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See detailThe inner debris disk of Vega as seen by CHARA/FLUOR
Absil, Olivier ULg

Scientific conference (2006, May 25)

Using the FLUOR beam-combiner installed at the CHARA Array (Mt Wilson, CA), we have obtained high-precision visibility measurements of Vega, one of the prototypic debris-disk stars, known to be surrounded ... [more ▼]

Using the FLUOR beam-combiner installed at the CHARA Array (Mt Wilson, CA), we have obtained high-precision visibility measurements of Vega, one of the prototypic debris-disk stars, known to be surrounded by a large amount of cold dust in a ring-like structure at 80-100 AU. The combination of short and long baselines has allowed us to separately resolve the stellar photosphere and the close environment of the star (<8 AU). Our observations show a significant deficit in square visibility at short baselines with respect to the expected visibility of a simple UD stellar model (dV2 ~ 2%), suggesting the presence of an extended source of emission around Vega. The sparse (u,v) plane coverage does not allow the discrimination between a point source and an extended circumstellar emission as the source of the extended emission. However, we show that the presence of a point-like source within the FLUOR field-of-view (1" in radius, i.e., 7.8 AU at the distance of Vega) is highly unlikely. The excess emission is most likely due to the presence of hot circumstellar dust in the inner part of Vega's debris disk, with a flux ratio of 1.29 +/- 0.19% between the integrated dust emission and the stellar photosphere. Complementing this result with archival photometric data in the near- and mid-infrared and taking into account a realistic photospheric model for the rapidly rotating Vega, we derive the expected physical properties of the circumstellar dust by modelling its Spectral Energy Distribution. The inferred properties suggest that the Vega system could be currently undergoing major dynamical perturbations. [less ▲]

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See detailVega: the star with comets?
Absil, Olivier ULg

Scientific conference (2006, May 19)

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See detailHot circumstellar material in Vega’s inner planetary system
Absil, Olivier ULg

Scientific conference (2006, March 21)

Using the FLUOR beam-combiner installed at the CHARA Array (Mt Wilson, CA), we have obtained high-precision visibility measurements of Vega, one of the prototypic debris-disk stars, known to be surrounded ... [more ▼]

Using the FLUOR beam-combiner installed at the CHARA Array (Mt Wilson, CA), we have obtained high-precision visibility measurements of Vega, one of the prototypic debris-disk stars, known to be surrounded by large amounts of cold dust in a ring-like structure at 80-100 AU. The combination of short and long baselines has allowed us to separately resolve the stellar photosphere and the close environment of the star (< 8 AU). Our observations show a significant deficit in square visibility at short baselines with respect to the expected visibility of a simple uniform disk stellar model (DV2 ~ 2%), suggesting the presence of an extended source of emission around Vega. The sparse (u, v) plane coverage does not allow for discriminating between a point source and an extended circumstellar emission as the origin of the extended emission. However, we show that the presence of a point-like source within the FLUOR field-ofview (1 arcsec in radius = 7.7 AU at the distance of Vega) is highly unlikely, and propose that the excess emission is most likely due to the presence of hot circumstellar dust in the inner part of Vega’s debris disk, with a flux ratio of 1.29±0.19% between the integrated dust emission and the stellar photosphere. Using this information together with archival photometric measurements in the near- and mid-infrared, we derive the expected physical properties of the circumstellar dust by modelling its infrared Spectral Energy Distribution. The inferred properties suggest that the Vega system could be currently undergoing major dynamical perturbations. [less ▲]

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See detailAstrophysical studies of extrasolar planetary systems using infrared interferometric techniques
Absil, Olivier ULg

Doctoral thesis (2006)

The study of extrasolar planetary systems has thrived during the last two decades, stimulated by the discovery of circumstellar dust and extrasolar planets around main sequence stars. However, direct ... [more ▼]

