2013ApJ...763..130L

We present YJHK photometry, or a subset, for the six Y dwarfs discovered in Wide-field Infrared Survey Explorer (WISE) data by Cushing et al. The data were obtained using the Near-Infrared Imager 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 J154151.65225025.2; and 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. 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 µm 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 CO₂, decreasing the flux at 5 µm. Vertical mixing also decreases the abundance of NH₃, which would otherwise have strong absorption features at 1.03 µm and 1.52 µm that are not seen in the Y0 WISEPC J205628.90+145953.3. We find that the five Y0 to Y0.5 dwarfs have 300 ≲ T_eff K ≲ 450, 4.0 ≲ log g ≲ 4.5, and f_sed~ 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~ 325 K and log g ≲ 4.5 primary, and a T_eff~ 300 K and log g ≳ 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~ 300 K cloud-free model fluxes at K and W2 are too faint by 0.5-1.0 mag. We will address this discrepancy in our next generation of models, which will incorporate water clouds and mixing.