Context. It is now generally accepted that the near-infrared excess of Herbig AeBe stars originates in the dust of a circumstellar disk. Aims. 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. Methods. The program objects (51 in total) were observed with the H-band (1.6 mu m) 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. Results. We find that dust at the inner rim of the disk has a sublimation temperature T sub(sub)approximate 1800 K. A ring morphology is confirmed for approximately half the resolved objects; these rings are wide delta r/r> or = 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/rapproximate 0.2, flaring to z/rapproximate 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.
Aims. The prime motivation of this project was to design and build a state-of-art mm-wave heterodyne receiver system to enhance the observing throughput of the IRAM 30-m radiotelescope. More ...specifically, the requirements were i) state-of-art noise performance for spectroscopic observations; ii) simultaneous dual polarization and dual-frequency observing; iii) coverage of the atmospheric transmission windows from 83 to 360 GHz; iv) compact footprint and minimal maintenance. Methods. Key elements for low noise performance of heterodyne mixers are the superconducting Niobium junctions, operating at ≃4 K. These junctions are embedded in carefully designed coupling structures; furthermore, since atmospheric radiation is a significant contributor to the system noise budget, all mixers are either sideband separating or sideband rejecting. To achieve low noise, it is also essential to maximize the coupling of the receiver to the astronomical source, and to minimize the coupling to thermal radiation from the ground-based environment; this is achieved through mirror optics that realize a wavelength-independent coupling to the telescope. A flexible configuration of mirrors and frequency selective surfaces permits various combinations of frequency bands, as well as dual-load radiometric calibration. Low noise intermediate frequency amplifiers and bias electronics also play an important role in the system performance. Results. The EMIR receiver in operation at the 30 m telescope offers four frequency bands: B1: 83−117 GHz, B2: 129−174 GHz, B3: 200−267 GHz, and B4: 260−360 GHz. In each band, the two orthogonal polarizations are observed simultaneously. Dual-band combinations B1/2 B1/3, and B2/4 are available. Bands 1 and 4 (also 3 as of Nov.-2011) feature sideband separation. In dual-band configuration, including sideband separation and polarization diplexing, up to eight IF channels are delivered to the spectrometers, totaling up to 64 GHz of signal bandwidth (of which 32 GHz can be transported and processed by spectrometers, status Nov.-2011). The EMIR receiver has been in continuous operation for more than two years and has allowed, through a qualitative jump in performance, observations not possible before, as shown by a few selected examples of astronomical results.
Context. To understand the chemical composition of planets, it is important to know the chemical composition of the region where they form in protoplanetary disks. Because of its fundamental role in ...chemical and biological processes, carbon is a key element to trace. Aims. We identify the carriers and processes behind the extended near-infrared (NIR) flux observed around several Herbig stars. Methods. We compared the extended NIR flux from objects in the PIONIER Herbig Ae/Be survey with their flux in the policyclic aromatic hydrocarbon (PAH) features. HD 100453 is used as a benchmark case to investigate the influence of quantum heated particles, like PAHs or very small carbonaceous grains, in more detail. We use the Monte Carlo radiative transfer code MCMax to do a parameter study of the quantum heated particle (QHP) size and scale height and examine the influence of quantum heating on the amount of extended flux in the NIR visibilities. Results. There is a correlation between the PAH feature flux of a disk and the amount of its extended NIR flux. We find that very small carbonaceous grains create the observed extended NIR flux around HD 100453 and still lead to a realistic SED. These results cannot be achieved without using quantum heating effects, e.g. only with scattered light and grains in thermal equilibrium. Conclusions. It is possible to explain the extended NIR emission around Herbig stars with the presence of carbonaceous, quantum heated particles. Interferometric observations can be used to constrain the spatial distribution and typical size of carbonaceous material in the terrestrial planet forming region.
Context. The emergence of optical interferometers with three and more telescopes allows image reconstruction of astronomical objects at the milliarcsecond scale. However, some objects contain ...components with very different spectral energy distributions (SED; i.e. different temperatures), which produces strong chromatic effects on the interferograms that have to be managed with care by image reconstruction algorithms. For example, the gray approximation for the image reconstruction process results in a degraded image if the total (u,v)-coverage given by the spectral supersynthesis is used. Aims. The relative flux contribution of the central object and an extended structure changes with wavelength for different temperatures. For young stellar objects, the known characteristics of the central object (i.e., stellar SED), or even the fit of the spectral index and the relative flux ratio, can be used to model the central star while reconstructing the image of the extended structure separately. Methods. We present a new method, called SPARCO (semi-parametric algorithm for the image reconstruction of chromatic objects), which describes the spectral characteristics of both the central object and the extended structure to consider them properly when reconstructing the image of the surrounding environment. We adapted two image-reconstruction codes ( Macim , Squeeze , and MiRA ) to implement this new prescription. Results. SPARCO is applied using Macim , Squeeze , and MiRA on a young stellar object model and also on literature data on HR 5999 in the near-infrared with the VLTI. We obtain smoother images of the modeled circumstellar emission and improve the chi super(2) by a factor 9. Conclusions. This method paves the way to improved aperture-synthesis imaging of several young stellar objects with existing datasets. More generally, the approach can be used on astrophysical sources with similar features, such as active galactic nuclei, planetary nebulae, and asymptotic giant branch stars.
