The properties of the 500 K dwarf UGPS J072227.51-054031.2 and a study of the far-red flux of cold brown dwarfs

We present i and z photometry for 25Tdwarfs and 1Ldwarf. Combined with published photometry, the data show that the i - z, z - Y, and z - J colors of Tdwarfs are very red, and continue to increase through to the late-type Tdwarfs, with a hint of a saturation for the latest types with T _eff 600K. We present new 0.7-1.0 μm and 2.8-4.2 μm spectra for the very late type Tdwarf UGPS J072227.51-054031.2, as well as improved astrometry for this dwarf. Examination of the spectral energy distribution using new and published data, with Saumon & Marley models, shows that the dwarf has T _eff = 505 10K, a mass of 3-11 M _Jupiter, and an age between 60Myr and 1Gyr. This young age is consistent with the thin disk kinematics of the dwarf. The mass range overlaps with that usually considered to be planetary, despite this being an unbound object discovered in the field near the Sun. This apparently young rapid rotator is also undergoing vigorous atmospheric mixing, as determined by the IRAC and WISE 4.5 μm photometry and the Saumon & Marley models. The optical spectrum for this 500K object shows clearly detected lines of the neutral alkalis Cs and Rb, which are emitted from deep atmospheric layers with temperatures of 900-1200K.