/ 01 03 39 5 v 1 2 3 M ar 2 00 1 Unified Model Photospheres for Ultracool Dwarfs of the Types L and T ⋆

The presence of the two distinct groups of ultracool dwarfs (UCDs), ‘L’ and ‘T’ types, is now well established: L dwarfs are thought to be dusty while T dwarfs show strong bands of methane (CH4) but little evidence for dust in their spectra. So far, different model sequences, which we referred to as the dusty model (or case B) and dustsegregated model (or case C) have been considered for L and T dwarfs, respectively. We now propose instead that these two groups of UCDs may be understood as a temperature effect in a unique sequence of the model photospheres in which dust always exists but only in the restricted region (where 1800 ∼ < T ∼ < 2000K) referred to as an active dust zone (or this can be referred to as a dust cloud). This is a natural consequence of considering not only dust formation but also its segregation process in the photosphere. By this model sequence, the dust-column density in the observable photosphere first increases for cooler objects and the infrared colours become redder from late M to L dwarfs. On the other hand, the dust-column density in the observable photosphere decreases in the objects cooler than the latest L dwarfs as the active dust zone moves to the optically thick region deep in the photosphere and the infrared colours turn blueward towards the coolest T dwarfs. In this way, the observed colours and spectra of UCDs through L and T types can be explained consistently by a single grid of unified model photospheres. More generally, an important conclusion is that the photospheric dust formation is effective only in warmer deep regions. This is contrary to the general belief that dust forms in cooler surface regions. key words: dust-column density, L-type prototype GD165B, L/T transition objects, photospheric dust formation, quantitative spectroscopy, spectral classification, T-type prototype Gliese 229B