1,738 research outputs found
Towards a covariant canonical formulation for closed topological defects without boundaries
On the basis of the covariant description of the canonical formalism for
quantization, we present the basic elements of the symplectic geometry for a
restricted class of topological defects propagating on a curved background
spacetime. We discuss the future extensions of the present results.Comment: LaTeX, 12 pages, submitted to Phys. Lett. B. (2002
Variability of stellar granulation and convective blueshift with spectral type and magnetic activity. I. K and G main sequence stars
In solar-type stars, the attenuation of convective blueshift by stellar
magnetic activity dominates the RV variations over the low amplitude signal
induced by low mass planets. Models of stars that differ from the Sun will
require a good knowledge of the attenuation of the convective blueshift to
estimate its impact on the variations. It is therefore crucial to precisely
determine not only the amplitude of the convective blueshift for different
types of stars, but also the dependence of this convective blueshift on
magnetic activity, as these are key factors in our model producing the RV. We
studied a sample of main sequence stars with spectral types from G0 to K2 and
focused on their temporally averaged properties: the activity level and a
criterion allowing to characterise the amplitude of the convective blueshift.
We find the differential velocity shifts of spectral lines due to convection to
depend on the spectral type, the wavelength (this dependence is correlated with
the Teff and activity level), and on the activity level. This allows us to
quantify the dependence of granulation properties on magnetic activity for
stars other than the Sun. The attenuation factor of the convective blueshift
appears to be constant over the considered range of spectral types. We derive a
convective blueshift which decreases towards lower temperatures, with a trend
in close agreement with models for Teff lower than 5800 K, but with a
significantly larger global amplitude. We finally compare the observed RV
variation amplitudes with those that could be derived from our convective
blueshift using a simple law and find a general agreement on the amplitude. Our
results are consistent with previous results and provide, for the first time,
an estimation of the convective blueshift as a function of Teff, magnetic
activity, and wavelength, over a large sample of G and K main sequence stars
Interpreting the photometry and spectroscopy of directly imaged planets: a new atmospheric model applied to beta Pictoris b and SPHERE observations
We aim to interpret future photometric and spectral measurements from these
instruments, in terms of physical parameters of the planets, with an
atmospheric model using a minimal number of assumptions and parameters.
We developed Exoplanet Radiative-convective Equilibrium Model (Exo-REM) to
analyze the photometric and spectro- scopic data of directly imaged planets.
The input parameters are a planet's surface gravity (g), effective temperature
(Teff ), and elemental composition. The model predicts the equilibrium
temperature profile and mixing ratio profiles of the most important gases.
Opacity sources include the H2-He collision-induced absorption and molecular
lines from eight compounds (including CH4 updated with the Exomol line list).
Absorption by iron and silicate cloud particles is added above the expected
condensation levels with a fixed scale height and a given optical depth at some
reference wavelength. Scattering was not included at this stage.
We applied Exo-REM to photometric and spectral observations of the planet
beta Pictoris b obtained in a series of near-IR filters. We derived Teff = 1550
+- 150 K, log(g) = 3.5 +- 1, and radius R = 1.76 +- 0.24 RJup (2-{\sigma} error
bars from photometric measurements). These values are comparable to those found
in the literature, although with more conservative error bars, consistent with
the model accuracy. We were able to reproduce, within error bars, the J- and
H-band spectra of beta Pictoris b. We finally investigated the precision to
which the above parameterComment: 15 pages, 14 figures, accepted by A&
Visibility Contractors: Application to Mobile Robot Localization
Visibility is studied and used in several fields: computer graphics, telecommunication, robotics... For instance, in Computer-aided design (CAD) synthesis images are created by simulating light propagation in a scene. Visibility notions are then necessary to compute the visible objects from a point of view, and the shadow of those objects. In mobile robotics the visibility is used for path planning (visibility graph) and localization problems. This presentation is about visibility information for mobile robot localization. The objective is twofold. First a visibility notion based on segment intersections is presented. By considering a set-membership approach it is possible to develop contractors associated to this visibility relation. Then two applications of those visibility contractors to mobile robot localization are presented. The first one corresponds to the pose tracking of a team of robots. The idea is to use a Boolean information (the visibility between two robots: two robots are visible or not) in order to avoid the drifting of those robots (in order to maintain the precision of their position estimations). The second application corresponds to the processing of an original constraint for a set-membership global localization algorithm. This global localization algorithm is based on a CSP approach (Constraint Satisfaction Problem). Adding a visibility constraint to this CSP improves the accuracy of the algorithm
On Sufficient Conditions of the Injectivity: Development of a Numerical Test Algorithm via Interval Analysis
This paper presents a new numerical algorithm based on interval analysis able to verify that a continuously differentiable function is injective. The efficiency of the method is demonstrated by illustrative examples. These examples have been treated by a C++ solver which is made available
Confirmation of Lagrange Hypothesis for Twisted Elastic Rod
The history of structural optimization as an exact science begins possibly
with the celebrated Lagrange problem: to find a curve which by its revolution
about an axis in its plane determines the rod of greatest efficiency. The
Lagrange hypothesis, that the optimal rod possesses the constant cross-section
was abandoned for Euler buckling problem. In this Article the Lagrange
hypothesis is proved to be valid for Greenhill's problem of torque buckling.
