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Beyond the T Dwarfs: Theoretical Spectra, Colors, and Detectability of the Coolest Brown Dwarfs

Beyond the T Dwarfs: Theoretical Spectra, Colors, and Detectability of the Coolest Brown Dwarfs,10.1086/377709,Astrophysical Journal,Adam Burrows,Davi

Beyond the T Dwarfs: Theoretical Spectra, Colors, and Detectability of the Coolest Brown Dwarfs   (Citations: 30)
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We explore the spectral and atmospheric properties of brown dwarfs cooler than the latest known T dwarfs. Our focus is on the yet-to-be-discovered free-floating brown dwarfs in the Teff range from ~800 to ~130 K and with masses from 25 to 1 MJ. This study is in anticipation of the new characterization capabilities enabled by the launch of the Space Infrared Telescope Facility (SIRTF) and the eventual launch of the James Webb Space Telescope (JWST). In addition, it is in support of the continuing ground-based searches for the coolest substellar objects. We provide spectra from ~0.4 to 30 mum, highlight the evolution and mass dependence of the dominant H2O, CH4, and NH3 molecular bands, consider the formation and effects of water ice clouds, and compare our theoretical flux densities with the putative sensitivities of the instruments on board SIRTF and JWST. The latter can be used to determine the detection ranges from space of cool brown dwarfs. In the process, we determine the reversal point of the blueward trend in the near-infrared colors with decreasing Teff (a prominent feature of the hotter T dwarf family), the Teff's at which water and ammonia clouds appear, the strengths of gas-phase ammonia and methane bands, the masses and ages of the objects for which the neutral alkali metal lines (signatures of L and T dwarfs) are muted, and the increasing role as Teff decreases of the mid-infrared fluxes longward of 4 mum. These changes suggest physical reasons to expect the emergence of at least one new stellar class beyond the T dwarfs. Furthermore, studies in the mid-infrared could assume a new, perhaps transformational, importance in the understanding of the coolest brown dwarfs. Our spectral models populate, with cooler brown dwarfs having progressively more planet-like features, the theoretical gap between the known T dwarfs and the known giant planets. Such objects likely inhabit the Galaxy, but their numbers are as yet unknown.
Journal: Astrophysical Journal - ASTROPHYS J , vol. 596, no. 1, pp. 587-596, 2003
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    • ...The latest suggestion for addition to the spectral sequence is the letter “Y” for objects that lack water absorption bands because the outer atmospheres of Y dwarfs should be cool enough (<~500 K) to sequester water into clouds (Ackerman & Marley 2001, Burrows et al. 2003)...
    • ...The models by Burrows et al. (2003) predict observable amounts of NH3 at 1.5, 1.95 µm (H band) and 2.95 µm (K-band) for substellar dwarfs with effective temperatures below 600 K. These expectations are consistent with the observations by Knapp et al. (2004) and Golimowski et al. (2004) that NH3 absorptions are absent in the H and K bands for the coolest known T dwarf 2MASS J04151954-0935066 with Teff ~700 K...

    Katharina Lodderset al. Chemistry of Low Mass Substellar Objects

    • ...Sφ √ B CAO + Sφ(Chalo + B2/12) , (11) where B is the fractional receiver bandwidth...

    J. L. Codonaet al. A high-contrast coronagraph for the MMT using phase apodization: desig...

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