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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 ($<$ 200 Myr) and in the low-mass regime ($\mathrm{<1~M_{\odot}}$). GJ 2060 AB and TWA 22 AB are two rare astrometric M-dwarf binaries respectively members of the AB Doradus and Beta Pictoris moving groups. As their dynamical mass can be measured within a few years, they can be used to calibrate the evolutionary tracks and set new constraints on the age of young moving groups. We find a total mass of $\mathrm{0.18\pm 0.02~M_\odot}$ for TWA 22. That mass is in good agreement with model predictions at the age of the Beta Pic moving group. We obtain a total mass of $\mathrm{1.09 \pm 0.10~M_{\odot}}$ for GJ 2060. We estimate a spectral type of M$1\pm0.5$, $\mathrm{L/L_{\odot}=-1.20\pm0.05}$ dex, and $\mathrm{T_{eff}=3700\pm100}$ K for GJ 2060 A. The B component is a M$3\pm0.5$ dwarf with $\mathrm{L/L_{\odot}=-1.63\pm0.05}$ dex and $\mathrm{T_{eff}=3400\pm100}$ K. The dynamical mass of GJ 2060 AB is inconsistent with the most recent models predictions (BCAH15, PARSEC) for an ABDor age in the range 50-150 Myr. It is 10 to 20\% (1-2 sigma, depending on the assumed age) above the models predictions, corresponding to an underestimation of $0.10$ to $0.20~\mathrm{M_\odot}$. Coevality suggests a young age for the system ($\sim$ 50 Myr) according to most evolutionary models. TWA 22 validates the predictions of recent evolutionary tracks at $\sim$20 Myr. On the other hand, we evidence a 1-2 sigma mismatch between the predicted and observed mass of GJ 2060 AB. This slight departure may indicate that one of the star hosts a tight companion. Alternatively, this would confirm the models tendency to underestimate the mass of young low-mass stars., Comment: Accepted for publication in A&A. 26 pages, 22 figures, 12 tables

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., Comment: 64 pages, 33 figures, 12 tables, accepted for publication in A&A

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.

(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., Comment: Accepted in A&A

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., Comment: 4 pages, 3 figures. A&A Letter

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., Comment: Accepted for publication in A&A, 7 pages, 6 figures, 5 tables

The number of publications of aperture-synthesis images based on optical long-baseline interferometry measurements has recently increased due to easier access to visible and infrared interferometers. The interferometry technique has now reached a technical maturity level that opens new avenues for numerous astrophysical topics requiring milli-arcsecond model-independent imaging. In writing this paper our motivation was twofold: 1) review and publicize emblematic excerpts of the impressive corpus accumulated in the field of optical interferometry image reconstruction; 2) discuss future prospects for this technique by selecting four representative astrophysical science cases in order to review the potential benefits of using optical long baseline interferometers. For this second goal we have simulated interferometric data from those selected astrophysical environments and used state-of-the-art codes to provide the reconstructed images that are reachable with current or soon-to-be facilities. The image reconstruction process was "blind" in the sense that reconstructors had no knowledge of the input brightness distributions. We discuss the impact of optical interferometry in those four astrophysical fields. We show that image reconstruction software successfully provides accurate morphological information on a variety of astrophysical topics and review the current strengths and weaknesses of such reconstructions. We investigate how to improve image reconstruction and the quality of the image possibly by upgrading the current facilities. We finally argue that optical interferometers and their corresponding instrumentation, existing or to come, with 6 to 10 telescopes, should be well suited to provide images of complex sceneries., Comment: Acccepted to Astronomy and Astrophysics Review

