Evolutionary Models for Very Low‐Mass Stars and Brown Dwarfs with Dusty Atmospheres

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Abstract
We present evolutionary calculations for very low-mass stars and brown dwarfs based on synthetic spectra and nongray atmosphere models which include dust formation and opacity, i.e., objects with Teff 2800 K. The interior of the most massive brown dwarfs is shown to develop a conductive core after ~2 Gyr which slows down their cooling. Comparison is made in optical and infrared color-magnitude diagrams with recent late-M and L dwarf observations. The saturation in optical colors and the very red near-infrared colors of these objects are well explained by the onset of dust formation in the atmosphere. Comparison of the faintest presently observed L dwarfs with these dusty evolutionary models suggests that dynamical processes such as turbulent diffusion and gravitational settling are taking place near the photosphere. As the effective temperature decreases below Teff ≈ 1300-1400 K, the colors of these objects move to very blue near-infrared colors, a consequence of the ongoing methane absorption in the infrared. We suggest the possibility of a brown dwarf dearth in J, H, and K color-magnitude diagrams around this temperature.
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