arXiv subjects

Fabien Malbet

Publications and source records attributed to Fabien Malbet.

At least 55 records · Page 3Linked to original sources

arXiv2007

Inner disk regions revealed by infrared interferometry

I review the results obtained by long-baseline interferometry at infrared wavelengths on the innermost regions around young stars. These observations directly probe the location of the dust and gas in the disks. The characteristic sizes of these regions found are larger than previously thought. These results have motivated in part a new class of models of the inner disk structure. However the precise understanding of the origin of these low visibilities is still in debate. Mid-infrared observations have probed disk emission over a larger range of scales revealing mineralogy gradients in the disk. Recent spectrally resolved observations allow the dust and gas to be studied separately. The few results shows that the Brackett gamma emission can find its origin either in a wind or in a magnetosphere but there are no definitive answers yet. In a certain number of cases, the very high spatial resolution seems to reveal very close companions. In any case, these results provide crucial information on the structure and physical properties of disks surrounding young stars especially as initial conditions for planet formation.

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arXiv2007

AMBER on the VLTI: data processing and calibration issues

We present here the current performances of the AMBER / VLTI instrument for standard use and compare these with the offered modes of the instrument. We show that the instrument is able to reach its specified precision only for medium and high spectral resolution modes, differential observables and bright objects. For absolute observables, the current achievable accuracy is strongly limited by the vibrations of the Unit Telescopes, and also by the observing procedure which does not take into account the night-long transfer function monitoring. For low-resolution mode, the current limitation is more in the data reduction side, since several effects negligible at medium spectral resolution are not taken into account in the current pipeline. Finally, for faint objects (SNR around 1 per spectral channel), electromagnetic interferences in the VLTI interferometric laboratory with the detector electronics prevents currently to get unbiased measurements. Ideas are under study to correct in the data processing side this effect, but a hardware fix should be investigated seriously since it limits seriously the effective limiting magnitude of the instrument.

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arXiv2006

VSI: a milli-arcsec spectro-imager for the VLTI

VLTi Spectro-Imager (VSI) is a proposition for a second generation VLTI instrument which is aimed at providing the ESO community with the capability of performing image synthesis at milli-arcsecond angular resolution. VSI provides the VLTI with an instrument able to combine 4 telescopes in a baseline version and optionally up to 6 telescopes in the near-infrared spectral domain with moderate to high spectral resolution. The instrument contains its own fringe tracker in order to relax the constraints onto the VLTI infrastructure. VSI will do imaging at the milli-arcsecond scale with spectral resolution of: a) the close environments of young stars probing the initial conditions for planet formation; b) the surfaces of stars; c) the environment of evolved stars, stellar remnants and stellar winds, and d) the central region of active galactic nuclei and supermassive black holes. The science cases allowed us to specify the astrophysical requirements of the instrument and to define the necessary studies of the science group for phase A.

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arXiv2006

First astrophysical results from AMBER/VLTI

The AMBER instrument installed at the Very Large Telescope (VLT) combines three beams from as many telescopes to produce spectrally dispersed fringes from milli-arcsecond angular scale in the near infrared. Two years after installation, first scientific observations have been carried out during the Science Demonstration Time and the Guaranteed Time mostly on bright sources due to some VLTI limitations. In this paper, we review these first astrophysical results and we show which types of completely new information is brought by AMBER. The first astrophysical results have been mainly focusing on stellar wind structure, kinematics, and its interaction with dust usually concentrated in a disk. Because AMBER has dramatically increased the number of measures per baseline, this instrument brings strong constraints on morphology and models despite a relatively poor (u, v) coverage for each object.

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arXiv2006

First direct detection of a Keplerian rotating disk around the Be star $α$ Arae using the VLTI/AMBER instrument

Aims. We aim to study the geometry and kinematics of the disk around the Be star $α$ Arae as a function of wavelength, especially across the Br$γ$ emission line. The main purpose of this paper is to answer the question about the nature of the disk rotation around Be stars. Methods. We use the VLTI/AMBER instrument operating in the K band which provides a gain by a factor 5 in spatial resolution compared to previous VLTI/MIDI observations. Moreover, it is possible to combine the high angular resolution provided with the (medium) spectral resolution of AMBER to study the kinematics of the inner part of the disk and to infer its rotation law. Results. We obtain for the first time the direct evidence that the disk is in keplerian rotation, answering a question that occurs since the discovery of the first Be star $γ$ Cas by father Secchi in 1866. We also present the global geometry of the disk showing that it is compatible with a thin disk + polar enhanced winds modeled with the SIMECA code. We found that the disk around $α$ Arae is compatible with a dense equatorial matter confined in the central region whereas a polar wind is contributing along the rotational axis of the central star. Between these two regions the density must be low enough to reproduce the large visibility modulus (small extension) obtained for two of the four VLTI baselines. Moreover, we obtain that $α$ Arae is rotating very close to its critical rotation. This scenario is also compatible with the previous MIDI measurements.