NGC 6823 is a young open cluster that lies at a distance of ~2 kpc in the Vulpecula OB1 association. Previous studies using CCD photometry and spectroscopy have identified a Trapezium system of bright O- and B-type stars at its center. We present optical, near-infrared and Spitzer photometric observations of the cluster. Our survey reaches down to I~22 mag and Ks~18 mag. There is significant differential reddening within the cluster. We find a bimodal distribution for Av, with a peak at ~3 mag and a broader peak at ~10 mag. We find a ~20% fraction of Class I/Class II young stellar objects (YSOs) in the cluster, while a large 80% fraction of the sources have a Class III classification. We have made use of the IPHAS survey in order to probe the strength in Halpha emission for this large population of Class III sources. Nearly all of the Class III objects have photospheric (r'-Halpha) colors, implying an absence of Halpha in emission. This large population of Class III sources is thus likely the extincted field star population rather than the diskless YSOs in the cluster. There is a higher concentration of the Class I/II systems in the eastern region of the cluster and close to the central Trapezium. The western part of the cluster mostly contains Class III/field stars and seems devoid of disk sources. We find evidence of a pre-main sequence population in NGC 6823, in addition to an upper main-sequence population. The pre-main sequence population mainly consists of young disk sources with ages between ~1-5 Myr, and at lower masses of ~0.1-0.4 Msun. There may be a possible mass dependent age spread in the cluster, with the older stars being more massive than the younger ones. The presence of young disk sources in NGC 6823 indicates similar star formation properties in the outer regions of the Galaxy as observed for young clusters in the solar neighborhood., Comment: Accepted in MNRAS

PIONIER stands for Precision Integrated-Optics Near-infrared Imaging ExpeRiment. It combines four 1.8m Auxilliary Telescopes or four 8m Unit Telescopes of the Very Large Telescope Interferometer (ESO, Chile) using an integrated optics combiner. The instrument has been integrated at IPAG starting in December 2009 and commissioned at the Paranal Observatory in October 2010. It provides scientific observations since November 2010. In this paper, we detail the instrumental concept, we describe the standard operational modes and the data reduction strategy. We present the typical performance and discuss how to improve them. This paper is based on laboratory data obtained during the integrations at IPAG, as well as on-sky data gathered during the commissioning at VLTI. We illustrate the imaging capability of PIONIER on the binaries deltaSco and HIP11231. PIONIER provides 6 visibilities and 3 independent closure phases in the H band, either in a broadband mode or with a low spectral dispersion (R=40), using natural light (i.e. unpolarized). The limiting magnitude is Hmag=7 in dispersed mode under median atmospheric conditions (seeing<1", tau0>3ms) with the 1.8m Auxiliary Telescopes. We demonstrate a precision of 0.5deg on the closure phases. The precision on the calibrated visibilities ranges from 3 to 15% depending on the atmospheric conditions. PIONIER has been installed and successfully tested as a visitor instrument for the VLTI. It permits high angular resolution imaging studies at an unprecedented level of sensitivity. The successful combination of the four 8m Unit Telescopes in March 2011 demonstrates that VLTI is ready for 4-telescope operation.

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., Comment: 11 pages, 10 figures

The transition between massive Class II circumstellar disks and Class III debris disks, with dust residuals, has not yet been clearly understood. Disks are expected to dissipate with time, and dust clearing in the inner regions can be the consequence of several mechanisms. Planetary formation is one of them that will possibly open a gap inside the disk. According to recent models based on photometric observations, T Cha is expected to present a large gap within its disk, meaning that an inner dusty disk is supposed to have survived close to the star. We investigate this scenario with new near-infrared interferometric observations. We observed T Cha in the H and K bands using the AMBER instrument at VLTI and used the MCFOST radiative transfer code to model the SED of T Cha and the interferometric observations simultaneously and to test the scenario of an inner dusty structure. We also used a toy model of a binary to check that a companion close to the star can reproduce our observations. The scenario of a close (few mas) companion cannot satisfactorily reproduce the visibilities and SED, while a disk model with a large gap and an inner ring producing the bulk of the emission (in H and K-bands) close to 0.1 AU is able to account for all the observations. With this study, the presence of an optically thick inner dusty disk close to the star and dominating the H and K- bands emission is confirmed. According to our model, the large gap extends up to ~ 7.5 AU. This points toward a companion (located at several AU) gap-opening scenario to explain the morphology of T Cha.

