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Evolutionary models are widely used to infer the mass of stars, brown dwarfs, and giant planets. Their predictions are thought to be less reliable at young ages ($, Comment: Accepted for publication in A&A. 26 pages, 22 figures, 12 tables

Recent studies of the substellar population in the Taurus cloud have revealed a deficit of brown dwarfs compared to the Trapezium cluster population (Briceno et al. 1998; Luhman 2000; Luhman et al. 2003a; Luhman 2004) However, these works have concentrated on the highest stellar density regions of the Taurus cloud. We have performed a large scale optical survey of this region, covering a total area of ≃ 28 deg2, and encompassing the densest parts of the cloud as well as their surroundings. In this paper, we present optical spectroscopic follow-up observations of 97 photometrically selected potential new low-mass Taurus members. We derive spectral types, visual absorption and luminosity class estimates and discuss our criteria to assess Taurus membership. These observations reveal 5 new very low-mass stars (VLMs) and 12 new Brown Dwarfs (BDs) on the Taurus cloud. We derive a new substellar to stellar ratio in Taurus of ℛss = 0.23 ± 0.05. This revisited ratio appears consistent with the value of 0.26 ± 0.04 previously derived in the Trapezium cluster under similar assumptions. We find indications that the relative numbers of BDs with respect to stars is decreased by a factor 2 in the central regions of the aggregates with respect to the more distributed population. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The currently known substellar population of the Taurus cloud now approaches three dozen; two third of this population has been published in the literature before 2005, while a remaining 12 were recently found by Guieu et al. (2005a, b). With such a large number of brown dwarfs at hand, a series of statistical studies can be tackled. In this paper, we examine the state of the art of the current studies for brown dwarfs in Taurus and present some general properties of this population. We discuss the number of substellar objects with respect to stars, as well as some preliminary implications for substellar formation models in Taurus. We find that the embryo-ejection model seems to reproduce both qualitatively and quantitatively the spatial distribution of stars and BDs in Taurus as well as the evolution of the brown dwarfs to stars ratio RSS , and does not appear incompatible with the accretion properties of the Taurus substellar population. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present near infrared (J,H and Ks) observations of a 5 � × 10 � sample field in the Serpens Star Formation region obtained with SOFI at the NTT. These observations are sensitive enough to detect a 20 MJup brown dwarf through an extinction of AV ∼ 16 and are used to build an infrared census of this field in the cluster. From photometry and mass-luminosity models, we have developed a detailed methodology to extract quantitative parameters (distance modulus, extinction, spectral type, masses) for objects observed towards and inside the Serpens molecular cloud. An extinction map of the region is derived allowing us to disentangle cloud members from background field objects. Luminosities and masses for 14 low-mass stars and substellar object candidate members of the cluster are derived. Three of these objects have masses compatible with the brown dwarf regime and one of them (BD-Ser 1) was observed spectroscopically with ISAAC at the VLT, confirming its substellar status. Long-term photometric variability of BD-Ser 1 could be consistent with signs of accretion.

We obtained linear polarization observations of 82 A/B-type stars in the young cluster NGC 6611, in order to probe the circumstellar material and to search for any evidence of intracluster or interstellar material that could also contribute to the polarization. We found linear polarization values that reach up to 14%. We consider the distribution of the polarization, its position angle, correlations with extinction and membership probability, polarization variability and wavelength distribution to identify the origin of the polarization toward NGC 6611. The polarization is found to be dominated by interstellar polarization, although some stars also have some circumstellar polarization. There is no evidence for intracluster dust. Rather, the dust must be located in a low density cloud toward the general line of sight to NGC 6611 and in front of it. The depth of that cloud along the line of sight increases slowly from the south–east to the north–west. The cloud is threaded by a very uniform magnetic field.

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., 19 pages 10 figures Important conceptual changes concerning the flaring of the irradiated surface

We report the discovery of the first young brown dwarf in the Serpens cloud (BD-Ser 1). It is obscured by more than ten magnitudes of visual absorption as indicated by near infrared (NIR) photometric survey at the NTT and confirmed by NIR spectroscopy at the VLT. We estimate the mass of this brown dwarf to be M ~ 0.05 M⊙ and its age to be ~ 3.5 Myr. Available NIR indices in the literature (designed for field brown dwarfs) fail to provide its current spectral type but using a model they correctly determine its future spectral type to be T. This is the first young brown dwarf ever found deeply embedded in the Serpens star formation region

In this letter, we report the discovery of the rst young early L-dwarf in the Serpens cloud. It is obscured by more than ten magnitudes of visual absorption and was found during a near infrared (NIR) photometric survey of a 50 square arcmin area in the Serpens cloud, deep enough to probe the substellar domain. After selection from NIR colour-magnitude and colour-colour diagrams, its substellar nature has been conrmed by NIR spectroscopy at the VLT. We estimate the mass of this brown dwarf to be 0.05M and its age to be 3.5 Myr. From its NIR indices, we estimate its present spectral type to be L0{L3, and using a model its future spectral type to be T. This is the rst young brown dwarf ever found deeply embedded in a star formation region.

