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 Teff of the alkali metal atoms to larger pressures and depths. Moreover, we surmise that those extrasolar giant planets (EGPs) that achieve Teffvalues 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.