| 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 >100 M⊙ 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.
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