A&A 508, 923-932 (2009)

Imaging the spotty surface of Betelgeuse in the H band

¹ LESIA, Observatoire de Paris, 5 place Jules Janssen, 92190 Meudon, France e-mail: xavier.haubois@obspm.fr
² Instituut voor Sterrenkunde, K.U. Leuven, 3001 Leuven, Belgium
³ Office National d'Études et de Recherches Aéronautiques, DOTA, 92322 Chatillon, France
⁴ Centre de Recherche Astrophysique de Lyon, CNRS/UMR 5574, 69561 Saint Genis Laval, France
⁵ Laboratoire d'Astrophysique de Grenoble, CNRS/UMR 5571, 38041 Grenoble, France
⁶ Kitt Peak National Observatory, National Optical Astronomy Observatories, PO Box 26732, Tucson, AZ 85726-6732, USA
⁷ University of Michigan, 941 Dennison Building, 500 Church Street, Ann Arbor, MI 48109-1090, USA
⁸ Caltech/Michelson Science Center, Pasadena, CA, USA
⁹ Jet Propulsion Laboratory, California Institute of Technology, M/S 301-451, 4800 Oak Grove Dr., Pasadena CA, 91109, USA

Received: 18 July 2009
Accepted: 7 October 2009

Abstract

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 ± 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 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.