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Context: Betelgeuse is one the largest stars in the sky in terms of angulardiameter. Structures on the stellar photosphere have been detected in thevisible and near-infrared as well as a compact molecular environment called theMOLsphere. Mid-infrared observations have revealed the nature of some of themolecules in the MOLsphere, some being the precursor of dust. Aims: Betelgeuseis an excellent candidate to understand the process of mass loss in redsupergiants. Using diffraction-limited adaptive optics (AO) in thenear-infrared, we probe the photosphere and close environment of Betelgeuse tostudy the wavelength dependence of its extension, and to search forasymmetries. Methods: We obtained AO images with the VLT/NACO instrument,taking advantage of the "cube" mode of the CONICA camera to record separately alarge number of short-exposure frames. This allowed us to adopt a "luckyimaging" approach for the data reduction, and obtain diffraction-limited imagesover the spectral range 1.04-2.17 $\mu$m in 10 narrow-band filters. Results: Inall filters, the photosphere of Betelgeuse appears partly resolved. We identifyan asymmetric envelope around the star, with in particular a relatively bright"plume" extending in the southwestern quadrant up to a radius of approximatelysix times the photosphere. The CN molecule provides an excellent match to the1.09 mic bandhead in absorption in front of the stellar photosphere, but theemission spectrum of the plume is more difficult to interpret. Conclusions: OurAO images show that the envelope surrounding Betelgeuse has a complex andirregular structure. We propose that the southwestern plume is linked either tothe presence of a convective hot spot on the photosphere, or to the rotation ofthe star.

The close circumstellar environment of Betelgeuse - Adaptive optics spectro-imaging in the near-IR with VLT/NACO