A survey for low-mass stars and brown dwarfs in the Upper Scorpius OB association

Upper Scorpius association is the OB association nearest to the Sun (145 pc ). Its young age (similar to 5 Myr) makes it an ideal place to search for l ow-mass stars and brown dwarfs, as these objects should be relatively brigh t. We have performed a photometric search for the low-mass members of the a ssociation, using the R, I, and Z filters. The completeness limit is I simi lar to 18.5, and the saturation limit is I similar to 13. We obtain 138 can didate members, covering nearly the entire M spectral type range. We find a n excess of brown dwarf candidates over the number predicted by a Miller-Sc ale initial mass function. In addition, we have performed infrared imaging and low-resolution optical spectroscopy of selected candidates. We find tha t the infrared observations confirm the spectral types obtained with the op tical photometry. Furthermore, we find Ha in emission in 20 of the 22 objec ts observed spectroscopically. As H alpha is an indicator of youth, we beli eve that these 20 objects may belong to the association. One of them, UScoC TIO 128, has a very strong and constant H alpha line (equivalent width -130 Angstrom), and its position in the color-magnitude diagram suggests that i t is a brown dwarf with mass 0.02 M.. Confirmation of this and the other ca ndidates will have to wait for higher resolution observations that can reve al spectroscopic mass indicators such as Li I and gravity indicators such a s K I and the subordinate lines of Na I.