Discovery of a wide, low-mass binary system in Upper Scorpius

Using the near-infrared spectrometer SpeX and its slit-viewing camera at the Infrared Telescope Facility, I have resolved a low-mass member of the Upper Scorpius OB association into a double star. From K-band images of the pair, DENIS-P J161833.2-251750.4 A and B, I measure a separation of 0″.96 and a magnitude difference of ΔK = 0.42 mag. I present resolved 0.8-2.5 μm spectroscopy of the two objects, both of which exhibit signatures of youth in the shape of their H- and K-band continua, demonstrating that both are members of Upper Scorpius, rather than field stars. In addition, through a comparison with optically classified pre-main-sequence objects, I derive a spectral type near M5 for each component, corresponding to a mass of ∼0.15 M _⊙ with the evolutionary models of Chabrier & Baraffe. The probability that this pair is composed of unrelated M-type members of Upper Scorpius is ∼10^-5. When added to the recent discoveries of other wide, easily disrupted low-mass binaries, this new system further establishes that the formation of low-mass stars and brown dwarfs does not require ejection from multiple systems. These observations also indicate that wide, low-mass binaries can form in OB associations as well as in smaller clusters where the previously known wide pairs have been found. Thus, the available data show no perceptible effect of star-forming environment on the prevalence of loosely bound low-mass systems.