  The study of the atmosphere of red supergiant stars in general and of
Betelgeuse (alpha Orionis) in particular is of prime importance to understand
dust formation and how mass is lost to the interstellar medium in evolved
massive stars. A molecular shell, the MOLsphere (Tsuji, 2000a), in the
atmosphere of Betelgeuse has been proposed to account for the near- and
mid-infrared spectroscopic observations of Betelgeuse. The goal is to further
test this hypothesis and to identify some of the molecules in this MOLsphere.
We report on measurements taken with the mid-infrared two-telescope beam
combiner of the VLTI, MIDI, operated between 7.5 and 13.5 $\mu$m. The data are
compared to a simple geometric model of a photosphere surrounded by a warm
absorbing and emitting shell. Physical characteristics of the shell are
derived: size, temperature and optical depth. The chemical constituents are
determined with an analysis consistent with available infrared spectra and
interferometric data. We are able to account for the measured optical depth of
the shell in the N band, the ISO-SWS spectrum and K and L band interferometric
data with a shell whose inner and outer radii are given by the above range and
with the following species: H2O, SiO and Al2O3. These results confirm the
MOLsphere model. We bring evidence for more constituents and for the presence
of species participating in the formation of dust grains in the atmosphere of
the star, i.e. well below the distance at which the dust shell is detected. We
believe these results bring key elements to the understanding of mass loss in
Betelgeuse and red supergiants in general and bring support to the dust-driven
scenario.
