A search for planetary-mass objects and brown dwarfs in the Upper Scorpius association

Abstract

We report the results of a deep photometric search for planets and brown dwarfs in the nearby young OB Upper Scorpius association. We obtained optical (I) and near-infrared (JKs) images around nine very low-mass stars and brown dwarf member candidates of the association, covering a total area of 113 arcmin^2. Using a point spread function subtraction technique, we have searched for planetary-mass companions (0.002 Msol 0.007 Msol) at physical distances larger than 600AU. From the I, I-J colour-magnitude diagrams and follow-up Ks-band photometry, we identify four very red objects (I-J > 2.5) in the area of the survey. According to their positions in the diagram, below the expected theoretical sequence of the cluster, three are probable late M-dwarfs in the field. The faintest and reddest object (J-Ks = 2.39 +/- 0.06) may be either a field intermediate L-dwarf with anomalously red J-Ks colour at a distance of approx. 135pc, an extremely red distant galaxy, or a reddened planetary-mass object in the Upper Scorpius association.