Physical parameters of two very cool T dwarfs

We present new infrared spectra of the T8 brown dwarf 2MASS J04151954-0935066: 2.9-4.1 μm spectra obtained with the Infrared Camera and Spectrograph on the Subaru Telescope, and 5.2-14.5 μm spectra obtained with the Infrared Spectrograph on the Spitzer Space Telescope. We use these data and models to determine an accurate bolometric luminosity of log L _bol/L_⊙ = -5.67 and to constrain the effective temperature, gravity, mass, and age to 725-775 K, log g = 5.00-5.37, M = 33-58 M_Jup, and age = 3-10 Gyr. We perform the same analysis using published 0.6-15 μm spectra for the T7.5 dwarf 2MASSJ12171110-0311131, for which we find a metal-rich composition ([Fe/H] ∼ 0.3), and log L _bol/L_⊙ = -5.31, T_eff = 850-950 K, log g = 4.80-5.42, M = 25-66 M_Jup, and age = 1-10 Gyr. These luminosities and effective temperatures straddle those determined with the same method and models for Gl 570D by Saumon et al. and make 2MASS J04151954-0935066 the coolest and least luminous T dwarf with well-determined properties. We find that synthetic spectra generated by the models reproduce the observed red through mid-infrared spectra of 2MASS J04151954-0935066 and 2MASS J12171110-0311131 very well, except for known discrepancies that are most likely due to the incomplete CH₄ opacities. Both objects show evidence of departures from strict chemical equilibrium, and we discuss this result in the context of other late T dwarfs in which disequilibrium phenomena have been observed.