912 research outputs found
Testing the validity of the ray-tracing code GYOTO
In the next few years, the near-infrared interferometer GRAVITY will be able
to observe the Galactic center. Astrometric data will be obtained with an
anticipated accuracy of 10 as. To analyze these future data, we have
developed a code called GYOTO to compute orbits and images. We want to assess
the validity and accuracy of GYOTO in a variety of contexts, in particular for
stellar astrometry in the Galactic center. Furthermore, we want to tackle and
complete a study made on the astrometric displacements that are due to lensing
effects of a star of the central parsec with GYOTO. We first validate GYOTO in
the weak-deflection limit (WDL) by studying primary caustics and primary
critical curves obtained for a Kerr black hole. We compare GYOTO results to
available analytical approximations and estimate GYOTO errors using an
intrinsic estimator. In the strong-deflection limit (SDL), we choose to compare
null geodesics computed by GYOTO and the ray-tracing code named Geokerr.
Finally, we use GYOTO to estimate the apparent displacements of a star for
different angles from Sagittarius A* (Sgr A*). We have demonstrated that GYOTO
is accurate to a very high level, orders of magnitude better than the GRAVITY
requirements. GYOTO is also valid in weak- and strong-deflection regimes and
for very long integrations. At the astrometric precision that GRAVITY is aiming
for, lensing effects must always be taken into account when fitting stellar
orbits in the central parsec of the Galaxy.Comment: 11 pages, 12 figure
Exploring the water and carbon monoxide shell around Betelgeuse with VLTI/AMBER
We present the results of the analysis of our recent interferometric
observations of Betelgeuse, using the AMBER instrument of the VLTI. Using the
medium spectral resolution mode () we detected the presence of the
water vapour and carbon monoxide (CO) molecules in the H and K bands. We also
derived the photospheric angular diameter in the continuum. By analysing the
depth of the molecular lines and the interferometric visibilities, we derived
the column densities of the molecules, as well as the temperature and the size
of the corresponding regions in the atmosphere of Betelgeuse (the MOLsphere)
using a single shell model around the photosphere. Our results confirm the
findings by Perrin \et al\ (\cite{Perrin2004}) and Ohnaka \et al\
(\cite{Ohnaka2011}) that the HO and CO molecules are distributed around
Betelgeuse in a MOLsphere extending to approximately 1.3 times the star's
photospheric radius.Comment: Betelgeuse Workshop, November 2012, Paris. To be published in the
European Astronomical Society Publications Series, 2013, Editors: Pierre
Kervella, Thibaut Le Bertre \& Guy Perri
The convection of close red supergiant stars observed with near-infrared interferometry
Our team has obtained observations of the photosphere of the two closest red
supergiant stars Betelgeuse ( Ori) and Antares ( Sco) using
near infrared interferometry. We have been monitoring the photosphere of
Betelgeuse with the VLTI/PIONIER instrument for three years. On Antares, we
obtained an unprecedented sampling of the visibility function. These data allow
us to probe the convective photosphere of massive evolved stars.Comment: 5 pages, 3 figures. Published in the proceedings of the Physics Of
Evolved Stars conference, dedicated to the memory of Olivier Chesneau (Nice,
France, 2015
Comparison of fringe-tracking algorithms for single-mode near-infrared long-baseline interferometers
To enable optical long baseline interferometry toward faint objects, long
integrations are necessary despite atmospheric turbulence. Fringe trackers are
needed to stabilize the fringes and thus increase the fringe visibility and
phase signal-to-noise ratio (SNR), with efficient controllers robust to
instrumental vibrations, and to subsequent path fluctuations and flux
drop-outs.
We report on simulations, analysis and comparison of the performances of a
classical integrator controller and of a Kalman controller, both optimized to
track fringes under realistic observing conditions for different source
magnitudes, disturbance conditions, and sampling frequencies. The key
parameters of our simulations (instrument photometric performance, detection
noise, turbulence and vibrations statistics) are based on typical observing
conditions at the Very Large Telescope observatory and on the design of the
GRAVITY instrument, a 4-telescope single-mode long baseline interferometer in
the near-infrared, next in line to be installed at VLT Interferometer.
We find that both controller performances follow a two-regime law with the
star magnitude, a constant disturbance limited regime, and a diverging detector
and photon noise limited regime. Moreover, we find that the Kalman controller
is optimal in the high and medium SNR regime due to its predictive commands
based on an accurate disturbance model. In the low SNR regime, the model is not
accurate enough to be more robust than an integrator controller. Identifying
the disturbances from high SNR measurements improves the Kalman performances in
case of strong optical path difference disturbances.Comment: Accepted for publication in A&A. 17 pages 15 figure
Properties of the CO and HO MOLsphere of the red supergiant Betelgeuse from VLTI/AMBER observations
Context. Betelgeuse is the closest red supergiant (RSG); therefore, it is
well suited for studying the complex processes in its atmosphere that lead to
the chemical enrichment of the interstellar medium. Aims. We intend to
investigate the shape and composition of the close molecular layer (also known
as the MOLsphere) that surrounds the star. This analysis is part of a wider
program that aims at understanding the dynamics of the circumstellar envelope
of Betelgeuse. Methods. On January and February 2011, Betelgeuse was observed
using the Astronomical Multi-BEam combineR (AMBER) instrument of the Very Large
Telescope Interferometer (VLTI) in the H and K bands. Using the medium spectral
resolution of the instrument (R 1500), we were able to investigate the
carbon monoxide band heads and the water-vapor bands. We used two different
approaches to analyse our data: a model fit in both the continuum and
absorption lines and then a fit with a Radiative HydroDynamics (RHD)
simulation. Results. Using the continuum data, we derive a uniform disk
diameter of ~mas, a power law type limb-darkened disk diameter
of ~mas and a limb-darkening exponent of .
