References of "Defrere, Denis"
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See detailExozodiacal clouds: Hot and warm dust around main sequence stars
Kral, Quentin; Krivov, Alexander V.; Defrere, Denis ULg et al

in Astronomical Review (in press)

A warm/hot dust component (at temperature $>$ 300K) has been detected around $\sim$ 20% of stars. This component is called "exozodiacal dust" as it presents similarities with the zodiacal dust detected in ... [more ▼]

A warm/hot dust component (at temperature $>$ 300K) has been detected around $\sim$ 20% of stars. This component is called "exozodiacal dust" as it presents similarities with the zodiacal dust detected in our Solar System, even though its physical properties and spatial distribution can be significantly different. Understanding the origin and evolution of this dust is of crucial importance, not only because its presence could hamper future detections of Earth-like planets in their habitable zones, but also because it can provide invaluable information about the inner regions of planetary systems. In this review, we present a detailed overview of the observational techniques used in the detection and characterisation of exozodiacal dust clouds ("exozodis") and the results they have yielded so far, in particular regarding the incidence rate of exozodis as a function of crucial parameters such as stellar type and age, or the presence of an outer cold debris disc. We also present the important constraints that have been obtained, on dust size distribution and spatial location, by using state-of-the-art radiation transfer models on some of these systems. Finally, we investigate the crucial issue of how to explain the presence of exozodiacal dust around so many stars (regardless of their ages) despite the fact that such dust so close to its host star should disappear rapidly due to the coupled effect of collisions and stellar radiation pressure. Several potential mechanisms have been proposed to solve this paradox and are reviewed in detail in this paper. The review finishes by presenting the future of this growing field. [less ▲]

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See detailCoronagraphy and Interferometry: Imaging the sky at high contrast and high-angular resolution
Defrere, Denis ULg; Absil, Olivier ULg

Scientific conference (2017, September 15)

Short overview of the main activities related to exoplanet imaging at the STAR Institute

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See detailCharacterizing exoplanetary atmospheres with a mid-infrared nulling interferometer
Defrere, Denis ULg; Léger, Alain; Absil, Olivier ULg

Speech/Talk (2017)

The discovery of an increasing number of terrestrial planets around nearby stars marks the dawn of a new era in the exoplanet field: the characterization and understanding of their atmospheres. To make ... [more ▼]

The discovery of an increasing number of terrestrial planets around nearby stars marks the dawn of a new era in the exoplanet field: the characterization and understanding of their atmospheres. To make significant progress, it becomes clear that a large number of exoplanetary atmospheres have to be studied at various wavelengths. This is particularly relevant for identifying possible bio-signatures. In this talk, we present a concept of a space-based mid-infrared nulling spectrograph that can characterize a large number of exoplanetary atmospheres and provide key information on their size, surface temperature, and the presence of key molecules such as CO2, H2O, CH4 and O3. The proposed mission concept would be particularly suited to characterize Proxima Cen b. [less ▲]

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See detailProxima Cen b: theoretical spectral signatures for different atmospheric scenarios
Defrere, Denis ULg; Léger, Alain; Grenfell, John Lee et al

Speech/Talk (2017)

Proxima Cen b is possibly the nearest rocky exoplanet orbiting in the habitable zone of its star and might represent by consequent a formidable opportunity for astrobiology. In this presentation, we ... [more ▼]

Proxima Cen b is possibly the nearest rocky exoplanet orbiting in the habitable zone of its star and might represent by consequent a formidable opportunity for astrobiology. In this presentation, we discuss several possible atmospheric compositions and present the corresponding infrared spectrum computed with modern planetary atmosphere models. To be specific, we consider (1) a bare planet, which has lost its atmosphere; (2) a water-ocean planet; (3) an Earth-analog planet; and (4) a planet similar to Earth but with a lower O2 pressure (< 1mbar) that produces a false positive for the triple signature (H20, O3, and CO2). We discuss the information contained in each infrared spectrum and the possibility to constrain the nature of the planet by remote sensing. We end this presentation by describing an instrumental concept recently proposed to ESA and optimised for this task. [less ▲]

