New H-2 Collision-Induced Absorption And Nh3 Opacity And The Spectra Of The Coolest Brown Dwarfs
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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 (H-2). We compare the new synthetic spectra with models based on an earlier description of the H-2 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.