Dynamical masses for the nearest brown dwarf binary: ε Indi Ba, Bb

Abstract

We present preliminary astrometric results for the closest known brown dwarf binary to the Sun: e Indi Ba, Bb at a distance of 3.626 pc. With ongoing monitoring of the relative separation of the two brown dwarfs (spectral types T1 and T6) with VLT NACO near‐IR adaptive optics system since June 2004, we obtain a model‐independent dynamical total mass for the system of 121 MJup, some 60% larger than the one obtained by McCaughrean et al., implying that the system may be as old as 5 Gyr. We have also been monitoring the absolute astrometric motions of the system using the VLTFORS2 optical imager since August 2005 to determine the individual masses. We predict a periastron passage in early 2010, by which time the system mass will be constrained to <1 MJup and we will be able to determine the individual masses accurately in a dynamical, model‐independent manner.