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The study of the atmosphere of red supergiant stars in general and ofBetelgeuse (alpha Orionis) in particular is of prime importance to understanddust formation and how mass is lost to the interstellar medium in evolvedmassive stars. A molecular shell, the MOLsphere (Tsuji, 2000a), in theatmosphere of Betelgeuse has been proposed to account for the near- andmid-infrared spectroscopic observations of Betelgeuse. The goal is to furthertest this hypothesis and to identify some of the molecules in this MOLsphere.We report on measurements taken with the mid-infrared two-telescope beamcombiner of the VLTI, MIDI, operated between 7.5 and 13.5 $\mu$m. The data arecompared to a simple geometric model of a photosphere surrounded by a warmabsorbing and emitting shell. Physical characteristics of the shell arederived: size, temperature and optical depth. The chemical constituents aredetermined with an analysis consistent with available infrared spectra andinterferometric data. We are able to account for the measured optical depth ofthe shell in the N band, the ISO-SWS spectrum and K and L band interferometricdata with a shell whose inner and outer radii are given by the above range andwith the following species: H2O, SiO and Al2O3. These results confirm theMOLsphere model. We bring evidence for more constituents and for the presenceof species participating in the formation of dust grains in the atmosphere ofthe star, i.e. well below the distance at which the dust shell is detected. Webelieve these results bring key elements to the understanding of mass loss inBetelgeuse and red supergiants in general and bring support to the dust-drivenscenario.

The molecular and dusty composition of Betelgeuse's inner circumstellar environment