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Context. The inner regions of the discs of high-mass young stellar objects (HMYSOs) are still poorly known due to the small angular scales and the high visual extinction involved. Aims. We deploy near-infrared spectro-interferometry to probe the inner gaseous disc in HMYSOs and investigate the origin and physical characteristics of the CO bandhead emission (2.3-2.4 m). Methods. We present the first GRAVITY/VLTI observations at high spectral (R = 4000) and spatial (mas) resolution of the CO overtone transitions in NGC2024 IRS 2. Results. The continuum emission is resolved in all baselines and is slightly asymmetric, displaying small closure phases (≤8°). Our best ellipsoid model provides a disc inclination of 34° ± 1° a disc major axis position angle (PA) of 166° ± 1° and a disc diameter of 3:99 ± 0:09 mas (or 1.69 ± 0.04 au, at a distance of 423 pc). The small closure phase signals in the continuum are modelled with a skewed rim, originating from a pure inclination effect. For the first time, our observations spatially and spectrally resolve the first four CO bandheads. Changes in visibility, as well as differential and closure phases across the bandheads are detected. Both the size and geometry of the CO-emitting region are determined by fitting a bidimensional Gaussian to the continuum-compensated CO bandhead visibilities. The CO-emitting region has a diameter of 2.74±0:08 0:07 mas (1.16 ± 0.03 au), and is located in the inner gaseous disc, well within the dusty rim, with inclination and PA matching the dusty disc geometry, which indicates that both dusty and gaseous discs are coplanar. Physical and dynamical gas conditions are inferred by modelling the CO spectrum. Finally, we derive a direct measurement of the stellar mass of M• ~ 14:7+2 -3:6 M⊙ by combining our interferometric and spectral modelling results. [ABSTRACT FROM AUTHOR]

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. Aims. 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. Methods. 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). Results. Thirteen disks are resolved well and the visibility curves are fullysampled 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(<10 Mλ) in 8 resolved disks. This is indicative of a significant contribution from an extended (R > 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. Conclusions. 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. [ABSTRACT FROM AUTHOR]

Context. 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". Aims. 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. Methods. It is possible to give an analytical formula for how the baselines and tip-tilt errors affect the astrometric measurement. This formula depends on the limit points of three type of baselines: the wide-angle baseline, the narrow-angle baseline, and the imaging baseline. We also numerically include non-common path, higher order aberrations, whose amplitudes were measured during technical time at the Very Large Telescope Interferometer (VLTI). We end by simulating the influence of high-order, common-path aberrations due to atmospheric residuals calculated from a Monte-Carlo simulation tool for adaptive optics (AO) systems. Results. The result of this work is an error budget of the biases caused by the multiple optical imperfections, including optical dispersion. We show that the beam stabilization through both focal and pupil tracking is crucial to the GRAVITY system. Assuming the instrument pupil is stabilized at a 4 cm level on M1 and a field tracking below 0.2 λ/D, we show that GRAVITY will be able to reach its objective of 10 μas accuracy. [ABSTRACT FROM AUTHOR]

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 X2 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. [ABSTRACT FROM AUTHOR]

Context. 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. However, the geometry and physical properties of their environments on larger scales (up to a few stellar radii) are currently only predicted by simulations but have never been directly observed. Aims. We aim to explore the environment of Vela X-1 at a few stellar radii (R⋆) 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. Methods. We observed Vela X-1 in 2010 and 2012 using near-infrared long baseline interferometry at the Very Large Telescope Interferometer (VLTI), respectively with the AMBER instrument in the K band (medium spectral resolution), and the PIONIER instrument in the H band (low spectral resolution). The PIONIER observations span one orbital period to monitor possible evolutions in the geometry of the system. Results. We resolved a structure of 8 ± 3 R⋆ from the AMBER K-band observations, and 2.0-1.2+0.7 R⋆ from the PIONIER H-band data. From the closure phase observable, we found that the circumstellar environment of Vela X-1 is symmetrical in the near-infrared. We observed comparable interferometric measurements between the continuum and the spectral lines in the K band, meaning that both emissions originate from the same forming region. From the monitoring of the system over one period in the H band in 2012, we found the signal to be constant with the orbital phase within the error bars. Conclusions. 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 K 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. [ABSTRACT FROM AUTHOR]

