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 lambda 6708 absorption line and Ha in emission. The infrared spectrum of Kelu-1 has strong H2O absorption and also shows the effect of pressure-induced H-2 opacity. Kelu-1 was discovered through its large proper motion of 0 ".35 yr(-1), with position angle theta = 265 degrees. A distance of similar to 10 pc is estimated astrometrically for the object, which implies absolute magnitudes of M-V similar to 22 and M-K similar to 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 T-eff = 1900 +/- 100 K and a mass below 0.075 M/M..