Theoretical Spectral Models of T Dwarfs at Short Wavelengths and Their Comparison with Data

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 , 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) 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 . 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 , in a reversal of the normal pattern, optical spectra become bluer with further decreases in . Moreover, we conclude that T dwarf subtype is not a function of alone, but that it is a non-trivial function of gravity and metallicity as well. As do Marley et al. (2001), we see evidence in early T dwarf atmospheres of a residual effect of clouds. With cloudless models, we obtain spectral fits to the two late T dwarfs with known parallaxes, but a residual effect of clouds on the emergent spectra of even late T dwarfs can not yet be discounted.