With Herschel/PACS 134 low mass members of the Taurus star-forming region spanning the M4-L0 spectral type range and covering the transition from low mass stars to brown dwarfs were observed. Combining the new Herschel results with other programs, a total of 150 of the 154 M4-L0 Taurus members members have observations with Herschel. Among the 150 targets, 70um flux densities were measured for 7 of the 7 ClassI objects, 48 of the 67 ClassII members, and 3 of the 76 ClassIII targets. For the detected ClassII objects, the median 70um flux density level declines with spectral type, however, the distribution of excess relative to central object flux density does not change across the stellar/substellar boundary in the M4-L0 range. Connecting the 70um TBOSS values with the results from K0-M3 ClassII members results in the first comprehensive census of far-IR emission across the full mass spectrum of the stellar and substellar population of a star-forming region, and the median flux density declines with spectral type in a trend analogous to the flux density decline expected for the central objects. SEDs were constructed for all TBOSS targets covering the optical to far-IR range and extending to the submm/mm for a subset of sources. Based on an initial exploration of the impact of different physical parameters; inclination, scale height and flaring have the largest influence on the PACS flux densities. From the 24um to 70um spectral index of the SEDs, 5 new candidate transition disks were identified. The steep 24um to 70um slope for a subset of 8 TBOSS targets may be an indication of truncated disks in these systems.Two examples of mixed pair systems that include secondaries with disks were measured. Finally, comparing the TBOSS results with a Herschel study of Ophiuchus brown dwarfs reveals a lower fraction of disks around the Taurus substellar population., 64 pages, 33 figures, 12 tables, accepted for publication in A&A

We review correlations of spectral accretion diagnostics in young binary star systems. Hydrogen emission lines at visual (Hα) and near-infrared (Brγ) wavelengths indicate the presence of active accretion flows onto young stars. We examine the simultaneity of this process in both components of close binaries (separations 100–1500 AU) and find that active accretion is not a random process in these systems: the two stars are typically both active or both inactive. Only 19% of our sample consists of mixed pairs in which just one component displays evidence of accretion, usually the primary. In systems with two active stars, the equivalent width and in particular the line flux of the primary star is generally dominant, indicating stronger accretion activity. We discuss accretion and disk dissipation processes in close binaries and propose that a circumbinary envelope model accounts for the correlated evolution of circumstellar disks around young binaries.

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., 10 pages

A new 1-5 μm high-resolution camera dedicated to the ESO adaptive optics system ADONIS has been developed as a collaborative project of Observatoire de Paris-Meudon and Observatoire de Grenoble, under ESO contract. Since this camera has been designed to correctly sample the diffraction, two focal plate scales are available: 36 mas pixel–1 for the 1-2.5 μm range and 100 mas pixel–1 for the 3-5 μm range, yielding fields of view of and , respectively. Several broadband and narrowband filters are available as well as two circular variable filters, allowing low spectral resolution (R ~ 60-120) imagery between 1.2 and 4.8 μm. This camera is equipped with a 128 × 128 HgCdTe/CCD array detector built by the CEA-LETI-LIR (Grenoble, France). Among its main characteristics, this detector offers a remarkably high storage capacity (more than 106 electrons) with a total system readout noise of ≈1000 electrons rms, making it particularly well suited for long integration time imagery in the 3-5 μm range of the near-infrared domain. The measured dark current is 2000 electrons s–1 pixel–1 at the regular operating temperature of 77 K, allowing long exposure times at short wavelengths (λ 3 μm), the performances are background-noise limited. We have estimated the ADONIS + COMIC imaging performances using a method specially dedicated to high angular resolution cameras.

International audience; We aim to refine the dynamical masses of the individual component of the low-mass pre-main sequence binary Haro 1-14 C. We combine the data of the preliminary orbit presented previously with new interferometric observations obtained with the four 8m telescopes of the Very Large Telescope Interferometer. The derived masses are $M_a=0.905\pm0.043\,\Msun$ and $M_b=0.308\pm0.011\,\Msun$ for the primary and secondary components, respectively. This is about five times better than the uncertainties of the preliminary orbit. Moreover, the possibility of larger masses is now securely discarded. The new dynamical distance, $d=96\pm\,9\,$pc, is smaller than the distance to the Ophiuchus core with a significance of $2.6\,\sigma$. Fitting the spectral energy distribution yields apparent diameters of $\phi_a=0.13\pm0.01\mas$ and $\phi_b=0.10\pm0.01\mas$ (corresponding to $\Ra=1.50\,\Rsun$ and $\Rb=1.13\,\Rsun$) and a visual extinction of $A_v\approx1.75$. Although the revised orbit has a nearly edge-on geometry, the system is unlikely to be a long-period eclipsing binary. The secondary in Haro~1-14C is one of the few low-mass, pre-main sequence stars with an accurately determined dynamical mass and distance.

We present optical direct and polarimetric imaging, optical polarimetry, and infrared direct imaging of the Herbig Ae/Be stars V633 Cas (=LkHα 198) and V376 Cas. Both stars are associated with extended reflection nebulosities.V633 Cas appears as a single object associated with an extended optical nebula oriented northwest-southeast with a mean position angle of 128°. This nebula, having the form of a large "loop," traces the redshifted lobe of a CO outflow. Various features are present within the circumstellar medium of V633 Cas. The position of the star does not change with wavelength within the uncertainties. We suspect its linear polarization to be time variable. The polarization pattern close to V633 Cas suggests that another source, possibly the infrared source 6'' north of V633 Cas, V633 Cas B, is the most probable candidate to drive the large-scale outflow.For V376 Cas, high-resolution images with very good seeing in R and [S II] filters show for the first time two peaks in the isophote contours, about 1'' apart. Also, V376 Cas has the largest linear polarization observed so far in a young stellar object, ≈23% at a position angle of ≈26°. This is well explained if an edge-on circumstellar disk is present. Polarization maps of V376 Cas showing a pattern of aligned vectors and two null points support this explanation. Further support comes from the detection of two intensity peaks that can be explained by the presence of a reflection nebula whose center is hidden by an optically thick disk seen edge-on. A less preferable explanation would be the presence of enhanced line emission regions due to shocks in recollimating flows. We exclude the possibility that V376 Cas is a binary star with a 1'' separation.