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arXiv2006

The Circumstellar Environments of Young Stars at AU Scales

We review recent advances in our understanding of the innermost regions of the circumstellar environment around young stars, made possible by the technique of long baseline interferometry at infrared wavelengths. Near-infrared observations directly probe the location of the hottest dust. The characteristic sizes found are much larger than previously thought, and strongly correlate with the luminosity of the central young stars. This relation has motivated in part a new class of models of the inner disk structure. The first mid-infrared observations have probed disk emission over a larger range of scales, and spectrally resolved interferometry has for the first time revealed mineralogy gradients in the disk. These new measurements provide crucial information on the structure and physical properties of young circumstellar disks, as initial conditions for planet formation.

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arXiv2005

Integrated optics for astronomical interferometry - VI. Coupling the light of the VLTI in K band

Our objective is to prove that integrated optics (IO) is not only a good concept for astronomical interferometry but also a working technique with high performance. We used the commissioning data obtained with the dedicated K-band integrated optics two-telescope beam combiner which now replaces the fiber coupler MONA in the VLTI/VINCI instrument. We characterize the behaviour of this IO device and compare its properties to other single mode beam combiner like the previously used MONA fiber coupler. The IO combiner provides a high optical throughput, a contrast of 89% with a night-to-night stability of a few percent. Even if a dispersive phase is present, we show that it does not bias the measured Fourier visibility estimate. An upper limit of 0.005 for the cross-talk between linear polarization states has been measured. We take advantage of the intrinsic contrast stability to test a new astronomical prodecure for calibrating diameters of simple stars by simultaneously fitting the instrumental contrast and the apparent stellar diameters. This method reaches an accuracy with diameter errors of the order of previous ones but without the need of an already known calibrator. These results are an important step of integrated optics and paves the road to incoming imaging interferometer projects.

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arXiv2005

Planar Integrated Optics and astronomical interferometry

Integrated optics (IO) is an optical technology that allows to reproduce optical circuits on a planar substrate. Since 1996, we have investigated the potentiality of IO in the framework of astronomical single mode interferometry. We review in this paper the principles of IO, the requirements for interferometry and the corresponding solutions offered by IO, the results of component characterization and the possible fields of application.

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arXiv2005

VITRUV - Science Cases

VITRUV is a second generation spectro-imager for the PRIMA enabled Very Large Telescope Interferometer. By combining simultaneously up to 8 telescopes VITRUV makes the VLTI up to 6 times more efficient. This operational gain allows two novel scientific methodologies: 1) massive surveys of sizes; 2) routine interferometric imaging. The science cases presented concentrate on the qualitatively new routine interferometric imaging methodology. The science cases are not exhaustive but complementary to the PRIMA reference mission. The focus is on: a) the close environment of young stars probing for the initial conditions of planet formation and disk evolution; b) the surfaces of stars tackling dynamos, activity, pulsation, mass-loss and evolution; c) revealing the origin of the extraordinary morphologies of Planetary Nebulae and related stars; d) studying the accretion-ejection structures of stellar black-holes (microquasars) in our galaxy; e) unveiling the different interacting components (torus, jets, BLRs) of Active Galactic Nuclei; and f) probing the environment of nearby supermassive black-holes and relativistic effects in the Galactic Center black-hole.

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arXiv2005

AMBER : a near infrared focal instrument for the VLTI

AMBER 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.

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arXiv2005

VITRUV - Imaging close environments of stars and galaxies with the VLTI at milli-arcsec resolution

The VITRUV project has the objective to deliver milli-arcsecond spectro-images of the environment of compact sources like young stars, active galaxies and evolved stars to the community. This instrument of the VLTI second generation based on the integrated optics technology is able to combine from 4 to 8 beams from the VLT telescopes. Working primarily in the near infrared, it will provide intermediate to high spectral resolutions and eventually polarization analysis. This paper summarizes the result from the concept study led within the Joint Research Activity advanced instruments of the OPTICON program.

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arXiv2005

Disentangling the wind and the disk in the close surrounding of the young stellar object MWC297 with AMBER/VLTI

The young stellar object MWC297 is a B1.5Ve star exhibiting strong hydrogen emission lines. This object has been observed by the AMBER/VLTI instrument in 2-telescope mode in a sub-region of the K spectral band centered around the Br gamma line at 2.1656 microns. The object has not only been resolved in the continuum with a visibility of 0.50+/-0.10, but also in the Br gamma line, where the flux is about twice larger, with a visibility about twice smaller (0.33+/-0.06). The continuum emission is consistent with the expectation of an optically thick thermal emission from dust in a circumstellar disk. The hydrogen emission can be understood by the emission of a halo above the disk surface. It can be modelled as a latitudinal-dependant wind model and it explains the width, the strength and the visibibility through the emission lines. The AMBER data associated with a high resolution ISAAC spectrum constrains the apparent size of the wind but also its kinematics.

