3.6-7.9 μm Photometry of L and T Dwarfs and the Prevalence of Vertical Mixing in their Atmospheres

We present new L^' (3.75 μm) photometry of six L and T dwarfs, and M^' (4.70 μm) photometry of 10 L and T dwarfs, observed at the Gemini (North) Observatory, and new 3.55, 4.49, 5.73, and 7.87 μm photometry of nine L and T dwarfs, obtained with the Spitzer Space Telescope. The sample includes unusually blue and red dwarfs selected from our near-infrared studies. The data are combined with previously published L^', M^', and Spitzer photometry of L and T dwarfs, and trends of colors with spectral type and other colors are examined. Recent model atmospheres by Marley and Saumon are used to generate synthetic colors for ranges of effective temperature, gravity, grain sedimentation efficiency, metallicity, and vertical mixing efficiency. We explore how these parameters affect the mid-infrared colors of L and T dwarfs and find that the data are modeled satisfactorily only if substantial vertical mixing occurs in both L and T dwarf atmospheres. The location and range of the L and T dwarf sequences in IRAC color-color and color-magnitude diagrams is also only reproduced if this mixing occurs, with a range of efficiency described by K_zz~10²-10⁶ cm² s^-1. The colors of the unusually red dwarfs are best reproduced by nonequilibrium models with low sedimentation efficiency, i.e., thick cloud decks, and the colors of the unusually blue dwarfs by nonequilibrium models with high sedimentation efficiency, i.e., thin cloud decks. The K-L^' and Spitzer [3.55]-[4.49] colors can be used as indicators of effective temperature for L and T dwarfs, but care must be taken to include gravity and metallicity effects for late-T dwarfs and vertical mixing for both late-L and T dwarfs.