Deep near-infrared interferometric search for low-mass companions around β Pictoris

Aims. We search for low-mass companions in the innermost region (<300 mas, i.e., 6 AU) of the beta Pic planetary system. Methods. We obtained interferometric closure phase measurements in the K-band with the VLTI/AMBER instrument used in its medium spectral resolution mode. Fringe stabilization was provided by the FINITO fringe tracker. Results. In a search region of between 2 and 60 mas in radius, our observations exclude at 3 sigma significance the presence of companions with K-band contrasts greater than 5 x 10(-3) for 90% of the possible positions in the search zone (i.e., 90% completeness). The median 1 sigma error bar in the contrast of potential companions within our search region is 1.2 x 10(-3). The best fit to our data set using a binary model is found for a faint companion located at about 14.4 mas from beta Pic, which has a contrast of 1.8 x 10(-3) +/- 1.1 x 10(-3) (a result consistent with the absence of companions). For angular separations larger than 60 mas, both time smearing and field-of-view limitations reduce the sensitivity. Conclusions. We can exclude the presence of brown dwarfs with masses higher than 29 M(Jup) (resp. 47 M(Jup)) at a 50% (resp. 90%) completeness level within the first few AUs around beta Pic. Interferometric closure phases offer a promising way to directly image low-mass companions in the close environment of nearby young stars.