Studying the inner regions of protoplanetary disks (1-10 AU) is of importance to understand the formation of planets and the accretion process feeding the forming central star. Herbig AeBe stars are bright enough to be routinely observed by Near IR interferometers. The data for the fainter T Tauri stars is much more sparse. In this contribution we present the results of our ongoing survey at the VLTI. We used the PIONIER combiner that allows the simultaneous use of 4 telescopes, yielding 6 baselines and 3 independent closure phases at once. PIONIER's integrated optics technology makes it a sensitive instrument. We have observed 22 T Tauri stars so far, the largest survey for T Tauri stars to this date.Our results demonstrate the very significant contribution of an extended component to the interferometric signal. The extended component is different from source to source and the data, with several baselines, offer a way to improve our knowledge of the disk geometry and/or composition. These results validate an earlier study by Pinte et al. 2008 and show that the dust inner radii of T Tauri disks now appear to be in better agreement with the expected position of the dust sublimation radius, contrary to previous claims. [ABSTRACT FROM PUBLISHER]

In the course of our VLTI young stellar object PIONIER imaging program, we have identified a strong visibility chromatic dependency that appeared in certain sources. This effect, rising value of visibilities with decreasing wavelengths over one base, is also present in previous published and archival AMBER data. For Herbig AeBe stars, the H band is generally located at the transition between the star and the disk predominance in flux for Herbig AeBe stars. We believe that this phenomenon is responsible for the visibility rise effect. We present a method to correct the visibilities from this effect in order to allow "gray" image reconstruction software, like Mira, to be used. In parallel we probe the interest of carrying an image reconstruction in each spectral channel and then combine them to obtain the final broadband one. As an illustration we apply these imaging methods to MWC158, a (possibly Herbig) B[e] star intensively observed with PIONIER. Finally, we compare our result with a parametric model fitted onto the data. [ABSTRACT FROM AUTHOR]

Context. Optical interferometers are subject to many atmospheric and instrumental artifacts that contribute to the degradation of their instrumental contrast, hence their performances. The differential birefringence is, among these effects, one of the trickiest to control, in particular for instrument using fibers, where it can be far larger than the one arising in the optical mirror trains. Several solutions have been tested in the past, ranging from polarization splitting to fiber tweaking. We adopt a new solution for the PIONIER instrument, a four-telescope (4T) combiner at the Very Large Telescope Interferometer (VLTI). Aims. We present a method to cancel the instrumental birefringence in an optical interferometer, allowing the joint detection of the fringe patterns of both polarizations, and substantial gains to be made in both signal-to-noise ratio and readout speed. Methods. A thin (2 mm) plate of birefringent material (LiNbO3) is inserted in each of the four beams. The incidence angle of each plate is adjustable. This allows us to introduce a controlled amount of birefringence in each beam and to cancel the instrumental differential birefringence. We present our derivation of the induced birefringence versus incidence angle and discuss the design choices. Results. Our proposed solution is implemented in the Pionier instrument. Before correction, the instrumental birefringence was of order 5 μm (path length). The adjustment takes about one hour, results in a birefringence of less than 0.1 μm, and is stable for at least the duration of an observing run (several days). Conclusions. We demonstrate on an operational near-infrared interferometer a novel, simple, low-cost, and effective technique to control the differential birefringence. The predictability and stability of the correction make this technique ideal for an automated correction in the VLTI second generation instruments. [ABSTRACT FROM AUTHOR]

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 ) 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 30m 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. [ABSTRACT FROM AUTHOR]

