New H₂ collision-induced absorption and NH₃ opacity and the spectra of the coolest brown dwarfs

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 NH₃ opacities. 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.