  Context: Betelgeuse is one the largest stars in the sky in terms of angular
diameter. Structures on the stellar photosphere have been detected in the
visible and near-infrared as well as a compact molecular environment called the
MOLsphere. Mid-infrared observations have revealed the nature of some of the
molecules in the MOLsphere, some being the precursor of dust. Aims: Betelgeuse
is an excellent candidate to understand the process of mass loss in red
supergiants. Using diffraction-limited adaptive optics (AO) in the
near-infrared, we probe the photosphere and close environment of Betelgeuse to
study the wavelength dependence of its extension, and to search for
asymmetries. Methods: We obtained AO images with the VLT/NACO instrument,
taking advantage of the "cube" mode of the CONICA camera to record separately a
large number of short-exposure frames. This allowed us to adopt a "lucky
imaging" approach for the data reduction, and obtain diffraction-limited images
over the spectral range 1.04-2.17 $\mu$m in 10 narrow-band filters. Results: In
all filters, the photosphere of Betelgeuse appears partly resolved. We identify
an asymmetric envelope around the star, with in particular a relatively bright
"plume" extending in the southwestern quadrant up to a radius of approximately
six times the photosphere. The CN molecule provides an excellent match to the
1.09 mic bandhead in absorption in front of the stellar photosphere, but the
emission spectrum of the plume is more difficult to interpret. Conclusions: Our
AO images show that the envelope surrounding Betelgeuse has a complex and
irregular structure. We propose that the southwestern plume is linked either to
the presence of a convective hot spot on the photosphere, or to the rotation of
the star.
