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Title:
Multiplicity of Nearby Free-Floating Ultracool Dwarfs: A Hubble Space Telescope WFPC2 Search for Companions
Authors:
Bouy, Hervé; Brandner, Wolfgang; Martín, Eduardo L.; Delfosse, Xavier; Allard, France; Basri, Gibor
Affiliation:
AA(European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany; ), AB(Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany; ), AC(Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822; ), AD(Laboratoire d'Astrophysique, Observatoire de Grenoble, B.P. 53, 414 rue de la Piscine, F-38400 Saint Martin d'Hère, France; ), AE(Centre de Recherche Astronomique de Lyon (UML 5574), Ecole Normale Supérieure, 46 Allée d'Italie, Lyon, F-69364, Cedex 07, France; ), AF(Department of Astronomy, MC 3411, University of California, Berkeley, Berkeley, CA 94720; )
Publication:
The Astronomical Journal, Volume 126, Issue 3, pp. 1526-1554. (AJ Homepage)
Publication Date:
09/2003
Origin:
UCP
AJ Keywords:
Stars: Binaries: General, Stars: Low-Mass, Brown Dwarfs
DOI:
10.1086/377343
Bibliographic Code:
2003AJ....126.1526B

Abstract

We present Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) observations of a sample of 134 ultracool objects (spectral types later than M7) coming from the Deep Near Infrared Survey (DENIS), Two Micron All Sky Survey (2MASS), and Sloan Digital Sky Survey (SDSS), with distances estimated to range from 7 to 105 pc. Fifteen new ultracool binary candidates are reported here. Eleven known binaries are confirmed, and orbital motion is detected in some of them. We estimate that the closest binary systems in this sample have periods between 5 and 20 yr, and thus dynamical masses will be derived in the near future. For the calculation of binary frequency, we restrict ourselves to systems with distances less than 20 pc. After correction of the binaries bias, we find a ratio of visual binaries (at the HST limit of detection) of around 10%, and that ~15% of the 26 objects within 20 pc are binary systems with separations between 1 and 8 AU. The observed frequency of ultracool binaries is similar to that of binaries with G-type primaries in the separation range from 2.1 to 140 AU. There is also a clear deficit of ultracool binaries with separations greater than 15 AU, and a possible tendency for the binaries to have mass ratios near unity. Most systems have indeed visual and near-infrared brightness ratios between 1 and 0.3. We discuss our results in the framework of current scenarios for the formation and evolution of free-floating brown dwarfs.
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