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 Å as the source of anomalous absorption seen in the near-infrared spectra of Gliese 229B and, by extension, of methane dwarfs in general. In broad outline, this conclusion is supported by the 1999 work of Tsuji et al. 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 Å, and, perhaps, the Li I line at 6708 Å 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. 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 T_eff of the alkali metal atoms to larger pressures and depths. Moreover, we surmise that those extrasolar giant planets (EGPs) that achieve T_effvalues in the 800-1300 K range because of stellar insolation will show signatures of the neutral alkali metals in their albedo and reflection spectra. We estimate that, predominantly because of absorption by Na D lines, the geometric albedo of the EGP τ Boo b at λ=0.48 μm is less than 0.1, which is consistent with the new (and low) upper limit of 0.3 recently obtained by Charbonneau et al. in 1999.