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 References of "Berger, J.-P"      in Complete repository Arts & humanities   Archaeology   Art & art history   Classical & oriental studies   History   Languages & linguistics   Literature   Performing arts   Philosophy & ethics   Religion & theology   Multidisciplinary, general & others Business & economic sciences   Accounting & auditing   Production, distribution & supply chain management   Finance   General management & organizational theory   Human resources management   Management information systems   Marketing   Strategy & innovation   Quantitative methods in economics & management   General economics & history of economic thought   International economics   Macroeconomics & monetary economics   Microeconomics   Economic systems & public economics   Social economics   Special economic topics (health, labor, transportation…)   Multidisciplinary, general & others Engineering, computing & technology   Aerospace & aeronautics engineering   Architecture   Chemical engineering   Civil engineering   Computer science   Electrical & electronics engineering   Energy   Geological, petroleum & mining engineering   Materials science & engineering   Mechanical engineering   Multidisciplinary, general & others Human health sciences   Alternative medicine   Anesthesia & intensive care   Cardiovascular & respiratory systems   Dentistry & oral medicine   Dermatology   Endocrinology, metabolism & nutrition   Forensic medicine   Gastroenterology & hepatology   General & internal medicine   Geriatrics   Hematology   Immunology & infectious disease   Laboratory medicine & medical technology   Neurology   Oncology   Ophthalmology   Orthopedics, rehabilitation & sports medicine   Otolaryngology   Pediatrics   Pharmacy, pharmacology & toxicology   Psychiatry   Public health, health care sciences & services   Radiology, nuclear medicine & imaging   Reproductive medicine (gynecology, andrology, obstetrics)   Rheumatology   Surgery   Urology & nephrology   Multidisciplinary, general & others Law, criminology & political science   Civil law   Criminal law & procedure   Criminology   Economic & commercial law   European & international law   Judicial law   Metalaw, Roman law, history of law & comparative law   Political science, public administration & international relations   Public law   Social law   Tax law   Multidisciplinary, general & others Life sciences   Agriculture & agronomy   Anatomy (cytology, histology, embryology...) & physiology   Animal production & animal husbandry   Aquatic sciences & oceanology   Biochemistry, biophysics & molecular biology   Biotechnology   Entomology & pest control   Environmental sciences & ecology   Food science   Genetics & genetic processes   Microbiology   Phytobiology (plant sciences, forestry, mycology...)   Veterinary medicine & animal health   Zoology   Multidisciplinary, general & others Physical, chemical, mathematical & earth Sciences   Chemistry   Earth sciences & physical geography   Mathematics   Physics   Space science, astronomy & astrophysics   Multidisciplinary, general & others Social & behavioral sciences, psychology   Animal psychology, ethology & psychobiology   Anthropology   Communication & mass media   Education & instruction   Human geography & demography   Library & information sciences   Neurosciences & behavior   Regional & inter-regional studies   Social work & social policy   Sociology & social sciences   Social, industrial & organizational psychology   Theoretical & cognitive psychology   Treatment & clinical psychology   Multidisciplinary, general & others     Showing results 1 to 20 of 24 1 2     The path towards high-contrast imaging with the VLTI: the Hi-5 projectDefrere, Denis ; Absil, Olivier ; Berger, J.-P. et alin Experimental Astronomy: Astrophysical Instrumentation and Methods (in press), 1801The development of high-contrast capabilities has long been recognized as one of the top priorities for the VLTI. As of today, the VLTI routinely achieves contrasts of a few 10$^{-3}$ in the near-infrared ... [more ▼]The development of high-contrast capabilities has long been recognized as one of the top priorities for the VLTI. As of today, the VLTI routinely achieves contrasts of a few 10$^{-3}$ in the near-infrared with PIONIER (H band) and GRAVITY (K band). Nulling interferometers in the northern hemisphere and non-redundant aperture masking experiments have, however, demonstrated that contrasts of at least a few 10$^{-4}$ are within reach using specific beam combination and data acquisition techniques. In this paper, we explore the possibility to reach similar or higher contrasts on the VLTI. After reviewing the state-of-the-art in high-contrast infrared interferometry, we discuss key features that made the success of other high-contrast interferometric instruments (e.g., integrated optics, nulling, closure