This contribution is intended to provide a brief overview of mm-wave heterodyne instrumentation, of the main categories of components, and of the challenges for continued development. The scope of this overview includes, beyond the receiver itself, the antenna and the atmosphere, that jointly contribute to the achievable sensitivity. It does not include spectrometers, whose technology is not specific to the mm-wave domain, and with which a simple interface (power level, bandwidth) can be defined. This review reports the progress of the community of engineers and scientists devoting themselves to the improvement of mm-wave heterodyne instrumentation; in the course of its preparation, I have benefitted from the experience accumulated during years of collaboration with that community at large, and with my colleagues at IRAM in particular. [ABSTRACT FROM AUTHOR]

The techniques used to set and stabilise the surface of the IRAM 30 m radio telescope to a final root mean square accuracy of about 50 µm are described. This involved both phase retrieval and phase coherent holography using a variety of radiation sources at several frequencies. A finite-element model was utilised in improving the temperature control system for the telescope structure. The factors influencing the ultimate surface accuracy are discussed. [ABSTRACT FROM AUTHOR]

A fully electronically tuned 283–365 GHz local oscillator developed for radio astronomical applications is presented. The source is constructed with a 15.7–20.3 GHz YIG oscillator followed by multipliers and amplifiers. It is shown that eliminating spurious harmonic products is essential in achieving low-noise operation of mixers pumped by this LO. This is done with suitable filters, the design of which is described. [ABSTRACT FROM AUTHOR]

This paper presents the design and characterization of a fixed tuned 220-320 GHz harmonic mixer employing commercial Schottky beam lead diodes in an antiparallel configuration. A diplexer provides distinct ports and matching for the subharmonic LO (26-40GHz) and the IF (20 MHz). The conversion loss was measured using a Gunn + tripler or quadrupler source calibrated with an absolute bolometer. We have built a 220-320 GHz full band sweep Vector Network Analyzer (VNA) using two WR3 waveguide couplers and three identical harmonic mixers (transmitter, reference, and receiver) pumped by two synthesizers (13-20GHz) followed by active doublers. The available dynamic range is 60 dBc/Hz over most of the band. [ABSTRACT FROM AUTHOR]

We have studied a series of flares in McMath 11482, 1972 August 19-22, with particular reference to the basis for the flares and comparison with dekameter radio data. We find that the flares were produced by rapid (∼ 1000 km h) westward motion of a large new p spot. Many flares occur just in front of the spot, and they cease when the motion stops. All flares occurring in front of the spot produce type III bursts, while even strong flares elsewhere in the region produce little or no type III. The time of type III emission agrees perfectly with the start of the Hα flare. Thus type III bursts are only produced in favorable configurations. Simultaneous K-line movies are compared with Hα films and show little difference in flare appearance. [ABSTRACT FROM AUTHOR]

A single photomixer generated the local oscillator signal for a four- element focal plane array of niobium superconducting heterodyne receivers operating at 4 K. The photomixer incorporated an InGaAsP photodiode, driven by two 1.55 µm laser beams, that operated at an ambient temperature of 77 K. Its output signal propagated through a stainless steel waveguide to the 4 K cryostat stage. Distribution of LO power to each superconducting junction was accomplished via 17 dB broadwall waveguide couplers. Double sideband noise temperatures of the array were 60±15 K across the 129–174 GHz local oscillator band. [ABSTRACT FROM AUTHOR]

The close environment of Herbig stars starts to be revealed step by step and it appears to be quite complex. Many physical phenomena interplay: the dust sublimation causing a puffed-up inner rim, a dusty halo, a dusty wind or an inner gaseous component. To investigate more deeply these regions, getting images at the first Astronomical Unit scale is necessary. This has become possible with near infrared instruments on the VLTI. We have developed a new imaging method adapted to young stellar objects where we process separately the stellar component from the rest of the image to reveal the environment by using the spectral differences between these two components. We present the result of this method on the first imaging survey of Herbig stars carried out by PIONIER on the VLTI. [ABSTRACT FROM PUBLISHER]