56,028 research outputs found
BILIPROTEINS FROM THE BUTTERFLY Pieris brassicae STUDIED BY TIME-RESOLVED FLUORESCENCE AND COHERENT ANTI-STOKES RAMAN SPECTROSCOPY
The fluorescence decay time of the biliverdin IX7 chromophore present in biliproteins isolated from Pieris brassicae is determined to be 44 ± 3 ps. This value suggests a cyclic helical chromophore structure. The vibrational frequencies determined by CARS-spectroscopy are compared with those of model compounds. The data confirm that the chromophore in the protein-bound state adopts a cyclic-helical, flexible conformation
A systematic approach to atomicity decomposition in Event-B
Event-B is a state-based formal method that supports a refinement process in which an abstract model is elaborated towards an implementation in a step-wise manner. One weakness of Event-B is that control flow between events is typically modelled implicitly via variables and event guards. While this fits well with Event-B refinement, it can make models involving sequencing of events more difficult to specify and understand than if control flow was explicitly specified. New events may be introduced in Event-B refinement and these are often used to decompose the atomicity of an abstract event into a series of steps. A second weakness of Event-B is that there is no explicit link between such new events that represent a step in the decomposition of atomicity and the abstract event to which they contribute. To address these weaknesses, atomicity decomposition diagrams support the explicit modelling of control flow and refinement relationships for new events. In previous work,the atomicity decomposition approach has been evaluated manually in the development of two large case studies, a multi media protocol and a spacecraft sub-system. The evaluation results helped us to develop a systematic definition of the atomicity decomposition approach, and to develop a tool supporting the approach. In this paper we outline this systematic definition of the approach, the tool that supports it and evaluate the contribution that the tool makes
Superluminal Caustics of Close, Rapidly-Rotating Binary Microlenses
The two outer triangular caustics (regions of infinite magnification) of a
close binary microlens move much faster than the components of the binary
themselves, and can even exceed the speed of light. When , where
is the caustic speed, the usual formalism for calculating the lens
magnification breaks down. We develop a new formalism that makes use of the
gravitational analog of the Li\'enard-Wiechert potential. We find that as the
binary speeds up, the caustics undergo several related changes: First, their
position in space drifts. Second, they rotate about their own axes so that they
no longer have a cusp facing the binary center of mass. Third, they grow larger
and dramatically so for . Fourth, they grow weaker roughly in
proportion to their increasing size. Superluminal caustic-crossing events are
probably not uncommon, but they are difficult to observe.Comment: 12 pages, 7 ps figures, submitted to Ap
Formation of nanocrystals based on decomposition in the amorphous Zr41.2Ti13.8Cu12.5Ni10Be22.5 alloy
Primary crystallization and decomposition in the bulk amorphous alloy Ar41.2Ti13.8Cu12.5Ni10Be22.5 have been studied by small angle neutron scattering (SANS), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). SANS data of samples annealed isothermally at 623 K exhibit an interference peak centered at q=0.46 nm(^-1) after an incubation time of about 100 min. TEM and DSC investigations confirm that the respective periodic variation in the scattering length density is due to the formation of nanocrystals embedded in the amorphous matrix. These observations suggest that during the incubation time a chemical decomposition process occurs and the related periodic composition fluctuations give rise to the observed periodic arrangement of the nanocrystals
On aberration in gravitational lensing
It is known that a relative translational motion between the deflector and
the observer affects gravitational lensing. In this paper, a lens equation is
obtained to describe such effects on actual lensing observables. Results can be
easily interpreted in terms of aberration of light-rays. Both radial and
transverse motions with relativistic velocities are considered. The lens
equation is derived by first considering geodesic motion of photons in the
rest-frame Schwarzschild spacetime of the lens, and, then, light-ray detection
in the moving observer's frame. Due to the transverse motion images are
displaced and distorted in the observer's celestial sphere, whereas the radial