phase, and statistical data reduction) and address possible avenues to improve the contrast of the VLTI by at least one order of magnitude. In particular, we discuss the possibility to use integrated optics, proven in the near-infrared, in the thermal near-infrared (L and M bands, 3-5 $\mu$m), a sweet spot to image and characterize young extra-solar planetary systems. Finally, we address the science cases of a high-contrast VLTI imaging instrument and focus particularly on exoplanet science (young exoplanets, planet formation, and exozodiacal disks), stellar physics (fundamental parameters and multiplicity), and extragalactic astrophysics (active galactic nuclei and fundamental constants). Synergies and scientific preparation for other potential future instruments such as the Planet Formation Imager are also briefly discussed. [less ▲]Detailed reference viewed: 19 (10 ULiège) Resolved astrometric orbits of ten O-type binariesLe Bouquin, J.-B.; Sana, H.; Gosset, Eric et alin Astronomy and Astrophysics (2017), 601Our long term aim is to derive model-independent stellar masses and distances for long period massive binaries by combining apparent astrometric orbit with double-lined radial velocity amplitudes (SB2 ... [more ▼]Our long term aim is to derive model-independent stellar masses and distances for long period massive binaries by combining apparent astrometric orbit with double-lined radial velocity amplitudes (SB2). We follow-up ten O+O binaries with AMBER, PIONIER and GRAVITY at the VLTI. Here, we report about 130 astrometric observations over the last 7 years. We combine this dataset with distance estimates to compute the total mass of the systems. We also compute preliminary individual component masses for the five systems with available SB2 radial velocities. Nine over the ten binaries have their three dimensional orbit well constrained. Four of them are known colliding wind, non-thermal radio emitters, and thus constitute valuable targets for future high angular resolution radio imaging. Two binaries break the correlation between period and eccentricity tentatively observed in previous studies. It suggests either that massive star formation produce a wide range of systems, or that several binary formation mechanisms are at play. Finally, we found that the use of existing SB2 radial velocity amplitudes can lead to unrealistic masses and distances. If not understood, the biases in radial velocity amplitudes will represent an intrinsic limitation for estimating dynamical masses from SB2+interferometry or SB2+Gaia. Nevertheless, our results can be combined with future Gaia astrometry to measure the dynamical masses and distances of the individual components with an accuracy of 5 to 15\%, completely independently of the radial velocities. [less ▲]Detailed reference viewed: 34 (10 ULiège) Structure of Herbig AeBe disks at the milliarcsecond scale. A statistical survey in the H band using PIONIER-VLTILazareff, B.; Berger, J.-P.; Kluska, J. et alin Astronomy and Astrophysics (2017), 599Context. It is now generally accepted that the near-infrared excess of Herbig AeBe stars originates in the dust of a circumstellar disk. Aims. The aims of this article are to infer the radial and vertical ... [more ▼]Context. It is now generally accepted that the near-infrared excess of Herbig AeBe stars originates in the dust of a circumstellar disk. Aims. The aims of this article are to infer the radial and vertical structure of these disks at scales of order one au, and the properties of the dust grains. Methods. The program objects (51 in total) were observed with the H-band (1.6micron) PIONIER/VLTI interferometer. The largest baselines allowed us to resolve (at least partially) structures of a few tenths of an au at typical distances of a few hundred parsecs. Dedicated UBVRIJHK photometric measurements were also obtained. Spectral and 2D geometrical parameters are extracted via fits of a few simple models: ellipsoids and broadened rings with azimuthal modulation. Model bias is mitigated by parallel fits of physical disk models. Sample statistics were evaluated against similar statistics for the physical disk models to infer properties of the sample objects as a group. Results. We find that dust at the inner rim of the disk has a sublimation temperature Tsub~1800K. A ring morphology is confirmed for approximately half the resolved objects; these rings are wide delta_r>=0.5. A wide ring favors a rim that, on the star-facing side, looks more like a knife edge than a doughnut. The data are also compatible with a the combination of a narrow ring and an inner disk of unspecified nature inside the dust sublimation radius. The disk inner part has a thickness z/r~0.2, flaring to z/r~0.5 in the outer part. We confirm the known luminosity-radius relation; a simple physical model is consistent with both the mean luminosity-radius relation and the ring relative width; however, a significant spread around the mean relation is present. In some of the objects we find a halo component, fully resolved at the shortest interferometer spacing, that is related to the HAeBe class. [less ▲]Detailed reference viewed: 24 (5 ULiège) A near-infrared interferometric survey of debris-disc stars. V. PIONIER search for variabilityErtel, S.; Defrere, Denis ; Absil, Olivier et alin Astronomy and Astrophysics (2016), 595Context. 