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See detailLatest results with LBTI's Vortex coronagraph: real-time tip/tilt sensing, new data reduction algorithms, and YSO observations
Defrere, Denis ULg; Hinz, Philip; Absil, Olivier ULg

Speech/Talk (2017)

Vortex coronagraphs are among the most promising solutions to perform high contrast imaging at small angular separations from bright stars. They enhance the dynamic range at very small inner working angle ... [more ▼]

Vortex coronagraphs are among the most promising solutions to perform high contrast imaging at small angular separations from bright stars. They enhance the dynamic range at very small inner working angle (down to the diffraction limit of the telescope) and provide a clear 360 degree discovery space for high-contrast direct imaging of exoplanets. In 2013, we installed and commissioned an L-band coronagraph in LBTI/LMIRCam and obtained outstanding images of the four planets around HR8799 during the first hours on sky. In this presentation, we will present the results of the latest data reduction performed with the VIP software that is developed at the University of Liège and that features state-of-the-art image processing algorithms inherited from the field of background subtraction in computer vision (including machine learning algorithms and low rank modeling algorithms). We will also present the results obtained with the second L- and M-band coronagraph that was recently installed in LMIRCam to enable binocular Vortex observations. During the first observations (October 2016), we tested and validated a new real-time post-coronagraphic tip-tilt sensing technique (called QACITS) to quickly align each beam on the center of their respective Vortex coronagraph and obtained observations of a young star showing disk features near the resolution limit of each aperture. Finally, we will present some exciting prospects for the Vortex coronagraph that will be installed on VISIR and ELT/METIS. [less ▲]

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See detailMulti-phase volcanic resurfacing at Loki Patera on Io
de Kleer, K.; Skrutskie, M.; Leisenring, J. et al

in Nature (La) (2017), 545

The Jovian moon Io hosts the most powerful persistently active volcano in the Solar System, Loki Patera. The interior of this volcanic, caldera-like feature is composed of a warm, dark floor covering 21 ... [more ▼]

The Jovian moon Io hosts the most powerful persistently active volcano in the Solar System, Loki Patera. The interior of this volcanic, caldera-like feature is composed of a warm, dark floor covering 21,500 square kilometres surrounding a much cooler central ‘island’. The temperature gradient seen across areas of the patera indicates a systematic resurfacing process, which has been seen to occur typically every one to three years since the 1980s. Analysis of past data has indicated that the resurfacing progressed around the patera in an anti-clockwise direction at a rate of one to two kilometres per day, and that it is caused either by episodic eruptions that emplace voluminous lava flows or by a cyclically overturning lava lake contained within the patera. However, spacecraft and telescope observations have been unable to map the emission from the entire patera floor at sufficient spatial resolution to establish the physical processes at play. Here we report temperature and lava cooling age maps of the entire patera floor at a spatial sampling of about two kilometres, derived from ground-based interferometric imaging of thermal emission from Loki Patera obtained on 8 March 2015 UT as the limb of Europa occulted Io. Our results indicate that Loki Patera is resurfaced by a multi-phase process in which two waves propagate and converge around the central island. The different velocities and start times of the waves indicate a non-uniformity in the lava gas content and/or crust bulk density across the patera. [less ▲]

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See detailCharacterizing exoplanetary atmospheres with a mid-infrared nulling spectrograph
Defrere, Denis ULg; Léger, Alain; Absil, Olivier ULg et al

Poster (2017, March 07)

The discovery of an increasing number of terrestrial planets around nearby stars marks the dawn of a new era in the exoplanet field: the characterization and understanding of their atmospheres. To make ... [more ▼]