We report the detection of a molecular outflow driven by the brown dwarf binary FU Tau. Using the IRAM 30 m telescope we observed the $^{12}$CO(2-1) (CO) emission in the vicinity of FU Tau and detected a bipolar outflow by examining the wings of the CO(2-1) line as we moved away from the source position. An integrated map of the wing emission between 3 kms$^{-1}$ and 5 kms$^{-1}$ reveals a blue-shifted lobe at a position of $\sim$ 20 \arcsec\ from the FU Tau system and at a position angle of $\sim$ 20$^{\circ}$. The beam size of the observations is $11\arcsec$\ hence it is not possible to distinguish between the two components of the FU Tau binary. However as optical forbidden emission, a strong tracer of the shocks caused by outflow activity, has been detected in the spectrum of FU Tau A we assume this component to be the driving source of the molecular outflow. We estimate the mass and mass outflow rate of the outflow at 4 $\times$ 10$^{-6}$ \Msun\ and 6 $\times$ 10$^{-10}$ \Msun/yr respectively. These results agree well with previous estimates for BD molecular outflows. FU Tau A is now the third BD found to be associated with molecular outflow activity and this discovery adds to the already extensive list of the interesting properties of FU Tau., 4 pages, 3 figures. A&A Letter

(abridged) We present a near-infrared (NIR) photometric variability study of the candidate protoplanet, TMR-1C, located at a separation of about 10" (~1000 AU) from the Class I protobinary TMR-1AB in the Taurus molecular cloud. Our campaign was conducted between October, 2011, and January, 2012. We were able to obtain 44 epochs of observations in each of the H and Ks filters. Based on the final accuracy of our observations, we do not find any strong evidence of short-term NIR variability at amplitudes of >0.15-0.2 mag for TMR-1C or TMR-1AB. Our present observations, however, have reconfirmed the large-amplitude long-term variations in the NIR emission for TMR-1C, which were earlier observed between 1998 and 2002, and have also shown that no particular correlation exists between the brightness and the color changes. TMR-1C became brighter in the H-band by ~1.8 mag between 1998 and 2002, and then fainter again by ~0.7 mag between 2002 and 2011. In contrast, it has persistently become brighter in the Ks-band in the period between 1998 and 2011. The (H-Ks) color for TMR-1C shows large variations, from a red value of 1.3+/-0.07 and 1.6+/-0.05 mag in 1998 and 2000, to a much bluer color of -0.1+/-0.5 mag in 2002, and then again a red color of 1.1+/-0.08 mag in 2011. The observed variability from 1998 to 2011 suggests that TMR-1C becomes fainter when it gets redder, as expected from variable extinction, while the brightening observed in the Ks-band could be due to physical variations in its inner disk structure. The NIR colors for TMR-1C obtained using the high precision photometry from 1998, 2000, and 2011 observations are similar to the protostars in Taurus, suggesting that it could be a faint dusty Class I source. Our study has also revealed two new variable sources in the vicinity of TMR-1AB, which show long-term variations of ~1-2 mag in the NIR colors between 2002 and 2011., Accepted in A&A

This poster advertizes the Jean-Marie Mariotti Center software tools, databases and services aimed at facilitating the use of optical interferometry worldwide such as preparation of observations, data reduction and data analysis. Its mission and organization are presented before listing the current software suite. Finally some facts and perspectives are mentioned.

Context. Spectrally resolved long-baseline optical/IR interferometry of rotating stars opens perspectives to investigate their fundamental parameters and the physical mechanisms that govern their interior, photosphere, and circumstellar envelope structures.Aims. Based on the signatures of stellar rotation on observed interferometric wavelength-differential phases, we aim to measure angular diameters, rotation velocities, and orientation of stellar rotation axes.Methods. We used the AMBER focal instrument at ESO-VLTI in its high-spectral resolution mode to record interferometric data on the fast rotator Achernar. Differential phases centered on the hydrogen Br γ line (K band) were obtained during four almost consecutive nights with a continuous Earth-rotation synthesis during ~5 h/night, corresponding to ~60° position angle coverage per baseline. These observations were interpreted with our numerical code dedicated to long-baseline interferometry of rotating stars.Results. By fitting our model to Achernar’s differential phases from AMBER, we could measure its equatorial radius R eq = 11.6 ± 0.3 R ⊙ , equatorial rotation velocity V eq = 298 ± 9 km s-1 , rotation axis inclination angle i = 101.5 ± 5.2°, and rotation axis position angle (from North to East) PArot = 34.9 ± 1.6°. From these parameters and the stellar distance, the equatorial angular diameter ⌀eq of Achernar is found to be 2.45 ± 0.09 mas, which is compatible with previous values derived from the commonly used visibility amplitude. In particular, ⌀eq and PArot measured in this work with VLTI/AMBER are compatible with the values previously obtained with VLTI/VINCI.Conclusions. The present paper, based on real data, demonstrates the super-resolution potential of differential interferometry for measuring sizes, rotation velocities, and orientation of rotating stars in cases where visibility amplitudes are unavailable and/or when the star is partially or poorly resolved. In particular, we showed that differential phases allow the measurement of sizes up to ~4 times smaller than the diffraction-limited angular resolution of the interferometer.