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 ▲]Detailed reference viewed: 21 (10 ULiège) Planet Formation Imager (PFI): Science vision and key requirementsKraus, S.; Monnier, J. D.; Ireland, M. J. et alin Proceedings of SPIE (2016)The Planet Formation Imager (PFI) project aims to provide a strong scientific vision for ground-based optical astronomy beyond the upcoming generation of Extremely Large Telescopes. We make the case that ... [more ▼]The Planet Formation Imager (PFI) project aims to provide a strong scientific vision for ground-based optical astronomy beyond the upcoming generation of Extremely Large Telescopes. We make the case that a breakthrough in angular resolution imaging capabilities is required in order to unravel the processes involved in planet formation. PFI will be optimised to provide a complete census of the protoplanet population at all stellocentric radii and over the age range from 0.1 to ∼100 Myr. Within this age period, planetary systems undergo dramatic changes and the final architecture of planetary systems is determined. Our goal is to study the planetary birth on the natural spatial scale where the material is assembled, which is the "Hill Sphere" of the forming planet, and to characterise the protoplanetary cores by measuring their masses and physical properties. Our science working group has investigated the observational characteristics of these young protoplanets as well as the migration mechanisms that might alter the system architecture. We simulated the imprints that the planets leave in the disk and study how PFI could revolutionise areas ranging from exoplanet to extragalactic science. In this contribution we outline the key science drivers of PFI and discuss the requirements that will guide the technology choices, the site selection, and potential science/technology tradeoffs. © 2016 SPIE. [less ▲]Detailed reference viewed: 24 (1 ULiège) The VLTI/PIONIER near-infrared interferometric survey of southern T Tauri stars. I. First resultsAnthonioz, F.; Menard, F.; Pinte, C. et alin Astronomy and Astrophysics (2015), 574Context. The properties of the inner disks of bright Herbig AeBe stars have been studied with near-infrared (NIR) interferometry and high resolution spectroscopy. The continuum (dust) and a few molecular ... [more ▼]Context. The properties of the inner disks of bright Herbig AeBe stars have been studied with near-infrared (NIR) interferometry and high resolution spectroscopy. The continuum (dust) and a few molecular gas species have been studied close to the central star; however, sensitivity problems limit direct information about the inner disks of the fainter T Tauri stars.
Aims: Our aim is to measure some of the properties (inner radius, brightness profile, shape) of the inner regions of circumstellar disk surrounding southern T Tauri stars.
Methods: We performed a survey with the VLTI/PIONIER recombiner instrument at H-band of 21 T Tauri stars. The baselines used ranged from 11 m to 129 m, corresponding to a maximum resolution of ~3 mas (~0.45 au at 150 pc).
Results: Thirteen disks are resolved well and the visibility curves are fully sampled as a function of baseline in the range 45-130 m for these 13 objects. A simple qualitative examination of visibility profiles allows us to identify a rapid drop-off in the visibilities at short baselines(<10 Mlambda) in 8 resolved disks. This is indicative of a significant contribution from an extended (R> 3 au, at 150 pc) contribution of light from the disk. We demonstrate that this component is compatible with scattered light, providing strong support to an earlier prediction. The amplitude of the drop-off and the amount of dust thermal emission changes from source to source suggesting that each disk is different. A by-product of the survey is the identification of a new milli-arcsec separation binary: WW Cha. Spectroscopic and interferometric data of AK Sco have also been fitted with a binary + disk model.
Aims: In this paper, we explain the instrumental concept and describe the standard operational modes and the data reduction strategy. We present the typical performance and discuss how to improve them.
Methods: This paper is based on laboratory data obtained during the integrations at IPAG, as well as on-sky data gathered during the commissioning at VLTI. We illustrate the imaging capability of PIONIER on the binaries δ Sco and HIP11231.