The discovery of an increasing number of terrestrial planets around nearby stars marks the dawn of a new era in the exoplanet field: the characterization and understanding of their atmospheres. To make significant progress, it becomes clear that a large number of exoplanetary atmospheres have to be studied at various wavelengths. This is particularly relevant for identifying possible bio-signatures. In this poster, we present a concept of a space-based mid-infrared nulling spectrograph that can characterize a large number of exoplanetary atmospheres and provide key information on their size, surface temperature, and the presence of key molecules such as CO2, H2O, CH4 and O3. The proposed mission concept would be particularly suited to characterize Proxima Cen b. [less ▲]

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See detailThe W. M. Keck Observatory infrared vortex coronagraph and a first image of HIP79124 B
Serabyn, Eugene; Huby, Elsa ULg; Matthews, Keith et al

in Astronomical Journal (The) (2017), 153(1), 43

An optical vortex coronagraph has been implemented within the NIRC2 camera on the Keck II telescope and used to carry out on-sky tests and observations. The development of this new L'-band observational ... [more ▼]

An optical vortex coronagraph has been implemented within the NIRC2 camera on the Keck II telescope and used to carry out on-sky tests and observations. The development of this new L'-band observational mode is described, and an initial demonstration of the new capability is presented: a resolved image of the low-mass companion to HIP79124, which had previously been detected by means of interferometry. With HIP79124 B at a projected separation of 186.5 mas, both the small inner working angle of the vortex coronagraph and the related imaging improvements were crucial in imaging this close companion directly. Due to higher Strehl ratios and more relaxed contrasts in L' band versus H band, this new coronagraphic capability will enable high-contrast small-angle observations of nearby young exoplanets and disks on a par with those of shorter-wavelength extreme adaptive optics coronagraphs. [less ▲]

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See detailCharacterization of the inner disk around HD 141569 A from Keck/NIRC2 L-band vortex coronagraphy
Mawet, Dimitri; Choquet, Élodie; Absil, Olivier ULg et al

in Astronomical Journal (The) (2017), 153(1), 44

HD 141569 A is a pre-main sequence B9.5 Ve star surrounded by a prominent and complex circumstellar disk, likely still in a transition stage from protoplanetary to debris disk phase. Here, we present a ... [more ▼]

HD 141569 A is a pre-main sequence B9.5 Ve star surrounded by a prominent and complex circumstellar disk, likely still in a transition stage from protoplanetary to debris disk phase. Here, we present a new image of the third inner disk component of HD 141569 A made in the L' band (3.8 micron) during the commissioning of the vector vortex coronagraph recently installed in the near-infrared imager and spectrograph NIRC2 behind the W.M. Keck Observatory Keck II adaptive optics system. We used reference point spread function subtraction, which reveals the innermost disk component from the inner working distance of $\simeq 23$ AU and up to $\simeq 70$ AU. The spatial scale of our detection roughly corresponds to the optical and near-infrared scattered light, thermal Q, N and 8.6 micron PAH emission reported earlier. We also see an outward progression in dust location from the L'-band to the H-band (VLT/SPHERE image) to the visible (HST/STIS image), likely indicative of dust blowout. The warm disk component is nested deep inside the two outer belts imaged by HST NICMOS in 1999 (respectively at 406 and 245 AU). We fit our new L'-band image and spectral energy distribution of HD 141569 A with the radiative transfer code MCFOST. Our best-fit models favor pure olivine grains, and are consistent with the composition of the outer belts. While our image shows a putative very-faint point-like clump or source embedded in the inner disk, we did not detect any true companion within the gap between the inner disk and the first outer ring, at a sensitivity of a few Jupiter masses. [less ▲]

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See detailHigh dynamic range thermal infrared imager for the VLTI
Defrere, Denis ULg; Absil, Olivier ULg; Ertel, Steve et al

in Pott, Jörg-Uwe; Surdej, Jean (Eds.) Future of otpical-infrared interferometry in Europe (2017)

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See detailProspects for the characterization of exozodiacal dust with the VLTI
Ertel, Steve; Absil, Olivier ULg; Augereau, Jean-Charles et al

in Pott, Jörg-Uwe; Surdej, Jean (Eds.) Future of optical-infrared interferometry in Europe (2017)

