The Near‐Infrared and Optical Spectra of Methane Dwarfs and Brown Dwarfs

We identify the pressure--broadened red wings of the saturated potassiumresonance lines at 7700 \AA as the source of anomalous absorption seen in thenear-infrared spectra of Gliese 229B and, by extension, of methane dwarfs ingeneral. This conclusion is supported by the recent work of Tsuji {\it et al.}1999, though unlike them we find that dust need not be invoked to explain thespectra of methane dwarfs shortward of 1 micron. We find that a combination ofenhanced alkali abundances due to rainout and a more realistic non-Lorentziantheory of resonant line shapes may be all that is needed to properly accountfor these spectra from 0.5 to 1.0 microns. The WFPC2 $I$ measurement of Gliese229B is also consistent with this theory. Furthermore, a combination of theblue wings of this K I resonance doublet, the red wings of the Na D lines at5890 \AA, and, perhaps, the Li I line at 6708 \AA can explain in a natural waythe observed WFPC2 $R$ band flux of Gliese 229B. Hence, we conclude that theneutral alkali metals play a central role in the near-infrared and opticalspectra of methane dwarfs and that their lines have the potential to providecrucial diagnostics of brown dwarfs. We speculate on the systematics of thenear-infrared and optical spectra of methane dwarfs, for a given mass andcomposition, that stems from the progressive burial with decreasing \teff ofthe alkali metal atoms to larger pressures and depths.