THE 0.8-14.5 mu m SPECTRA OF MID-L TO MID-T DWARFS: DIAGNOSTICS OF EFFECTIVE TEMPERATURE, GRAIN SEDIMENTATION, GAS TRANSPORT, AND SURFACE GRAVITY

We present new 5.2-14.5 mu m low-resolution spectra of 14 mid-L to mid-T dwarfs. We also present new 3.0-4.1 mu m spectra for five of these dwarfs. These data are supplemented by existing red and near-infrared spectra (similar to 0.6-2.5 mu m), 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 mu m and the CO band at 2.3 mu m 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 greater than or similar to T(eff) greater than or similar to 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.