Multiplicity of Nearby Free-floating Ultracool Dwarfs: AHubble Space TelescopeWFPC2 Search for Companions

Abstract

We present HST/WFPC2 observations of a sample of 134 ultra-cool objects (spectral types later than M7) coming from the DENIS, 2MASS and SDSS surveys, with distances estimated to range from 7 pc 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 years, 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 parsecs are binary systems with separations between 1 and 8 A.U. The observed frequency of ultra-cool binaries is similar than that of binaries with G-type primaries in the separation range from 2.1 A.U. to 140 A.U. There is also a clear deficit of ultra-cool binaries with separations greater than 15 A.U., 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.

Spectroscopic characterization of 78 DENIS ultracool dwarf candidates in the solar neighborhood and the Upper Scorpii OB association

“…Three of them, namely DENIS J0314352−462341, DENIS J0301488−590302, and DENIS J1216121−125731 had spectral types estimated from I − J colors (Bouy et al 2003). Our spectral type determination supersedes theirs, and underscores the limitations of using color-spectral class relations for ultracool dwarfs.…”

The ultracool-field dwarf luminosity-function and space density from the Canada-France Brown Dwarf Survey

“…This also affects the colour of the objects. Direct imaging surveys (Gizis et al 2003;Bouy et al 2003) that probe separations of >2 AU show that ∼10−15% of field brown dwarfs are binaries with separations that peak between 2 and 4 AU. Lower separations probed by spectroscopic surveys (see Joergens 2008, and references therein) contribute a further 7 +5 −3 % (separations <0.3 AU) or 10 +18 −8 % (separations <3 AU).…”

Resolving the L/T transition binary SDSS J2052-1609 AB

“…For determining the relative photometry (flux ratio) and astrometry (separation and position angle (PA)) of the new system, we used the IDL -based simultaneous PSF -fitting algorithm from Bouy et al (2003), adapted to the VLT/NACO data. As PSF reference star, we used the nearby point source 2MASS J205234-160913, which was observed in the same FoV as our target, hence under the same observing conditions.…”

“…Similar to the VLT/NACO data, we applied the PSF -fitting algorithm from Bouy et al (2003) to derive the astrometric parameters of the binary, as well as the flux ratio of the two components. This time we built and used a library of 6 reference PSFs: one TinyTim synthetic PSF and five natural PSFs from other HST/NICMOS programs that targeted brown dwarfs of similar spectral types (L7 -T1.5) in the corresponding filters (GO 10143 and 10879, PI: I. Reid).…”

“…After the discovery of the first binary brown dwarfs (Martín et al 1999a,b), systematic surveys have led to the discovery of more than 70 brown dwarfs in binary or multiple systems (e.g., Bouy et al 2003;Burgasser et al 2003;Gizis et al 2003). Astrometric follow-up observations of some of the shorter period binaries provided first system-mass estimates (e.g.…”