  We present new infrared spectra of the T8 brown dwarf 2MASS
J04151954-0935066: 2.9-4.1 micron spectra obtained with the Infrared Camera and
Spectrograph on the Subaru Telescope, and 5.2-14.5 micron 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_sun=-5.67
and to constrain the effective temperature, gravity, mass and age to 725-775K,
log g=5.00-5.37, M=33-58 M_Jupiter and age=3-10Gyr. We perform the same
analysis using published 0.6-15 micron spectra for the T7.5 dwarf 2MASS
J12171110-0311131, for which we find a metal-rich composition ([Fe/H]~0.3) and
log L_bol/L_sun=-5.31, T_eff=850-950K, log g=4.80-5.42, M=25-66M_Jupiter and
age=1-10Gyr. These luminosities and effective temperatures straddle those
determined with the same method and models for Gl 570D by Saumon et al. (2006)
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 which 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 where disequilibrium
phenomena have been observed.
