Interpretation of the Spectra Originating from the Photospheres Contaminated with Dust - Experience in L and T Dwarfs
Abstract
Recent observations revealed that the infrared colors of L and T dwarfs show a large variation at a given T eff and this result may imply that the thickness of the dust cloud is changing at a given T eff. Our Unified Cloudy Model (UCM), in which the dust cloud forms between the condensation temperature, Tcond, and the critical temperature, Tcr, is already prepared for such a new situation and can be used by simply changing Tcr at a given Teff, instead of assuming a constant value of Tcr=1800K throughout as in our previous applications of the UCMs. However, the effects of Teff and Tcr on individual spectra are difficult to discriminate, since the effect of Teff at a fixed Tcr on one hand and the effect of Tcr at a fixed Teff on the other have essentially the same effect on the spectra. For this reason, spectra of dusty dwarfs were in fact misinterpreted in general, also by ourselves. We show that the degeneracy of Teff and Tcr can be removed to some extent by the analysis of the spectra transformed to the SEDs on an absolute scale. Our reanalysis of some spectra revealed that the L-T spectral sequence is not a sequence of Teff, at least between middle L and early T types, but should be interpreted as a sequence of Tcr. In other words, the L-T spectral sequence has little to do with Teff but reflects a change of the thickness of the dust cloud. Although this unexpected result is based on a limited sample, an odd ``brightening'' of the absolute magnitudes (e.g., MJ and Mbol) plotted against the L-T spectral types may also be a manifestation that the L-T spectral sequence is not a temperature sequence. :
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