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See detailA recent history of science cases for interferometry
Defrere, Denis ULg; Aerts, Conny; Kishimoto, Makoto et al

in Surdej, Jean; Pott, Jörg-Uwe (Eds.) Future of optical-infrared interferometry in Europe (2017)

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See detail20 Hypertelescope concepts: from Carlina prototypes into space
Labeyrie, Antoine; Mourard, Denis; Le Coroller, Hervé et al

in Pott, Jörg-Uwe; Surdej, Jean (Eds.) Future of optical-infrared interferometry in Europe (2017)

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See detailVIP: Vortex Image Processing package for high-contrast direct imaging
Gómez González, Carlos ULg; Wertz, Olivier; Absil, Olivier ULg et al

in Astronomical Journal (The) (2017)

We present the Vortex Image Processing (VIP) library, a python package dedicated to astronomical high-contrast imaging. Our package relies on the extensive python stack of scientific libraries and aims to ... [more ▼]

We present the Vortex Image Processing (VIP) library, a python package dedicated to astronomical high-contrast imaging. Our package relies on the extensive python stack of scientific libraries and aims to provide a flexible framework for high-contrast data and image processing. In this paper, we describe the capabilities of VIP related to processing image sequences acquired using the angular di↵erential imaging (ADI) observing technique. VIP implements functionalities for building high-contrast data processing pipelines, encompass- ing pre- and post-processing algorithms, potential sources position and flux estimation, and sensitivity curves generation. Among the reference point-spread function subtraction techniques for ADI post-processing, VIP includes several flavors of principal component analysis (PCA) based algorithms, such as annular PCA and incremental PCA algorithm capable of processing big datacubes (of several gigabytes) on a computer with limited memory. Also, we present a novel ADI algorithm based on non-negative matrix factorization (NMF), which comes from the same family of low-rank matrix approximations as PCA and provides fairly similar results. We showcase the ADI capabilities of the VIP library using a deep sequence on HR8799 taken with the LBTI/LMIRCam and its recently commissioned L-band vortex coronagraph. Using VIP we investigated the presence of additional companions around HR8799 and did not find any significant additional point source beyond the four known planets. VIP is available at http://github.com/vortex-exoplanet/VIP and is accompanied with Jupyter notebook tutorials illustrating the main functionalities of the library. [less ▲]

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See detailResolving Io's Volcanoes from a Mutual Event Observation at the Large Binocular Telescope
de Kleer, K.; Skrutskie, M.; Leisenring, J. et al

Conference (2016, December 01)

Near-infrared observations of Io during occultation by Jupiter and the other Galilean satellites have been central to ground-based studies of Io's volcanism for decades. When such observations are made ... [more ▼]

Near-infrared observations of Io during occultation by Jupiter and the other Galilean satellites have been central to ground-based studies of Io's volcanism for decades. When such observations are made using adaptive optics on 8-10m telescopes, the infrared emission from individual features can be resolved at a resolution approaching a few km on Io's surface. On March 8, 2015, the Large Binocular Telescope Interferometer (LBTI) observed Io during a Europa mutual occultation event. Images were obtained at a wavelength of 4.8 microns every 123 milliseconds, corresponding to 2 km on Io's surface. The thermal emission from four hot spots including Loki Patera, Pillan Patera, and Kurdalagon Patera is clearly resolved. The latter two hot spots hosted bright eruptions in early 2015; the thermal emission from these sites likely represents the aftermath of those eruptions. The occultation light curves are used to construct a brightness temperature map for each of the four hot spots, from which the lava age is estimated using a model for cooling basaltic lavas. The thermal mapping of Loki Patera has produced the first-ever temperature map of the entire patera floor at high (10 km) spatial resolution, and the corresponding age distribution yields the resurfacing rate. For each hot spot, the age and spatial extent of the lava is interpreted in the context of its activity during the surrounding months. [less ▲]

