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 Lbol/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 MJup, and age = 3-10 Gyr. We perform the same analysis using published 0.6-15 μm spectra for the T7.5 dwarf 2MASS J12171110-0311131, for which we find a metal-rich composition ([Fe/H] ~ 0.3), and log Lbol/L☉ = -5.31, Teff = 850-950 K, log g = 4.80-5.42, M = 25-66 MJup, 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.