We present the results of a new series of non-gray calculations of the atmospheres, spectra, colors, and evolution of extrasolar giant planets (EGPs) and brown dwarfs for effective temperatures below 1300 K. This theory encompasses most of the mass/age parameter space occupied by substellar objects and is the first spectral study down to 100 K. These calculations are in aid of the multitude of searches being conducted or planned around the world for giant planets and brown dwarfs and reveal the exotic nature of the class. Generically, absorption by H 2 at longer wavelengths and H 2 O opacity windows at shorter wavelengths conspire to redistribute flux blueward. Below 1200 K, methane is the dominant carbon bearing molecule and is a universal diagnostic feature of EGP and brown dwarf spectra. We find that the primary bands in which to search are Z (∼1.05 µm), J (∼1.2 µm), H (∼1.6 µm), K (∼2.2 µm), M (∼5 µm), and N (∼10 µm), that enhancements of the emergent flux over blackbody values, in particular in the near infrared, can be by many orders of magnitude, and that the infrared colors of EGPs and brown dwarfs are much bluer than previously believed. In particular, relative to J and H, the K band flux is reduced by CH 4 and H 2 absorption. Furthermore, we derive that for T eff s below 1200 K most or all true metals are sequestered below the photosphere, that an interior radiative zone is a generic feature of substellar objects, and that clouds of H 2 O and NH 3 are formed for T eff s below ∼400 K and ∼200 K, respectively. This study is done for solar-metallicity objects in isolation and does not include the effects of stellar insolation. Nevertheless, it is a comprehensive attempt to bridge the gap between the planetary and stellar realms and to develop a non-gray theory of objects from 0.3 M J ("saturn") to 70 M J (∼0.07 M ). We find that the detection ranges for brown dwarf/EGP -3discovery of both ground-and space-based telescopes are larger than previously estimated.

We calculate detailed chemical abundance profiles for a variety of brown dwarf and extrasolar giant planet atmosphere models, focusing in particular on Gliese 229B, and derive the systematics of the changes in the dominant reservoirs of the major elements with altitude and temperature. We assume an Anders and Grevesse (1989) solar composition of 27 chemical elements and track 330 gas-phase species, including the monatomic forms of the elements, as well as about 120 condensates. We address the issue of the formation and composition of clouds in the cool atmospheres of substellar objects and explore the rain out and depletion of refractories. We conclude that the opacity of clouds of low-temperature (≤900 K), small-radius condensibles (specific chlorides and sulfides, not silicates), may be responsible for the steep spectrum of Gliese 229B observed in the near infrared below 1 µm . Furthermore, we assemble a temperature sequence of chemical transitions in substellar atmospheres that may be used to anchor and define a sequence of spectral types for substellar objects with T eff s from ∼2200 K to ∼100 K.

We present a comprehensive description of the theory and practice of opacity calculations from the infrared to the ultraviolet needed to generate models of the atmospheres of brown dwarfs and extrasolar giant planets. Methods for using existing line lists and spectroscopic databases in disparate formats are presented, and plots of the resulting absorptive opacities versus wavelength for the most important molecules and atoms at representative temperature/ pressure points are provided. Electronic, rovibrational, bound-free, bound-bound, free-free, and collision-induced transitions and monochromatic opacities are derived, discussed, and analyzed. The species addressed include the alkali metals, iron, heavy metal oxides, metal hydrides, H 2 , H 2 O, CH 4 , CO, NH 3 , H 2 S, PH 3 , and representative grains. Once monochromatic absorption cross sections for all constituents have been derived, chemical abundances have to be obtained before the resulting product can be summed to obtain total opacities. Hence, we include a review of the thermochemistry, techniques, and databases needed to derive equilibrium abundances and provide some sample results.

We identify the pressure-broadened red wings of the saturated potassium resonance lines at 7700Å as the source of anomalous absorption seen in the near-infrared spectra of Gliese 229B and, by extension, of methane dwarfs in general. In broad outline, this conclusion is supported by the recent work of Tsuji et al. 1999. The WFPC2 I band measurement of Gliese 229B is also consistent with this hypothesis. Furthermore, a combination of the blue wings of this K I resonance doublet, the red wings of the Na D lines at 5890Å, and, perhaps, the Li I line at 6708Å can explain in a natural way the observed WFPC2 R band flux of Gliese 229B. Hence, we conclude that the neutral alkali metals play a central role in the near-infrared and optical spectra of methane dwarfs and that their lines have the potential to provide crucial diagnostics of brown dwarf properties.The slope of the spectrum from 0.8 µm to 0.9 µm for the Sloan methane dwarf, SDSS 1624+00, is shallower than that for Gliese 229B and its Cs lines are weaker. From this, we conclude that its atmosphere is tied to a lower core entropy or that its K and Cs abundances are smaller, with a preference for the former hypothesis. We speculate on the systematics of the near-infrared and optical spectra of methane dwarfs, for a given mass and composition, that stems from the progressive burial with decreasing T eff of the alkali metal atoms to larger pressures and depths.Moreover, we surmise that those extrasolar giant planets (EGPs) that achieve T eff s in the 800-1300 K range due to stellar insolation will show signatures of the neutral alkali metals in their albedo and reflection spectra. We estimate that, due predominantly to absorption by Na D lines, the geometric albedo of the EGP τ Boo b at λ = 0.48 µm is < 0.1, consistent with the new (and low) upper limit of 0.3 recently obtained by Charbonneau et al. (1999).

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