The study of extrasolar planetary systems has thrived during the last two decades, stimulated by the discovery of circumstellar dust and extrasolar planets around main sequence stars. However, direct imaging of planetary systems has been possible in only very special cases so far due to the huge contrast and to the small angular separation between stars and their environments. Even for these favourable cases, the inner regions where terrestrial planets are expected to be forming and where life could develop have not been investigated yet due to the lack of appropriate tools. Infrared interferometry is a very promising technique in this context, as it provides the required angular resolution to separate the emissions from the star and its immediate neighbourhood. The present work aims at developing the high dynamic range capabilities of interferometric techniques for the characterisation of planetary systems. As a first step, we demonstrate that current interferometric facilities have the potential of detecting dust in the first few astronomical units of massive debris disks around nearby stars. Our observations of Vega with the FLUOR near-infrared beam combiner at the CHARA Array reveal the presence of warm dust responsible for a K-band flux 78 times fainter than the stellar photospheric emission. In order to extend the imaging of planetary systems to fainter disks and to giant extrasolar planets, we investigate in a second step the performance of future ground-based nulling interferometers, taking into account atmospheric eff ects in a realistic way. Our simulations show that a nulling instrument installed at the ESO Very Large Telescope Interferometer would detect circumstellar features as faint as a few 10^-4 of the stellar flux. Finally, the third part of this work focuses on the implementation of nulling interferometry on future space-borne missions, the goal being to characterise extrasolar planets with sizes down to that of the Earth. [less ▲]

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See detailPerformance study of ground-based infrared Bracewell interferometers. Application to the detection of exozodiacal dust disks with GENIE
Absil, Olivier ULg; den Hartog, R.; Gondoin, P. et al

in Astronomy and Astrophysics (2006), 448

Nulling interferometry, a powerful technique for high-resolution imaging of the close neighbourhood of bright astrophysical objets, is currently considered for future space missions such as Darwin or the ... [more ▼]

Nulling interferometry, a powerful technique for high-resolution imaging of the close neighbourhood of bright astrophysical objets, is currently considered for future space missions such as Darwin or the Terrestrial Planet Finder Interferometer (TPF-I), both aiming at Earth-like planet detection and characterization. Ground-based nulling interferometers are being studied for both technology demonstration and scientific preparation of the Darwin/TPF-I missions through a systematic survey of circumstellar dust disks around nearby stars. In this paper, we investigate the influence of atmospheric turbulence on the performance of ground-based nulling instruments, and deduce the major design guidelines for such instruments. End-to-end numerical simulations allow us to estimate the performance of the main subsystems and thereby the actual sensitivity of the nuller to faint exozodiacal disks. Particular attention is also given to the important question of stellar leakage calibration. This study is illustrated in the context of GENIE, the Ground-based European Nulling Interferometer Experiment, to be installed at the VLTI and working in the L' band. We estimate that this instrument will detect exozodiacal clouds as faint as about 50 times the Solar zodiacal cloud, thereby placing strong constraints on the acceptable targets for Darwin/TPF-I. [less ▲]

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See detailInterferometric Gravity Darkening Observations of Vega with the CHARA Array
Aufdenberg, J. P.; Merand, A.; Coude Foresto, V. et al

in Coudé du Foresto, V; Rouan, D; Rousset, G (Eds.) Visions for Infrared Astronomy (2006, March)

We have obtained high-precision interferometric measurements of the A0 V standard star Vega with the Center for High Angular Resolution Astronomy (CHARA) Array and the Fiber Linked Unit for Optical ... [more ▼]

We have obtained high-precision interferometric measurements of the A0 V standard star Vega with the Center for High Angular Resolution Astronomy (CHARA) Array and the Fiber Linked Unit for Optical Recombination (FLUOR) beam combiner in the K' band at projected baselines between 103 m and 273 m. The measured squared visibility amplitudes beyond the first lobe are significantly weaker than expected for a slowly rotating star and provide strong evidence for the model of Vega as a rapidly rotating star viewed very nearly pole on. We have constructed a Roche-von Zeipel gravity-darkened model atmosphere which is in generally good agreement with both our interferometric data and archival spectrophotometry. Our model indicates Vega is rotating at Ë 92% of its angular break-up rate with an equatorial velocity of Ë 275 km s[SUP]-1[/SUP]. We find a polar effective temperature of Ë 10150 K and a pole-to-equator effective temperature difference of Ë 2500 K, much larger than the Ë 300 K derived by Gulliver, Hill, and Adelman. Our model suggests that Vega's cool equatorial atmosphere may have significant convective flux and predicts a significantly cooler spectral energy distribution for Vega as seen by its surrounding debris disk. This work was performed in part under contract with the Jet Propulsion Laboratory (JPL) funded by NASA through the Michelson Fellowship Program. JPL is managed for NASA by the California Institute of Technology. The CHARA Array is operated by the Center for High Angular Resolution Astronomy with support from Georgia State University and the National Science Foundation, the Keck Foundation and the Packard Foundation. [less ▲]

