We present YJHK photometry , or a subset , for the six Y dwarfs discovered in WISE data by Cushing et al..
The data were obtained using NIRI on the Gemini North telescope ; YJHK were obtained for WISEP J041022.71 + 150248.5 , WISEP J173835.52 + 273258.9 and WISEPC J205628.90 + 145953.3 ; YJH for WISEPC J140518.40 + 553421.5 and WISEP J154151.65 – 225025.2 ; YJK for WISEP J182831.08 + 265037.8 .
We also present a far-red spectrum obtained using GMOS-North for WISEPC J205628.90 + 145953.3 .
We compare the data to Morley et al .
( 2012 ) models , which include cloud decks of sulfide and chloride condensates .
We find that the models with these previously neglected clouds can reproduce the energy distributions of T9 to Y0 dwarfs quite well , other than near 5 \mu m where the models are too bright .
This is thought to be because the models do not include departures from chemical equilibrium caused by vertical mixing , which would enhance the abundance of CO and CO _ { 2 } , decreasing the flux at 5 \mu m. Vertical mixing also decreases the abundance of NH _ { 3 } , which would otherwise have strong absorption features at 1.03 \mu m and 1.52 \mu m that are not seen in the Y0 WISEPC J205628.90 + 145953.3 .
We find that the five Y0 to Y0.5 dwarfs have 300 \lesssim T _ { eff } K \lesssim 450 , 4.0 \lesssim \log g \lesssim 4.5 and f _ { sed } \approx 3 .
These temperatures and gravities imply a mass range of 5 – 15 M _ { Jupiter } and ages around 5 Gyr .
We suggest that WISEP J182831.08 + 265037.8 is a binary system , as this better explains its luminosity and color .
We find that the data can be made consistent with observed trends , and generally consistent with the models , if the system is composed of a T _ { eff } \approx 325 K and \log g \lesssim 4.5 primary , and a T _ { eff } \approx 300 K and \log g \gtrsim 4.0 secondary , corresponding to masses of 10 and 7 M _ { Jupiter } and an age around 2 Gyr .
If our deconvolution is correct , then the T _ { eff } \approx 300 K cloud-free model fluxes at K and W2 are too faint by 0.5 – 1.0 magnitudes .
We will address this discrepancy in our next generation of models , which will incorporate water clouds and mixing .