Results: PIONIER provides six visibilities and three independent closure phases in the H band, either in a broadband mode or with a low spectral dispersion (R = 40), using natural light (i.e. unpolarized). The limiting magnitude is Hmag = 7 in dispersed mode under median atmospheric conditions (seeing < 1, τ[SUB]0[/SUB] > 3ms) with the 1.8m Auxiliary Telescopes. We demonstrate a precision of 0.5deg on the closure phases. The precision on the calibrated visibilities ranges from 3% to 15% depending on the atmospheric conditions.
Conclusions: PIONIER was installed and successfully tested as a visitor instrument for the VLTI. It permits high angular resolution imaging studies at an unprecedented level of sensitivity. The successful combination of the four 8m Unit Telescopes in March 2011 demonstrates that VLTI is ready for four-telescope operation. Based on observations collected at the European Southern Observatory, Paranal, Chile (commissioning data and 087.C-0709). [less ▲]Detailed reference viewed: 23 (0 ULiège) Searching for faint companions with VLTI/PIONIER. I. Method and first resultsAbsil, Olivier ; Le Bouquin, J.-B.; Berger, J.-P. et alin Astronomy and Astrophysics (2011), 535Context. A new four-telescope interferometric instrument called PIONIER has recently been installed at VLTI. It provides improved imaging capabilities together with high precision.
Aims: We search ... [more ▼]Context. A new four-telescope interferometric instrument called PIONIER has recently been installed at VLTI. It provides improved imaging capabilities together with high precision.
Aims: We search for low-mass companions around a few bright stars using different strategies, and determine the dynamic range currently reachable with PIONIER.
Methods: Our method is based on the closure phase, which is the most robust interferometric quantity when searching for faint companions. We computed the χ[SUP]2[/SUP] goodness of fit for a series of binary star models at different positions and with various flux ratios. The resulting χ[SUP]2[/SUP] cube was used to identify the best-fit binary model and evaluate its significance, or to determine upper limits on the companion flux in case of non-detections.
Results: No companion is found around Fomalhaut, tau Cet and Regulus. The median upper limits at 3σ on the companion flux ratio are respectively of 2.3 × 10[SUP]-3[/SUP] (in 4 h), 3.5 × 10[SUP]-3[/SUP] (in 3 h) and 5.4 × 10[SUP]-3[/SUP] (in 1.5 h) on the search region extending from 5 to 100 mas. Our observations confirm that the previously detected near-infrared excess emissions around Fomalhaut and tau Cet are not related to a low-mass companion, and instead come from an extended source such as an exozodiacal disk. In the case of del Aqr, in 30 min of observation, we obtain the first direct detection of a previously known companion, at an angular distance of about 40 mas and with a flux ratio of 2.05 × 10[SUP]-2[/SUP] ± 0.16 × 10[SUP]-2[/SUP]. Due to the limited u,v plane coverage, its position can, however, not be unambiguously determined.
Conclusions: After only a few months of operation, PIONIER has already achieved one of the best dynamic ranges world-wide for multi-aperture interferometers. A dynamic range up to about 1:500 is demonstrated on unresolved targets, but significant improvements are still required to reach the ultimate goal of directly detecting hot giant extrasolar planets. Based on observations obtained at the European Southern Observatory (ESO) Very Large Telescope Interferometer (VLTI), Paranal, Chile. [less ▲]Detailed reference viewed: 40 (4 ULiège) Hot exozodiacal dust resolved around Vega with IOTA/IONICDefrere, Denis ; Absil, Olivier ; Augereau, J.-C. et alin Astronomy and Astrophysics (2011), 534Context. Although debris discs have been detected around a significant number of main-sequence stars, only a few of them are known to harbour hot dust in their inner part where terrestrial planets may ... [more ▼]Context. Although debris discs have been detected around a significant number of main-sequence stars, only a few of them are known to harbour hot dust in their inner part where terrestrial planets may have formed. Thanks to infrared interferometric observations, it is possible to obtain a direct measurement of these regions, which are of prime importance for preparing future exo-Earth characterisation missions.
Aims: We resolve the exozodiacal dust disc around Vega with the help of infrared stellar interferometry and estimate the integrated H-band flux originating from the first few AUs of the debris disc.
Methods: Precise H-band interferometric measurements were obtained on Vega with the 3-telescope IOTA/IONIC interferometer (Mount Hopkins, Arizona). Thorough modelling of both interferometric data (squared visibility and closure phase) and spectral energy distribution was performed to constrain the nature of the near-infrared excess emission.
