SYNTHETIC SPECTRA AND MASS DETERMINATION OF THE BROWN DWARF GLIESE 229B

We present preliminary nongray model atmospheres and interiors for coo l brown dwarfs. The resulting synthetic spectra are compared to availa ble spectroscopic and photometric observations of the coolest brown dw arf yet discovered, G1 229B (Nakajima et al.). Despite the grainless n ature of the present models, we find that the resulting synthetic spec tra provide an excellent fit to most of the spectral features of the b rown dwarf. We confirm the presence of methane absorption and the subs tellar nature of G1 229B. 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 M(.). While uncertainties in the age of the system yield some indeterm inateness for the mass of G1 229B, the most likely solution is m appro ximate to 0.04 - 0.055 M(.). In any case, we can set an upper limit m = 0.065 M(.) for a very unlikely age, t = 10 Gyr.