High angular resolution imaging and spectroscopy in the near infrared using the adaptive optics at the ESO Very Large Telescope (VLT) have shown that the nearest brown dwarf is binary. Such multiple brown dwarfs are particularly important because their physical parameters can be accurately determined, and they can serve as key templates in the understanding of sub-stellar sources. Unit Telescope 4 "Yepun" of the VLT on Cerro Paranal, Chile, was pointed to ε Indi B on August 13, 2003. The adaptive optics system of the telescope corrects image distortions from atmospheric bubbles by adjusting one of the mirrors a hundred times a second.

NAOS/CONICA broad-band near-infrared adaptive optics images of the ε Indi Ba,Bb system. The images are taken in the J, H, and Ks bands, from left to right. Ks covers the longest wavelengths ("reddest") of the three, where adaptive optics works best. The angular resolution of the right image is 0.08 arc seconds which corresponds to resolving a head of a match 10 km away.

Composite image of the three wavelength bands where J is assigned blue, H is green and Ks is red. Under the colour balance chosen, the Sun would turn out slightly yellow, i.e. as we are used to seeing it in the optical. The rather different colours of ε Indi Ba and Bb reflect the substantially different infrared colours of the sources: these are not due to temperature differences as normal, but to deep absorption bands in the atmospheres. Indeed, the lower-mass, cooler object Bb looks bluer than Ba because of this.

H band spectra of ε Indi Ba and Bb. Both objects show signatures of spectral class T which is associated with brown dwarfs. Combining the spectral classification of the sub-stars with their distance from us and the approximate age of the ε Indi system, surface temperatures and masses can be estimated. These are 1200-1300 K and 40-50 Jupiter masses for the brighter of the two and 800-900 K and 25-30 Jupiter masses for the fainter one. They are orbiting each other with a period of roughly 15 years.

Paper accepted for publication in Astronomy & Astrophysics.