Results: Resolved circumstellar emission within ~6 AU from Vega is identified at the 3-σ level. The most straightforward scenario consists in a compact dust disc producing a thermal emission that is largely dominated by small grains located between 0.1 and 0.3 AU from Vega and accounting for 1.23 ± 0.45% of the near-infrared stellar flux for our best-fit model. This flux ratio is shown to vary slightly with the geometry of the model used to fit our interferometric data (variations within ± 0.19%).
Conclusions: The presence of hot exozodiacal dust in the vicinity of Vega, initially revealed by K-band CHARA/FLUOR observations, is confirmed by our H-band IOTA/IONIC measurements. Whereas the origin of the dust is still uncertain, its presence and the possible connection with the outer disc suggest that the Vega system is currently undergoing major dynamical perturbations. [less ▲]Detailed reference viewed: 26 (1 ULiège) The Non-thermal Radio Emitter HD 93250 Resolved by Long Baseline InterferometrySana, H.; Le Bouquin, J.-B.; De Becker, Michaël et alin Astrophysical Journal Letters (2011), 740As the brightest O-type X-ray source in the Carina nebula, HD 93250 (O4 III(fc)) is X-ray overluminous for its spectral type and has an unusually hard X-ray spectrum. Two different scenarios have been ... [more ▼]As the brightest O-type X-ray source in the Carina nebula, HD 93250 (O4 III(fc)) is X-ray overluminous for its spectral type and has an unusually hard X-ray spectrum. Two different scenarios have been invoked to explain its X-ray properties: wind-wind interaction and magnetic wind confinement. Yet, HD 93250 shows absolutely constant radial velocities over timescales of years suggesting either a single star, a binary system seen pole-on view or a very long period, and/or highly eccentric system. Using the ESO Very Large Telescope Interferometer, we resolved HD 93250 as a close pair with similar components. We measured a near-infrared flux ratio of 0.8 ± 0.1 and a separation of (1.5 ± 0.2) × 10-3 arcsec. At the distance of Carina, this corresponds to a projected physical distance of 3.5 AU. While a quantitative investigation would require a full characterization of the orbit, the binary nature of HD 93250 allows us to qualitatively explain both its X-ray flux and hardness and its non-thermal radio emission in the framework of a colliding wind scenario. We also discuss various observational biases. We show that, due to line blending of two similar spectral components, HD 93250 could have a period as short as 1 to several years despite the lack of measurable radial velocity variations. [less ▲]Detailed reference viewed: 18 (1 ULiège) Optimized fringe sensors for the VLTI next generation instrumentsBlind, N.; Absil, Olivier ; Le Bouquin, J.-B. et alin Astronomy and Astrophysics (2011), 530Context. With the arrival of the next generation of ground-based imaging interferometers combining from four to possibly six telescopes simultaneously, there is also a strong need for a new generation of ... [more ▼]Context. With the arrival of the next generation of ground-based imaging interferometers combining from four to possibly six telescopes simultaneously, there is also a strong need for a new generation of fringe trackers able to cophase these arrays. These instruments have to be very sensitive and to provide robust operations in quickly varying observational conditions.
Aims: We aim at defining the optimal characteristics of fringe sensor concepts operating with four or six telescopes. The current detector limitations lead us to consider solutions based on co-axial pairwise combination schemes.
Methods: We independently study several aspects of the fringe sensing process: 1) how to measure the phase and the group delay, and 2) how to combine the telescopes to ensure a precise and robust fringe tracking in real conditions. Thanks to analytical developments and numerical simulations, we define the optimal fringe-sensor concepts and compute the expected performance of the four-telescope one with our dedicated end-to-end simulation tool sim2GFT.
