44 research outputs found
High spectral resolution imaging of the dynamical atmosphere of the red supergiant Antares in the CO first overtone lines with VLTI/AMBER
We present high spectral resolution aperture-synthesis imaging of the red
supergiant Antares (alpha Sco) in individual CO first overtone lines with
VLTI/AMBER. The reconstructed images reveal that the star appears differently
in the blue wing, line center, and red wing and shows an asymmetrically
extended component. The appearance of the star within the CO lines changes
drastically within one year, implying a significant change in the velocity
field in the atmosphere. Our modeling suggests an outer atmosphere (MOLsphere)
extending to 1.2--1.4 stellar radii with CO column densities of
(0.5--1)x10^{20} cm^{-2} and a temperature of ~2000 K. While the velocity field
in 2009 is characterized by strong upwelling motions at 20--30 km/s, it changed
to strong downdrafts in 2010. On the other hand, the AMBER data in the
continuum show only a slight deviation from limb-darkened disks and only
marginal time variations. We derive a limb-darkened disk diameter of
37.38+/-0.06 mas and a power-law-type limb-darkening parameter of
(8.7+/-1.6)x10^{-2} (2009) and 37.31+/-0.09 mas and (1.5+/-0.2)x10^{-1} (2010).
We also obtain Teff = 3660+/-120 K and log L/Lsun = 4.88+/-0.23, which suggests
a mass of 15+/-5 Msun with an age of 11-15 Myr. This age is consistent with the
recently estimated age for the Upper Scorpius OB association. The properties of
the outer atmosphere of Antares are similar to those of another well-studied
red supergiant, Betelgeuse. The density of the extended outer atmosphere of
Antares and Betelgeuse is higher than predicted by the current 3-D convection
simulations by at least six orders of magnitude, implying that convection alone
cannot explain the formation of the extended outer atmosphere.Comment: 18 pages, 16 figures, accepted for publication in Astronomy and
Astrophysics, short discussion on the age of Antares and the Upper Scorpius
OB association added, movies of the reconstructed images available at
http://www.mpifr-bonn.mpg.de/staff/kohnaka
Fresnel diffraction in an interferometer: application to MATISSE
While doing optical study in an instrument similar to the interferometers
dedicated to the Very Large Telescope (VLT), we have to take care of the pupil
and focus conjugations. Modules with artificial sources are designed to
simulate the stellar beams, in terms of collimation and pupil location. They
constitute alignment and calibration tools. In this paper, we present such a
module in which the pupil mask is not located in a collimated beam thus
introducing Fresnel diffraction. We study the instrumental contrast taking into
account the spatial coherence of the source, and the pupil diffraction. The
considered example is MATISSE, but this study can apply to any other instrument
concerned with Fresnel diffraction.Comment: 8 pages- to appear in Proceedings of SPIE Astronomical Telescopes and
Instrumentation 201
Parasitic Interference in Long Baseline Optical Interferometry: Requirements for Hot Jupiter-like Planet Detection
International audienceThe observable quantities in optical interferometry, which are the modulus and the phase of the complex visibility, may be corrupted by parasitic fringes superimposed on the genuine fringe pattern. These fringes are due to an interference phenomenon occurring from stray light effects inside an interferometric instrument. We developed an analytical approach to better understand this phenomenon when stray light causes cross talk between beams. We deduced that the parasitic interference significantly affects the interferometric phase and thus the associated observables including the differential phase and the closure phase. The amount of parasitic flux coupled to the piston between beams appears to be very influential in this degradation. For instance, considering a point-like source and a piston ranging from λ/500 to λ/5 in the L band (λ = 3.5 μm), a parasitic flux of about 1% of the total flux produces a parasitic phase reaching at most one-third of the intrinsic phase. The piston, which can have different origins (instrumental stability, atmospheric perturbations, etc.), thus amplifies the effect of parasitic interference. According to the specifications of piston correction in space or at ground level (respectively λ/500 ≈ 2 nm and λ/30 ≈ 100 nm), the detection of hot Jupiter-like planets, one of the most challenging aims for current ground-based interferometers, limits parasitic radiation to about 5% of the incident intensity. This was evaluated by considering different types of hot Jupiter