Synthetic Spectra and Mass Determination of the Brown Dwarf Gl229B

We present preliminary non-grey model atmospheres and interiors for cool brown dwarfs. The resulting synthetic spectra are compared to available spectroscopic and photometric observations of the coolest brown dwarf yet discovered, Gl229B (Nakajima \etal, 1995). Despite recognized shortcomings of current methane opacities, we find that the model spectra provide an excellent fit to most of the spectral features of the brown dwarf. We confirm the presence of methane absorption and the substellar nature of Gl229B. These preliminary models set an upper limit for the effective temperature of 1000~K. We also compute the evolution of brown dwarfs with solar composition and masses from 0.02 to 0.065 $\msol$. While uncertainties in the age of the system yield some undetermination for the mass of Gl229B, the most likely solution is $m\approx 0.04-0.055 \msol$. In any case, we can set an upper limit $m= 0.065\msol$ for a very unlikely age $t=10$ Gyr.