velocity along the line of sight causes an effective re-scaling of the lens
mass. The Einstein ring is distorted to an ellipse whereas the caustics in the
source plane are still point-like. Either for null transverse motion or up to
linear order in velocities, the critical curve is still a circle with its
radius corrected by a factor (1+z_d) with respect to the static case, z_d being
the relativistic Doppler shift of the deflector. From the observational point
of view, the orbital motion of the Earth can cause potentially observable
corrections of the order of the microarcsec in lensing towards the
super-massive black hole at the Galactic center. On a cosmological scale,
tangential peculiar velocities of cluster of galaxies bring about a typical
flexion in images of background galaxies in the weak lensing regime but future
measurements seem to be too much challenging.Comment: 8 pages, 2 figures, in press on PR
Direct Detection of Giant Close-In Planets Around the Source Stars of Caustic-Crossing Microlensing Events
We propose a direct method to detect close-in giant planets orbiting stars in
the Galactic bulge. This method uses caustic-crossing binary microlensing
events discovered by survey teams monitoring the bulge to measure light from a
planet orbiting the source star. When the planet crosses the caustic, it is
more magnified than the source star; its light is magnified by two orders of
magnitude for Jupiter size planets. If the planet is a giant close to the star,
it may be bright enough to make a significant deviation in the light curve of
the star. Detection of this deviation requires intensive monitoring of the
microlensing light curve using a 10-meter class telescope for a few hours after
the caustic. This is the only method yet proposed to directly detect close-in
planets around stars outside the solar neighborhood.Comment: 4 pages, 2 figures. Submitted to ApJ Letter
Angular Radii of Stars via Microlensing
We outline a method by which the angular radii of giant and main sequence
stars in the Galactic bulge can be measured to a few percent accuracy. The
method combines ground-based photometry of caustic-crossing bulge microlensing
events, with a handful of precise astrometric measurements of the lensed star
during the event, to measure the angular radius of the source, theta_*. Dense
photometric coverage of one caustic crossing yields the crossing timescale dt.
Less frequent coverage of the entire event yields the Einstein timescale t_E
and the angle phi of source trajectory with respect to the caustic. The
photometric light curve solution predicts the motion of the source centroid up
to an orientation on the sky and overall scale. A few precise astrometric
measurements therefore yield theta_E, the angular Einstein ring radius. Then
the angular radius of the source is obtained by theta_*=theta_E(dt/t_E)
sin(phi). We argue that theta_* should be measurable to a few percent accuracy
for Galactic bulge giant stars using ground-based photometry from a network of
small (1m-class) telescopes, combined with astrometric observations with a
precision of ~10 microarcsec to measure theta_E. We find that a factor of ~50
times fewer photons are required to measure theta_E to a given precision for
binary-lens events than single-lens events. Adopting parameters appropriate to
the Space Interferometry Mission (SIM), ~7 min of SIM time is required to
measure theta_E to ~5% accuracy for giant sources in the bulge. For
main-sequence sources, theta_E can be measured to ~15% accuracy in ~1.4 hours.
With 10 hrs of SIM time, it should be possible to measure theta_* to ~5% for
\~80 giant stars, or to 15% for ~7 main sequence stars. A byproduct of such a
campaign is a significant sample of precise binary-lens mass measurements.Comment: 13 pages, 3 figures. Revised version, minor changes, required SIM
integration times revised upward by ~60%. Accepted to ApJ, to appear in the
March 20, 2003 issue (v586
Implications of the isotope effects on the magnetization, magnetic torque and susceptibility
We analyze the magnetization, magnetic torque and susceptibility data of
La2-xSrxCu(16,18)O4 and YBa2(63,65)CuO7-x near Tc in terms of the universal
3D-XY scaling relations. It is shown that the isotope effect on Tc mirrors that
on the anisotropy. Invoking the generic behavior of the anisotropy the doping
dependence of the isotope effects on the critical properties, including Tc,
correlation lengths and magnetic penetration depths are traced back to a change
of the mobile carrier concentration.Comment: 5 pages, 3 figure
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