ε Indi Ba,Bb: The nearest binary brown dwarf

We have carried out high angular resolution near-infrared imaging and low-resolution ($R\sim1000$) spectroscopy of the nearest known brown dwarf, e Indi B, using the ESO VLT NAOS/CONICA adaptive optics system. We find it to be a close binary (as also noted by Volk et al. 2003), with an angular separation of 0.732 arcsec, corresponding to 2.65 AU at the 3.626 pc distance of the e Indi system. In our discovery paper (Scholz et al. 2003), we concluded that e Indi B was a ~50 M Jup T2.5 dwarf: our revised finding is that the two system components ( e Indi Ba and e Indi Bb) have spectral types of T1 and T6, respectively, and estimated masses of 47 and 28 M Jup , respectively, assuming an age of 1.3 Gyr. Errors in the masses are ± 10 and ± 7 M Jup , respectively, dominated by the uncertainty in the age determination (0.8–2 Gyr range). This uniquely well-characterised T dwarf binary system should prove important in the study of low-mass, cool brown dwarfs. The two components are bright and relatively well-resolved: e Indi B is the only T dwarf binary in which spectra have been obtained for both components. The system has a well-established distance and age. Finally, their orbital motion can be measured on a fairly short timescale (nominal orbital period ~15 yrs), permitting an accurate determination of the true total system mass, helping to calibrate brown dwarf evolutionary models.