  The physical mechanism through which the outgoing material of massive red
supergiants is accelerated above the escape velocity is unclear. Thanks to the
transparency of its circumstellar envelope, the nearby red supergiant
Betelgeuse gives the opportunity to probe the innermost layers of the envelope
of a typical red supergiant down to the photosphere, i.e. where the
acceleration of the wind is expected to occur. We took advantage of the
SPHERE/ZIMPOL adaptive optics imaging polarimeter to resolve the visible
photosphere and close envelope of Betelgeuse. We detect an asymmetric gaseous
envelope inside a radius of 2 to 3 times the near-infrared photospheric radius
of the star (R*), and a significant Halpha emission mostly contained within 3
R*. From the polarimetric signal, we also identify the signature of dust
scattering in an asymmetric and incomplete dust shell located at a similar
radius. The presence of dust so close to the star may have a significant impact
on the wind acceleration through radiative pressure on the grains. The 3 R*
radius emerges as a major interface between the hot gaseous and dusty
envelopes. The detected asymmetries strengthen previous indications that the
mass loss of Betelgeuse is likely tied to the vigorous convective motions in
its atmosphere.