synthetic spectra. Otherwise, if no fringe tracking is used, the detection of a typical hot Jupiter-like system with a solar-like star would admit a maximum level of parasitic intensity of 0.01% for piston errors equal to λ/15. If the fringe tracking specifications are not precisely observed, it thus appears that the allowed level of parasitic intensity dramatically decreases and may prevent the detection. In parallel, the calibration of the parasitic phase by a reference star, at this accuracy level, seems very difficult. Moreover, since parasitic phase is an object-dependent quantity, the use of a hypothetical phase abacus, directly giving the parasitic phase from a given parasitic flux level, is also impossible. Some instrumental solutions, implemented at the instrument design stage for limiting or preventing this parasitic interference, appear to be crucial and are presented in this paper
Apodized Lyot Coronagraph for VLT-SPHERE: Laboratory tests and performances of a first prototype in the visible
We present some of the High Dynamic Range Imaging activities developed around
the coronagraphic test-bench of the Laboratoire A. H. Fizeau (Nice). They
concern research and development of an Apodized Lyot Coronagraph (ALC) for the
VLT-SPHERE instrument and experimental results from our testbed working in the
visible domain. We determined by numerical simulations the specifications of
the apodizing filter and searched the best technological process to manufacture
it. We present the results of the experimental tests on the first apodizer
prototype in the visible and the resulting ALC nulling performances. The tests
concern particularly the apodizer characterization (average transmission radial
profile, global reflectivity and transmittivity in the visible), ALC nulling
performances compared with expectations, sensitivity of the ALC performances to
misalignments of its components
Prototyping coronagraphs for exoplanet characterization with SPHERE
The detection and characterization of extrasolar planets with SPHERE (Spectro
Polarimetric High contrast Exoplanet REsearch) is challenging and in particular
relies on the ability of a coronagraph to attenuate the diffracted starlight.
SPHERE includes 3 instruments, 2 of which can be operated simultaneously in the
near IR from 0.95 to 1.8 microns. This requirements is extremely critical for
coronagraphy. This paper briefly introduces the concepts of 2 coronagraphs, the
Half-Wave Plate Four Quadrant Phase Masks and the Apodized Pupil Lyot
Coronagraph, prototyped within the SPHERE consortium by LESIA (Observatory of
Paris) and FIZEAU (University of Nice) respectively. Then, we present the
measurements of contrast and sensitivity analysis. The comparison with
technical specifications allows to validate the technology for manufacturing
these coronagraphs.Comment: 10 pages, will be published in the proceeding of the SPIE conference
Volume 7015 "Adaptive Optics", held in Marseille from 23 to 28 june 200
Study of the atmospheric refraction in a single mode instrument - Application to AMBER/VLTI
International audienceThis paper presents a study of the atmospheric refraction and its effect on the light coupling efficiency in an instrument using single-mode optical fibers. We show the analytical approach which allowed us to assess the need to correct the refraction in J- and H-bands while observing with an 8-m Unit Telescope. We then developed numerical simulations to go further in calculations. The hypotheses on the instrumental characteristics are those of AMBER (Astronomical Multi BEam combineR), the near infrared focal beam combiner of the Very Large Telescope Interferometric mode (VLTI), but most of the conclusions can be generalized to other single-mode instruments. We used the software package caos (Code for Adaptive Optics Systems) to take into account the atmospheric turbulence effect after correction by the ESO system MACAO (Multi-Application Curvature Adaptive Optics). The opto-mechanical study and design of the system correcting the atmospheric refraction on AMBER is then detailed. We showed that the atmospheric refraction becomes predominant over the atmospheric turbulence for some zenith angles z and spectral conditions: for z larger than 30° in J-band for example. The study of the optical system showed that it allows to achieve the required instrumental performance in terms of throughput in J- and H-bands. First observations in J-band of a bright star, alpha Cir star, at more than 30° from zenith clearly showed the gain to control the atmospheric refraction in a single mode instrument, and validated the operating law
A binary engine fuelling HD87643' s complex circumstellar environment, using AMBER/VLTI
