paper

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

arXiv:astro-ph/9908078 · doi:10.1086/308462

Abstract

We identify the pressure--broadened red wings of the saturated potassium resonance lines at 7700 Åas the source of anomalous absorption seen in the near-infrared spectra of Gliese 229B and, by extension, of methane dwarfs in general. 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 the spectra of methane dwarfs shortward of 1 micron. We find that a combination of enhanced alkali abundances due to rainout and a more realistic non-Lorentzian theory of resonant line shapes may be all that is needed to properly account for these spectra from 0.5 to 1.0 microns. The WFPC2 measurement of Gliese 229B is also consistent with this theory. 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 Åcan explain in a natural way the observed WFPC2 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 dwarfs. 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.

Revised and accepted to Ap.J. volume 531, March 1, 2000, also available at http://jupiter.as.arizona.edu/~burrows/papers/BMS.ps

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