123 research outputs found
Light deflection in Weyl gravity: critical distances for photon paths
The Weyl gravity appears to be a very peculiar theory. The contribution of
the Weyl linear parameter to the effective geodesic potential is opposite for
massive and nonmassive geodesics. However, photon geodesics do not depend on
the unknown conformal factor, unlike massive geodesics. Hence light deflection
offers an interesting test of the Weyl theory.
In order to investigate light deflection in the setting of Weyl gravity, we
first distinguish between a weak field and a strong field approximation.
Indeed, the Weyl gravity does not turn off asymptotically and becomes even
stronger at larger distances.
We then take full advantage of the conformal invariance of the photon
effective potential to provide the key radial distances in Weyl gravity.
According to those, we analyze the weak and strong field regime for light
deflection. We further show some amazing features of the Weyl theory in the
strong regime.Comment: 20 pages, 9 figures (see published version for a better resolution,
or online version at stacks.iop.org/CQG/21/1897
The Sun Asphericities: Astrophysical Relevance
Of all the fundamental parameters of the Sun (diameter, mass,
temperature...), the gravitational multipole moments (of degree l and order m)
that determine the solar moments of inertia, are still poorly known. However,
at the first order (l=2), the quadrupole moment is relevant to many
astrophysical applications. It indeed contributes to the relativistic
perihelion advance of planets, together with the post-Newtonian (PN)
parameters; or to the precession of the orbital plane about the Sun polar axis,
the latter being unaffected by the purely relativistic PN contribution. Hence,
a precise knowledge of the quadrupole moment is necessary for accurate orbit
determination, and alternatively, to obtain constraints on the PN parameters.
Moreover, the successive gravitational multipole moments have a physical
meaning: they describe deviations from a purely spherical mass distribution.
Thus, their precise determination gives indications on the solar internal
structure. Here, we explain why it is difficult to compute these parameters,
how to derive the best values, and how they will be determined in a near future
by means of space experiments.Comment: 14 pages, 9 figures (see published version for a better resolution),
submited to Proceedings of the Royal Society: Mathematical, Physical and
Engineering Science
Solar gravitational energy and luminosity variations
Due to non-homogeneous mass distribution and non-uniform velocity rate inside
the Sun, the solar outer shape is distorted in latitude. In this paper, we
analyze the consequences of a temporal change in this figure on the luminosity.
To do so, we use the Total Solar Irradiance (TSI) as an indicator of
luminosity. Considering that most of the authors have explained the largest
part of the TSI modulation with magnetic network (spots and faculae) but not
the whole, we could set constraints on radius and effective temperature
variations (dR, dT). However computations show that the amplitude of solar
irradiance modulation is very sensitive to photospheric temperature variations.
In order to understand discrepancies between our best fit and recent
observations of Livingston et al. (2005), showing no effective surface
temperature variation during the solar cycle, we investigated small effective
temperature variation in irradiance modeling. We emphasized a phase-shift
(correlated or anticorrelated radius and irradiance variations) in the (dR,
dT)-parameter plane. We further obtained an upper limit on the amplitude of
cyclic solar radius variations, deduced from the gravitational energy
variations. Our estimate is consistent with both observations of the
helioseismic radius through the analysis of f-mode frequencies and observations
of the basal photospheric temperature at Kitt Peak. Finally, we suggest a
mechanism to explain faint changes in the solar shape due to variation of
magnetic pressure which modifies the granules size. This mechanism is supported
by our estimate of the asphericity-luminosity parameter, which implies an
effectiveness of convective heat transfer only in very outer layers of the Sun.Comment: 17 pages, 2 figure, 1 table, published in New Astronom
Are non-magnetic mechanisms such as temporal solar diameter variations conceivable for an irradiance variability?
