3.6–7.9μm 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, andM ′ (4.70 μm) photometry of ten L and T dwarfs, observed at 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 modelled satisfactorily only if substantial vertical mixing occurs in both Land 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 Kzz ∼ 10 –10 cm s. The colors of the unusually red dwarfs are best reproduced by non-equilibrium models with low sedimentation efficiency, i.e. thick cloud decks, and the colors of the unusually blue dwarfs by non-equilibrium 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. Subject headings: infrared: stars — stars: low-mass, brown dwarfs 1