An outline of the paper: Neglected clouds in T and Y dwarf atmospheres. by C. Morley et al.
As brown dwarfs slowly cool, a variety of species condense in their atmospheres forming clouds. The atmospheres of L dwarfs are dominated by a significant layer of iron and silicate clouds. When the brown dwarfs reach an effective temperature of about 1400 K, methane absorption features become visible in the near-infrared spectra and we enter the domain of the T dwarf. The presence of the methane absorption is thought to be due to the clouds sinking below the photosphere, allowing the flux to emerge from hotter atmospheric layers, making the brown dwarf much bluer in colour (figure below).
As the T-dwarfs continue to cool, optically thinner clouds made up of mainly Na2S and MnS are thought to emerge. These clouds have not been included in pervious atmosphere models and it is here that Morley et al. set out to examine the effect these clouds have on model T and Y dwarf atmospheres.
To do this Morley et al. calculate the total amount of condensate at each layer in the atmosphere by using a modified Ackerman & Marley (2001) cloud model by including Cr, MnS, Na2S, ZnS, and KCl (figure below). For the reader interested in cloud modelling it is worth mentioning the Helling at al. (2008) paper were five different cloud models were compared in an effort to make objective comparisons between cloud models easier. The Marley, Ackerman & Lodders model is mentioned in in section 2.2.3. Morley et al. generate a plethora of model grids spanning the full range of effective temperatures and surface gravities of T dwarfs and find that cloudy models match pervious observational data much better than the corresponding cloud-free models (last figure).
It is the emergence of sulfide clouds at effective temperatures cooler than 900 K which are thought to change the observed spectra making the T dwarf appear redder.The authors do caution that they have not yet done any investigation into whether the sulfur clouds will have identifiable spectral features which can confirm the presence of sulfide clouds in the atmospheres of T dwarfs. Judging by the features in the sulfide indices of refraction, they estimate the features to be in the mid-infrared.
Feature Image: Morley et al. (2012)
Precipitating Condensation Clouds in Substellar Atmospheres by Ackerman & Marley 2001
A comparison of chemistry and dust cloud formation in ultracool dwarf model atmospheres by Helling et al. 2008