Irradiance variability has been monitored from space for more than two
decades. Even if data are coming from different sources, it is well established
that a temporal variability exists which can be set to as approximately 0.1%,
in phase with the solar cycle. Today, one of the best explanation for such an
irradiance variability is provided by the evolution of the solar surface
magnetic fields. But if some 90 to 95% can be reproduced, what would be the
origin of the 10 to 5% left? Non magnetic effects are conceivable. In this
paper we will consider temporal variations of the diameter of the Sun as a
possible contributor for the remaining part. Such an approach imposes strong
constraints on the solar radius variability. We will show that over a solar
cycle, variations of no more than 20 mas of amplitude can be considered. Such a
variability (far from what is reported by observers conducting measurements by
means of ground-based solar astrolabes) may explain a little part of the
irradiance changes not explained by magnetic features. Further requirements are
needed that may help to reach a conclusion. Dedicated space missions are
necessary (for example PICARD, GOLF-NG or SDO, scheduled for a launch around
2008); it is also proposed to reactivate SDS flights for such a purpose.Comment: 8 pages, 2 eps figures, published in Solar Physic
The Slowly Formed Guiselin Brush
We study polymer layers formed by irreversible adsorption from a polymer
melt. Our theory describes an experiment which is a ``slow'' version of that
proposed by Guiselin [Europhys. Lett., v. 17 (1992) p. 225] who considered
instantaneously irreversibly adsorbing chains and predicted a universal density
profile of the layer after swelling with solvent to produce the ``Guiselin
brush.'' Here we ask what happens when adsorption is not instantaneous. The
classic example is chemisorption. In this case the brush is formed slowly and
the final structure depends on the experiment's duration, . We find
the swollen layer consists of an inner region of thickness with approximately constant density and an outer region
extending up to height which has the same density decay as for the Guiselin case.Comment: 7 pages, submitted to Europhysics Letter
Schwarzschild black hole surrounded by quintessence: Null geodesics
We have studied the null geodesics of the Schwarzschild black hole surrounded
by quintessence matter. Quintessence matter is a candidate for dark energy.
Here, we have done a detailed analysis of the geodesics and exact solutions are
presented in terms of Jacobi-elliptic integrals for all possible energy and
angular momentum of the photons. The circular orbits of the photons are studied
in detail. As an application of the null geodesics, the angle of deflection of
the photons are computed.Comment: 25 pages, 20 figures. typos corrected and some of the notation
change
Non-Equilibrium in Adsorbed Polymer Layers
High molecular weight polymer solutions have a powerful tendency to deposit
adsorbed layers when exposed to even mildly attractive surfaces. The
equilibrium properties of these dense interfacial layers have been extensively
studied theoretically. A large body of experimental evidence, however,
indicates that non-equilibrium effects are dominant whenever monomer-surface
sticking energies are somewhat larger than kT, a common case. Polymer
relaxation kinetics within the layer are then severely retarded, leading to
non-equilibrium layers whose structure and dynamics depend on adsorption
kinetics and layer ageing. Here we review experimental and theoretical work
exploring these non-equilibrium effects, with emphasis on recent developments.
The discussion addresses the structure and dynamics in non-equilibrium polymer
layers adsorbed from dilute polymer solutions and from polymer melts and more
concentrated solutions. Two distinct classes of behaviour arise, depending on
whether physisorption or chemisorption is involved. A given adsorbed chain
belonging to the layer has a certain fraction of its monomers bound to the
surface, f, and the remainder belonging to loops making bulk excursions. A
natural classification scheme for layers adsorbed from solution is the
distribution of single chain f values, P(f), which may hold the key to
quantifying the degree of irreversibility in adsorbed polymer layers. Here we
calculate P(f) for equilibrium layers; we find its form is very different to
the theoretical P(f) for non-equilibrium layers which are predicted to have
infinitely many statistical classes of chain. Experimental measurements of P(f)
are compared to these theoretical predictions.Comment: 29 pages, Submitted to J. Phys.: Condens. Matte
The Relativistic Factor in the Orbital Dynamics of Point Masses
There is a growing population of relativistically relevant minor bodies in
the Solar System and a growing population of massive extrasolar planets with
orbits very close to the central star where relativistic effects should have
some signature. Our purpose is to review how general relativity affects the
orbital dynamics of the planetary systems and to define a suitable relativistic
correction for Solar System orbital studies when only point masses are
considered. Using relativistic formulae for the N body problem suited for a
planetary system given in the literature we present a series of numerical
orbital integrations designed to test the relevance of the effects due to the
general theory of relativity in the case of our Solar System. Comparison
between different algorithms for accounting for the relativistic corrections
are performed. Relativistic effects generated by the Sun or by the central star
are the most relevant ones and produce evident modifications in the secular
dynamics of the inner Solar System. The Kozai mechanism, for example, is
modified due to the relativistic effects on the argument of the perihelion.
Relativistic effects generated by planets instead are of very low relevance but
detectable in numerical simulations
X-ray fluorescence study of organic-inorganic polymer conversion into ceramics induced by ion irradiation
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