Beyond the T dwarfs: Theoretical spectra, colors, and detectability of the coolest brown dwarfs

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 T-eff range from similar to800 to similar to130 K and with masses from 25 to 1 M-J. 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 similar to0.4 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 T-eff (a prominent feature of the hotter T dwarf family), the T-eff'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 T-eff 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.