Within the absorption lines, using a single layer model, we obtain parameters
of the MOLsphere. Using a RHD simulation, we unveil the convection pattern in
the visibilities. Conclusions. We derived a new value of the angular diameter
of Betelgeuse in the K band continuum. Our observations in the absorption lines
are well reproduced by a molecular layer at 1.2 stellar radii containing both
CO and HO. The visibilities at higher spatial frequencies are matching a
convection pattern in a RHD simulation.Comment: 13 pages, 11 figures, accepted for publication in Astronomy &
Astrophysics; Language editin
A self-calibration approach for optical long baseline interferometry imaging
Current optical interferometers are affected by unknown turbulent phases on
each telescope. In the field of radio-interferometry, the self-calibration
technique is a powerful tool to process interferometric data with missing phase
information. This paper intends to revisit the application of self-calibration
to Optical Long Baseline Interferometry (OLBI). We cast rigorously the OLBI
data processing problem into the self-calibration framework and demonstrate the
efficiency of the method on real astronomical OLBI dataset
10 um wavefront spatial filtering: first results with chalcogenide fibers
Wavefront cleaning by single-mode fibers has proved to be efficient in
optical-infrared interferometry to improve calibration quality. For instance,
the FLUOR instrument has demonstrated the capability of fluoride glass
single-mode fibers in this respect in the K and L bands. New interferometric
instruments developped for the mid-infrared require the same capability for the
8-12 um range. We have initiated a program to develop single-mode fibers in the
prospect of the VLTI mid-infrared instrument MIDI and of the ESA/DARWIN and
NASA/TPF missions that require excellent wavefront quality. In order to
characterize the performances of chalcogenide fibers we are developping, we
have set up an experiment to measure the far-field pattern radiated at 10 um.
In this paper, we report the first and promising results obtained with this new
component.Comment: Conference "Interferometry for Optical Astronomy II", SPIE 200
An edge-on translucent dust disk around the nearest AGB star L2 Puppis - VLT/NACO spectro-imaging from 1.04 to 4.05 microns and VLTI interferometry
As the nearest known AGB star (d=64pc) and one of the brightest (mK-2), L2
Pup is a particularly interesting benchmark object to monitor the final stages
of stellar evolution. We report new lucky imaging observations of this star
with the VLT/NACO adaptive optics system in twelve narrow band filters covering
the 1.0-4.0 microns wavelength range. These diffraction limited images reveal
an extended circumstellar dust lane in front of the star, that exhibits a high
opacity in the J band and becomes translucent in the H and K bands. In the L
band, extended thermal emission from the dust is detected. We reproduce these
observations using Monte-Carlo radiative transfer modeling of a dust disk with
the RADMC-3D code. We also present new interferometric observations with the
VLTI/VINCI and MIDI instruments. We measure in the K band an upper limit to the
limb-darkened angular diameter of theta_LD = 17.9 +/- 1.6 mas, converting to a
maximum linear radius of R = 123 +/- 14 Rsun. Considering the geometry of the
extended K band emission in the NACO images, this upper limit is probably close
to the actual angular diameter of the star. The position of L2 Pup in the
Herzsprung-Russell diagram indicates that this star has a mass around 2 Msun
and is probably experiencing an early stage of the asymptotic giant branch. We
do not detect any stellar companion of L2 Pup in our adaptive optics and
interferometric observations, and we attribute its apparent astrometric wobble
in the Hipparcos data to variable lighting effects on its circumstellar
material. We however do not exclude the presence of a binary companion, as the
large loop structure extending to more than 10 AU to the North-East of the disk
in our L band images may be the result of interaction between the stellar wind
of L2 Pup and a hidden secondary object. The geometric configuration that we
propose, with a large dust disk seen almost edge-on, appears particularly
favorable to test and develop our understanding of the formation of bipolar
nebulae.Comment: 16 pages, 15 figure
A catalog of reference stars for long baseline stellar interferometry
The calibration process of long baseline stellar interferometers requires the
use of reference stars with accurately determined angular diameters. We present
a catalog of 374 carefully chosen stars among the all-sky network of infrared
sources provided by Cohen et al. 1999. The catalog benefits from a very good
sky coverage and a median formal error on the angular diameters of only 1.2%.
Besides, its groups together in a homogeneous handy set stellar coordinates,
uniform and limb-darkened angular diameters, photometric measurements, and
other parameters relevant to optical interferometry. In this paper, we describe
the selection criteria applied to qualify stars as reference sources. Then, we
discuss the catalog's statistical properties such as the sky coverage or the
distributions of magnitudes and angular diameters. We study the number of
available reference stars as a function of the baseline and the precision
needed on the visibility measurements. Finally, we compare the angular
diameters predicted in Cohen et al. 1999 with existing determinations in the
literature, and find a very good agreement.Comment: Conference "Interferometry for Optical Astronomy II", SPIE 200
Les X astronomes
Avec la découverte des lois de la gravitation newtonienne à la fin du XVIIeme siècle, l’Univers proche semble désormais accessible. La mécanique céleste renaît, se développe à grands pas et entraîne l’astronomie dans sa lancée. L’enjeu principal de l’astronomie est alors d’expliquer la dynamique des corps du système solaire (planètes, satellites et comètes). Les succès rencontrés sont éclatants, à l’image de la redécouverte prédite de la comète de Halley en 1759, et ils alimentent et encourag..
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