(Abridged) Dust particles evolve in size and lattice structure in protoplanetary disks, due to coagulation, fragmentation and crystallization, and are radially and vertically mixed in disks. This paper aims at determining the mineralogical composition and size distribution of the dust grains in disks around 58 T Tauri stars observed with Spitzer/IRS. We present a spectral decomposition model that reproduces the IRS spectra over the full spectral range. The model assumes two dust populations: a warm component responsible for the 10\mu m emission arising from the disk inner regions and a colder component responsible for the 20-30\mu m emission, arising from more distant regions. We show evidence for a significant size distribution flattening compared to the typical MRN distribution, providing an explanation for the usual boxy 10\mu m feature profile generally observed. We reexamine the crystallinity paradox, observationally identified by Olofsson et al. (2009), and we find a simultaneous enrichment of the crystallinity in both the warm and cold regions, while grain sizes in both components are uncorrelated. Our modeling results do not show evidence for any correlations between the crystallinity and either the star spectral type, or the X-ray luminosity (for a subset of the sample). The size distribution flattening may suggests that grain coagulation is a slightly more effective process than fragmentation in disk atmospheres, and that this imbalance may last over most of the T Tauri phase. This result may also point toward small grain depletion via strong stellar winds or radiation pressure in the upper layers of disk. The non negligible cold crystallinity fractions suggests efficient radial mixing processes in order to distribute crystalline grains at large distances from the central object, along with possible nebular shocks in outer regions of disks that can thermally anneal amorphous grains.

Aims. The brown dwarf (BD) formation process has not yet been completely understood. To shed more light on the differences and similarities between star and BD formation processes, we study and compare the disk fraction among both kinds of objects over a large angular region in the Taurus cloud. In addition, we examine the spatial distribution of stars and BD relative to the underlying molecular gas Methods. In this paper, we present new and updated photometry data from the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope on 43 BDs in the Taurus cloud, and recalculate of the BD disk fraction in this region. We also useed recently available CO mm data to study the spatial distribution of stars and BDs relative to the cloud's molecular gas. Results. We find that the disk fraction among BDs in the Taurus cloud is 41 \pm 12%, a value statistically consistent with the one among TTS (58 \pm 9%). We find that BDs in transition from a state where they have a disk to a diskless state are rare, and we study one isolated example of a transitional disk with an inner radius of \approx 0.1 AU (CFHT BD Tau 12, found via its relatively small mid-IR excess compared to most members of Taurus that have disks. We find that BDs are statistically found in regions of similar molecular gas surface density to those associated with stars. Furthermore, we find that the gas column density distribution is almost identical for stellar and substellar objects with and without disks., Comment: 8 page, 6 figures, Accepted in Astronomy & Astrophysics.

The Fourier-Kelvin Stellar Interferometer (FKSI) is a mission concept for a nulling interferometer for the near-to-mid-infrared spectral region. FKSI is conceived as a mid-sized strategic or Probe class mission. FKSI has been endorsed by the Exoplanet Community Forum 2008 as such a mission and has been costed to be within the expected budget. The current design of FKSI is a two-element nulling interferometer. The two telescopes, separated by 12.5 m, are precisely pointed (by small steering mirrors) on the target star. The two path lengths are accurately controlled to be the same to within a few nanometers. A phase shifter/beam combiner (Mach-Zehnder interferometer) produces an output beam consisting of the nulled sum of the target planet's light and the host star's light. When properly oriented, the starlight is nulled by a factor of 10^-4, and the planet light is undiminished. Accurate modeling of the signal is used to subtract the residual starlight, permitting the detection of planets much fainter than the host star. The current version of FKSI with 0.5-m apertures and waveband 3-8 microns has the following main capabilities: (1) detect exozodiacal emission levels to that of our own solar system (1 Solar System Zodi) around nearby F, G, and K, stars; (2) characterize spectroscopically the atmospheres of a large number of known non-transiting planets; (3) survey and characterize nearby stars for planets down to 2 Earth radii from just inside the habitable zone and inward. An enhanced version of FKSI with 1-m apertures separated by 20 m and cooled to 40 K, with science waveband 5-15 microns, allows for the detection and characterization of 2 Earth-radius super-Earths and smaller planets in the habitable zone around stars within about 30 pc., Comment: Pathways Conference, 6 pages, 7 figures

