The Role of Dust Clouds in the Atmospheres of Brown Dwarfs

The new spectroscopic classes, L and T, are defined by the role of dust clouds in their atmospheres, the former by their presence and the latter by their removal and near absence. Moreover, the M to L and L to T transitions are intimately tied to the condensation and character of silicate and iron grains, and the associated clouds play pivotal roles in the colors and spectra of such brown dwarfs. Spanning the effective temperature range from ∼2200 K to ∼600 K, these objects are being found in abundance and are a new arena in which condensation chemistry and the optical properties of grains is assuming astronomical importance. In this short paper, I summarize the role played by such refractories in determining the properties of these “stars” and the complexities of their theoretical treatment. 1. The Importance of Dust Clouds in Cool Atmospheres As the temperatures of a stellar atmosphere decrease below ∼4000 K, molecules form and begin to dominate. Water and carbon monoxide are two of the first to make their mark, but despite the low elemental abundance of titanium (∼10) and vanadium, TiO and VO too emerge in the M dwarf range as distinctive signatures in the optical. Figure 1 portrays the elements in order of abundance, shows the major molecules into which these elements partition as the temperature decreases, and suggests which species predominate due to their relative abundance. However, as Teff decreases further and approaches ∼2200 K, many of the more refractory elements condense out into clouds of dust. Figure 2 depicts many of the corresponding condensation curves at solar metallicity. Titanium begins to form perovskite (CaTiO3) and higher oxides, followed near ∼1700−1800 K by vanadium, which first forms condensed VO. Importantly, calcium-aluminum and calcium-magnesium silicates (such as akermanite, diopside, hibonite, and grossite) form and sequester many refractory elements into grains, whose depletion is indirectly manifest by the gradual disappearance of atomic lines of titanium, calcium, aluminum, and silicon. More importantly, a haze is formed that thickens into formidable clouds whose continuum opacity begins to redden the object’s near-infrared spectrum. This reddening signals the appearance of the new spectroscopic class of L dwarfs. Indeed, the M to L transition is caused by the formation of dust (and the simultaneous disappearance of TiO and VO). The entire L dwarf sequence is dominated by the prevalence of dust clouds and is defined by red J −K colors. Since the calcium and aluminum abundances are ∼10× smaller than the magnesium and silicon abundances, trapping the former into the most refractory compounds in stoichiometric ratios leaves plenty of the