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R. V. Yelle (Northern Arizona University), H. Partridge (NASA Ames Research Cntr.), C. A. Griffith (Northern Arizona University)
The visible and near IR spectra of brown dwarfs are effected by the far wings of alkali resonance lines. The spectrum of Gl229B, the best studied brown dwarf, exhibits a strong positive slope in the 600-1100 nm wavelength region, that is partly due to the red wing of the KI resonance lines at 766.5 and 769.9 nm. These lines are broadened by collisions with ambient H2, but the distances from line center are so large that the usual Lorentzian line shapes can not be used to describe the absorption coefficient. We therefore calculate the absorption line shape using the quasi-static theory of collisional broadening in conjunction with accurate SDCI+Q calculations of the K-H2 interatomic potential. Our calculations indicate that the KI line shape differs significantly from that derived from prior studies and presently used to study the optical spectra of brown dwarfs. We discuss the implications of the KI opacity on the structure of Gl229B's atmosphere, in particular the particulate content of the visible atmosphere.