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

We explore the spectral and atmospheric properties of brown dwarfs coolerthan the latest known T dwarfs. Our focus is on the yet-to-be-discoveredfree-floating brown dwarfs in the \teff range from $\sim$800 K to $\sim$130 Kand with masses from 25 to 1 \mj. This study is in anticipation of the newcharacterization capabilities enabled by the launch of SIRTF and the eventuallaunch of JWST. We provide spectra from $\sim$0.4 \mic to 30 \mic, highlightthe evolution and mass dependence of the dominant H$_2$O, CH$_4$, and NH$_3$molecular bands, consider the formation and effects of water-ice clouds, andcompare our theoretical flux densities with the sensitivities of theinstruments on board SIRTF and JWST. The latter can be used to determine thedetection ranges from space of cool brown dwarfs. In the process, we determinethe reversal point of the blueward trend in the near-infrared colors withdecreasing \teff, the \teffs at which water and ammonia clouds appear, thestrengths of gas-phase ammonia and methane bands, the masses and ages of theobjects for which the neutral alkali metal lines are muted, and the increasingrole as \teff decreases of the mid-infrared fluxes longward of 4 \mic. Thesechanges suggest physical reasons to expect the emergence of at least one newstellar class beyond the T dwarfs. Our spectral models populate, with coolerbrown dwarfs having progressively more planet-like features, the theoreticalgap between the known T dwarfs and the known giant planets. Such objects likelyinhabit the galaxy, but their numbers are as yet unknown.Comment: Includes 14 figures, most in color; accepted to the Astrophysical Journa