M. Benisty

Few Skewed Disks Found in First Closure-Phase Survey of Herbig Ae/Be Stars

Using the three-telescope IOTA interferometer on Mount Hopkins, we report results from the first near-infrared (lambda = 1.65 mu m) closure-phase survey of young stellar objects (YSOs). These closure phases allow us to unambiguously detect departures from centrosymmetry (i.e., skew) in the emission pattern from YSO disks on the scale of similar to 4 mas, expected from generic flared disk models. Six of 14 targets showed small, yet statistically significant nonzero closure phases, with largest values from the young binary system MWC 361-A and the (pre-main-sequence?) Be star HD 45677. Our observations are quite sensitive to the vertical structure of the inner disk, and we confront the predictions of the puffed-up inner wall models of Dullemond, Dominik, & Natta (DDN). Our data support disk models with curved inner rims because the expected emission appears symmetrically distributed around the star over a wide range of inclination angles. In contrast, our results are incompatible with the models possessing vertical inner walls because they predict extreme skewness (i.e., large closure phases) from the near-IR disk emission that is not seen in our data. In addition, we also present the discovery of mysterious H-band halos (similar to 5%-10% of light on scales 0.01-0.50) around a few objects, a preliminary parametric imaging study for HD 45677, and the first astrometric orbit for the young binary MWC 361-A.

An integrated optics beam combiner for the second generation VLTI instruments

Context. Recently, an increasing number of scientific publications making use of images obtained with near-infrared long-baseline interferometry have been produced. The technique has reached, at last, a technical maturity level that opens new avenues for numerous astrophysical topics requiring milli-arc-second model-independent imaging. The Very Large Telescope Interferometer (VLTI) will soon be equipped with instruments able to combine between four and six telescopes. Aims. In the framework of the VLTI second generation instruments Gravity and VSI, we propose a new beam combining concept using integrated optics (IO) technologies with a novel ABCD-like fringe encoding scheme. Our goal is to demonstrate that IO-based combinations bring considerable advantages in terms of instrumental design and performance. We therefore aim at giving a full characterization of an IOxa0beam combiner in order to establish its performance and check its compliance with the specifications of an imaging instrument. Methods. For this purpose, prototype IO beam combiners have been manufactured and laboratory measurements were made in the H xa0band with a dedicated testbed, simulating a four-telescope interferometer. We studied the beam combiners through the analysis of throughput, instrumental visibilities, phases and closure phases in wide band as well as with spectral dispersion. Study of the polarization properties was also carried out. Results. We obtain competitive throughput (65%), high and stable instrumental contrasts (from 80% in wide band up to 100%xa0 ± 1% with spectral dispersion), stable but non-zero closure phases (e.g.xa0115°xa0 ± 2°) which we attribute to internal optical path differences (OPD) that can be calibrated. We validate a new static and an achromatic phase shifting IOxa0function close to the nominal 90°xa0value (e.g.xa080°xa0 ± 1°). All these observables show limited chromaticity over the H xa0band range. Conclusions. Our results demonstrate that such ABCD-like beam combiners are particularly well suited for interferometric combination of multiple beams to achieve aperture synthesis imaging. This opens the way to extending this technique to all near infrared wavelengths and in particular, the K xa0band.

Imaging diagnostics for transitional discs

Transitional discs are a special type of protoplanetary disc, where planet formation is thought to be taking place. These objects feature characteristic inner cavities and/or gaps of a few tens of AUs in sub-millimetre images of the disc. This signature suggests a localised depletion of matter in the disc that could be caused by planet formation processes. However, recent observations have revealed differences in the structures imaged at different wavelengths in some of these discs. In this paper, we aim to explain these observational differences using self-consistent physical 2D hydrodynamical and dust evolution models of these objects, assuming their morphology is indeed generated by the presence of a planet. We use these models to derive the distribution of gas and dust in a theoretical planet-hosting disc for various planet masses and orbital separations. We then simulate observations of the emitted and scattered light from these models with Very Large Telescope (VLT)/SPHERE-ZIMPOL, Subaru/HiCIAO, VLT/VISIR, and ALMA. We do this by first computing the full resolution images of the models at different wavelengths and then simulating the observations while accounting for the characteristics of each particular instrument. The presence of the planet generates pressure bumps in the gas distribution of the disc, whose characteristics strongly depend on the planet mass and position. These bumps cause large grains to accumulate, while small grains are allowed into inner regions. This spatial differentiation of the grain sizes explains the differences in the observations, since different wavelengths and observing techniques trace different parts of the dust size distribution. Based on this effect, we conclude that the combination of visible/near-infrared polarimetric and sub-mm images is the best strategy to constrain the properties of the unseen planet responsible for the disc structure.

