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

Abstract

The Upper Scorpius association is the OB association nearest to the Sun (145 pc). Its young age (∼5 Myr) makes it an ideal place to search for low-mass stars and brown dwarfs, as these objects should be relatively bright. We have performed 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 the saturation limit is I ∼ 13. We obtain 138 candidate members, covering nearly the entire M spectral type range. We find an excess of brown dwarf candidates over the number predicted by a Miller-Scalo initial mass function. In addition, we have performed infrared imaging and low-resolution optical spectroscopy of selected candidates. We find that the infrared observations confirm the spectral types obtained with the optical photometry. Furthermore, we find Hα in emission in 20 of the 22 objects observed spectroscopically. As Hα is an indicator of youth, we believe that these 20 objects may belong to the association. One of them, UScoCTIO 128, has a very strong and constant Hα line (equivalent width -130 Å), and its position in the color-magnitude diagram suggests that it is a brown dwarf with mass 0.02 M⊙. Confirmation of this and the other candidates will have to wait for higher resolution observations that can reveal spectroscopic mass indicators such as Li I and gravity indicators such as K I and the subordinate lines of Na I.