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 ( \sim 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 .
( 2008 ) .
We compare these spectra to those generated from the model atmospheres of Saumon & Marley ( 2008 ) .
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 _ { \mathrm { eff } } ) , surface gravity , grain sedimentation efficiency ( f _ { \mathrm { 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 \gtrsim T _ { \mathrm { eff } } \gtrsim 1000 K , with an L-T transition ( spectral types L7–T4 ) that lies between 1400 K 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 _ { \mathrm { eff } } values as they tend to have earlier optical spectral types .
In this model analysis , f _ { \mathrm { 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 .