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

We identify the pressure--broadened red wings of the saturated potassium resonance lines at 7700 \AA as the source of anomalous absorption seen in the near-infrared spectra of Gliese 229B and of methane dwarfs in general. The WFPC2 $I$ band measurement of Gliese 229B is also consistent with this hypothesis. Furthermore, a combination of the blue wings of this K I resonance doublet, the red wings of the Na D lines at 5890 \AA, and, perhaps, the Li I line at 6708 \AA can explain in a natural way the observed WFPC2 $R$ band flux of Gliese 229B. Hence, we conclude that the neutral alkali metals play a central role in the near-infrared and optical spectra of methane dwarfs and that their lines have the potential to provide crucial diagnostics of brown dwarf properties. The slope of the spectrum from 0.8 \mic to 0.9 \mic for the Sloan methane dwarf, SDSS 1624+00, is shallower than that for Gliese 229B and its Cs lines are weaker. From this, we conclude that its atmosphere is tied to a lower core entropy or that its K and Cs abundances are smaller, with a preference for the former hypothesis. We speculate on the systematics of the near-infrared and optical spectra of methane dwarfs, for a given mass and composition, that stems from the progressive burial with decreasing \teff of the alkali metal atoms to larger pressures and depths. Moreover, we surmise that those extrasolar giant planets (EGPs) that achieve \teffs in the 800--1300 K range due to stellar insolation will show signatures of the neutral alkali metals in their albedo and reflection spectra. We estimate that, due predominantly to absorption by Na D lines, the geometric albedo of the EGP $\tau$ Boo b at $\lambda=0.48$ \mic is $<0.1$, consistent with the new (and low) upper limit of 0.3 recently obtained by Charbonneau et al (1999).