2012ApJ...750...74S

We present new cloudy and cloudless model atmospheres for brown dwarfs using recent ab initio calculations of the line list of ammonia (NH₃) and of the collision-induced absorption of molecular hydrogen (H₂). We compare the new synthetic spectra with models based on an earlier description of the H₂ and NH₃opacities. We find a significant improvement in fitting the nearly complete spectral energy distribution of the T7p dwarf Gliese 570D and in near-infrared color-magnitude diagrams of field brown dwarfs. We apply these new models to the identification of NH₃ absorption in the H-band peak of very late T dwarfs and the new Y dwarfs and discuss the observed trend in the NH₃-H spectral index. The new NH₃ line list also allows a detailed study of the medium-resolution spectrum of the T9/T10 dwarf UGPS J072227.51-054031.2 where we identify several specific features caused by NH₃.