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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.

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.

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.

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

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

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.

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.

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.

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

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

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.