Kelu-1 : A Free-floating Brown Dwarf in the Solar Neighborhood

We present the discovery of a field brown dwarf, Kelu-1. The object has a peculiar optical spectrum that, in spite of its extremely red color, shows no sign of the TiO and VO absorption bands typical of cool, low-mass stars. The optical spectrum does show the Li I λ6708 absorption line and Hα in emission. The infrared spectrum of Kelu-1 has strong H2O absorption and also shows the effect of pressure-induced H2 opacity. Kelu-1 was discovered through its large proper motion of 035 yr-1, with position angle θ=265°. A distance of ~10 pc is estimated astrometrically for the object, which implies absolute magnitudes of MV~22 and MK~12. Comparison of the observed energy distribution of Kelu-1 to synthetic spectra generated by atmosphere models of Allard et al., and structural and evolutionary models of Chabrier & Baraffe, implies that the object has Teff=1900±100 K and a mass below 0.075 M/M.