Context :

Aims : This paper reports on H-band interferometric observations of Betelgeuse made at the three-telescope interferometer IOTA .
We image Betelgeuse and its asymmetries to understand the spatial variation of the photosphere , including its diameter , limb darkening , effective temperature , surrounding brightness , and bright ( or dark ) star spots .

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 \pm 0.15 mas with linear and quadratic models and a Rosseland diameter of 45.03 \pm 0.12 mas with a MARCS model .
These measurements lead us to derive an updated effective temperature of 3600 \pm 66 K. We detect a fully-resolved environment to which the silicate dust shell is likely to contribute .
By using two imaging reconstruction algorithms , we unveiled two bright spots on the surface of Betelgeuse .
One spot has a diameter of about 11 mas and accounts for about 8.5 % of the total flux .
The second one is unresolved ( diameter < 9 mas ) with 4.5 % of the total flux .

Conclusions : Resolved images of Betelgeuse in the H band are asymmetric at the level of a few percent .
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 potential signatures of convective cells .