41 research outputs found
Gravitational Waves in the Nonsymmetric Gravitational Theory
We prove that the flux of gravitational radiation from an isolated source in
the Nonsymmetric Gravitational Theory is identical to that found in Einstein's
General Theory of Relativity.Comment: 10 Page
Enhanced statistical stability in coherent interferometric imaging
http://iopscience.iop.org/0266-5611/International audienc
Nyquist method for Wigner-Poisson quantum plasmas
By means of the Nyquist method, we investigate the linear stability of
electrostatic waves in homogeneous equilibria of quantum plasmas described by
the Wigner-Poisson system. We show that, unlike the classical Vlasov-Poisson
system, the Wigner-Poisson case does not necessarily possess a Penrose
functional determining its linear stability properties. The Nyquist method is
then applied to a two-stream distribution, for which we obtain an exact,
necessary and sufficient condition for linear stability, as well as to a
bump-in-tail equilibrium.Comment: 6 figure
The effects of disk and dust structure on observed polarimetric images of protoplanetary disks
Imaging polarimetry is a powerful tool for imaging faint circumstellar
material. For a correct analysis of observations we need to fully understand
the effects of dust particle parameters, as well as the effects of the
telescope, atmospheric seeing, and assumptions about the data reduction and
processing of the observed signal. Here we study the major effects of dust
particle structure, size-dependent grain settling, and instrumental properties.
We performed radiative transfer modeling using different dust particle models
and disk structures. To study the influence of seeing and telescope diffraction
we ran the models through an instrument simulator for the ExPo dual-beam
imaging polarimeter mounted at the 4.2m William Herschel Telescope (WHT).
Particle shape and size have a strong influence on the brightness and
detectability of the disks. In the simulated observations, the central
resolution element also contains contributions from the inner regions of the
protoplanetary disk besides the unpolarized central star. This causes the
central resolution element to be polarized, making simple corrections for
instrumental polarization difficult. This effect strongly depends on the
spatial resolution, so adaptive optics systems are needed for proper
polarization calibration. We find that the commonly employed homogeneous sphere
model gives results that differ significantly from more realistic models. For a
proper analysis of the wealth of data available now or in the near future, one
must properly take the effects of particle types and disk structure into
account. The observed signal depends strongly on the properties of these more
realistic models, thus providing a potentially powerful diagnostic. We conclude
that it is important to correctly understand telescope depolarization and
calibration effects for a correct interpretation of the degree of polarization.Comment: Accepted for publication in A&
Modern optical astronomy: technology and impact of interferometry
The present `state of the art' and the path to future progress in high
spatial resolution imaging interferometry is reviewed. The review begins with a
treatment of the fundamentals of stellar optical interferometry, the origin,
properties, optical effects of turbulence in the Earth's atmosphere, the
passive methods that are applied on a single telescope to overcome atmospheric
image degradation such as speckle interferometry, and various other techniques.
These topics include differential speckle interferometry, speckle spectroscopy
and polarimetry, phase diversity, wavefront shearing interferometry,
phase-closure methods, dark speckle imaging, as well as the limitations imposed
by the detectors on the performance of speckle imaging. A brief account is
given of the technological innovation of adaptive-optics (AO) to compensate
such atmospheric effects on the image in real time. A major advancement
involves the transition from single-aperture to the dilute-aperture
interferometry using multiple telescopes. Therefore, the review deals with
recent developments involving ground-based, and space-based optical arrays.
Emphasis is placed on the problems specific to delay-lines, beam recombination,
polarization, dispersion, fringe-tracking, bootstrapping, coherencing and
cophasing, and recovery of the visibility functions. The role of AO in
enhancing visibilities is also discussed. The applications of interferometry,
such as imaging, astrometry, and nulling are described. The mathematical
intricacies of the various `post-detection' image-processing techniques are
examined critically. The review concludes with a discussion of the
astrophysical importance and the perspectives of interferometry.Comment: 65 pages LaTeX file including 23 figures. Reviews of Modern Physics,
2002, to appear in April issu
A new concept for the combination of optical interferometers and high-resolution spectrographs
The combination of high spatial and spectral resolution in optical astronomy
enables new observational approaches to many open problems in stellar and
circumstellar astrophysics. However, constructing a high-resolution
spectrograph for an interferometer is a costly and time-intensive undertaking.
Our aim is to show that, by coupling existing high-resolution spectrographs to
existing interferometers, one could observe in the domain of high spectral and
spatial resolution, and avoid the construction of a new complex and expensive
instrument. We investigate in this article the different challenges which arise
from combining an interferometer with a high-resolution spectrograph. The
requirements for the different sub-systems are determined, with special
attention given to the problems of fringe tracking and dispersion. A concept
study for the combination of the VLTI (Very Large Telescope Interferometer)
with UVES (UV-Visual Echelle Spectrograph) is carried out, and several other
specific instrument pairings are discussed. We show that the proposed
combination of an interferometer with a high-resolution spectrograph is indeed
feasible with current technology, for a fraction of the cost of building a
whole new spectrograph. The impact on the existing instruments and their
ongoing programs would be minimal.Comment: 27 pages, 9 figures, Experimental Astronomy; v2: accepted versio
Generalized Whittle-Matrn random field as a model of correlated fluctuations
This paper considers a generalization of Gaussian random field with
covariance function of Whittle-Matrn family. Such a random
field can be obtained as the solution to the fractional stochastic differential
equation with two fractional orders. Asymptotic properties of the covariance
functions belonging to this generalized Whittle-Matrn family
are studied, which are used to deduce the sample path properties of the random
field. The Whittle-Matrn field has been widely used in
modeling geostatistical data such as sea beam data, wind speed, field
temperature and soil data. In this article we show that generalized
Whittle-Matrn field provides a more flexible model for wind
speed data.Comment: 22 pages, 10 figures, accepted by Journal of Physics