We report on the properties of pre-main-sequence objects in the Taurus molecular clouds as observed in 7 mid- and far-infrared bands with the Spitzer Space Telescope. There are 215 previously-identified members of the Taurus star-forming region in our ~44 square degree map; these members exhibit a range of Spitzer colors that we take to define young stars still surrounded by circumstellar dust (noting that ~20% of the bonafide Taurus members exhibit no detectable dust excesses). We looked for new objects in the survey field with similar Spitzer properties, aided by extensive optical, X-ray, and ultraviolet imaging, and found 148 candidate new members of Taurus. We have obtained follow-up spectroscopy for about half the candidate sample, thus far confirming 34 new members, 3 probable new members, and 10 possible new members, an increase of 15-20% in Taurus members. Of the objects for which we have spectroscopy, 7 are now confirmed extragalactic objects, and one is a background Be star. The remaining 93 candidate objects await additional analysis and/or data to be confirmed or rejected as Taurus members. Most of the new members are Class II M stars and are located along the same cloud filaments as the previously-identified Taurus members. Among non-members with Spitzer colors similar to young, dusty stars are evolved Be stars, planetary nebulae, carbon stars, galaxies, and AGN., Comment: Accepted to ApJS. Two large online-only figures available with the preprint here: http://web.ipac.caltech.edu/staff/rebull/research.html

Dust grains in the planet forming regions around young stars are expected to be heavily processed due to coagulation, fragmentation and crystallization. This paper focuses on the crystalline silicate dust grains in protoplanetary disks. As part of the Cores to Disks Legacy Program, we obtained more than a hundred Spitzer/IRS spectra of TTauri stars. More than 3/4 of our objects show at least one crystalline silicate emission feature that can be essentially attributed to Mg-rich silicates. Observational properties of the crystalline features seen at lambda > 20 mu correlate with each other, while they are largely uncorrelated with the properties of the amorphous silicate 10 mu feature. This supports the idea that the IRS spectra essentially probe two independent disk regions: a warm zone (< 1 AU) emitting at lambda ~ 10 mu and a much colder region emitting at lambda > 20 mu (< 10 AU). We identify a crystallinity paradox, as the long-wavelength crystalline silicate features are 3.5 times more frequently detected (~55 % vs. ~15%) than the crystalline features arising from much warmer disk regions. This suggests that the disk has an inhomogeneous dust composition within ~10 AU. The abundant crystalline silicates found far from their presumed formation regions suggests efficient outward radial transport mechanisms in the disks. The analysis of the shape and strength of both the amorphous 10 mu feature and the crystalline feature around 23 mu provides evidence for the prevalence of micron-sized grains in upper layers of disks. Their presence in disk atmospheres suggests efficient vertical diffusion, likely accompanied by grain-grain fragmentation to balance the efficient growth expected. Finally, the depletion of submicron-sized grains points toward removal mechanisms such as stellar winds or radiation pressure.

