Molecular Abundances in the Atmosphere of the T Dwarf GL 229B

We present new high-resolution infrared spectra of the T dwarf Gliese 229B in the J, H, and K bandpasses. We analyze each of these as well as previously published spectra to determine its metallicity and the abundances of NH₃ and CO in terms of the surface gravity of Gl 229B, which remains poorly constrained. The metallicity increases with increasing gravity and is below the solar value unless Gl 229B is a high-gravity brown dwarf with logg (cgs)~5.5. The NH₃ abundance is determined from both the H- and the K-band spectra, which probe two different levels in the atmosphere. We find that the abundance from the K-band data is well below that expected from chemical equilibrium, which we interpret as strong evidence for dynamical transport of NH₃ in the atmosphere. This is consistent with the previous detection of CO and provides additional constraints on the dynamics of the atmosphere of this T dwarf.