Dust in the Photospheric Environment III. A Fundamental Element in the   Characterization of Ultracool Dwarfs
  Recent photometry of L and T dwarfs revealed that the infrared colors show a large variation at a given Teff and, within the framework of our Unified Cloudy Model (UCM), this result can be interpreted as due to a variation of the critical temperature (Tcr) which is essentially a measure of the thickness of the dust cloud. It appears that the L/T transition takes place at around Teff = 1400K, where Tcr shows a particularly large change. Thus the L/T transition is associated with a drastic change of the thickness of the dust cloud at around Teff = 1400K, but the reason for this change is unknown. Once we allow Tcr to vary at given Teff and log g, the two-color and color-magnitude diagrams can be well explained as the effect of Teff, log g, and Tcr, but not by that of Teff and log g alone. In general, the effects of Teff and Tcr are difficult to discriminate on individual spectra, but this degeneracy of Teff and Tcr can be removed to some extent by the analysis of the SED on an absolute scale. The reanalysis of a selected sample of spectra revealed that the L-T spectral sequence may not necessarily be a sequence of Teff, but may reflect a change of the thickness of the dust cloud, represented by Tcr in our UCM. Also, an odd 'brightening' of the absolute J magnitudes plotted against the L-T spectral types may also be a manifestation that the L-T spectral sequence is not a temperature sequence, since Mbol also shows a similar 'brightening'. Then, the 'J-brightening' may not be due to any atmospheric effect and hence should not be a problem to be solved by model atmospheres including the UCMs. Thus, almost all the available observed data are reasonably well interpreted with the UCMs in which the cloud thickness varies, and the problem now is how to understand why the cloud thickness (or Tcr) changes independently of Teff and log g. 
Takashi Tsuji