A Survey for Low-Mass Stars and Brown Dwarfs in the Upper Scorpius OB Association

The Upper-Scorpius association is the OB association nearest to the Sun (145pc). Its young age (5 Myr) makes it an ideal place to search for low-mass starsand brown dwarfs, as these objects should be relatively bright. We haveperformed a photometric search for the low-mass members of the association,using the R, I, and Z filters. The completeness limit is I=18.5 and thesaturation limit is I=13. We obtain 138 candidate members, covering nearly theentire M spectral type range. We find an excess of brown dwarf candidates overthe number predicted by a Miller-Scalo Initial Mass Function. In addition, wehave performed infrared imaging and low resolution optical spectroscopy ofselected candidates. We find that the infrared observations confirm thespectral types obtained with the optical photometry. Furthermore, we findH_alpha in emission in 20 of the 22 objects observed spectroscopically. AsH_alpha is an indicator of youth, we believe that these 20 objects may belongto the association. One of them, UScoCTIO 128 has a very strong and constantH_alpha line (equivalent width: -130 A), and its position in the colormagnitude diagram suggests that it is a brown dwarf with mass equal to 0.02msun. Confirmation of this and the other candidates will have to wait forhigher resolution observations that can reveal spectroscopic mass indicatorslike Li I and gravity indicators, such as K I and the subordinate lines of NaI.Comment: 20 pages, 7 figures, 3 tables, accepted in the Astronomical Journa