@article {2014A\&A...567A..21S, title = {{Properties of bow-shock sources at the Galactic center}}, journal = {Astronomy and Astrophysics}, volume = {567}, year = {2014}, pages = {A21}, abstract = {Context. There exists an enigmatic population of massive stars around the Galactic center (GC) that were formed some Myr ago. A fraction of these stars has been found to orbit the supermassive black hole, Sgr A*, in a projected clockwise disk-like structure, which suggests that they were formed in a formerly existing dense disk around Sgr A*.
Aims: We focus on a subgroup of objects, the extended, near-infrared (NIR) bright sources IRS 1W, IRS 5, IRS 10W, and IRS 21, that have been suggested to be young, massive stars that form bow shocks through their interaction with the interstellar medium (ISM). Their nature has impeded accurate determinations of their orbital parameters. We aim at establishing their nature and kinematics to test whether they form part of the clockwise disk.
Methods: We performed NIR multiwavelength imaging with NACO/VLT using direct adaptive optics (AO) and AO-assisted sparse aperture masking (SAM). We introduce a new method for self-calibration of the SAM point spread function in dense stellar fields. The emission mechanism, morphology, and kinematics of the targets were examined via 3D models, combined with existing models of the gas flow in the central parsec.
Results: We confirm previous findings that IRS 21, IRS 1W, and IRS 5 are bow-shocks created by the interaction between mass-losing stars and the interstellar gas. The nature of IRS 10W remains unclear. Our modeling shows that the bow-shock emission is caused by thermal emission, while the scattering of stellar light does not play a significant role. IRS 1W shows a morphology that is consistent with a bow shock produced by an anisotropic stellar wind or by locally inhomogeneous ISM density. Our best-fit models provide estimates of the local proper motion of the ISM in the Northern Arm that agree with previously published models that were based on radio interferometry and NIR spectroscopy. Assuming that all of the sources are gravitationally tied to Sagittarius A*, their orbital planes were obtained via a Monte Carlo simulation.
Conclusions: Our sources appear to be Wolf-Rayet stars associated to the last starburst at the GC. Our orbital analysis suggests that they are not part of any of the previously suggested coherent stellar structures, in particular the clockwise disk. We thus add more evidence to recent findings that a large proportion of the massive stars show apparently random orbital orientations, suggesting either that not all of them were formed in the clockwise disk, or that their orbits were randomized rapidly after formation in the disk. Appendix A is available in electronic form at http://www.aanda.org}, keywords = {circumstellar matter, Galaxy: center, outflows, stars: winds, techniques: high angular resolution, techniques: interferometric}, doi = {10.1051/0004-6361/201423657}, url = {http://adsabs.harvard.edu/abs/2014A\%26A...567A..21S}, author = {J. Sanchez-Bermudez and Sch{\"o}del, R. and Alberdi, A. and Muzi{\'c}, K. and Hummel, C.~A. and Pott, J.-U.} } @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 {2012JPhCS.372a2025S, title = {{NaCo/SAM observations of sources at the Galactic Center}}, journal = {Journal of Physics Conference Series}, volume = {372}, number = {1}, year = {2012}, month = {jul}, pages = {012025}, abstract = {Sparse aperture masking (SAM) interferometry combined with Adaptive Optics (AO) is a technique that is uniquely suited to investigate structures near the diffraction limit of large telescopes. The strengths of the technique are a robust calibration of the Point Spread Function (PSF) while maintaining a relatively high dynamic range. We used SAM+AO observations to investigate the circumstellar environment of several bright sources with infrared excess in the central parsec of the Galaxy. For our observations, unstable atmospheric conditions as well as significant residuals after the background subtraction presented serious problems for the standard approach of calibrating SAM data via interspersed observations of reference stars. We circumvented these difficulties by constructing a synthesized calibrator directly from sources within the field-of-view. When observing crowded fields, this novel method can boost the efficiency of SAM observations because it renders interspersed calibrator observations unnecessary. Here, we presented the first NaCo/SAM images reconstructed using this method.}, doi = {10.1088/1742-6596/372/1/012025}, url = {http://adsabs.harvard.edu/abs/2012JPhCS.372a2025S}, author = {J. Sanchez-Bermudez and Sch{\"o}del, R. and Alberdi, A. and Pott, J.~U.} } @article {2011A\&A...532A..83S, title = {{The mean infrared emission of Sagittarius A*}}, journal = {Astronomy and Astrophysics}, volume = {532}, year = {2011}, month = {aug}, pages = {A83}, abstract = {Context. The massive black hole at the center of the Milky Way, Sagittarius A* (Sgr A*) is, in relative terms, the weakest accreting black hole accessible to observations. It has inspired the theoretical models of radiatively inefficient accretion. Unfortunately, our knowledge of the mean SED and source structure of Sgr A* is very limited owing to numerous observational difficulties. At the moment, the mean SED of Sgr A* is only known reliably in the radio to mm regimes.
Aims: The goal of this paper is to provide constraints on the mean emission from Sgr A* in the near-to-mid infrared.
Methods: Sensitive images of the surroundings of Sgr A* at 8.6 {$μ$}m, 4.8 {$μ$}m , and 3.8 {$μ$}m were produced by combining large quantities of imaging data. Images were produced for several observing epochs. Excellent imaging quality was reached in the MIR by using speckle imaging combined with holographic image reconstruction, a novel technique for this kind of data.
Results: No counterpart of Sgr A* is detected at 8.6 {$μ$}m. At this wavelength, Sgr A* is located atop a dust ridge, which considerably complicates the search for a potential point source. An observed 3{$\sigma$} upper limit of \~{}10 mJy is estimated for the emission of Sgr A* at 8.6 {$μ$}m, a tighter limit at this wavelength than in previous work. The de-reddened 3{$\sigma$} upper limit, including the uncertainty of the extinction correction, is \~{}84 mJy . Based on the available data, it is argued that, with currently available instruments, Sgr A* cannot be detected in the MIR, not even during flares. At 4.8 {$μ$}m and 3.8 {$μ$}m, on the other hand, Sgr A* is detected at all times, at least when considering timescales of a few up to 13 min. We derive well-defined time-averaged, de-reddened flux densities of 3.8 {\plusmn} 1.3 mJy at 4.8 {$μ$}m and 5.0 {\plusmn} 0.6 mJy at 3.8 {$μ$}m. Observations with NIRC2/Keck and NaCo/VLT from the literature provide good evidence that Sgr A* also has a fairly well-defined de-reddened mean flux of 0.5-2.5 mJy at wavelengths of 2.1-2.2 {$μ$}m.
Conclusions: We present well-constrained anchor points for the SED of Sgr A* on the high-frequency side of the Terahertz peak. The new data are in general agreement with published theoretical SEDs of the mean emission from Sgr A*, but we expect them to have an appreciable impact on the model parameters in future theoretical work.}, keywords = {black hole physics, Galaxy: center, instrumentation: adaptive optics, instrumentation: high angular resolution}, doi = {10.1051/0004-6361/201116994}, url = {http://adsabs.harvard.edu/abs/2011A\%26A...532A..83S}, author = {Sch{\"o}del, R. and Morris, M.~R. and Muzi{\'c}, K. and Alberdi, A. and Meyer, L. and Eckart, A. and Gezari, D.~Y.} }