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 Teff ≈ 350–400 K. At Teff ∼ 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 Teff ∼ 350 K, the discrepancy may be resolved by incorporating thick water clouds. The onset of these clouds might occur over a narrow range in Teff, 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.