International audienceContext. The star HD 87643, exhibiting the “B[e] phenomenon”, has one of the most extreme infrared excesses for this object class. It harbours a large amount of both hot and cold dust, and is surrounded by an extended reflection nebula. Aims. One of our major goals was to investigate the presence of a companion in HD87643. In addition, the presence of close dusty material was tested through a combination of multi-wavelength high spatial resolution observations. Methods. We observed HD 87643 with high spatial resolution techniques, using the near-IR AMBER/VLTI interferometer with baselines ranging from 60 m to 130 m and the mid-IR MIDI/VLTI interferometer with baselines ranging from 25 m to 65 m. These observations are complemented by NACO/VLT adaptive-optics-corrected images in the K and L-bands, ESO-2.2m optical Wide-Field Imager large-scale images in the B, V and R-bands, Results. We report the direct detection of a companion to HD 87643 by means of image synthesis using the AMBER/VLTI instrument. The presence of the companion is confirmed by the MIDI and NACO data, although with a lower confidence. The companion is separated by ∼ 34 mas with a roughly north-south orientation. The period must be large (several tens of years) and hence the orbital parameters are not determined yet. Binarity with high eccentricity might be the key to interpreting the extreme characteristics of this system, namely a dusty circumstellar envelope around the primary, a compact dust nebulosity around the binary system and a complex extended nebula witnessing past violent ejections
AMBER : a near infrared focal instrument for the VLTI
10 pagesInternational audienceAMBER is the General User near-infrared focal instrument of the Very Large Telescope interferometer. Its specifications are based on three key programs on Young Stellar Objects, Active Galactic Nuclei central regions, masses and spectra of hot Extra Solar Planets. It has an imaging capacity because it combines up to three beams and very high accuracy measurement are expected from the spatial filtering of beams by single mode fibers and the comparison of measurements made simultaneously in different spectral channels
An asymmetry detected in the disk of Kappa CMa with the AMBER/VLTI
International audienceAims. We study the geometry and kinematics of the circumstellar environment of the Be star Kappa CMa in the Br gamma emission line and its nearby continuum. Methods. We use the VLTI/AMBER instrument operating in the K band which provides a spatial resolution of about 6 mas with a spectral resolution of 1500 to study the kinematics within the disk and to infer its rotation law. In order to obtain more kinematical constraints we also use an high spectral resolution Pa beta line profile obtain in December 2005 at the Observatorio do Pico do Dios, Brazil and we compile V/R line profile variations and spectral energy distribution data points from the literature. Results. Using differential visibilities and differential phases across the Br gamma line we detect an asymmetry in the disk. Moreover, we found that kappa CMa seems difficult to fit within the classical scenario for Be stars, illustrated recently by alpha Arae observations, i.e. a fast rotating B star close to its breakup velocity surrounded by a Keplerian circumstellar disk with an enhanced polar wind. Finally we discuss the possibility for kappa CMa to be a critical rotator with a Keplerian rotating disk and try to see if the detected asymmetry can be interpreted within the "one-armed" viscous disk framework
The GRAVITY+ Project: Towards All-sky, Faint-Science, High-Contrast Near-Infrared Interferometry at the VLTI
The GRAVITY instrument has been revolutionary for near-infrared
interferometry by pushing sensitivity and precision to previously unknown
limits. With the upgrade of GRAVITY and the Very Large Telescope Interferometer
(VLTI) in GRAVITY+, these limits will be pushed even further, with vastly
improved sky coverage, as well as faint-science and high-contrast capabilities.
This upgrade includes the implementation of wide-field off-axis
fringe-tracking, new adaptive optics systems on all Unit Telescopes, and laser
guide stars in an upgraded facility. GRAVITY+ will open up the sky to the
measurement of black hole masses across cosmic time in hundreds of active
galactic nuclei, use the faint stars in the Galactic centre to probe General
Relativity, and enable the characterisation of dozens of young exoplanets to
study their formation, bearing the promise of another scientific revolution to
come at the VLTI.Comment: Published in the ESO Messenge