10,607 research outputs found
Functional Methods and Effective Potentials for Nonlinear Composites
A formulation of variational principles in terms of functional integrals is
proposed for any type of local plastic potentials. The minimization problem is
reduced to the computation of a path integral. This integral can be used as a
starting point for different approximations. As a first application, it is
shown how to compute to second-order the weak-disorder perturbative expansion
of the effective potentials in random composite. The three-dimensional results
of Suquet and Ponte-Casta\~neda (1993) for the plastic dissipation potential
with uniform applied tractions are retrieved and extended to any space
dimension, taking correlations into account. In addition, the viscoplastic
potential is also computed for uniform strain rates.Comment: 20 pages, accepted for publication in JMP
Pupil remapping for high contrast astronomy: results from an optical testbed
The direct imaging and characterization of Earth-like planets is among the
most sought-after prizes in contemporary astrophysics, however current optical
instrumentation delivers insufficient dynamic range to overcome the vast
contrast differential between the planet and its host star. New opportunities
are offered by coherent single mode fibers, whose technological development has
been motivated by the needs of the telecom industry in the near infrared. This
paper presents a new vision for an instrument using coherent waveguides to
remap the pupil geometry of the telescope. It would (i) inject the full pupil
of the telescope into an array of single mode fibers, (ii) rearrange the pupil
so fringes can be accurately measured, and (iii) permit image reconstruction so
that atmospheric blurring can be totally removed. Here we present a laboratory
experiment whose goal was to validate the theoretical concepts underpinning our
proposed method. We successfully confirmed that we can retrieve the image of a
simulated astrophysical object (in this case a binary star) though a pupil
remapping instrument using single mode fibers.Comment: Accepted in Optics Expres
Updated results on prototype chalcogenide fibers for 10-um wavefront spatial filtering
The detection of terrestrial planets by Darwin/TPF missions will require
extremely high quality wavefronts. Single-mode fibers have proven to be
powerful beam cleaning components in the near-infrared, but are currently not
available in the mid-infrared where they would be critically needed for
Darwin/TPF. In this paper, we present updated measurements on the prototype
chalcogenide fibers we are developing for the purpose of mid-infrared spatial
filtering. We demonstrate the guiding property of our 3rd generation component
and we characterize its filtering performances on a 4 mm length: the far-field
radiation pattern matches a Gaussian profile at the level of 3% rms and 13%
pk-pk.Comment: 4 pages, 5 figures, to appear in the proceedings of the conference
"Toward Other Earths, Darwin/TPF and the search for extrasolar terrestrial
planets", held in Heidelberg, Germany, 22-25 April 2003, ESA SP-53
Distinguishing an ejected blob from alternative flare models at the Galactic centre with GRAVITY
The black hole at the Galactic centre exhibits regularly flares of radiation,
the origin of which is still not understood. In this article, we study the
ability of the near-future GRAVITY infrared instrument to constrain the nature
of these events. We develop realistic simulations of GRAVITY astrometric data
sets for various flare models. We show that the instrument will be able to
distinguish an ejected blob from alternative flare models, provided the blob
inclination is >= 45deg, the flare brightest magnitude is 14 <= mK <= 15 and
the flare duration is >= 1h30.Comment: 11 pages, 9 figures, accepted by MNRA
Maximizing the community exploitation of the VLTI 2nd-generation instruments
This is the author accepted manuscript. The final version is available from Springer Verlag via the DOI in this recordThe Very Large Telescope Interferometer (VLTI, Figure 1) is the European flagship
interferometric facility and allows European astronomers to study the universe on milliarcsecond
scale, enabling fundamentally new opportunities in planet formation to stellar and
extragalactic astronomy. The facility was conceived with the goal of making optical
interferometry available to the whole European astronomy community and to serve the needs
both of expert as well as non-expert users. The VINCI commissioning instrument (Kervella et
al. 2000), the 1st generation
instruments MIDI (Leinert et al. 2003) and AMBER (Petrov et al.
