Neglected clouds in T and Y dwarf atmospheres


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).

Colour-magnitude diagram of M, L and T dwarfs. Observational data is from Dupuy & Liu (2012) and the models are plottet as solid lines. Red lines are cloudy models whilst blue lines are cloud free.

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).

Presure-temperature profiles of model atmospheres. Shown are cloudless models as well as cloudy models with optically thin and optically thick clouds. Condensation curves for each condensate species are shown as dotted lines. The cloudy models include the condensates Cr, MnS, Na2S, ZnS, and KCl. As the cloud thickness increases so does the temperature at a given atmospheric pressure. Thick lines indicate the position of the 1-6 μm photosphere.

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.

A colour-magnitude diagram with the Morley et al. 2012 models overlain. The blue line shows a cloud free model whilst the red lines show cloud models with different sedimentation efficiencies. A higher fsed corresponds to optically thicker clouds. The hottest cloudy models have very similar near-infrared colours as the cloudless models. As the effective temperature decreases cloud material condenses and the cloudy model has a redder photometric colour shown by the red lines curving of to the right as compared to the cloudless model. More optically thick clouds have a redder photometric colour.


Feature Image: Morley et al. (2012)

Further Reading:

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


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