@article {2014A\&A...572L...1S, title = {{Resolving the stellar components of the massive multiple system Herschel 36 with AMBER/VLTI}}, journal = {Astronomy and Astrophysics}, volume = {572}, year = {2014}, pages = {L1}, abstract = {Context. Massive stars are extremely important for the evolution of the galaxies; there are large gaps in our understanding of their properties and formation, however, mainly because they evolve rapidly, are rare, and distant. Recent findings suggest that most O-stars belong to multiple systems. It may well be that almost all massive stars are born as triples or higher multiples, but their large distances require very high angular resolution to directly detect the companions at milliarcsecond scales.
Aims: Herschel 36 is a young massive system located at 1.3 kpc. It has a combined smallest predicted mass of 45 M$_{\&sun;}$. Multi-epoch spectroscopic data suggest the existence of at least three gravitationally bound components. Two of them, system Ab, are tightly bound in a spectroscopic binary, and the third one, component Aa, orbits in a wider orbit. Our aim was to image and obtain astrometric and photometric measurements of components Aa and Ab using, for the first time, long-baseline optical interferometry to further constrain its nature.
Methods: We observed Herschel 36 with the near-infrared instrument AMBER attached to the ESO VLT Interferometer, which provides an angular resolution of \~{}2 mas. We used the code BSMEM to perform the interferometric image reconstruction. We fitted the interferometric observables using proprietary IDL routines and the code LitPro.
Results: We imaged the Aa + Ab components of Herschel 36 in H and K filters. Component Ab is located at a projected distance of 1.81 mas, at a position angle of \~{}222{\deg} east of north, the flux ratio between components Aa and Ab is close to one. These findings agree with previous predictions about the properties of Herschel 36. The small measured angular separation indicates that system Ab and Ab may be approaching the periastron of their orbits. These results, only achievable with long-baseline near-infrared interferometry, constitute the first step toward a thorough understanding of this massive triple system.}, keywords = {binaries: close, instrumentation: high angular resolution, instrumentation: interferometers, stars: massive}, doi = {10.1051/0004-6361/201424901}, url = {http://adsabs.harvard.edu/abs/2014A\%26A...572L...1S}, author = {J. Sanchez-Bermudez and Alberdi, A. and Sch{\"o}del, R. and Hummel, C.~A. and Arias, J.~I. and Barb{\'a}, R.~H. and Ma{\'\i}z Apell{\'a}niz, J. and Pott, J.-U.} } @article {2013A\&A...554L...4S, title = {{Direct detection of the tertiary component in the massive multiple HD 150136 with VLTI}}, journal = {Astronomy and Astrophysics}, volume = {554}, year = {2013}, pages = {L4}, abstract = {Context. Massive stars are of fundamental importance for almost all aspects of astrophysics, but there still exist large gaps in our understanding of their properties and formation because they are rare and therefore distant. It has been found that most O-stars are multiples. It may well be that almost all massive stars are born as triples or higher multiples, but their large distances require milliarcsecond angular resolution for a direct detection of the companions.
Aims: HD 150136 is the nearest system to Earth with {\gt}100 M$_{\&sun;}$ and provides a unique opportunity to study an extremely massive system. Recently, evidence for the existence of a third component in HD 150136, in addition to the tight spectroscopic binary that forms the main component, was found in spectroscopic observations. Our aim was to image and obtain astrometric and photometric measurements of this component using long-baseline optical interferometry to further constrain the nature of this component.
Methods: We observed HD 150136 with the near-infrared instrument AMBER attached to the ESO VLT Interferometer, which provides an angular resolution of 2 mas. The recovered closure phases are robust to systematic errors and provide unique information on the source asymmetry. Therefore, they are of crucial relevance for both image reconstruction and model fitting of the source structure.
Results: The third component in HD 150136 is clearly detected in the high-quality data from AMBER. It is located at a projected angular distance of 7.3 mas, or about 13 AU at the line-of-sight distance of HD 150136, at a position angle of 209 degrees east of north, and has a flux ratio of 0.25 with respect to the inner binary. Our findings agree with previous results and have permitted us to improve the orbital solutions of the tertiary around the inner system.