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See detailALADDIN: a DARWIN Pathfinder Mission on the Antarctic Plateau
Coudé Du Foresto, V.; Absil, Olivier ULg; Barillot, M.

in Coudé du Foresto, Vincent; Rouan, Daniel; Rousset, Gérard (Eds.) Visions for Infrared Astronomy (2006, March)

We propose an innovative concept for a DARWIN pathfinder experiment. It involves an integrated nulling interferometer of relatively modest dimensions (two 1 m class collectors on a 40 m truss), which ... [more ▼]

We propose an innovative concept for a DARWIN pathfinder experiment. It involves an integrated nulling interferometer of relatively modest dimensions (two 1 m class collectors on a 40 m truss), which would take advantage of a privileged location (at Dome C on the Antarctic plateau) to perform better than a nulling instrument on a large interferometer (such as GENIE at the VLTI). [less ▲]

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See detailHot Circumstellar Material around Vega
Absil, Olivier ULg; Di Folco, Emmanuel; Mérand, Antoine et al

in Coudé du Foresto, Vincent; Rouan, Daniel; Rousset, Gérard (Eds.) Visions for Infrared Astronomy (2006, March)

Using the FLUOR beam-combiner at the CHARA Array, we have obtained highprecision visibility measurements of Vega, a prototypic debris-disk star. The combination of long and short baselines has allowed us ... [more ▼]

Using the FLUOR beam-combiner at the CHARA Array, we have obtained highprecision visibility measurements of Vega, a prototypic debris-disk star. The combination of long and short baselines has allowed us to separately resolve the stellar photosphere and the close environment of the star (< 8 AU). Our observations show a significant deficit in square visibility at short baselines with respect to the expected visibility of a simple uniform disk stellar model, suggesting the presence of an extended source around Vega. We propose that the excess emission is most likely due to the presence of hot circumstellar dust in the inner part of Vega's debris disk, with a flux ratio of 1.29 ± 0.19% between the integrated dust emission and the stellar photosphere. Using this information together with archival photometric measurements in the nearand mid-infrared, we derive the expected physical properties of the circumstellar dust by modelling its infrared Spectral Energy Distribution. The inferred properties suggest that the Vega system could be currently undergoing major dynamical perturbations. [less ▲]

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See detailInfrared Interferometric Gravity Darkening Observations of Vega with CHARA/FLUOR
Aufdenberg, J.; Mérand, A.; Coudé Du Foresto, V. et al

in Coudé du Foresto, Vincent; Rouan, Daniel; Rousset, Gérard (Eds.) Visions for Infrared Astronomy (2006, March)

We have obtained high-precision measurements of Vega at projected baselines between 103 m and 273 m with the CHARA Array using the (FLUOR) beam combiner in the K' band. A strongly gravity-darkened model ... [more ▼]

We have obtained high-precision measurements of Vega at projected baselines between 103 m and 273 m with the CHARA Array using the (FLUOR) beam combiner in the K' band. A strongly gravity-darkened model atmosphere is in general agreement with both our interferometric data and archival spectrophotometry. This model indicates that Vega is rotating at 91%ofitsangular break-uprateandthatthepole-to-equator effectivetemperaturedifference is 2250 K. [less ▲]