We describe results from a program of speckle interferometry with two-dimensional infrared array detectors. Analysis of observations of eta Carinae made with 58 x 62 InSb detector are discussed. The data have been analyzed with both the Labeyrie autocorrelation, a deconvolution of shift-and-add data, and a phase restoration process. Development of a new camera based on a much lower noise HgCdTe detector will lead to a significant improvement i limiting magnitude for IR speckle interferometry.

We investigate the environment of the very low-luminosity object L1521F IRS using data from the Taurus Spitzer Legacy Survey. The MIPS 160 micron image shows both extended emission from the Taurus cloud as well as emission from multiple cold cores over a 1 X 2 deg region. Analysis shows that the cloud dust temperature is 14.2 +- 0.4 K and the extinction ratio is A_160/A_K = 0.010 +- 0.001 up to A_V ~ 4 mag. We find kappa_160 = 0.23 +- 0.046 cm^2/g for the specific opacity of the gas-dust mixture. Therefore, for dust in the Taurus cloud we find the 160 um opacity is significantly higher than that measured for the diffuse ISM, but not too different from dense cores, even at modest extinction values. Furthermore, the 160 um image shows features that do not appear in the IRAS 100 um image. We identify six regions as cold cores, i.e. colder than 14.2 K, all of which have counterparts in extinction maps or C18O maps. We compare the effects of L1521F IRS on its natal core and find there is no evidence for dust heating at 160 or 100 um by the embedded source. From the infrared luminosity L_TIR = 0.024 Lo we find L_bol = 0.034 - 0.046L_o, thus confirming the source's low-luminosity. Comparison of L1521F IRS with theoretical simulations for the very early phases of star formation appears to rule out the first core collapse phase. The evolutionary state appears similar to or younger than the class 0 phase, and the estimated mass is likely to be substellar., 39 pages, 11 figures, accepted by Astrophysical Journal

We use disk sizes published on the “www.circumstellardisks.org” Web site to compare these sizes with the central object spectral type, used as a proxy for the mass. Contrary to our first guess, we find that the disk size actually increases with later spectral type, hence when the central object’s mass decreases. This surprising result could be explained by recent models of star & brown dwarf formation where objects are ejected after close encounters.

We present interferometric near-infrared observations of the Luminous Blue Variable (LBV) η Car using the Very Large Telescope Interferometer (VLTI) and the AMBER instrument of the European Southern Observatory (ESO). A high spatial resolution of 5 mas (~11.5 AU) and a high spectral resolution R = λ/Δλ=1500 and 12000 were obtained. Some of the data was recorded using the fringe tracker FINITO. The observations were obtained in the wavelength range around both the He I 2.059 μm and the Brγ 2.166 μm emission lines. The AMBER interferograms allow the investigation of the wavelength dependence of η Car's visibility, wavelength-differential phase, and closure phase. If we fit Hillier et al. model visibilities to the observations, we obtain 50% encircled-energy diameters of 4.2, 6.5 and 9.6 mas in the 2.17 μm continuum, the He I, and the Brγ emission lines, respectively. In the continuum, an elongation along a position angle of 120° ± 15° was derived from the visibilities. The VLTI observations support theoretical models of anisotropic winds from fast-rotating, luminous hot stars with enhanced high-velocity mass loss near the pole.

We present a study of the circumstellar environment of IRAS 04158+2805 based on multi-wavelength observations and models. Images in the optical and near-infrared, a polarisation map in the optical, and mid-infrared spectra were obtained with VLT-FORS1, CFHT-IR, and Spitzer-IRS. Additionally we used an X-ray spectrum observed with Chandra. We interpret the observations in terms of a central star surrounded by an axisymmetric circumstellar disc, but without an envelope, to test the validity of this simple geometry. We estimate the structural properties of the disc and its gas and dust content. We modelled the dust disc with a 3D continuum radiative transfer code, MCFOST, based on a Monte-Carlo method that provides synthetic scattered light images and polarisation maps, as well as spectral energy distributions. We find that the disc images and spectral energy distribution narrowly constrain many of the disc model parameters, such as a total dust mass of 1.0-1.75x10^-4 sollar masses and an inclination of 62-63 degrees. The maximum grain size required to fit all available data is of the order of 1.6-2.8 microns although the upper end of this range is loosely constrained. The observed optical polarisation map is reproduced well by the same disc model, suggesting that the geometry we find is adequate and the optical properties are representative of the visible dust content. We compare the inferred dust column density to the gas column density derived from the X-ray spectrum and find a gas-to-dust ratio along the line of sight that is consistent with the ISM value. To our knowledge, this measurement is the first to directly compare dust and gas column densities in a protoplanetary disc., 8 figures, 11 pages, accepted by A&A