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See detailProxima Cen b: theoretical spectral signatures for different atmospheric scenarios
Defrere, Denis ULg; Leger, A.; Grenfell, J.l. et al

Speech/Talk (2016)

We consider several possible atmospheric compositions for our nearest neighboring planet, a.k.a. Proxima Can b, and compute the infrared spectrum with modern planetary atmosphere models. To be specific ... [more ▼]

We consider several possible atmospheric compositions for our nearest neighboring planet, a.k.a. Proxima Can b, and compute the infrared spectrum with modern planetary atmosphere models. To be specific, we consider (1) a bare planet, which has lost its atmosphere; (2) a water-ocean planet; (3) an Earth-analog planet; and (4) a planet similar to Earth but with a lower O2 pressure (< 1mbar) that produces a false positive for the triple signature (H20, O3, and CO2). We discuss the information in each infrared spectrum and the possibility to identify the nature of the atmospheres by remote sensing. The same exercise is performed for a rocky planet in the habitable zone of a K2V star, such as epsilon Eri. [less ▲]

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See detailA near-infrared interferometric survey of debris-disc stars. V. PIONIER search for variability
Ertel, S.; Defrere, Denis ULg; Absil, Olivier ULg et al

in Astronomy and Astrophysics (2016), 595

Context. Extended circumstellar emission has been detected within a few 100 milli-arcsec around ≳10% of nearby main sequence stars using near-infrared interferometry. Follow-up observations using other ... [more ▼]

Context. Extended circumstellar emission has been detected within a few 100 milli-arcsec around ≳10% of nearby main sequence stars using near-infrared interferometry. Follow-up observations using other techniques, should they yield similar results or non-detections, can provide strong constraints on the origin of the emission. They can also reveal the variability of the phenomenon. Aims: We aim to demonstrate the persistence of the phenomenon over the timescale of a few years and to search for variability of our previously detected excesses. Methods: Using Very Large Telescope Interferometer (VLTI)/Precision Integrated Optics Near Infrared ExpeRiment (PIONIER) in H band we have carried out multi-epoch observations of the stars for which a near-infrared excess was previously detected using the same observation technique and instrument. The detection rates and distribution of the excesses from our original survey and the follow-up observations are compared statistically. A search for variability of the excesses in our time series is carried out based on the level of the broadband excesses. Results: In 12 of 16 follow-up observations, an excess is re-detected with a significance of > 2σ, and in 7 of 16 follow-up observations significant excess (> 3σ) is re-detected. We statistically demonstrate with very high confidence that the phenomenon persists for the majority of the systems. We also present the first detection of potential variability in two sources. Conclusions: We conclude that the phenomenon responsible for the excesses persists over the timescale of a few years for the majority of the systems. However, we also find that variability intrinsic to a target can cause it to have no significant excess at the time of a specific observation. [less ▲]

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See detailVizieR Online Data Catalog: Planet candidates discovered using K2's 1st yr (Crossfield+, 2016)
Crossfield, I. J. M.; Ciardi, D. R.; Petigura, E. A. et al

in Planet candidates discovered using K2's 1st yr (Crossfield+, 2016) (2016)

We select our FGK stellar sample from the all-sky Transiting Exoplanet Survey Satellite (TESS) Dwarf Catalog (TDC; Stassun et al. 2014, arXiv:1410.6379). We extract time-series photometry from the target ... [more ▼]

