Context. This paper reports on H band interferometric observations of Betelgeuse made at the three-telescope interferometer IOTA.
Aims. We aimed at imaging Betelgeuse and its asymmetries to understand their nature.
Methods. We used different theoretical simulations of the photosphere and dusty environment to model the visibility data. We made images with parametric modeling and two image reconstruction algorithms: MIRA and WISARD.
Results. We measure an average limb-darkened diameter of 44.28 +/- 0.15 mas with linear and quadratic models and a Rosseland diameter of 45.03 +/- 0.12 mas with a MARCS model. These measurements lead us to derive an updated effective temperature of 3600 +/-66 K. We detect a fully-resolved environment to which the silicate dust shell is likely to contribute. By the use of two imaging reconstruction algorithms, we finally unveiled the presence of two bright spots on the surface of Betelgeuse. One spot has an extension of about 11 mas and accounts for about 8.5% of the total flux. The second one is unresolved with 4.5% of the total flux.
Conclusions. Resolved images of Betelgeuse in the H band are asymmetric at the few percent level. The MOLsphere is not detected in this wavelength range. The amount of measured limb-darkening is in good agreement with model predictions. The two spots imaged at the surface of the star are likely to be interpreted as the signature of convective cells.