@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 {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.} }