Results: We first show that measuring the phase and the group delay by obtaining the data in several steps (i.e. by temporally modulating the optical path difference) is extremely sensitive to atmospheric turbulence and therefore conclude that it is better to obtain the fringe position with a set of data obtained simultaneously. Subsequently, we show that among all co-axial pairwise schemes, moderately redundant concepts increase the sensitivity as well as the robustness in various atmospheric or observing conditions. Merging all these results, end-to-end simulations show that our four-telescope fringe sensor concept is able to track fringes at least 90% of the time up to limiting magnitudes of 7.5 and 9.5 for the 1.8- and 8.2-meter VLTI telescopes respectively. [less ▲]Detailed reference viewed: 17 (1 ULiège) PEGASE, an infrared interferometer to study stellar environments and low mass companions around nearby starsOllivier, M.; Absil, Olivier ; Allard, F. et alin Experimental Astronomy (2009), 23PEGASE is a mission dedicated to the exploration of the environment (including habitable zone) of young and solar-type stars (particularly those in the DARWIN catalogue) and the observation of low mass ... [more ▼]PEGASE is a mission dedicated to the exploration of the environment (including habitable zone) of young and solar-type stars (particularly those in the DARWIN catalogue) and the observation of low mass companions around nearby stars. It is a space interferometer project composed of three free flying spacecraft, respectively featuring two 40 cm siderostats and a beam combiner working in the visible and near infrared. It has been proposed to ESA as an answer to the first Cosmic Vision'' call for proposals, as an M mission. The concept also enables full-scale demonstration of space nulling interferometry operation for DARWIN. [less ▲]Detailed reference viewed: 69 (9 ULiège) Milli-arcsecond Astrophysics with VSI, the VLTI Spectro-imager in the ELT EraMalbet, F.; Buscher, D.; Weigelt, G. et alin Moorwood, Alan (Ed.) Science with the VLT in the ELT Era (2009)Nowadays, compact sources relatively warm like surfaces of nearby stars, circumstellar environments of stars from early stages to the most evolved ones and surroundings of active galactic nuclei can be ... [more ▼]Nowadays, compact sources relatively warm like surfaces of nearby stars, circumstellar environments of stars from early stages to the most evolved ones and surroundings of active galactic nuclei can be investigated at milli-arcsecond scales only with the VLT in its interferometric mode. We propose a spectro-imager, named VSI (VLTI spectro-imager), which is capable to probe these sources both over spatial and spectral scales in the near-infrared domain. This instrument will provide information complementary to what is obtained at the same time with ALMA at different wavelengths and the extreme large telescopes. [less ▲]Detailed reference viewed: 65 (12 ULiège) VSI: the VLTI spectro-imagerMalbet, F.; Buscher, D.; Weigelt, G. et alin Schöller, Markus; Danchi, William; Delplancke, Françoise (Eds.) Optical and Infrared Interferometry (2008, July 01)The VLTI Spectro Imager (VSI) was proposed as a second-generation instrument of the Very Large Telescope Interferometer providing the ESO community with spectrally-resolved, near-infrared images at ... [more ▼]The VLTI Spectro Imager (VSI) was proposed as a second-generation instrument of the Very Large Telescope Interferometer providing the ESO community with spectrally-resolved, near-infrared images at angular resolutions down to 1.1 milliarcsecond and spectral resolutions up to R = 12000. Targets as faint as K = 13 will be imaged without requiring a brighter nearby reference object; fainter targets can be accessed if a suitable reference is available. The unique combination of high-dynamic-range imaging at high angular resolution and high spectral resolution enables a scientific program which serves a broad user community and at the same time provides the opportunity for breakthroughs in many areas of astrophysics. The high level specifications of the instrument are derived from a detailed science case based on the capability to obtain, for the first time, milliarcsecond-resolution images of a wide range of targets including: probing the initial conditions for planet formation in the AU-scale environments of young stars; imaging convective cells and other phenomena on the surfaces of stars; mapping the chemical and physical environments of evolved stars, stellar remnants, and stellar winds; and disentangling the central regions of active galactic nuclei and supermassive black holes. VSI will provide these new capabilities using technologies which have been extensively tested in the past and VSI requires little in terms of new infrastructure on the VLTI. At the same time, VSI will be able to make maximum use of new infrastructure as it becomes available; for example, by combining 4, 6 and eventually 8 telescopes, enabling rapid imaging through the measurement of up to 28 visibilities in every wavelength channel within a few minutes. The current studies are focused on a 4-telescope version with an upgrade to a 6-telescope one. The instrument contains its own fringe tracker and tip-tilt control in order to reduce the constraints on the VLTI infrastructure and maximize the scientific return. [less ▲]Detailed reference viewed: 70 (7 ULiège) 1 2