Sculpting the disk around T Chamaeleontis: an interferometric view

Context. Circumstellar disks are believed to be the birthplace of planets and are expected to dissipate on a timescale of a few Myr. The processes responsible for the removal of the dust and gas will strongly modify the radial distribution of the circumstellar matter and consequently the spectral energy distribution. In particular, a young planet will open a gap, resulting in an inner disk dominating the near-IR emission and an outer disk emitting mostly in the far-infrared. Aims. We analyze a full set of data involving new near-infrared data obtained with the 4-telescope combiner (VLTI/PIONIER), new mid-infrared interferometric VLTI/MIDI data, literature photometric and archival data from VLT/NaCo/SAM to constrain the structure of the transition disk around T Cha. Methods. After a preliminary analysis with a simple geometric model, we used the MCFOST radiative transfer code to simultaneously model the SED and the interferometric observables from raytraced images in the H-, L 0 -, and N-bands. Results. We find that the dust responsible for the strong emission in excess in the near-IR must have a narrow temperature distribution with a maximum close to the silicate sublimation temperature. This translates into a narrow inner dusty disk (0.07‐0.11 AU), with a significant height (H=r 0:2) to increase the geometric surface illuminated by the central star. We find that the outer disk starts at about 12 AU and is partially resolved by the PIONIER, SAM, and MIDI instruments. We discuss the possibility of a self-shadowed inner disk, which can extend to distances of several AU. Finally, we show that the SAM closure phases, interpreted as the signature of a candidate companion, may actually trace the asymmetry generated by forward scattering by dust grains in the upper layers of the outer disk. These observations help constrain the inclination and position angle of the disk to about +58 and 70 , respectively. Conclusions. The circumstellar environment of T Cha appears to be best described by two disks spatially separated by a large gap. The presence of matter (dust or gas) inside the gap is, however, di cult to assess with present-day observations. Our model suggests the outer disk contaminates the interferometric signature of any potential companion that could be responsible for the gap opening, and such a companion still has to be unambiguously detected. We stress the di culty to observe point sources in bright massive disks, and the consequent need to account for disk asymmetries (e.g. anisotropic scattering) in model-dependent search for companions.

Three Radial Gaps in the Disk of TW Hydrae Imaged with SPHERE

We present scattered light images of the TW Hya disk performed with the Spectro‐Polarimetric High‐contrast Exoplanet REsearch instrument in Polarimetric Differential Imaging mode at 0.63, 0.79, 1.24, and 1.62 mu m. We also present H2/H3‐band angular differential imaging (ADI) observations. Three distinct radial depressions in the polarized intensity distribution are seen, around approximate to 85, approximate to 21, and less than or similar to 6 au.(21) The overall intensity distribution has a high degree of azimuthal symmetry; the disk is somewhat brighter than average toward the south and darker toward the north‐west. The ADI observations yielded no signifiant detection of point sources in the disk. Our observations have a linear spatial resolution of 1‐2 au, similar to that of recent ALMA dust continuum observations. The submicron‐ sized dust grains that dominate the light scattering in the disk surface are strongly coupled to the gas. We created a radiative transfer disk model with self‐consistent temperature and vertical structure iteration and including grain size‐dependent dust settling. This method may provide independent constraints on the gas distribution at higher spatial resolution than is feasible with ALMA gas line observations. We find that the gas surface density in the ``gaps is reduced by approximate to 50% to approximate to 80% relative to an unperturbed model. Should embedded planets be responsible for carving the gaps then their masses are at most a few 10M(circle plus). The observed gaps are wider, with shallower flanks, than expected for planet‐disk interaction with such low‐mass planets. If forming planetary bodies have undergone collapse and are in the ``detached phase, then they may be directly observable with future facilities such as the Mid‐Infrared E‐ELT Imager and Spectrograph at the E‐ELT.

