Near-infrared photometry of Y dwarfs: Low ammonia abundance and the onset of water clouds

We present new near-infrared photometry for seven late-type T dwarfs and nine Y-type dwarfs, and lower limit magnitudes for a tenth Y dwarf, obtained at Gemini Observatory. We also present a reanalysis of H-band imaging data from the Keck Observatory Archive, for an 11th Y dwarf. These data are combined with earlier MKO-system photometry, Spitzer and WISE mid-infrared photometry, and available trigonometric parallaxes, to create a sample of late-type brown dwarfs that includes 10 T9-T9.5 dwarfs or dwarf systems, and 16 Y dwarfs. We compare the data to our models, which include updated H₂ and NH₃ opacity, as well as low-temperature condensate clouds. The models qualitatively reproduce the trends seen in the observed colors; however, there are discrepancies of around a factor of two in flux for the Y0-Y1 dwarfs, with T_eff ≈ 350-400 K. At T_eff ∼ 400 K, the problems could be addressed by significantly reducing the NH₃ absorption, for example by halving the abundance of NH₃ possibly by vertical mixing. At T_eff ∼ 350 K, the discrepancy may be resolved by incorporating thick water clouds. The onset of these clouds might occur over a narrow range in T_eff, as indicated by the observed small change in 5 μm flux over a large change in J - W2 color. Of the known Y dwarfs, the reddest in J - W2 are WISEP J182831.08 + 265037.8 and WISE J085510.83-071442.5. We interpret the former as a pair of identical 300-350 K dwarfs, and the latter as a 250 K dwarf. If these objects are ∼3 Gyr old, their masses are ∼10 and ∼5 Jupiter-masses, respectively.