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., Comment: 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), Comment: 17 pages, 10 figures, 7 tables, accepted for A&A

We report on accretion- and outflow-related X-rays from T Tauri stars, based on results from the "XMM-Newton Extended Survey of the Taurus Molecular Cloud." X-rays potentially form in shocks of accretion streams near the stellar surface, although we hypothesize that direct interactions between the streams and magnetic coronae may occur as well. We report on the discovery of a "soft excess" in accreting T Tauri stars supporting these scenarios. We further discuss a new type of X-ray source in jet-driving T Tauri stars. It shows a strongly absorbed coronal component and a very soft, weakly absorbed component probably related to shocks in microjets. The excessive coronal absorption points to dust-depletion in the accretion streams., Comment: 8 pages, 9 figures, in press, Proc. IAUS 243, Star-Disk Interactions in Young Stars

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., Comment: 11 pages, 5 figures, A&A accepted

In this work, we present the first AMBER observations, of the Wolf-Rayet and O (WR+O) star binary system y2 Velorum. The AMBER instrument was used with the telescopes UT2, UT3, and UT4 on baselines ranging from 46m to 85m. It delivered spectrally dispersed visibilities, as well as differential and closure phases, with a resolution R = 1500 in the spectral band 1.95-2.17 micron. We interpret these data in the context of a binary system with unresolved components, neglecting in a first approximation the wind-wind collision zone flux contribution. We show that the AMBER observables result primarily from the contribution of the individual components of the WR+O binary system. We discuss several interpretations of the residuals, and speculate on the detection of an additional continuum component, originating from the free-free emission associated with the wind-wind collision zone (WWCZ), and contributing at most to the observed K-band flux at the 5% level. The expected absolute separation and position angle at the time of observations were 5.1±0.9mas and 66±15° respectively. However, we infer a separation of 3.62+0.11-0.30 mas and a position angle of 73+9-11°. Our analysis thus implies that the binary system lies at a distance of 368+38-13 pc, in agreement with recent spectrophotometric estimates, but significantly larger than the Hipparcos value of 258+41-31 pc.

(abridged:) The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST) surveys the most populated ~5 square degrees of the Taurus star formation region, using the XMM-Newton X-ray observatory to study the thermal structure, variability, and long-term evolution of hot plasma, to investigate the magnetic dynamo, and to search for new potential members of the association. Many targets are also studied in the optical, and high-resolution X-ray grating spectroscopy has been obtained for selected bright sources. The X-ray spectra have been coherently analyzed with two different thermal models (2-component thermal model, and a continuous emission measure distribution model). We present overall correlations with fundamental stellar parameters that were derived from the previous literature. A few detections from Chandra observations have been added. The present overview paper introduces the project and provides the basic results from the X-ray analysis of all sources detected in the XEST survey.Comprehensive tables summarize the stellar properties of all targets surveyed. The survey goes deeper than previous X-ray surveys of Taurus by about an order of magnitude and for the first time systematically accesses very faint and strongly absorbed TMC objects. We find a detection rate of 85% and 98% for classical and weak-line T Tau stars (CTTS resp. WTTS), and identify about half of the surveyed protostars and brown dwarfs. Overall, 136 out of 169 surveyed stellar systems are detected. We describe an X-ray luminosity vs. mass correlation, discuss the distribution of X-ray-to-bolometric luminosity ratios, and show evidence for lower X-ray luminosities in CTTS compared to WTTS. Detailed analysis (e.g., variability, rotation-activity relations, influence of accretion on X-rays) will be discussed in a series of accompanying papers., Comment: 75 pg, 77 figs. Accepted by A&A, to appear in a special section/issue dedicated to the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST). V2: ASCII Table 14 added. Version with higher resolution figures at http://www.issibern.ch/teams/Taurus/papers.html or http://www.astro.phys.ethz.ch/papers/guedel/guedel_p_nf.html

