We present new 5.2-14.5 μm low-resolution spectra of 14 mid-L to mid-T dwarfs. We also present new 3.0-4.1 μm spectra for five of these dwarfs. These data are supplemented by existing red and near-infrared spectra (∼0.6-2.5 μm), as well as red through mid-infrared spectroscopy of seven other L and T dwarfs presented by Cushing et al. We compare these spectra to those generated from the model atmospheres of Saumon & Marley. The models reproduce the observed spectra well, except in the case of one very red L3.5 dwarf, 2MASS J22244381−0158521. The broad wavelength coverage allows us to constrain almost independently the four parameters used to describe these photospheres in our models: effective temperature (T eff ), surface gravity, grain sedimentation efficiency (f sed ), and vertical gas transport efficiency (K zz ). The CH 4 bands centered at 2.2, 3.3, and 7.65 μm and the CO band at 2.3 μm are sensitive to K zz , and indicates that chemical mixing is important in all L and T dwarf atmospheres. The sample of L3.5 to T5.5 dwarfs spans the range 1800 K T eff 1000 K, with an L-T transition (spectral types L7 to T4) that lies between 1400 and 1100 K for dwarfs with typical near-infrared colors; bluer and redder dwarfs can be 100 K warmer or cooler, respectively, when using infrared spectral types. When using optical spectral types, the bluer dwarfs have more typical T eff values as they tend to have earlier optical spectral types. In this model analysis, f sed increases rapidly between types T0 and T4, indicating that increased sedimentation can explain the rapid disappearance of clouds at this stage of brown dwarf evolution. There is a suggestion that the transition to dust-free atmospheres happens at lower temperatures for lower gravity dwarfs.

The reflected spectra of extrasolar giant planets are primarily influenced by Rayleigh scattering, molecular absorption, and atmospheric condensates. We present model geometric albedo and phase integral spectra and Bond albedos for planets and brown dwarfs with masses between 0.8 and 70 Jupiter masses. Rayleigh scattering predominates in the blue while molecular absorption removes most red and infrared photons. Thus cloud-free atmospheres, found on giant planets with effective temperatures exceeding about 400 K, are quite dark in reflected light beyond 0.6 µm.In cooler atmospheres first water clouds and then other condensates provide a bright reflecting layer. Only planets with cloudy atmospheres will be detectable in reflected light beyond 1 µm. Thermal emission dominates the near-infrared for warm objects with clear atmospheres. However the presence of other condensates, not considered here, may brighten some planets in reflected near-infrared light and darken them in the blue and UV. Bond albedos, the ratio of the total reflected to incident power, are sensitive to the spectral type of the primary. Most incident photons from early type stars will be Rayleigh scattered, while most incident photons from late type stars will be absorbed. The Bond albedo of a given planet thus may range from 0.4 to 0.05, depending on the primary type. Condensation of a water cloud may increase the Bond albedo of a planet by up to a factor of two. The spectra of cloudy planets are strongly influenced by poorly constrained cloud microphysical properties, particularly particle size and supersaturation. Both Bond and geometric albedos are comparatively less sensitive to variations in planet mass and effective temperature.

We present new infrared spectra of the T8 brown dwarf 2MASS J04151954À0935066: 2.9-4.1 m spectra obtained with the Infrared Camera and Spectrograph on the Subaru Telescope, and 5.2-14.5 m spectra obtained with the Infrared Spectrograph on the Spitzer Space Telescope. We use these data and models to determine an accurate bolometric luminosity of log L bol /L ¼ À5:67 and to constrain the effective temperature, gravity, mass, and age to 725-775 K, log g ¼ 5:00 5:37, M ¼ 33 58 M Jup , and age ¼ 3 10 Gyr. We perform the same analysis using published 0.6-15 m spectra for the T7.5 dwarf 2MASS J12171110À0311131, for which we find a metal-rich composition (½Fe/H $ 0:3), and log L bol /L ¼ À5:31, T eA ¼ 850 950 K, log g ¼ 4:80 5:42, M ¼ 25 66 M Jup , and age ¼ 1 10 Gyr. These luminosities and effective temperatures straddle those determined with the same method and models for Gl 570D by Saumon et al. and make 2MASS J04151954À0935066 the coolest and least luminous T dwarf with well-determined properties. We find that synthetic spectra generated by the models reproduce the observed red through mid-infrared spectra of 2MASS J04151954À0935066 and 2MASS J12171110À0311131 very well, except for known discrepancies that are most likely due to the incomplete CH 4 opacities. Both objects show evidence of departures from strict chemical equilibrium, and we discuss this result in the context of other late T dwarfs in which disequilibrium phenomena have been observed.

In the last few years a significant population of ultracool L and T dwarfs has been discovered. With effective temperatures ranging from ∼ 2200 to 700 K, these objects emit most of their radiation in the near infrared and their spectral energy distributions are dominated by strong molecular absorption bands. These highly structured energy distributions lead to JHK magnitudes that are extremely sensitive to the exact filter bandpass used. In the case of the T dwarfs, the differences between commonly used photometric systems can be as large as 0.4 mag at J and 0.5 mag at J − K.Near-infrared magnitudes have been published for L and T dwarfs using a variety of photometric systems. Currently, the data obtained with these systems cannot be accurately compared or combined as transformations based on the colors of hotter stars are not valid for L and T dwarfs. To address this problem, we have synthesized J, H, and K magnitudes for some of the common photometric systems and present transformation equations with respect to the most atmospheric-independent system, the Mauna Kea Observatory (MKO) filter set. If the spectral type of the dwarf is known, our transformations allow data to be converted between systems to 0.01 mag, which is better than the typical measurement uncertainty. Transforming on the basis of color alone is more difficult because of the degeneracy and intrinsic scatter in the near-infrared colors of L and T dwarfs; in this case J magnitudes can only be transformed to 0.05 mag and H and K to 0.02 mag.

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