1. What are the contributions in "C: " ?
The authors 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 ).. The authors 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.. The authors 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.
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2. What is the reason for the creation of the new Y spectral class?
The detection of NH3 in the near-infrared spectrum of several ultracool dwarfs is one of the factors that motivate the creation of the new Y spectral class (Cushing et al. 2011).
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3. At what temperature does the opacity of the NH3 line list change?
At lower Teff , the chemical equilibrium shifts to strongly favor NH3 over N2 as the dominant nitrogen bearing species and its opacity become more significant compared to the competing absorption from H2O and CH4.
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4. Why is the new sequence of models slightly bluer than the late T dwarf sequence?
The new sequence of models is still slightly bluer by 0.15 than the late T dwarf sequence, which may be due to missing CH4 opacity on the J-band peak.
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![Figure 8. Spectrum of Gliese 570D. The data are shown in black (Burgasser et al. 2003b; Geballe et al. 2001; Cushing et al. 2006; Geballe et al. 2009). The spectrum obtained with our previous models is shown in red (Saumon et al. 2006). The blue curve shows the spectrum computed with the new H2 CIA and the BYTe NH3 line list with the same model parameters: Teff = 820 K, log g = 5.23 (cgs), [M/H] = 0, and eddy diffusion coefficient logKzz (cm2 s−1) = 4.5. The models are plotted at a resolving power of R = 500 (top panel) and R = 100 (bottom panel), approximating that of the data. The models are not normalized to the data but are fluxes at Earth computed with the known distance of Gl 570D and the radius obtained from the evolution with the above parameters.](/figures/figure-8-spectrum-of-gliese-570d-the-data-are-shown-in-black-1mko98zk.webp)
