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 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 (Tcr) which is essentially a measure of the thickness of the dust cloud. In our previous applications of the UCMs, we assumed that Tcr is constant at about 1800K in all the L and T dwarfs but, in view of the new observing result, we now allow Tcr to vary between the surface temperature (T0) and the condensation temperature (Tcond) at given Teff and log g. Then, the two-color diagram and color-magnitude diagram can be well explained as the effect of Teff , log g, and Tcr, but not by the effect of Teff and log g alone. This result implies that Tcr will be one of the important parameters needed for characterization of dusty dwarfs. The effects of Teff and Tcr on individual spectra, however, are difficult to discriminate, since changing of Teff at a fixed Tcr on one hand and changing of Tcr at a fixed Teff on the other essentially have the same effect on the spectra. We show that the degeneracy of Teff and Tcr 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 Teff , but may reflect a change of the thickness of the dust cloud, represented by Tcr 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 Mbol based