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Published November 1, 2005 | Published + Submitted
Journal Article Open

Multiplicity at the Stellar/Substellar Boundary in Upper Scorpius


We present the results of a high-resolution imaging survey of 12 brown dwarfs and very low mass stars in the closest (~145 pc) young (~5 Myr) OB association, Upper Scorpius. We obtained images with the Advanced Camera for Surveys High Resolution Channel on HST through the F555W (V), F775W (i'), and F850LP (z') filters. This survey discovered three new binary systems, including one marginally resolved pair with a projected separation of only 4.9 AU, resulting in an observed binary fraction of 25% ± 14% at separations ≳4 AU. After correcting for detection biases assuming a uniform distribution of mass ratios for m_s/m_p > 0.6, the estimated binary fraction is 33% ± 17%. The binary fraction is consistent with that inferred for higher mass stars in Upper Sco, but the separation and mass ratio distributions appear to be different. All three low-mass binary systems in Upper Sco are tight (<18 AU) and of similar mass (m_s/m_p ≳ 0.6), consistent with 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 the distinct separation and mass ratio distributions of low-mass systems are set in the formation process or at very young ages, rather than by dynamical disruption of wide systems at ages ≳5 Myr. Finally, we combine the survey detection limits with low-mass evolutionary models to show that there are no planets or very low mass brown dwarfs with masses >10M_J at projected separations >20 AU or masses >5M_J at projected separations >40 AU orbiting any of the low-mass (0.04-0.10 M☉) objects in our sample.

Additional Information

© 2005 The American Astronomical Society. Received 2005 March 11; accepted 2005 July 11. We would like to thank Andrea Ghez for helpful comments, and we thank the anonymous referee for a thorough and helpful response, which improved the quality of this work. This work is based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program 9853.

Attached Files

Published - Kraus_2005_ApJ_633_452.pdf

Submitted - 0507453.pdf


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