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 eleventh 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 which includes ten T9 — T9.5 dwarfs or dwarf systems , and sixteen Y dwarfs .
We compare the data to our models which include updated H _ { 2 } and NH _ { 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 _ { eff } \approx 350 – 400 K. At T _ { eff } \sim 400 K , the problems could be addressed by significantly reducing the NH _ { 3 } absorption , for example by halving the abundance of NH _ { 3 } possibly by vertical mixing .
At T _ { eff } \sim 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 \mu m flux over a large change in J - W 2 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 \sim 3 Gyrs old , their masses are \sim 10 and \sim 5 Jupiter-masses respectively .