Abstract

Context. Over the past years, the ?-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 and other properties of low mass cluster members.Aims. In the continuity of our previous studies of young associations cores, we search for ultracool members and new multiple systems within the central 5'.3 (˜0.6 pc) of the cluster. Methods. We obtained deep seeing limited J,Ks-band images of the 5'.3 central part of the cluster with NTT/SofI and H-band images with CAHA/Omega2000. These images were complemented by multi-conjugate adaptive optics (MCAO) H and Ks images of the 1'.5 central region of the ?-Orionis cluster obtained with the prototype MCAO facility MAD at the VLT. The direct vicinity of the massive ?-Ori O8III-star was probed using NACO/SDI at the VLT. Finally, we also retrieved Spitzer IRAC images of the same area and used archival Subaru Suprime-Cam and CFHT CFHT12K i-band images. Results. We report the detection of 9 new member candidates selected from optical and near-IR color-color and color-magnitude diagrams and 7 previously known members. The high spatial resolution images resolve 3 new visual multiple systems. Two of them are most likely not members of the association. The third one is made of a brown dwarf candidate companion to the F8V star HD 36861C. The simultaneous differential images allow us to rule out the presence of visual companions more massive than M < 0.07 M? in the range 1-2.5?, and M < 0.25 M? in the range 0?5-2.5?. © 2009 ESO.