Recent photometry of L and T dwarfs revealed that the infrared colors show a large variation at a given effective temperature and , within the framework of our Unified Cloudy Model ( UCM ) , this result can be interpreted as due to a variation of the critical temperature ( T _ { cr } ) which is essentially a measure of the thickness of the dust cloud .
Especially , the variation is the largest at T _ { eff } \approx 1400 \pm 100 K , where the transition from L to T types takes place .
Thus the L/T transition is associated with the drastic change of the thickness of the dust cloud at T _ { eff } \approx 1400 K , but the reason for this change is unknown .
Once we allow T _ { cr } to vary at given T _ { eff } and log g in our UCMs , the two-color diagram and color-magnitude diagram can be well explained as the effect of T _ { eff } , log g , and T _ { cr } , but not by the effect of T _ { eff } and log g alone .
This result implies that T _ { cr } will be one of the important parameters needed for characterization of dusty dwarfs .
The effects of T _ { eff } and T _ { cr } on individual spectra , however , are difficult to discriminate , since changing of T _ { eff } at a fixed T _ { cr } on one hand and changing of T _ { cr } at a fixed T _ { eff } on the other essentially have the same effect on the spectra .
We show that the degeneracy of T _ { eff } and T _ { cr } can be removed to some extent by the analysis of the spectral energy distribution ( 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 T _ { eff } , but may reflect a change of the thickness of the dust cloud , represented by T _ { cr } in our UCM .
Although this unexpected result is based on a limited sample , 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 necessarily a temperature sequence .
This is because M _ { bol } based on the same photometry data 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 , whose full capability emerges once T _ { cr } is introduced as the fifth parameter in addition to the usual four parameters ( i.e. , chemical composition , T _ { eff } , log g , and micro-turbulent velocity ) needed to characterize stellar spectra in general .