Gaia detection capabilities of spectroscopic brown dwarf binaries.

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 very small group 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 Cha Halpha 8 and of 0.4-0.8 mas for the very low-mass binary CHXR 74. We take the luminosity of the companions into account for these estimates and present a relation for the astrometric signature of a companion with non-negligible luminosity.