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′-, 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, difficult 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 difficulty 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.
A basic principle of long baseline interferometry is that an optical path difference (OPD) directly translates into an astrometric measurement. In the simplest case, the OPD is equal to the scalar ...product between the vector that links the two telescopes and the normalized vector pointing toward the star. However, in some circumstances, too simple an interpretation of this scalar product leads to seemingly conflicting results, called here "the baseline paradox". For micro-arcsecond accuracy astrometry, we have to model the metrology measurement in full. It involves a complex system subject to many optical effects: from pure baseline errors to static, quasi-static, and high-order optical aberrations. The goal of this paper is to present the strategy used by the "General Relativity Analysis via VLT InTerferometrY" instrument (GRAVITY) to minimize the biases introduced by these defects. The result of this work is an error budget of the biases caused by the multiple optical imperfections, including optical dispersion.
Context. A new four-telescope interferometric instrument called PIONIER has recently been installed at VLTI. It provides improved imaging capabilities together with high precision. Aims: We search ...for low-mass companions around a few bright stars using different strategies, and determine the dynamic range currently reachable with PIONIER. Methods: Our method is based on the closure phase, which is the most robust interferometric quantity when searching for faint companions. We computed the χ2 goodness of fit for a series of binary star models at different positions and with various flux ratios. The resulting χ2 cube was used to identify the best-fit binary model and evaluate its significance, or to determine upper limits on the companion flux in case of non-detections. Results: No companion is found around Fomalhaut, tau Cet and Regulus. The median upper limits at 3σ on the companion flux ratio are respectively of 2.3 × 10-3 (in 4 h), 3.5 × 10-3 (in 3 h) and 5.4 × 10-3 (in 1.5 h) on the search region extending from 5 to 100 mas. Our observations confirm that the previously detected near-infrared excess emissions around Fomalhaut and tau Cet are not related to a low-mass companion, and instead come from an extended source such as an exozodiacal disk. In the case of del Aqr, in 30 min of observation, we obtain the first direct detection of a previously known companion, at an angular distance of about 40 mas and with a flux ratio of 2.05 × 10-2 ± 0.16 × 10-2. Due to the limited u,v plane coverage, its position can, however, not be unambiguously determined. Conclusions: After only a few months of operation, PIONIER has already achieved one of the best dynamic ranges world-wide for multi-aperture interferometers. A dynamic range up to about 1:500 is demonstrated on unresolved targets, but significant improvements are still required to reach the ultimate goal of directly detecting hot giant extrasolar planets. Based on observations obtained at the European Southern Observatory (ESO) Very Large Telescope Interferometer (VLTI), Paranal, Chile.
Context. It is now generally accepted that the near-infrared excess of Herbig AeBe stars originates in the dust of a circumstellar disk. Aims. 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. Methods. 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. Results. We find that dust at the inner rim of the disk has a sublimation temperature Tsub ≈ 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.
Recent improvements in the sensitivity and spectral resolution of X-ray observations have led to a better understanding of the properties of matter in the near vicinity of high-mass X-ray binaries ...(HMXB) hosting a supergiant star and a compact object. We aim to explore the environment of Vela X-1 at a few stellar radii (Rsub *) of the supergiant using spatially resolved observations in the near-infrared, and to study its dynamical evolution along the nine-day orbital period of the system. We propose three possible scenarios for this discrepancy between the two measurements: 1) there is a strong temperature gradient in the supergiant wind, leading to a hot component that is much more compact than the cool part of the wind observed in the Kappa band; 2) we observed a diffuse shell in 2010, possibly triggered by an off-state in the accretion rate of the neutron star that was dissolved in the interstellar medium in 2012 during our second observations; or 3) the structure observed in the H band was the stellar photosphere instead of the supergiant wind.
We describe the technical concept, properties, and performance of HERA (HEterodyne Receiver Array) at the IRAM 30 m telescope. HERA is a multibeam, waveguide SIS receiver that greatly improves ...mapping speed in various observing modes and also provides possibilities for new high-sensitivity observing of small sources. Future extensions with a second polarization module will permit spectro-polarimetry. We present some examples of astronomical maps with HERA.
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