Transition from L to T Dwarfs on the Color-Magnitude Diagram

The color-magnitude (CM) diagram of cool dwarfs and brown dwarfs based on the recent astrometry data is compared with the CM diagram transformed from the theoretical evolutionary tracks via the unified cloudy models (UCMs) of L and T dwarfs. A reasonable agreement between the models and observations is shown for the whole regime of ultracool dwarfs covering L and T dwarfs, and this is achieved, for the first time, with the use of a single grid of self-consistent nongray model photospheres accommodating dust cloud (UCMs; 700 . Teff . 2600 K). A distinct brightening at the J band in the early T dwarfs revealed by the recent parallax measurements is explained as a natural consequence of the migration of the thin dust cloud to the inner region of the photosphere and should not necessarily be evidence for Burgasser et al.’s proposition that the dust cloud breaks up in the L/T dwarf transition. Also, the rapid bluing from the late L to the early T dwarfs is a direct result of the transition of the thin dust cloud from the optically thin (τ < 1) to thick (τ & 1) regimes while Lbol and Teff lower only slightly. Thus, the theoretical evolutionary models, the cloudy models of the photospheres (UCMs), and the observed fundamental stellar parameters are brought into a consistent picture of the newly defined L and T dwarfs. Subject headings: molecular processes — stars: atmospheres — stars: fundamental parameters —stars: late-type — stars: low-mass, brown dwarfs —