Multiplicity of very low-mass objects in the Upper Scorpius OB association:: a possible wide binary population

Aims. Properties of multiplicity, such as the multiplicity frequency, the distribution of mass ratios and periods, provide important informations on the formation processes at work. Studying these properties in different environments and at different ages allows us to constrain the models of formation and to understand the influence of the environment on the star formation. Methods. We report the initial results of a VLT/NACO high spatial resolution imaging survey for multiple systems among 58 M-type members of the nearby Upper Scorpius OB association. Results. Nine pairs with separations below 1" have been resolved. Their small angular separations and the similarity in the brightness of the components (Delta Mag(K) < 1 for all of them), indicate there is a reasonable likelihood that several of them are true binaries rather than chance projections. Follow-up imaging observations with WHT/LIRIS of the two widest binaries confirm that their near-infrared colours are consistent with physical very low-mass binaries. For one of these two binaries, WHT/LIRIS spectra of each component were obtained. We find that the two components have similar M6-M7 spectral types and signatures of low-gravity, as expected for a young brown dwarf binary in this association. Conclusions. Our preliminary results indicate a possible population of very low-mass binaries with semimajor axis in the range 100-150 AU, which has not been seen in the Pleiades open cluster. If these candidates are confirmed ( one is confirmed by this work), these results would indicate that the binary properties of very low-mass stars and brown dwarfs may depend on the environment where they form.