Deep imaging survey of the environment of α Centauri I . Adaptive optics imaging of α Cen B with VLT - NACO

Context. α Centauri is our closest stellar neighbor, at a distance of only 1.3 pc, and its two main components have spectral types comparable to the Sun. This is therefore a favorable target for an imaging search for extrasolar planets. Moreover, indications exist that the gravitational mass of α Cen B is higher than its modeled mass, the difference being consistent with a substellar companion of a few tens of Jupiter masses. Aims. We searched for faint comoving companions to α Cen B. As a secondary objective, we built a catalogue of the detected background sources. Methods. We used the NACO adaptive optics system of the VLT in the J, H, and K s bands to search for companions to α Cen B. This instrument allowed us to achieve a very high sensitivity to point-like sources, with a limiting magnitude of m Ks ≈ 18 at 7 " from the star. We complemented this data set with archival coronagraphic images from the HST-ACS instrument to obtain an accurate astrometric calibration. Results. Over the observed area, we did not detect any comoving companion to α Cen B down to an absolute magnitude of 19-20 in the H and K s bands. However, we present a catalogue of 252 background objects within about 15 " of the star. This catalogue fills part of the large void area that surrounds α Cen in sky surveys due to the strong diffused light. We also present a model of the diffused light as a function of angular distance for the NACO instrument, that can be used to predict the background level for bright star observations. Conclusions. According to recent numerical models, the limiting magnitude of our search sets the maximum mass of possible companions to 20-30 times Jupiter, between 7 and 20 AU from α Cen B.