Neglected clouds in T and y dwarf atmospheres

As brown dwarfs cool, a variety of species condense in their atmospheres, forming clouds. Iron and silicate clouds shape the emergent spectra of Ldwarfs, but these clouds dissipate at the L/T transition. A variety of other condensates are expected to form in cooler Tdwarf atmospheres. These include Cr, MnS, Na₂S, ZnS, and KCl, but the opacity of these optically thinner clouds has not been included in previous atmosphere models. Here, we examine their effect on model T and Y dwarf atmospheres. The cloud structures and opacities are calculated using the Ackerman & Marley cloud model, which is coupled to an atmosphere model to produce atmospheric pressure-temperature profiles in radiative-convective equilibrium. We generate a suite of models between T _eff = 400 and 1300K, log g = 4.0 and 5.5, and condensate sedimentation efficiencies from f _sed = 2 to 5. Model spectra are compared to two red Tdwarfs, Ross 458C and UGPS 0722-05; models that include clouds are found to match observed spectra significantly better than cloudless models. The emergence of sulfide clouds in cool atmospheres, particularly Na₂S, may be a more natural explanation for the "cloudy" spectra of these objects, rather than the reemergence of silicate clouds that wane at the L-to-T transition. We find that sulfide clouds provide a mechanism to match the near- and mid-infrared colors of observed Tdwarfs. Our results indicate that including the opacity of condensates in Tdwarf atmospheres is necessary to accurately determine the physical characteristics of many of the observed objects.