In order to improve our understanding of substellar formation, we have performed a compositional and structural study of a brown dwarf disk. We present the result of photometric, spectroscopic and imaging observations of 2MASS J04442713+2512164, a young brown dwarf (M7.25) member of the Taurus association. Our dataset, combined with results from the literature, provides a complete coverage of the spectral energy distribution from the optical to the millimeter including the first photometric measurement of a brown dwarf disk at 3.7mm, and allows us to perform a detailed analysis of the disk properties. The target was known to have a disk. High resolution optical spectroscopy shows that it is intensely accreting, and powers a jet and an outflow. The disk structure is similar to that observed for more massive TTauri stars. Spectral decomposition models of Spitzer/IRS spectra suggest that the mid-infrared emission from the optically thin disk layers is dominated by grains with intermediate sizes (1.5micron). Crystalline silicates are significantly more abondant in the outer part and/or deeper layers of the disk, implying very efficient mixing and/or additional annealing processes. Sub-millimeter and millimeter data indicate that most of the disk mass is in large grains (>1mm), 17 pages, 10 figures, 7 tables, accepted for A&A

Aims : We want to investigate whether brown dwarfs (BDs) form like stars or are ejected embryos. We study the presence of disks around BDs in the Taurus cloud, and discuss implications for substellar formation models. Methods : We use photometric measurements from the visible to the far infrared to determine the spectral energy distributions (SEDs) of Taurus BDs. Results: We use Spitzer color indices, Halpha as an accretion indicator, and models fit to the SEDs in order to estimate physical parameters of the disks around these BDs. We study the spatial distribution of BDs with and without disks across the Taurus aggregates, and we find that BDs with and without disks are not distributed regularly across the Taurus cloud. Conclusions: We find that 48%+/- 14% of Taurus BDs have a circumstellar disk signature, a ratio similar to recent results from previous authors in other regions. We fit the SEDs and find that none of the disks around BDs in Taurus can be fitted convincingly with a flaring index beta = 0, indicating that heating by the central object is efficient and that the disks we observe retain a significant amount of gas. We find that BDs with disks are proportionally more numerous in the northern Taurus filament, possibly the youngest filament. We do not find such a clear segregation for classical T Tauri stars (CTTS) and weak-lined T Tauri stars (WTTS), suggesting that, in addition to the effects of evolution, any segregation effects could be related to the mass of the object. A by-product of our study is to propose a recalibration of the Barrado y Navascues & Martin (2003) accretion limit in the substellar domain. The global shape of the limit fits our data points if it is raised by a factor 1.25-1.30., 11 pages, 5 figures, A&A accepted

The IPOL mode of FORS on UT1 allows to measure high quality polarimetric images on a wide field of view. We have used this instrument during ESO observation period 65 to determine the respective orientation of circumstellar disks in pre Main Sequence (PMS) binaries. In this paper, we present our method and first results obtained with FORS/IPOL. Thanks to the top quality assistance of the VLT and FORS teams, our run has been remarkably successful and the instrument appears extremely well adapted to fine polarimetric measurements on a large field of view. This large field of view is essential to disentangle between intrinsic polarisation and interstellar polarisation. Our first results suggest that circumstellar disks tend to be aligned in PMS binaries.

We present the first interferometric NIR observations of the LBV ? Carinae with high spectral resolution. The observations were carried out with three 8.2 m VLTI Unit Telescopes in the K-band. The raw data are spectrally dispersed interferograms obtained with spectral resolutions of 1,500 (MR-K mode) and 12,000 (HR-K mode). The observations were performed in the wavelength range around both the He I 2.059 ?m and the Br? 2.166 ?m emission lines. The spectrally dispersed AMBER interferograms allow the investigation of the wavelength dependence of the visibility, differential phase, and closure phase of ? Car. In the K-band continuum, a diameter of 4.0+/-0.2 mas (Gaussian FWHM) was measured for ? Car's optically thick wind region, whereas the Br? and He I emission line regions are larger. If we fit Hillier et al. model visibilities to the observed AMBER visibilities, we obtain 50% encircled-energy diameters of 4.3, 6.5 and 9.6 mas in the 2.17 ?m continuum, the He I, and the Br?emission lines, respectively. In the continuum near the Br? line, an elongation along a position angle of 128° +/- 15° was found, consistent with previous VLTI/VINCI measurements. We find good agreement between the measured visibilities and the predictions of the radiative transfer model of Hillier et al. For the interpretation of the non-zero differential and closure phases measured within the Br? line, we present a simple geometric model of an inclined, latitude-dependent wind zone. Our observations support theoretical models of anisotropic winds from fast-rotating, luminous hot stars with enhanced high-velocity mass loss near the polar regions.

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., latex2e with AMS macros, 14 pages, 10 figures

We present optical linear polarimetry of 9 ultra cool field dwarfs. The linear polarisation of each L dwarf we measured is less than 0.2 percent. Three dwarfs have polarisations compatible with zero, two are marginal detections, and three have significant polarisation. Due to their small distance, an insterstellar origin for the detected polarisation can be safely ruled out. Our detections confirm that dust is present in the atmosphere of these L dwarfs and that the scattering geometry is not symmetric.

