Theoretical Spectral Models of T Dwarfs at Short Wavelengths
Abstract
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. By defining spectral indices and standard colors in the optical and 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 non-Lorentzian profiles for the Na D and K I (7700\AA) resonance lines and aggressive rainout of heavy metals are required to fit the new data shortward of 1.0 \mic. 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, there are theoretical indications that T dwarf subtype is not a function of \teff alone, but that it is a non-trivial function of gravity and metallicity as well. Importantly, as do Marley et al. (2001), we see evidence in early T dwarf atmospheres of the residual effect of clouds. With cloudless models, we obtain very good spectral fits to the two late T dwarfs with known parallaxes, but a residual effect of clouds on the emergent spectra of later T dwarfs can not yet be discounted.Keywords
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