Conclusions: We resolved the third component of HD 150136 in J, H and K filters. The luminosity and color of the tertiary agrees with the predictions and shows that it is also an O main-sequence star. The small measured angular separation indicates that the tertiary may be approaching the periastron of its orbit. These results, only achievable with long-baseline near-infrared interferometry, constitute the first step toward understanding the massive star formation mechanisms.}, keywords = {binaries: general, instrumentation: high angular resolution, instrumentation: interferometers, stars: massive}, doi = {10.1051/0004-6361/201321649}, url = {http://adsabs.harvard.edu/abs/2013A\%26A...554L...4S}, author = {J. Sanchez-Bermudez and Sch{\"o}del, R. and Alberdi, A. and Barb{\'a}, R.~H. and Hummel, C.~A. and Ma{\'\i}z Apell{\'a}niz, J. and Pott, J.-U.} } @article {2012A\&A...541A.135M, title = {{Over-resolution of compact sources in interferometric observations}}, journal = {Astronomy and Astrophysics}, volume = {541}, year = {2012}, month = {may}, pages = {A135}, abstract = {We review the effects of source size in interferometric observations and focus on the cases of very compact sources. If a source is extremely compact and/or weak (such that it is impossible to detect signatures of source structure in the visibilities), we describe a test of hypothesis that can be used to set a strong upper limit to the size of the source. We also estimate the minimum possible size of a source whose structure can still be detected by an interferometer (i.e., the maximum theoretical over-resolution power of an interferometer), which depends on the overall observing time, the compactness of the array distribution, and the sensitivity of the receivers. As a result, and depending on the observing frequency, the over-resolution power of forthcoming ultra-sensitive arrays, such as the Square Kilometer Array (SKA), may allow us to study the details of sources at angular scales as small as a few {$μ$}as.}, keywords = {instrumentation: interferometers, techniques: high angular resolution, techniques: interferometric}, doi = {10.1051/0004-6361/201118334}, url = {http://adsabs.harvard.edu/abs/2012A\%26A...541A.135M}, author = {Mart{\'\i}-Vidal, I. and M. A. P{\'e}rez-Torres and Lobanov, A.~P.} } @article {2012A\&A...537A..93R, title = {{VLBI imaging of M 81* at 43 GHz}}, journal = {Astronomy and Astrophysics}, volume = {537}, year = {2012}, month = {jan}, pages = {A93}, abstract = {Context. The nearby spiral galaxy M 81 harbours in its core a low-luminosity active galactic nucleus (LLAGN), and appears to be closely related to the more distant and powerful active galactic nuclei (AGNs) seen in quasars and radio galaxies. The intrinsic size of this object is unknown because of too much scattering, and it has shown a core-jet morphology with weak extended emission rotating with wavelength.
Aims: The proximity of M 81 (D = 3.63 Mpc) allows for a detailed investigation of its nucleus. The nucleus is four orders of magnitude more luminous than the Galactic centre, and is therefore considered to be a link between Sgr A$^{*}$ and the more powerful nuclei of radio galaxies and quasars. Our main goal was to determine the size of M 81{\l}owast} at a shorter wavelength to directly test whether the frequency-size dependent law {$\Theta$} {\prop} {$ν$}$^{-0.8}$ is still valid for wavelengths shorter than 1 cm. In addition, we also aimed to confirm the rotation of the source as a function of frequency.
Methods: We observed the continuum 7 mm radio emission of M 81{\l}owast} with the Very Long Baseline Array on Sep. 13, 2002, using nearby calibrators to apply their interferometric observables to the target source to increase the chances of detection. The source was detected on all baselines and hybrid mapping was possible.
Results: We present the first 7 mm VLBI image of the core of M 81{\l}owast}, which is the highest resolution image ever of this source. Modelling the interferometric visibilities with two Gaussian functions sets constraints on the angular size of its core down to 38 microarcseconds, corresponding to a maximum (projected) linear size of 138 AU, and shows extended emission towards the NE with a position angle of \~{}50{\deg}. A fit of a one Gaussian elliptical function yields a position angle of 28 {\plusmn} 8 degrees for its elongated, compact structure. Combining the 7 mm size with earlier measurements at other frequencies, we determine a frequency-size dependence of {$\Theta$} {\prop} {$ν$}$^{(-0.88 {\textpm} 0.04)}$.