Optical linear polarimetry of 8 ultra cool field dwarfs, with spectral types ranging from M9 to L8, is presented. The linear polarisation, P, of each dwarf we measured is P < 0.2%. Three dwarfs have polarisations compatible with zero, two are marginal detections, and three have significant polarisation. Due to their small distance, an insterstellar origin for the detected polarisation can be safely ruled out. Our detections confirm that dust is present in the atmosphere of these brown dwarfs and that the scattering geometry is not symmetric. Possibilities for asymmetry include the dwarfs rotating rapidly and being oblate, or the cloud coverage in the atmosphere being inhomogenous., 4 pages, 1 figure, accepted by A&A Letters

Young Stellar Objects (YSOs) are the builders of worlds. During its infancy, a star transforms ordinary interstellar dust particles into astronomical gold: planets to say the process is complex, and largely unknown to data. Yet, violent and spectacular events of mass ejection are witnessed, disks in keplerian rotation are detected, multiple stars dancing around each other are found. These are as many traces of the stellar and planet formation process. The high angular resolution provided by adaptive optics, and the related gain in sensitivity, have allowed major breakthrough discoveries to be made in each of these specific fields and our understanding of the various physical processes involved in the formation of a star has leaped forward tremendously over the last few years. In the following, meant as a report of the progress made recently in star formation due to adaptive optics, we will describe new results obtained at optical and near- infrared wavelengths, in imaging and spectroscopic modes. Our images of accretion disks and ionized stellar jets permit direct measurements of many physical parameters and shed light into the physics of the accretion and ejection processes. Although the accretion/ejection process so fundamental to star formation is usually studied around single objects, most of young stars form as part of multiple systems. We also present our findings on how the fraction of stars in binary systems evolves with age. The implications of these results on the conditions under which these stars must have formed are discussed.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Young stellar objects offer a rich circumstellar environment, indirectly revealed by photometry, spectroscopy and imaging, such as accretion disks, stellar companions, optical jets, stellar winds, etc. Most of these phenomena occur in a region whose size is less than 1 AU viewed at distances ranging from 140 pc to 450 pc depending on the star forming regions. Therefore optical interferometry in general and the VLTI in particular which provides milli- arcsecond spatial resolution is a powerful tool to probe the physics in these regions. Detecting the thermal emission of optically thick accretion disks is one of the clues to understand the still poorly known physics of close circumstellar environment. In this paper we focus on the results from a numerical code simulating the vertical structure of an accretion disk around a T Tauri star. The visibility and closure phase curves are interpreted in the framework of the VLTI with the AMBER instrument.

The direct detection of accretion disks surrounding young stellar objects has been the “holy grail” of observational star formation research over the last ten years or so. At stakes, there is a tremendous reward that includes the comprehension of the jet/outflow driving mechanism, an understanding of the viscous and turbulent dissipation processes within the disks, and important clues regarding the formation of our own solar system. In this contribution we will present recent observational results that proved, at last, the existence of the accretion disks and allowed a direct assessment of their physical parameters. We also present numerical simulations of circumstellar and circumbinary disk visibility curves and demonstrate that such disks are detectable with an instrument like the VLT Interferometer.

A 1-5 micrometers astronomical infrared imaging camera, COMIC, is currently being developed for the ADONIS adaptive optics system, as a collaborative project of Observatoire de Paris and Observatoire de Grenoble under ESO (European Space Observatory) contract. This camera is based on a 128 by 128 HgCdTe/CCD array built by the CEA-LETI-LIR (Grenoble, France). Among its main characteristics, this detector offers a very high storage capacity of 3 106 e- with a total system read-out noise of about 600 e- which makes it particularly optimized for the 3-5 mum. COMIC will be installed in the fall of 1995 at the output focus of the ADONIS AO system on the ESO 3.6-m telescope at La Silla (Chile).© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Aims. We investigate the structure of the circumstellar disk of the T Tauri star S CrA N and test whether the observations agree with the standard picture proposed for Herbig Ae stars. Methods. Our observations were carried out with the VLTI/AMBER instrument in the H and K bands with the low spectral resolution mode. For the interpretation of our near-infrared AMBER and archival mid-infrared MIDI visibilities, we employed both geometric and temperature-gradient models. Results. To characterize the disk size, we first fitted geometric models consisting of a stellar point source, a ring-shaped disk, and a halo structure to the visibilities. In the H and K bands, we measured ring-fit radii of 0.73 ± 0.03 mas (corresponding to 0.095 ± 0.018 AU for a distance of 130 pc) and 0.85 ± 0.07 mas (0.111 ± 0.026 AU), respectively. This K-band radius is approximately two times larger than the dust sublimation radius of ≈0.05 AU expected for a dust sublimation temperature of 1500 K and gray dust opacities, but approximately agrees with the prediction of models including backwarming (namely a radius of ≈0.12 AU). The derived temperature-gradient models suggest that the disk is approximately face-on consisting of two disk components with a gap between star and disk. The inner disk component has a temperature close to the dust sublimation temperature and a quite narrow intensity distribution with a radial extension from 0.11 AU to 0.14 AU. Conclusions. Both our geometric and temperature-gradient models suggest that the T Tauri star S CrA N is surrounded by a circumstellar disk that is truncated at an inner radius of ≈0.11 AU. The narrow extension of the inner temperature-gradient disk component implies that there is a hot inner rim.

A 1-5 micron astronomical infrared imaging facility is currently under development at Observatoire de Grenoble for the ADONIS adaptive optics system, a collaborative project of Observatoire de Paris and Observatoire de Grenoble under ESO (European Southern Observatory) contract. This imaging detector will equip the camera to be installed at the F/45 output focus of the 3.6 m telescope operated by ESO at La Silla, Chile. The detector is a 128 X 128 HgCdTe/CCD array optimized in the 3-5 micron range, built by the CEA-LETI-LIR (Infrared Laboratory). The measured readout noise is less 450 electrons at a pixel readout rate of 415 kHz. The detector has a high storage capacity of 6.8 X 10 6 electrons yielding a dynamic range of about 84 dB. Such a high capacity is very useful, because adaptive optics allows large integration times up to a few minutes without degradation of the images by the atmospheric turbulence. Dark current is not a limitation for these large integration times even at an operation temperature of 77 K. A compact and versatile control electronics is being developed at the Observatoire de Grenoble. We will discuss the overall performances of the detector as well as the data acquisition and control systems built at Grenoble in the framework of astronomical imaging with adaptative optics techniques.

