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Title: 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 H{sub 2} and NH{sub 3} 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 {sub eff} ≈ 350-400 K. At T {sub eff} ∼ 400 K, the problems could be addressed by significantly reducing the NH{sub 3} absorption, for example by halving the abundance of NH{sub 3} possibly by vertical mixing. At T {sub eff} ∼ 350 K, the discrepancy may be resolved by incorporating thick water clouds. The onset of these clouds might occur over a narrow range inmore » T {sub 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.« less
Authors:
 [1] ;  [2] ;  [3] ;  [4]
  1. Gemini Observatory, Northern Operations Center, 670 N. A'ohoku Place, Hilo, HI 96720 (United States)
  2. Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States)
  3. NASA Ames Research Center, Mail Stop 245-3, Moffett Field, CA 94035 (United States)
  4. Los Alamos National Laboratory, P.O. Box 1663, MS F663, Los Alamos, NM 87545 (United States)
Publication Date:
OSTI Identifier:
22364526
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 799; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABSORPTION; ABUNDANCE; AMMONIA; CLOUDS; COLOR; COMPARATIVE EVALUATIONS; DWARF STARS; HYDROGEN; JUPITER PLANET; MASS; NEAR INFRARED RADIATION; OPACITY; PHOTOMETRY; WATER