2007) and the visitor instrument PIONIER (LeBouquin et al. 2011) took major steps towards
this goal but revealed also challenges, for instance, related to attracting non-expert users to
interferometry. Currently, the VLTI undergoes a major transformation with the arrival of the
2nd-generation instruments GRAVITY (GRAVITY collaboration et al. 2017) and MATISSE (Matter
et al. 2016). GRAVITY has been offered to the community since October 2016, while the onsky
commissioning of MATISSE is due to start in early 2018, with a possible start of regular
science observations the following year. In this contribution, we reflect on how the 2ndgeneration
instruments might help in expanding the VLTI user community and we will discuss
steps that could be taken to support this process. The expert community, both inside and
outside of the instrument consortia, should coordinate in order to optimize the scientific
output of the new VLTI instruments.We acknowledge support from the OPTICON Networking Activity FP7 WP14 and H2020 WP11,
funded through the European Commission’s 7th Framework R&D program (grant number
312430) and Horizon 2020 R&D program (grant number 730890). S.K. acknowledges support
from an STFC Rutherford Fellowship (ST/J004030/1)
Long-term spectropolarimetric monitoring of the cool supergiant Betelgeuse
We report on a long-term monitoring of the cool supergiant Betelgeuse, using
the NARVAL and ESPaDOnS high-resolution spectropolarimeters, respectively
installed at Telescope Bernard Lyot (Pic du Midi Observatory, France) and at
the Canada-France-Hawaii Telescope (Mauna Kea Observatory, Hawaii). The data
set, constituted of circularly polarized (Stokes V) and intensity (Stokes I)
spectra, was collected between 2010 and 2012. We investigate here the temporal
evolution of magnetic field, convection and temperature at photospheric level,
using simultaneous measurements of the longitudinal magnetic field component,
the core emission of the Ca II infrared triplet, the line-depth ratio of
selected photospheric lines and the radial velocity of the star.Comment: Proceedings of the Betelgeuse Workshop, Paris, 26-29 Nov 201
GRAVITY: The AO-Assisted, Two-Object Beam-Combiner Instrument
We present the proposal for the infrared adaptive optics (AO) assisted,
two-object, high-throughput, multiple-beam-combiner GRAVITY for the VLTI. This
instrument will be optimized for phase-referenced interferometric imaging and
narrow-angle astrometry of faint, red objects. Following the scientific
drivers, we analyze the VLTI infrastructure, and subsequently derive the
requirements and concept for the optimum instrument. The analysis can be
summarized with the need for highest sensitivity, phase referenced imaging and
astrometry of two objects in the VLTI beam, and infrared wavefront-sensing.
Consequently our proposed instrument allows the observations of faint, red
objects with its internal infrared wavefront sensor, pushes the optical
throughput by restricting observations to K-band at low and medium spectral
resolution, and is fully enclosed in a cryostat for optimum background
suppression and stability. Our instrument will thus increase the sensitivity of
the VLTI significantly beyond the present capabilities. With its two fibers per
telescope beam, GRAVITY will not only allow the simultaneous observations of
two objects, but will also push the astrometric accuracy for UTs to 10
micro-arcsec, and provide simultaneous astrometry for up to six baselines.Comment: 12 pages, to be published in the Proceedings of the ESO Workshop on
"The Power of Optical/IR Interferometry: Recent Scientific Results and 2nd
Generation VLTI Instrumentation", eds. F. Paresce, A. Richichi, A. Chelli and
F. Delplancke, held in Garching, Germany, 4-8 April 200
Resolving asymmetries along the pulsation cycle of the Mira star X Hya
The mass-loss process in Mira stars probably occurs in an asymmetric way
where dust can form in inhomogeneous circumstellar molecular clumps. Following
asymmetries along the pulsation cycle can give us clues about these mass-loss
processes. We imaged the Mira star X Hya and its environnement at different
epochs to follow the evolution of the morphology in the continuum and in the
molecular bands. We observed X Hya with AMBER in J-H-K at low resolution at two
epochs. We modelled squared visibilities with geometrical and physical models.
We also present imaging reconstruction results obtained with MiRA and based on
the physical a priori images. We report on the angular scale change of X Hya
between the two epochs. 1D CODEX profiles allowed us to understand and model
the spectral variation of squared visibilities and constrain the stellar
parameters. Reconstructed model-dependent images enabled us to reproduce
closure phase signals and the azimuthal dependence of squared visibilities.
They show evidence for material inhomogeneities located in the immediate
environment of the star.Comment: Accepted for publication in A&A, 17 pages, 16 figure
Optical Spectroscopy of Galactic Cirrus Clouds: Extended Red Emission in the Diffuse Interstellar Medium
We present initial results from the first optical spectroscopic survey of
high latitude Galactic cirrus clouds. The observed shape of the cirrus spectrum
does not agree with that of scattered ambient Galactic starlight. This mismatch
can be explained by the presence of Extended Red Emission (ERE) in the diffuse
interstellar medium, as found in many other astronomical objects, probably
caused by photoluminescence of hydrocarbons. The integrated ERE intensity,
I_ERE \approx 1.2 x 10^{-5} erg s^{-1} cm^{-2} sr^{-1}, is roughly a third of
the scattered light intensity, consistent with recent color measurements of
diffuse Galactic light. The peak of the cirrus ERE (lambda_{0} \sim 6000 AA) is
shifted towards short (bluer) wavelengths compared to the ERE in sources
excited by intense ultraviolet radiation, such as HII regions (lambda_{0} sim
8000 AA); such a trend is seen in laboratory experiments on hydrogenated
amorphous carbon films.Comment: 7 pages, 2 figures. Accepted for publication in ApJ Letter
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