The XMM-Newton Extended Survey of the TMC (XEST) is a large program designed to systematically investigate the X-ray properties of young stellar/substellar objects in the TMC. In particular, the area surveyed by 15 XMM-Newton pointings (of which three are archival observations), supplemented with one archival Chandra observation, allows us to study 17 BDs with M spectral types. Half of this sample (9 out of 17 BDs) is detected; 7 BDs are detected here for the first time in X-rays. We observed a flare from one BD. We confirm several previous findings on BD X-ray activity: a log-log relation between X-ray and bolometric luminosity for stars (with L*<10 Lsun) and BDs detected in X-rays; a shallow log-log relation between X-ray fractional luminosity and mass; a log-log relation between X-ray fractional luminosity and effective temperature; a log-log relation between X-ray surface flux and effective temperature. We find no significant log-log correlation between the X-ray fractional luminosity and EW(Halpha). Accreting and nonaccreting BDs have a similar X-ray fractional luminosity. The median X-ray fractional luminosity of nonaccreting BDs is about 4 times lower than the mean saturation value for rapidly rotating low-mass field stars. Our TMC BDs have higher X-ray fractional luminosity than BDs in the Chandra Orion Ultradeep Project. The X-ray fractional luminosity declines from low-mass stars to M-type BDs, and as a sample, the BDs are less efficient X-ray emitters than low-mass stars. We thus conclude that while the BD atmospheres observed here are mostly warm enough to sustain coronal activity, a trend is seen that may indicate its gradual decline due to the drop in photospheric ionization degree (abridged)., Comment: 20 pages and 19 Figures. Accepted by A&A, to appear in a special section/issue dedicated to the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST). Preprint with higher resolution figures is available at http://hal.ccsd.cnrs.fr/ccsd-00090499

The formation of a binary system surrounded by disks is the most common outcome of stellar formation. Hence studying and understanding the formation and the evolution of binary systems and associated disks is a cornerstone of star formation science. Moreover, since the components within binary systems are coeval and the sizes of their disks are fixed by the tidal truncation of their companion, binary systems provide an ideal "laboratory" in which to study disk evolution under well defined boundary conditions. In this paper, we review observations of several inner disk diagnostics in multiple systems, including hydrogen emission lines (indicative of ongoing accretion), $K-L$ and $K-N$ color excesses (evidence of warm inner disks), and polarization (indicative of the relative orientations of the disks around each component). We examine to what degree these properties are correlated within binary systems and how this degree of correlation depends on parameters such as separation and binary mass ratio. These findings will be interpreted both in terms of models that treat each disk as an isolated reservoir and those in which the disks are subject to re-supply from some form of circumbinary reservoir, the observational evidence for which we will also critically review. The planet forming potential of multiple star systems is discussed in terms of the relative lifetimes of disks around single stars, binary primaries and binary secondaries. Finally, we summarize several potentially revealing observational problems and future projects that could provide further insight into disk evolution in the coming decade, Comment: 16 pages, 7 figures, chapter in Protostars and Planets V

Recent studies of the substellar population in the Taurus cloud have revealed a deficit of brown dwarfs (BD) 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 30 deg^2, and encompassing the densest part of the cloud as well as their surroundings, down to a mass detection limits of 15 Jupiter Masses (MJ). In this paper, we present the optical spectroscopic follow-up observations of 97 photometrically selected potential new low-mass Taurus members, of which 27 are strong late-M (SpT < M4V) candidates. These observations reveal 5 new very low mass (VLM) Taurus members and 12 new BDs. Combining our observations with previously published results, we derive an updated substellar to stellar ratio in Taurus of Rss =0.23 +/- 0.05. This ratio now appears consistent with the value previously derived in the Trapezium cluster under similar assumptions of 0.26 +/- 0.04. We find strong indication 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. Our findings are best explained in the context of the embryo-ejection model where brown dwarfs originate from dynamical interactions in small N unstable multiple systems., Comment: 20 pages, 15 figures

AMBER is the first near infrared focal instrument of the VLTI. It combines three telescopes and produces spectrally resolved interferometric measures. This paper discusses some preliminary results of the first scientific observations of AMBER with three Unit Telescopes at medium (1500) and high (12000) spectral resolution. We derive a first set of constraints on the structure of the circumstellar material around the Wolf Rayet Gamma2 Velorum and the LBV Eta Carinae.