Structure of Herbig AeBe disks at the milliarcsecond scale: A statistical survey in the H band using PIONIER-VLTI

Context. It is now generally accepted that the near-infrared excess of Herbig AeBe stars originates in the dust of a circumstellar disk. n nAims. The aims of this article are to infer the radial and vertical structure of these disks at scales of order 1 au, and the properties of the dust grains. n nMethods. The program objects (51 in total) were observed with the H-band (1.6 μm) PIONIER/VLTI interferometer. The largest baselines allowed us to resolve (at least partially) structures of a few tenths of an au at typical distances of a few hundred parsecs. Dedicated UBVRIJHK photometric measurements were also obtained. Spectral and 2D geometrical parameters are extracted via fits of a few simple models: ellipsoids and broadened rings with azimuthal modulation. Model bias is mitigated by parallel fits of physical disk models. Sample statistics were evaluated against similar statistics for the physical disk models to infer properties of the sample objects as a group. n nResults. We find that dust at the inner rim of the disk has a sublimation temperature T_(sub) ≈ 1800 K. A ring morphology is confirmed for approximately half the resolved objects; these rings are wide δr/r ≥ 0.5. A wide ring favors a rim that, on the star-facing side, looks more like a knife edge than a doughnut. The data are also compatible with the combination of a narrow ring and an inner disk of unspecified nature inside the dust sublimation radius. The disk inner part has a thickness z/r ≈ 0.2, flaring to z/r ≈ 0.5 in the outer part. We confirm the known luminosity-radius relation; a simple physical model is consistent with both the mean luminosity-radius relation and the ring relative width; however, a significant spread around the mean relation is present. In some of the objects we find a halo component, fully resolved at the shortest interferometer spacing, that is related to the HAeBe class.

Bright Localized Near-Infrared Emission at 1-4 AU in the AB Aurigae Disk Revealed by IOTA Closure Phases

We report on the detection of localized off-center emission at 1-4 AU in the circumstellar environment of the young stellar object AB Aurigae. We used closure-phase measurements in the near-infrared that were made at the long-baseline interferometer IOTA, the first obtained on a young stellar object using this technique. When probing sub-AU scales, all closure phases are close to zero degrees, as expected given the previously determined size of the AB Aurigae inner-dust disk. However, a clear closure-phase signal of -3.5d +/- 0.5d is detected on one triangle containing relatively short baselines, requiring a high degree of non-point symmetry from emission at larger (AU-sized) scales in the disk. We have not identified any alternative explanation for these closure-phase results, and we demonstrate that a disk hot spot model can fit our data. We speculate that such detected asymmetric near-infrared emission might arise as a result of localized viscous heating due to a gravitational instability in the AB Aurigae disk, or to the presence of a close stellar companion or accreting substellar object.

Multiple rings in the transition disk and companion candidates around RX J1615.3-3255 - High contrast imaging with VLT/SPHERE

Context. The effects of a planet sculpting the disk from which it formed are most likely to be found in disks that are in transition between being classical protoplanetary and debris disks. Recent direct imaging of transition disks has revealed structures such as dust rings, gaps, and spiral arms, but an unambiguous link between these structures and sculpting planets is yet to be found. Aims. We aim to find signs of ongoing planet-disk interaction and study the distribution of small grains at the surface of the transition disk around RXJ1615.3-3255 (RX J1615). Methods. We observed RXJ1615 with VLT/SPHERE. From these observations, we obtained polarimetric imaging with ZIMPOL (R-band) and IRDIS (J), and IRDIS (H2H3) dual-band imaging with simultaneous spatially resolved spectra with the IFS (YJ). Results. We image the disk for the first time in scattered light and detect two arcs, two rings, a gap and an inner disk with marginal evidence for an inner cavity. The shapes of the arcs suggest that they are probably segments of full rings. Ellipse fitting for the two rings and inner disk yield a disk inclination i = 47 +/- 2 degrees and find semi-major axes of 1.50 +/- 0.01 ` (278 au), 1.06 +/- 0.01 ` (196 au) and 0.30 +/- 0.01 ` (56 au), respectively. We determine the scattering surface height above the midplane, based on the projected ring center offsets. Nine point sources are detected between 2.1 ` and 8.0 ` separation and considered as companion candidates. With NACO data we recover four of the nine point sources, which we determine to be not co-moving, and therefore unbound to the system. Conclusions. We present the first detection of the transition disk of RXJ1615 in scattered light. The height of the rings indicate limited flaring of the disk surface, which enables partial self-shadowing in the disk. The outermost arc either traces the bottom of the disk or it is another ring with semi-major axis greater than or similar to 2.35 ` (435 au). We explore both scenarios, extrapolating the complete shape of the feature, which will allow us to distinguish between the two in future observations. The most attractive scenario, where the arc traces the bottom of the outer ring, requires the disk to be truncated at r approximate to 360 au. If the closest companion candidate is indeed orbiting the disk at 540 au, then it would be the most likely cause for such truncation. This companion candidate, as well as the remaining four, all require follow up observations to determine if they are bound to the system.

