@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 {2011A\&A...531A..52M, title = {{A model for the thermal radio-continuum emission from radiative shocks in colliding stellar winds}}, journal = {Astronomy and Astrophysics}, volume = {531}, year = {2011}, month = {jul}, pages = {A52}, abstract = {Context. In massive-star binary systems, the interaction of the strong stellar winds results in a wind collision region (WCR) between the stars, which is limited by two shock fronts. Besides the nonthermal emission resulting from the shock acceleration, these shocks emit thermal (free-free) radiation detectable at radio frequencies that increase the expected emission from the stellar winds. Observations and theoretical studies of these sources show that the shocked gas is an important, but not dominant, contributor to the total emission in wide binary systems, while it plays a very substantial role in close binaries.
Aims: The interaction of two isotropic stellar winds is studied in order to calculate the free-free emission from the WCR. The effects of the binary separation and the wind momentum ratio on the emission from the wind-wind interaction region are investigated.
Methods: We developed a semi-analytical model for calculating the thermal emission from colliding stellar winds. Assuming radiative shocks for the compressed layer, which are expected in close binaries, we obtained the emission measure of the thin shell. Then, we computed the total optical depth along each line of sight to obtain the emission from the whole configuration.
Results: Here, we present predictions of the free-free emission at radio frequencies from analytic, radiative shock models in colliding wind binaries. It is shown that the emission from the WCR mainly arises from the optically thick region of the compressed layer and scales as \~{}D$^{4/5}$, where D is the binary separation. The predicted flux density S$_{nu}$ from the WCR becomes more important as the frequency {$ν$} increases, showing higher spectral indices than the expected 0.6 value (S$_{nu}$ {\prop} {$ν$}$^{alpha}$, where {$\alpha$} = 0.6) from the unshocked winds. We also investigate the emission from short-period WR+O systems calculated with our analytic formulation. In particular, we apply the model to the binary systems WR 98 and WR 113 and compare our results with the observations. Our theoretical results are in good agreement with the observed thermal spectra from these sources.}, keywords = {binaries: close, radiation mechanisms: thermal, radio continuum: stars, stars: Wolf-Rayet}, doi = {10.1051/0004-6361/201015988}, url = {http://adsabs.harvard.edu/abs/2011A\%26A...531A..52M}, author = {Montes, G. and Gonz{\'a}lez, R.~F. and Cant{\'o}, J. and M. A. P{\'e}rez-Torres and Alberdi, A.} }