We present synthetic images of accretion disks around young stars computed from a model where the disk’s vertical structure is solved assuming hydrostatic equilibrium. The disk’s brightness results from three emission processes: (1) the reprocessing of stellar photons in the optically thick disk’s regions; (2) the scattering of stellar photons in the optically thin parts of the disk; and (3) the thermal emission of the disk due to viscous energy dissipation during the accretion process. We discuss the relative importance of these emission processes at wavelengths ranging from 1.2 to 20 μm.

Circumstellar disks surrounding young stars are known to be the birthplaces of planets, and the innermost astronomical unit is of particular interest. We present new long-baseline spectro-interferometric observations of the HerbigAe star, HR5999, obtained in the H and K bands with the AMBER instrument at the VLTI, and aim to produce near-infrared images at the sub-AU spatial scale. We spatially resolve the circumstellar material and reconstruct images using the MiRA algorithm. In addition, we interpret the interferometric observations using models that assume that the near-infrared excess is dominated by the emission of a circumstellar disk. We compare the images reconstructed from the VLTI measurements to images obtained using simulated model data. The K-band image reveals three main elements: a ring-like feature located at ~0.65 AU, a low surface brightness region inside, and a central spot. At the maximum angular resolution of our observations (1.3 mas), the ring is resolved while the central spot is only marginally resolved, preventing us from revealing the exact morphology of the circumstellar environment. We suggest that the ring traces silicate condensation, i.e., an opacity change, in a circumstellar disk around HR 5999. We build a model that includes a ring at the silicate sublimation radius and an inner disk of low surface brightness responsible for a large amount of the near-infrared continuum emission. The model successfully fits the SED, visibilities, and closure phases, and provides evidence of a low surface brightness region inside the silicate sublimation radius. This study provides additional evidence that in HerbigAe stars, there is material in a low surface brightness region, probably a low optical depth region, located inside the silicate sublimation radius and of unknown nature., 11 pages, 10 figures

A series of currently available IR detectors for astronomical observations are reviewed and various applications of the devices to instrumental astronomy are examined, emphasizing the parameters relevant to each of them. Emphasis is given to the 1-5 micron window critical to near-IR astronomy. Some recent results in this field are reviewed.

We present an experiment aimed at diffraction-limited imaging of astronomical sources in the 2-5 ,im range. It is based on a rotation shearing interferometer that produces fringes in the pupil plane that can be scanned through the zero optical path difference. The two-dimensional interferograms are recorded with an infrared camera looking at the recombined pupil image. The modulus and the phase of the Fourier transform of the object intensity distribution are derived from these interferograms The main advantage of this technique is its constant transfer function that makes it independent of seeing variations and instrumental aberrations. We describe the experiment set-up and discuss some simulation results which illustrate the operation of the interferometer. We present astronomical data recently obtained at the 4.20m William Herschel Telescope of the Royal Greenwich Observatory in La Palma, and results on the circuinstellar shell star NML Cygni. 1. INTRODUCTION Infrared astronomy has made fast and impressive progress in the last two decades due to the development of infrared mono-detectors and, more recently, detectors

We present the results of high angular resolution infrared observations of the compact H II region G5.89-0.39. Subarcsecond imaging at 1.65, 2.2, 3.6, and 4.8 microns and diffraction-limited scans at 50 and 100 microns have been obtained at NASA's Infrared Telescope Facility and Kupier Airborne Observatory and the Canada-France-Hawaii Telescope. We find a bipolar source morphology at the shortest infrared wavelengths, a compact single source at 4-5 microns, similar to that seen at 10 microns, and a very high optical depth, barely resolved far-infrared source. Spherical radiative transfer models have been used to estimate dust envelope properties; in spite of the obvious nonspherical geometry at short wavelengths, these models provide strong constraints on the overall properties of the circumnebular envelope. We find that the bulk of the circumstellar dust must be relatively close to the H II region, with a relatively steep gradient. The high column densities and steep density gradient imply densities at the inner edge of the cloud of order 10(exp 7)-10(exp 8)/cc. These results suggest that the compact H II region is surrounded by a very dense dust torus.

Subarcsecond KLM images of the pre-main-sequence star Haro 6-10 obtained with the CIRCUS IR camera attached to the Canada-France-Hawaii Telescope are presented. They are the first images where both components of the Haro 6-10 binary system are resolved. We confirm the results by Leinert & Haas (1989), obtained from speckle interferometry, that the projected separation of the two stars is 1″.25 at position angle 356°. We also present new polarization measurement and models that suggest the presence of a flat circumbinary envelope (possibly a disk) associated with the system

Near-infrared images (1-2.2 microns) of young stellar objects, where the light distribution results from diffusion and absorption of radiation of the central stellar object by circumstellar dust, are presented. The distribution of matter is modeled as a combination of a disk, a clump, and a wind-blown cavity. Various density laws for the disk are explored, and numerical experiments are used to test the respective role of the three constituents of the model in the overall image. The essential factor that determines the aspect of the image is the degree of central condensation of the disk. 29 refs.