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arXiv2005

New insights on the AU-scale circumstellar structure of FU Orionis

We report new near-infrared, long-baseline interferometric observations at the AU scale of the pre-main-sequence star FU Orionis with the PTI, IOTA and VLTI interferometers. This young stellar object has been observed on 42 nights over a period of 6 years from 1998 to 2003. We have obtained 287 independent measurements of the fringe visibility with 6 different baselines ranging from 20 to 110 meters in length, in the H and K bands. Our extensive (u,v)-plane coverage, coupled with the published spectral energy distribution data, allows us to test the accretion disk scenario. We find that the most probable explanation for these observations is that FU Ori hosts an active accretion disk whose temperature law is consistent with standard models. We are able to constrain the geometry of the disk, including an inclination of 55 deg and a position angle of 47 deg. In addition, a 10 percent peak-to-peak oscillation is detected in the data (at the two-sigma level) from the longest baselines, which we interpret as a possible disk hot-spot or companion. However, the oscillation in our best data set is best explained with an unresolved spot located at a projected distance of 10 AU at the 130 deg position angle and with a magnitude difference of DeltaK = 3.9 and DeltaH = 3.6 mag moving away from the center at a rate of 1.2 AU/yr. we propose to interpret this spot as the signature of a companion of the central FU Ori system on an extremely eccentric orbit. We speculate that the close encounter of this putative companion and the central star could be the explanation of the initial photometric rise of the luminosity of this object.

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arXiv2004

The vertical structure of T Tauri accretion disks II. Physical conditions in the disk

We present a self-consistent analytical model for the computation of the physical conditions in a steady quasi-Keplerian accretion disk. The method, based on the thin disk approximation, considers the disk as concentric cylinders in which we treat the vertical transfer as in a plane-parallel medium. The formalism generalizes a work by Hubeny (1990), linking the disk temperature distribution to the local energy dissipation and leads to analytical formulae for the temperature distribution which help to understand the behaviour of the radiation propagated inside the disks. One of the main features of our new model is that it can take into account many heating sources. We apply the method first to two sources: viscous dissipation and stellar irradiation. We show that other heating sources like horizontal transfer or irradiation from the ambiant medium can also be taken into account. Using the analytical formulation in the case of a modified Shakura & Sunyaev radial distribution that allow the accretion rate to be partly self-similar in the inner region, and, for an alpha and beta prescription of the viscosity, we obtain two-dimensional maps of the temperature, pressure and density in the close environment of low mass young stars. We use these maps to derive the observational properties of the disks such as spectral energy distributions, high resolution spatial images or visibilities in order to underline their different behaviours under different input models.

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arXiv2003

The vertical structure of T Tauri accretion discs III. Consistent interpretation of spectra and visibilities with a two-layer model

We present a two-layer accretion disc model developed to simultaneously fit optical long baseline visibilities and spectral energy distributions of T Tauri accretion discs. Our model includes viscous heating and absorption of stellar radiation. It analytically expresses the vertical structure of the disc using a two-layer approach: the outer layer is heated by the star and by the inner layer, and the inner layer by visous dissipation and by the outer layer. We compare three prescriptions for the flaring of the irradiated surface, and conclude that a smooth profile with r ~ r^1/8->1/7 has predictions close to those of comparable numerical models. The observables (spectra, images and visibilities) are numerically determined. We study the influence of disc parameters on the structure and observables. Then we apply it to three stars observed in IR interferometry. For T Tau and SU Aur we find a model fit consistent with both visibilities and spectrum but we could not in the case of T Tau North, which might come from caveats in the flux correction from the close South companion. We find that even a single interferometric measurement at one wavelength can bring a very strong constraint on disc models. We predict that future massive interferometric observations will provide a breakthrough in the understanding of accretion disc physics.

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arXiv2003

Probing the close environment of young stellar objects with interferometry

The study of Young Stellar Objects (YSOs) is one of the most exciting topics that can be undertaken by long baseline optical interferometry. The magnitudes of these objects are at the edge of capabilities of current optical interferometers, limiting the studies to a few dozen, but are well within the capability of coming large aperture interferometers like the VLT Interferometer, the Keck Interferometer, the Large Binocular Telescope or 'OHANA. The milli-arcsecond spatial resolution reached by interferometry probes the very close environment of young stars, down to a tenth of an astronomical unit. In this paper, I review the different aspects of star formation that can be tackled by interferometry: circumstellar disks, multiplicity, jets. I present recent observations performed with operational infrared interferometers, IOTA, PTI and ISI, and I show why in the next future one will extend these studies with large aperture interferometers.

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arXiv1995

High angular resolution coronography for adaptive optics

Recent adaptive optics systems in astronomy achieve high-angular resolution. With the extreme stability of the images, detection at very low fluxes can be reached using a coronograph at the diffraction limit of the telescopes. This paper is an overview of the issues of stellar coronography used at the diffraction limit. Image formation through such a system is illustrated by numerical simulations. The description of a coronograph implemented on the VLT adaptive optics prototype, COME-ON, is presented as well as the first observations.

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arXiv1995

Star Formation Studies at the VLT

We present an overview of what can be done in the field of star formation using the VLT and its advanced instrumentation, and especially with the very high angular resolution provided by the VLT in the VLTI mode. Herbig-Haro objects and early stellar evolution, pre-main sequence binaries, circumstellar disks and surface properties of young stars are investigated. We show that the VLT and VISA will be not only very useful to better understand star formation processes, but indeed necessary in order to advance in certain subjects.

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