Deep search for companions to probable young brown dwarfs - VLT/NACO adaptive optics imaging using IR wavefront sensing

Aims. We have obtained high contrast images of four nearby, faint, and very low mass objects 2MASS J04351455-1414468, SDSS J044337.61+000205.1, 2MASS J06085283-2753583 and 2MASS J06524851-5741376 (hereafter 2MASS0435-14, SDSS0443+00, 2MASS0608-27 and 2MASS0652-57), identified in the field as probable isolated young brown dwarfs. Our goal was to search for binary companions down to the planetary mass regime. Methods. We used the NAOS-CONICA adaptive optics instrument (NACO) and its unique capability to sense the wavefront in the near-infrared to acquire sharp images of the four systems in K_s, with a field of view of 28" x 28". Additional J and L' imaging and follow-up observations at a second epoch were obtained for 2MASS0652-57. Results. With a typical contrast ΔK_s = 4.0−7.0 mag, our observations are sensitive down to the planetary mass regime considering a minimum age of 10 to 120 Myr for these systems. No additional point sources are detected in the environment of 2MASS0435-14, SDSS0443+00 and 2MASS0608-27 between 0.1−12" (i.e. about 2 to 250 AU at 20 pc). 2MASS0652-57 is resolved as a ~230 mas binary. Follow-up observations reject a background contaminate, resolve the orbital motion of the pair, and confirm with high confidence that the system is physically bound. The J, K_s and L′ photometry suggest a q ~ 0.7−0.8 mass ratio binary with a probable semi-major axis of 5−6 AU. Among the four systems, 2MASS0652-57 is probably the less constrained in terms of age determination. Further analysis would be necessary to confirm its youth. It would then be interesting to determine its orbital and physical properties to derive the system’s dynamical mass and to test evolutionary model predictions.