AAS 205th Meeting, 9-13 January 2005
Session 11 Brown Dwarfs and Exoplanets
Poster, Monday, January 10, 2005, 9:20am-6:30pm, Exhibit Hall

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[11.15] Multiplicity at the Stellar/Substellar Boundary in Upper Scorpius

A. L. Kraus, R. J. White, L. A. Hillenbrand (Caltech), A. M. Ghez (UCLA)

We present the results of an imaging survey of 12 brown dwarfs and very low mass stars in the young (~5 Myr) OB association Upper Scorpius. We obtained high spatial resolution images with the Advanced Camera for Surveys/High Resolution Camera on HST in the optical filters V, i', and z'. This survey discovered three new binary brown dwarf systems with separations of <18 AU and flux ratios \DeltaIC<1.0. The resulting binary fraction is 25 ± 14%. After correcting for detection biases, the estimated binary fraction is 42 ±19%. This binary fraction is consistent with higher-mass stars in Upper Sco, but the separation and flux ratio distributions appear to be different. All three systems are close and have similar mass ratios (q>0.5), which matches expectations based on previous multiplicity studies of brown dwarfs and very low mass stars in the field and in open clusters. The implication is that low mass systems only form at close separations, rather than over a large range of separations with subsequent dynamical disruption of the widest systems. Finally, based on detection thresholds, we find that there are no planets with mass >5 MJ at projected separations >40 AU or >10 MJ at >20 AU.

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© 2004. The American Astronomical Society.