We have generated new, self-consistent spectral and atmosphere models for the effective temperature range 600 K to 1300 K thought to encompass the known T dwarfs. For the first time, theoretical models are compared with a family of measured T dwarf spectra at wavelengths shortward of ∼1.0 micron. By defining spectral indices and standard colors in the optical and very near-infrared, we explore the theoretical systematics with Teff, gravity, and metallicity. We conclude that the short-wavelength range is rich in diagnostics that complement those in the near-infrared now used for spectral subtyping. We also conclude that the wings of the Na D and K I (7700˚A) resonance lines and aggressive rainout of heavy metals (with the resulting enhancement of the sodium and potassium abundances at altitude) are required to fit the new data shortward of 1.0 µm. Furthermore, we find that the water bands weaken with increasing gravity, that modest decreases in metallicity enhance the effect in the optical of the sodium and potassium lines, and that at low Teffs, in a reversal of the normal pattern, optical spectra become bluer with further decreases in Teff. Moreover, we conclude that T dwarf subtype is no
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