A deep look into the cores of young clusters I. σ-Orionis

Context. Nearby young clusters are privileged places to study the star formation history. Over the last decade, the sigma-Orionis cluster has been a prime location for the study of young very low mass stars, substellar and isolated planetary mass objects and the determination of the initial mass function. Aims. To extend previous studies of this association to its core, we searched for ultracool members and new multiple systems within the 1.'5 x 1.'5 central region of the cluster. Methods. We obtained deep multi-conjugate adaptive optics (MCAO) images of the core of the sigma-Orionis cluster with the prototype MCAO facility MAD at the VLT using the H and K-s filters. These images allow us to detect companions fainter by Delta H approximate to 5 mag as close as 0 ''.2 on a typical source with H = 14.5 mag. These images were complemented by archival SofI K-s-band images and Spitzer IRAC and MIPS mid-infrared images Results. We report the detection of 2 new visual multiple systems, one being a candidate binary proplyd and the other one a low mass companion to the massive star sigma Ori E. Of the 36 sources detected in the images, 25 have a H-band luminosity lower than the expected planetary mass limit for members, and H - K-s color consistent with the latest theoretical isochrones. Nine objects have additional Spitzer photometry and spectral energy distribution consistent with them being cluster members. One of them has a spectral energy distribution from H to 3.6 mu m consistent with that of a 5.5 M-Jup cluster member. Complementary NTT/SofI and Spitzer photometry allow us to confirm the nature and membership of two L-dwarf planetary mass candidates.