The corresponding isoperimetric inequality is affirmed.Comment: 4 page
Sparse aperture masking at the VLT II. Detection limits for the eight debris disks stars Pic, AU Mic, 49 Cet, Tel, Fomalhaut, g Lup, HD181327 and HR8799
Context. The formation of planetary systems is a common, yet complex
mechanism. Numerous stars have been identified to possess a debris disk, a
proto-planetary disk or a planetary system. The understanding of such formation
process requires the study of debris disks. These targets are substantial and
particularly suitable for optical and infrared observations. Sparse Aperture
masking (SAM) is a high angular resolution technique strongly contributing to
probe the region from 30 to 200 mas around the stars. This area is usually
unreachable with classical imaging, and the technique also remains highly
competitive compared to vortex coronagraphy. Aims. We aim to study debris disks
with aperture masking to probe the close environment of the stars. Our goal is
either to find low mass companions, or to set detection limits. Methods. We
observed eight stars presenting debris disks ( Pictoris, AU
Microscopii, 49 Ceti, Telescopii, Fomalhaut, g Lupi, HD181327 and
HR8799) with SAM technique on the NaCo instrument at the VLT. Results. No close
companions were detected using closure phase information under 0.5 of
separation from the parent stars. We obtained magnitude detection limits that
we converted to Jupiter masses detection limits using theoretical isochrones
from evolutionary models. Conclusions. We derived upper mass limits on the
presence of companions in the area of few times the diffraction limit of the
telescope around each target star.Comment: 7 pages, All magnitude detection limits maps are only available in
electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr
(130.79.128.5
3 - 14 Micron Spectroscopy of Comets C/2002 O4 (Honig), C/2002 V1 (NEAT), C/2002 X5 (Kudo-Fujikawa), C/2002 Y1 (Juels-Holvorcem), 69P/Taylor, and the Relationships among Grain Temperature, Silicate Band Strength and Structure among Comet Families
We report 3 - 13 micron spectroscopy of 4 comets observed between August 2002
and February 2003: C/2002 O4 (Honig) on August 1, 2002, C/2002 V1 (NEAT) on
Jan. 9 and 10, 2003, C/2002 X5 (Kudo-Fujikawa) on Jan. 9 and 10, 2003, and
C/2002 Y1 (Juels-Holvorcem) on Feb. 20, 2003. In addition, we include data
obtained much earlier on 69P/Taylor (February 9, 1998) but not previously
published. For Comets Taylor, Honig, NEAT, and Kudo-Fujikawa, the silicate
emission band was detected, being approximately 23%, 12%, 15%, and 10%,
respectively, above the continuum. The data for Comet Juels-Holvorcem were of
insufficient quality to detect the presence of a silicate band of comparable
strength to the other three objects, and we place an upper limit of 24% on this
feature. The silicate features in both NEAT and Kudo-Fujikawa contained
structure indicating the presence of crystalline material. Combining these data
with those of other comets, we confirm the correlation between silicate band
strength and grain temperature of Gehrz & Ney (1992) and Williams et al. (1997)
for dynamically new and long period comets, but the majority of Jupiter family
objects may deviate from this relation. The limited data available on Jupiter
family objects suggest that they may have silicate bands that are slightly
different from the former objects. Finally, when compared to the silicate
emission bands observed in pre-main sequence stars, the dynamically new and
long period comets most closely resemble the more evolved stellar systems,
while the limited data (in quantity and quality) on Jupiter family objects seem
to suggest that these have spectra more like the less-evolved stars.Comment: 45 pages, 12 figure
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