However, while some atlases come with an
API, these are generally specific for individual atlases, and this hinders the development and
adoption of open-source neuroanatomy software. The BrainGlobe atlas API (BG-Atlas API)
overcomes this problem by providing a common interface for programmers to download and
process data across a variety of model organisms. By adopting the BG-Atlas API, software can
then be developed agnostic to the atlas, increasing adoption and interoperability of packages
in neuroscience and enabling direct integration of different experimental modalities and even
comparisons across model organisms
- …