We present a complete infrared camera system, based on a high electron capacity detector (HgCdTe/CCD), that we have used under high background conditions to do astronomical images. The performances of the system and some results are presented, and the use of such a detector in astronomy will be discussed.

AMBER is the near-infrared instrument of the Very Large Telescope Interferometer (VLTI). With a spectral resolution up to 10000 in the 1.2-2.4 micron wavelength range, AMBER will offer the possibility to combine 3 beams from the VLTI array either 8-m or 1.8m telescopes. The instrument has been designed to bring high precision measurement and high sensitivity and therefore opens the way to new domain of investigation in stellar physics and for the first time access to extragalactic sources. We show how the performance of the instrument can apply in these different astrophysical fields. We present the work of the Science Group and the AMBER consortium who defined precise astrophysical goals for the first years of operation.

AMBER had first light in March 2004. The guaranteed time observations of the AMBER consortium (LAOG, MPIfR, OAA, OCA, UNSA) consists of 87 proposals ranging from cosmology, extragalactic studies, star formation, planetary system, late stages of stellar evolution to physical properties of stars. Some examples, AGN, evolved stars and hot stars are discussed in this paper.

The Comet Nucleus Sounding Experiment by Radio-wave Transmission (CONSERT / Rosetta) was designed to scan the interior of the comet 67P / Churyumov-Gerasimenko. This instrument consists of two parts:one on board of Rosetta and the other on board of Philae. The data available from operational and calibration tests allowed a end-to-end calibration of the instrument. This calibration enabled to correct the amplitude imbalance at 5%, of the output power and to build a compensation law for the different models of flight and recharging. This increased the precession of the AGC and gain systemand finally we estimated the delay generated by the filters and electronics system for the different models. The results obtained on the calibration of the data on each model will consequently reduce the estimation errors.Finally, the calibration of CONSER contributed significantly to improve the measurements made during the landing of the Philae on the comet., L'expérience Comet Nucleus Sounding par Radio-wave Transmission (CONSERT / Rosetta) a été conçu pour scanner l'intérieur de la comète 67P / Churyumov-Gerasimenko. Cet instrument se compose de deux parties: l'une à bord de Rosetta et l'autre à bord de Philae. Les données disponibles à partir des tests de fonctionnement et d’étalonnage ont permis de faire une calibration de l’instrument de bout à bout. Cette calibration a permis de corriger le déséquilibre d’amplitude à 5%, de la puissance de sortie et de construire une loi de compensation pour les différents modèles de vol et de recharge. Celui-ci a augmenté la précession de l’AGC et gain système et enfin nous avons estimé le délai engendré par les filtres et l’électronique du système pour les différents modèles. Les résultats obtenus sur la calibration des données sur chaque modèle réduiront, en conséquence, les erreurs d’estimation. Enfin, l’étalonnage de CONSER a beaucoup contribué à améliorer les mesures effectuées lors de l’atterrissage du Philae sur la comète.

This thesis work provides a complete study of a 1-5 /lm infrared camera designed to be used with the Adaptive Optics system installed at the European Southern Observatory (ESO) 3.6m telescope, from the laboratory characterization of the IRCCD 128X128 HgCdTe Focal Plane Array (FPA). to astronomical results obtained on a sampIe of Pre-Main-Sequence binaries. This document includes three parts which follow the "natural" chronological sequence of the study. The first part is related to the laboratory characterization of the FPA, followed by its setting in operation at the telescope, within the COMIC camera. This part was started at the Laboratoire Infrarouge-LETI /DOPT /CEA-Grenoble) and finished at ESO-La Silla Observatory. The second part deals with the specificities of IR observations with Adaptive Optics. This part illustrates the experience acquired during a long stay at ESO. It makes the link between the technical work of the first part and the observer's point of view. To finish , an illustration of the astronomical capabilities of the instrument (COMIC+ADONIS) is given in the third part, in the framework of a well-defined astronomical program. This program concerns the study of the circumstellar material in Pre-Main-Sequence binaries. This last part was done at the Observatoire de Grenoble., Ce travail de thèse concerne le suivi d'une caméra infrarouge sensible dans la gamme 1-5 p.m (COMIC), depuis la caractérisation en laboratoire de la mosaïque de détecteurs, jusqu'à son utilisation dans le cadre d'observations astronomiques, sur le système d'Optique Adaptative ADONIS au télescope de 3.60 m de l'Observatoire Européen Austral (ESO), au Chili. Ce manuscrit comporte trois parties qui suivent l'enchainement chronologique "naturel" de l'étude, dans des lieux de travail différents. La première partie concerne la mosaïque de détecteurs COMIC (IRCCD 128X128 HgCdTe), en commençant par sa caractérisation en laboratoire, pour finir par sa mise en opération sous télescope, au sein de la caméra COMIC. Elle s'est d'abord déroulée au sein du Laboratoire Infrarouge (LETI/DOPT/CEA-Grenoble), puis à l'ESO, au Chili. La deuxième partie traite des spécificités de l'observation infrarouge avec un instrument d'Optique Adaptative. Cette partie est le résultat de l'expérience acquise au cours d'un long séjour à l'ESO. Elle unit le point de vue technique de la première partie à celui de l'astronome utilisateur. Enfin, une illustration du potentiel de l'instrument (COMIC+ADONIS) est donnée en troisième partie dans le cadre d'un programme astronomique bien défini: l'étude de la matière circumstellaire dans des systèmes binaires pré-Séquence Principale. Cette dernière étude a été menée à l'Observatoire de Grenoble.