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See detailInstrumental stability requirements for exoplanet detection with a nulling interferometer: variability noise as a central issue
Chazelas, Bruno; Brachet, Frank; Bordé, Pascal et al

in Applied Optics (2006), 45

We revisit the nulling interferometer performances that are needed for direct detection and the spectroscopic analysis of exoplanets, e.g., with the DARWIN [European Space Agency-SCI 12 (2000)] or TPF-I ... [more ▼]

We revisit the nulling interferometer performances that are needed for direct detection and the spectroscopic analysis of exoplanets, e.g., with the DARWIN [European Space Agency-SCI 12 (2000)] or TPF-I [JPL Publ. 05-5, (2005)] missions. Two types of requirement are found, one concerning the mean value of the instrumental nulling function <nl(lambda)> and another regarding its stability. The stress is usually put on the former. It is stringent at short wavelengths but somewhat relaxed at longer wavelengths. The latter, which we call the variability noise condition, does not usually receive enough attention. It is required regardless of telescope size and stellar distance. The results from three nulling experiments performed in laboratories around the world are reported and compared with the requirements. All three exhibit 1/f noise that is incompatible with the performances required by the mission. As pointed out by Lay [Appl. Opt. 43, 6100-6123 (2004)], this stability problem is not fully solved by modulation techniques. Adequate solutions must be found that are likely to include servo systems using the stellar signal itself as a reference and internal metrology with high stability. [less ▲]

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See detailThe prospects of detecting exo-planets with the Ground-based European Nulling Interferometer Experiment (GENIE)
den Hartog, R.; Absil, Olivier ULg; Gondoin, P. et al

in Aime, C.; Vakili, F. (Eds.) Direct Imaging of Exoplanets: Science & Techniques (2006)

The European Space Agency's Darwin and NASA's Terrestrial Planet Finder (TPF) are among the most challenging space science missions ever considered. Their principal objective is to detect Earth-like ... [more ▼]

The European Space Agency's Darwin and NASA's Terrestrial Planet Finder (TPF) are among the most challenging space science missions ever considered. Their principal objective is to detect Earth-like planets around nearby stars and to characterize their atmospheres. Darwin and TPF-I are currently conceived as nulling interferometers with free-flying telescopes. Within the frame of the Darwin program, the ESA and the European Southern Observatory (ESO), supported by European industries and scientific institutes, have performed two parallel Phase A studies of a ground-based nulling interferometry experiment (GENIE) at the site of ESO's Very Large Telescope Interferometer (VLTI) in Paranal, Chile. GENIE will demonstrate several key technologies required for the Darwin mission. Its science objectives include the detection and characterization of dust disks and low-mass companions around nearby stars. These studies have established detailed instrumental designs, in which GENIE will operate in the L' band around 3.8 microns as a single Bracewell nulling or constructive interferometer, using either two Auxiliary or two Unit Telescopes. The studies were supported by detailed numerical simulations which indicated the possibility of detection and low-resolution spectroscopy in nulling mode of extra-solar giant planets (EGPs) with atmospheric temperatures down to 700 K, provided that a proper calibration of instrumental effects is applied. Detection of circumstellar exo-zodiacal (EZ) dust clouds is possible down to 0.5 mJy, with interesting prospects for the characterization of planet-forming disks. [less ▲]

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See detailPEGASE: a DARWIN/TPF pathfinder
Ollivier, M.; Le Duigou, J.-M.; Mourard, D. et al

in Aime, C.; Vakili, F. (Eds.) Direct Imaging of Exoplanets: Science & Techniques (2006)

The space mission PEGASE, proposed to the CNES (Centre National d'Etudes Spatiales = French Space Agency) in the framework of its call for scientific proposals : "formation flying missions", is a 2 ... [more ▼]

