By using interferometry, an international team led by an astronomer of Paris Observatory (LESIA) obtained an unprecedented image of the surface of the red supergiant Betelgeuse, in the constellation of Orion. The image reveals the presence of two giant bright spots, whose size is equivalent to the distance Earth-Sun: they cover a large fraction of the surface. It is a first strong and direct indication of the presence of phenomena of convection, transport of heat by the moving matter, in a star other than the Sun. This result allows to better understand the structure and the evolution of supergiants.
Betelgeuse is a red supergiant located in the constellation of Orion. This star is quite different from our Sun: 600 times larger in dimension, it radiates approximately 100 000 times more energy. But following the Sun, this type of object also reveals a surface with bright and dark spots, i.e. hotter and colder spots. These structures would be mainly due to the phenomenon of convection, i.e. the transport of heat by the matter currents. This phenomenon is observed every day in the boiling water. On the surface of the Sun, these spots are rather well-known and visible. However, it is not at all the case for other stars and in particular the supergiants. The size, the physical characteristics and the life time of these dynamical structures remain unknown.
Figure 1: The surface of Betelgeuse in near infrared at 1.64 micron in wavelength, obtained with the interferometre IOTA (Arizona). The image has been re-constructed with two different algorithms, which yield the same details, of 9 mas (milli-arcsecond). The star diametre is about 40 mas. ReferenceX. Haubois, G. Perrin, S. Lacour, T. Verhoelst, S. Meimon, L. Mugnier, E. Thiebaut, J.P. Berger, S.T. Ridgway, J.D. Monnier, R. Millan-Gabet, W. Traub: Imaging the spotty surface of Betelgeuse in the H band, 2009, A&A, 508, 923ContactXavier Haubois (Observatoire de Paris, LESIA, et CNRS) Guy Perrin (Observatoire de Paris, LESIA, et CNRS)
Other images of less quality, of Betelgeuse surface had already been obtained in the past. They were primarily models of the surface constrained from the interferometric data. Now, the researchers have a true image whose richness exceeds what is possible to imagine from a model. For the first time, one can say that two spots are present and determine the size of the largest. Perhaps this difference in dimension correspond to different physical phenomena.
The analysis of the brightness of the spots shows a variation of 500 degrees compared to the average temperature of the star (3 600 Kelvins). The largest of the two structures has a dimension equivalent to the quarter of the star diametre (or one and a half the distance Earth-Sun). This marks a clear difference with the Sun where the cells of convection are much finer and reach hardly 1/20th of the solar radius (a few Earth radius). These characteristics are compatible with the idea of luminous spots produced by the convection. These results constitute a first strong and direct indication of the presence of convection on the surface of a star other than the Sun.