The VLTI/PIONIER near-infrared interferometric survey of southern T Tauri stars - I. First results

Context. The properties of the inner disks of bright Herbig AeBe stars have been studied with near-infrared (NIR) interferometry and high resolution spectroscopy. The continuum (dust) and a few molecular gas species have been studied close to the central star; however, sensitivity problems limit direct information about the inner disks of the fainter T Tauri stars. n nAims. Our aim is to measure some of the properties (inner radius, brightness profile, shape) of the inner regions of circumstellar disk surrounding southern T Tauri stars. n nMethods. We performed a survey with the VLTI/PIONIER recombiner instrument at H-band of 21 T Tauri stars. The baselines used ranged from 11 m to 129 m, corresponding to a maximum resolution of ~3 mas (~0.45 au at 150 pc). n nResults. Thirteen disks are resolved well and the visibility curves are fully sampled as a function of baseline in the range 45–130 m for these 13 objects. A simple qualitative examination of visibility profiles allows us to identify a rapid drop-off in the visibilities at short baselines( 3 au, at 150 pc) contribution of light from the disk. We demonstrate that this component is compatible with scattered light, providing strong support to an earlier prediction. The amplitude of the drop-off and the amount of dust thermal emission changes from source to source suggesting that each disk is different. A by-product of the survey is the identification of a new milli-arcsec separation binary: WW Cha. Spectroscopic and interferometric data of AK Sco have also been fitted with a binary + disk model. n nConclusions. The visibility data are reproduced well when thermal emission and scattering from dust are fully considered. The inner radii measured are consistent with the expected dust sublimation radii. The modelling of AK Sco suggests a likely coplanarity between the disk and the binary’s orbital plane.

MWC 297: A young high-mass star rotating at critical velocity

Context. MWC 297 is a nearby young massive B[e] star. The central star is attenuated by 8 mag in the optical and has a high projected rotational velocity of 350 km s −1 . Despite the wealth of published observations, the nature of this object and its circumstellar environment is not understood very well. Aims. With the present paper, we intend to shed light on the geometrical structure of the circumstellar matter that is responsible for the near- to mid-infrared flux excess. Methods. The H-band (1.6−2.0 μm), K-band (2.0−2.5 μm), and N-band (8−13 μm) brightness distribution of MWC 297 was probed with the ESO interferometric spectrographs AMBER and MIDI, mounted on the VLTI in Paranal, Chile. We obtained visibility measurements on 3 AMBER and 12 MIDI baselines, covering a wide range of spatial frequencies. Different models (parametrized circumstellar disks, a dusty halo) were invoked to fit the data, all of which fail to do so in a satisfying way. We approximated the brightness distribution in H, K ,a ndN with a geometric model consisting of three Gaussian disks with different extents and brightness temperatures. This model can account for the entire near- to mid-IR emission of MWC 297. Results. The circumstellar matter around MWC 297 is resolved on all baselines. The near- and mid-IR emission, including the silicate emission at 10 micron, emanates from a very compact region (FWHM < 1.5 AU) around the central star. Conclusions. We argue that the extinction towards the MWC 297 star+disk system is interstellar and most likely due to remnants of the natal cloud from which MWC 297 was formed. Furthermore, we argue that the circumstellar matter in the MWC 297 system is organized in a circumstellar disk, seen under moderate (i < 40 ◦ ) inclination. The disk displays no inner emission-free gap at the resolution of our interferometric observations. The low inclination of the disk implies that the already high projected rotational velocity of the star corresponds to an actual rotational velocity that exceeds the critical velocity of the star. This result shows that stars can obtain such high rotation rates at birth. We discuss the impact of this result in terms of the formation of high-mass stars and the main-sequence evolution of classical Be stars.