The space mission PEGASE, proposed to the CNES (Centre National d'Etudes Spatiales = French Space Agency) in the framework of its call for scientific proposals : "formation flying missions", is a 2-aperture interferometer, composed by 3 free flying satellites (2 siderostats and 1 beam combiner), allowing baselines from 50 to 500 m in both nulling and visibility modes. With an angular resolution of a few mas and a spectral resolution of several tens in the spectral range 2.5-5 microns, PEGASE has several goals:science : spectroscopy of hot jupiters (Pegasides) and brown dwarves, exploration of the inner part of protoplanetary diskstechnology : validation in real space conditions of formation flying, nulling and visibility interferometry concepts.PEGASE has been studied at a 0-level. In this paper, we summarize the scientific program and associated technological and mission trade-off coming from this 0-level study. We also discuss how PEGASE can be considered as a TPF/DARWIN pathfinder in an international roadmap towards more complex space interferometry missions such as DARWIN/TPF. [less ▲]

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See detailALADDIN: an optimized ground-based precursor for DARWIN
Coudé du Foresto, Vincent; Absil, Olivier ULg; Barillot, Marc et al

in Aime, C.; Vakili, F. (Eds.) Direct Imaging of Exoplanets: Science & Techniques (2006)

The ALADDIN concept is an integrated Antarctic-based L-band experiment whose purpose is to demonstrate nulling interferometry and to prepare the DARWIN mission. Because of their privileged location, the ... [more ▼]

The ALADDIN concept is an integrated Antarctic-based L-band experiment whose purpose is to demonstrate nulling interferometry and to prepare the DARWIN mission. Because of their privileged location, the relatively modest collectors (1 m) and baseline (up to 40 m) are sufficient to achieve a sensitivity (in terms of detectable zodi levels) which is about twice better than that of a nulling instrument on a large interferometer (such as GENIE at the VLTI), and to reach the 20-zodi threshold value identified to carry out the DARWIN precursor science. These numbers are based on a preliminary design study by Alcatel Alenia Space and were obtained using the same simulation software as the one employed for GENIE. The integrated design enables top-level optimization and full access to the light collectors for the duration of the experiment, while reducing the complexity of the nulling breadboard. [less ▲]

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See detailPEGASE... towards DARWIN
Ollivier, M.; Le Duigou, J.-M.; Mourard, D. et al

in Casoli, F.; Contini, T.; Hameury, J.-M. (Eds.) et al SF2A-2005: Semaine de l'Astrophysique Francaise (2005, December 01)

The space mission PEGASE, proposed to CNES in the framework of its call for scientific proposals on "formation flying", is a 2-aperture interferometer, composed by 3 free flying satellites. With an ... [more ▼]

The space mission PEGASE, proposed to CNES in the framework of its call for scientific proposals on "formation flying", is a 2-aperture interferometer, composed by 3 free flying satellites. With an angular resolution of a few mas and a spectral resolution of several tens in the spectral range 2.5-5 mum, PEGASE has several goals: - science: spectroscopy of hot jupiters (Pegasides) and brown dwarves, exploration of the inner part of protoplanetary disks; - technology: validation in real space conditions of formation flying, nulling and visibility interferometry concepts. PEGASE, presently in 0-phase study takes place in the context of DARWIN preparation. We detail in this paper the present situation of this project [less ▲]

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See detailAnnular Groove Phase Mask Coronagraph
Mawet, D.; Riaud, Pierre ULg; Absil, Olivier ULg et al

in Astrophysical Journal (2005), 633

We present a new phase mask coronagraph consisting in an optical vortex induced by a space-variant surface relief subwavelength grating. Phase mask coronagraphy is a recent technique aiming at ... [more ▼]

We present a new phase mask coronagraph consisting in an optical vortex induced by a space-variant surface relief subwavelength grating. Phase mask coronagraphy is a recent technique aiming at accommodating both high dynamic and high angular resolution imaging of faint sources around bright objects such as exoplanets orbiting their parent stars or host galaxies of active galactic nuclei. Subwavelength gratings are known to be artificially birefringent. Their unique dispersive characteristics can be controlled through the grating geometry in order to synthesize achromatic phase shifters. We show that implementing them in a ring-shaped way produces a fully symmetric and achromatic coronagraph without any gap or ``dead zone.'' The practical manufacturing of the device is also discussed. [less ▲]

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See detailFirst evidence for a resolved structure in the inner Vega system
Absil, Olivier ULg

Poster (2005, October 01)