Conclusions: Our VLBI imaging of M 81{\l}owast} has clearly detected its core-jet structure, and has allowed us to estimate a size for its core, with a minimum size of 138 AU ({\ap}100 Schwarzschild radii). Our work opens the way for future observations of M 81{\l}owast} at high-angular resolution, including the monitoring of its structure, given that much higher bandwidths are currently available on the interferometric networks. In particular, this would allow one to test for possible proper motions of the core or of its components in the inner jet of M 81{\l}owast}, and for the speed of the detected jet components.}, keywords = {galaxies: active, galaxies: individual: M 81, galaxies: nuclei, instrumentation: interferometers, radio continuum: galaxies}, doi = {10.1051/0004-6361/201118399}, url = {http://adsabs.harvard.edu/abs/2012A\%26A...537A..93R}, author = {Ros, E. and M. A. P{\'e}rez-Torres} } @article {2011A\&A...533C...1M, title = {{Coherence loss in phase-referenced VLBI observations (Corrigendum)}}, journal = {Astronomy and Astrophysics}, volume = {533}, year = {2011}, month = {sep}, pages = {C1}, abstract = {We report an erratum in an equation that models the relation of the limiting dynamic range to the angular distance between target and calibrator.}, keywords = {addenda, atmospheric effects, errata, instrumentation: interferometers, techniques: interferometric}, doi = {10.1051/0004-6361/201014203e}, url = {http://adsabs.harvard.edu/abs/2011A\%26A...533C...1M}, author = {Mart{\'\i}-Vidal, I. and Ros, E. and M. A. P{\'e}rez-Torres and Guirado, J.~C. and Jim{\'e}nez-Monferrer, S. and Marcaide, J.~M.} } @article {2010A\&A...515A..53M, title = {{Coherence loss in phase-referenced VLBI observations}}, journal = {Astronomy and Astrophysics}, volume = {515}, year = {2010}, month = {jun}, pages = {A53}, abstract = {Context. Phase-referencing is a standard calibration technique in radio interferometry, particularly suited for the detection of weak sources close to the sensitivity limits of the interferometers. However, effects from a changing atmosphere and inaccuracies in the correlator model may affect the phase-referenced images, and lead to wrong estimates of source flux densities and positions. A systematic observational study of signal decoherence in phase-referencing and its effects in the image plane has not been performed yet.
Aims: We systematically studied how the signal coherence in Very-Long-Baseline-Interferometry (VLBI) observations is affected by a phase-reference calibration at different frequencies and for different calibrator-to-target separations. The results obtained should be of interest for a correct interpretation of many phase-referenced observations with VLBI.
Methods: We observed a set of 13 strong sources (the S5 polar cap sample) at 8.4 and 15 GHz in phase-reference mode with 32 different calibrator/target combinations spanning angular separations between 1.5 and 20.5 degrees. We obtained phase-referenced images and studied how the dynamic range and peak flux-density depend on observing frequency and source separation.
Results: We obtained dynamic ranges and peak flux densities of the phase-referenced images as a function of frequency and separation from the calibrator. We compared our results with models and phenomenological equations previously reported.
Conclusions: The dynamic range of the phase-referenced images is strongly limited by the atmosphere at all frequencies and for all source separations. The limiting dynamic range is inversely proportional to the sine of the calibrator-to-target separation. Not surpriseingly, we also find that the peak flux densities decrease with source separation, relative to those obtained from the self-calibrated images.}, keywords = {atmospheric effects, instrumentation: interferometers, techniques: interferometric}, doi = {10.1051/0004-6361/201014203}, url = {http://adsabs.harvard.edu/abs/2010A\%26A...515A..53M}, author = {Mart{\'\i}-Vidal, I. and Ros, E. and M. A. P{\'e}rez-Torres and Guirado, J.~C. and Jim{\'e}nez-Monferrer, S. and Marcaide, J.~M.} }