We select our FGK stellar sample from the all-sky Transiting Exoplanet Survey Satellite (TESS) Dwarf Catalog (TDC; Stassun et al. 2014, arXiv:1410.6379). We extract time-series photometry from the target pixel files provided by the K2 project using circular, stationary, soft-edged apertures. See section 2 for further explanations. We obtained high-resolution optical spectra of 83 planet candidate hosts using the HIRES echelle spectrometer on the 10m Keck I telescope (R=55000). We obtained spectra of 27 candidate host stars using the Levy high-resolution optical spectrograph mounted at the Automated Planet Finder (APF; R~80000 or 100000). We obtained spectra of a small number of candidate stellar hosts using the FEROS fiber-fed echelle spectrograph at the 2.2m MPG telescope (R~48000). See section 3.1 for further details. We obtained high-resolution imaging (HRI) for 164 of our candidate systems. Our primary instrument for this work was NIRC2 at the 10m Keck II telescope, with which we observed 110 systems. As part of multi-semester program GN-2015B-LP-5 (PI Crossfield) at Gemini Observatory, we observed 40 systems with the NIRI camera in the K-band. We also observed 33 stars with PHARO/PALM-3000 at the 5m Hale Telescope and 14 systems with LMIRCam at LBT, all at the K-band. We observed 39 stars at visible wavelengths using the automated Robo-AO laser adaptive optics system at the Palomar 1.5m telescope. Through our Long-Term Gemini program we also acquired high-resolution speckle imaging of 32 systems in narrowband filters centered at 692 and 880nm using the DSSI camera at the Gemini North telescope. See section 3.3 for further explanations. (7 data files). [less ▲]

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See detailDelay Compensation for Real Time Disturbance Estimation at Extremely Large Telescopes
Bohm, Michael; Pott, Jorg-Uwe; Kurster, Martin et al

in IEEE Transactions on Control Systems Technology (2016), PP(99), 10

In ground-based astronomy, aberrations due to structural vibrations, such as piston, limit the achievable resolution and cannot be corrected using adaptive optics (AO) for large telescopes. We present a ... [more ▼]

In ground-based astronomy, aberrations due to structural vibrations, such as piston, limit the achievable resolution and cannot be corrected using adaptive optics (AO) for large telescopes. We present a model-free strategy to estimate and compensate piston aberrations due to the vibrations of optical components using accelerometer disturbance feed forward, eventually allowing the use of fainter guide stars both for the fringe detector and in the AO loop. Because the correction performance is very sensitive to signal delays, we present a strategy to add a delay compensation to the developed disturbance estimator, which can, in principle, be applied to many other applications outside of astronomy that lack observer performance due to a measurement delay or need a prediction to compensate for input delays. The ability to estimate vibration disturbances in the critical frequency range of 8-60 Hz is demonstrated with on sky data from the Large Binocular Telescope (LBT) Interferometer, an interferometer at the LBT. The experimental results are promising, indicating the ability to suppress differential piston induced by telescope vibrations by a factor of about 3 (rms), which is significantly better than any currently commissioned system. [less ▲]

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See detailMaking high-accuracy null depth measurements for the LBTI exozodi survey
Mennesson, Bertrand; Defrere, Denis ULg; Nowak, Matthias et al

in Malbet, F.; Creech-Eakman, M.; Tuthill, P. (Eds.) Optical and Infrared Interferometry and Imaging V (2016, August 04)

The characterization of exozodiacal light emission is both important for the understanding of planetary systems evolution and for the preparation of future space missions aiming to characterize low mass ... [more ▼]

The characterization of exozodiacal light emission is both important for the understanding of planetary systems evolution and for the preparation of future space missions aiming to characterize low mass planets in the habitable zone of nearby main sequence stars. The Large Binocular Telescope Interferometer (LBTI) exozodi survey aims at providing a ten-fold improvement over current state of the art, measuring dust emission levels down to a typical accuracy of 12 zodis per star, for a representative ensemble of 30+ high priority targets. Such measurements promise to yield a final accuracy of about 2 zodis on the median exozodi level of the targets sample. Reaching a 1 σ measurement uncertainty of 12 zodis per star corresponds to measuring interferometric cancellation ("null") levels, i.e visibilities at the few 100 ppm uncertainty level. We discuss here the challenges posed by making such high accuracy mid-infrared visibility measurements from the ground and present the methodology we developed for achieving current best levels of 500 ppm or so. We also discuss current limitations and plans for enhanced exozodi observations over the next few years at LBTI. [less ▲]

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