  Context. The mass-loss mechanism of cool massive evolved stars is poorly
understood. The proximity of Betelgeuse makes it an appealing target to study
its atmosphere, map the shape of its envelope, and follow the structure of its
wind from the photosphere out to the interstellar medium. Aims. A link is
suspected between the powerful convective motions in Betelgeuse and its mass
loss. We aim to constrain the spatial structure and temporal evolution of the
convective pattern on the photosphere and to search for evidence of this link.
Methods. We report new interferometric observations in the infrared H band
using the VLTI/PIONIER instrument. We monitored the photosphere of Betelgeuse
between 2012 January and 2014 November to look for evolutions that may trigger
the outflow. Results. Our interferometric observations at low spatial
frequencies are compatible with the presence of a hot spot on the photosphere
that has a characteristic width of one stellar radius. It appears to be
superposed on the smaller scale convective pattern. In the higher spatial
frequency domain, we observe a significant difference between the observations
and the predictions of 3D hydrodynamical simulations. Conclusions. We bring new
evidence for the presence of a convective pattern in the photosphere of red
supergiants. The inferred hot spot is probably the top of a giant convection
cell although an asymmetric extension of the star cannot be excluded by these
interferometric observations alone. The properties of the observed surface
features show a stronger contrast and inhomogeneity as predicted by 3D
radiative hydrodynamical simulations. We propose that the large observed
feature is modifying the signature of the convective pattern at the surface of
the star in a way that simulations cannot reproduce.
