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

Abstract

We present new L ′ (3.75 µm) photometry of six L and T dwarfs, and M ′ (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 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 Kzz ∼ 10 2 –10 6 cm 2 s −1. 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