Evolutionary models for very low-mass stars and brown dwarfs with dusty atmospheres

We present evolutionary calculations for very low-mass stars and brown dwar fs based on synthetic spectra and nongray atmosphere models which include d ust formation and opacity, i.e., objects with T-eff less than or similar to 2800 K. The interior of the most massive brown dwarfs is shown to develop a conductive core after similar to2 Gyr which slows down their cooling. Com parison is made in optical and infrared color-magnitude diagrams with recen t 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 o nset of dust formation in the atmosphere. Comparison of the faintest presen tly observed L dwarfs with these dusty evolutionary models suggests that dy namical processes such as turbulent diffusion and gravitational settling ar e taking place near the photosphere. As the effective temperature decreases below T-eff approximate to 1300-1400 K, the colors of these objects move t o very blue near-infrared colors, a consequence of the ongoing methane abso rption in the infrared. We suggest the possibility of a brown dwarf dearth in J, H, and K color-magnitude diagrams around this temperature.