We report on infrared interferometric observations of Vega obtained in May and June 2005 with the FLUOR instrument installed on the CHARA interferometer. The combination of short and long baselines has ... [more ▼]

We report on infrared interferometric observations of Vega obtained in May and June 2005 with the FLUOR instrument installed on the CHARA interferometer. The combination of short and long baselines has allowed us to study the brightness distribution of the stellar photosphere, and to detect a spatial component which is over-resolved at our lowest resolution (~15 milli-arcseconds). This would be attributed to structure much larger than Vega, but within the CHARA/FLUOR field-of-view (about 1 arcsecond in radius, i.e., 8 AU around Vega). The contrast between the resolved flux and the central star is only slightly above the 1% level in K band. This emission could be interpreted as the first direct detection of thermal and/or scattered light emanating from the innermost part of the Vega debris disk, which is thought to be continuously replenished by asteroid collisions and/or comet evaporation. The possibility of a close stellar or sub-stellar companion is also investigated. [less ▲]

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See detailThe four-quadrant ZOG and annular groove phase mask coronagraphs
Mawet, Dimitri; Riaud, Pierre ULg; Absil, Olivier ULg et al

in Coulter, Daniel R (Ed.) Techniques and Instrumentation for Detection of Exoplanets II (2005, August 01)

Phase mask coronagraphy is a technique aiming at accommodating both high dynamic and high angular resolution imaging of faint sources around bright objects such as exo-planets orbiting their parent stars ... [more ▼]

Phase mask coronagraphy is a technique aiming at accommodating both high dynamic and high angular resolution imaging of faint sources around bright objects such as exo-planets orbiting their parent stars or host galaxies around Active Galactic Nuclei. We present two new phase mask coronagraphs implemented with subwavelength diffractive optical elements consisting of optimized surface-relief gratings. The first one is an evolution of the Four Quadrant Phase Mask coronagraph, which resolves the Ï phase shift chromaticity problem: the Four Quadrant Zeroth Order Grating (4QZOG). The second one is a totally new design consisting of an optical vortex induced by a space-variant grating: the Annular Groove Phase Mask (AGPM) coronagraph is fully symmetric and free from any "shaded zones". Some manufacturing hints are given. [less ▲]

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See detailLimitation of the Pupil Replication Technique in the Presence of Instrumental Defects
Riaud, Pierre ULg; Mawet, Dimitri; Absil, Olivier ULg

in Astrophysical Journal Letters (2005), 628

Pupil replication has been proposed by Greenaway et al. as a new optical technique to improve the suppression of starlight in high dynamic imaging. This Letter extends numerical simulations in the two ... [more ▼]

Pupil replication has been proposed by Greenaway et al. as a new optical technique to improve the suppression of starlight in high dynamic imaging. This Letter extends numerical simulations in the two-dimensional case with various realistic imperfections (surface error, chromatic smearing, and pupil shift). These results demonstrate some strong limitations compared to single-pupil apodization techniques for exoplanet detection. [less ▲]

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See detailPerspectives for next generation ground- and space-based interferometers (plenary session, invited)
Absil, Olivier ULg; Mawet, Dimitri

Scientific conference (2005, July 07)

The advent of large interferometric facilities such as the VLTI has recently offered the opportunity to the astronomical community to study various targets with an unprecedented angular resolution in the ... [more ▼]

The advent of large interferometric facilities such as the VLTI has recently offered the opportunity to the astronomical community to study various targets with an unprecedented angular resolution in the near- and mid-infrared. In this talk, we shortly review the main scientific topics that have largely benefited from interferometric studies in the past years, and present the future scientific challenges for interferometry. Some of these challenges will already be (partially) within reach when the second generation of VLTI instrumentation comes online, but most of them require more ambitious facilities to be built in the future. In this review talk, we recall the basic concepts of optical/IR interferometry and present some perspectives for future ground- and space-based interferometric facilities. Both approaches lead to major technological challenges that need to be overcome in the next decade. [less ▲]

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See detailDétection des exoplanètes avec le VLTI (invited)
Absil, Olivier ULg

Scientific conference (2005, May 30)

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