The astrometric space mission Gaia is expected to detect a large number of brown dwarf binary systems with close orbits and determine astrometric orbit solutions. This will provide key information for the formation and evolution of brown dwarfs, such
as the binary frequency and dynamical masses. Known brown dwarf binaries with orbit constraints from other techniques will play an important role. We are carrying out one of the most precise and long-lasting radial velocity surveys for brown dwarf binaries in the Cha I star-forming region at the VLT. We were able to add two orbit determinations to the small group of a handful of brown dwarf and very low-mass binaries with characterized RV orbits. We show here that the astrometric motion of both systems can be detected with Gaia. We predict an astrometric signal of about 1.2 - 1.6 milliarcseconds (mas) for the brown dwarf binary ChaHa8 and of 0.4 - 0.8 mas for the very low-mass binary CHXR74. We take the luminosity of the companion into account for these estimates and present a relation for the astrometric signature of a companion with non-negligible luminosity.
We report the discovery of a 16-20 Jupiter mass radial velocity companion around the very young (~3 Myr) brown dwarf candidate ChaHa8 (M5.75-M6.5). Based on high-resolution echelle spectra of ChaHa8 taken between 2000 and 2007 with UVES at the VLT, a
companion was detected through RV variability with a semi-amplitude of 1.6 km/s. A Kepler fit to the data yields an orbital period of the companion of 1590 days and an eccentricity of e=0.49. A companion minimum mass M2sini between 16 and 20 Jupiter masses is derived when using model-dependent mass estimates for the primary. The mass ratio q= M2/M1 might be as small as 0.2 and, with a probability of 87%, it is less than 0.4. ChaHa8 harbors most certainly the lowest mass companion detected so far in a close (~ 1 AU) orbit around a brown dwarf or very low-mass star. From the uncertainty in the orbit solution, it cannot completely be ruled out that the companion has a mass in the planetary regime. Its discovery is in any case an important step towards RV planet detections around BDs. Further, ChaHa8 is the fourth known spectroscopic brown dwarf or very low-mass binary system with an RV orbit solution and the second known very young one.
