MODEL ATMOSPHERES OF VERY-LOW MASS STARS AND BROWN DWARFS

As progressively cooler stellar and substellar objects are discovered, the presence first of molecules and then of condensed particulates gr eatly complicates the understanding of their physical properties. Accu rate model atmospheres that include these processes are the key to est ablishing their atmospheric parameters. They play a crucial role in de termining structural characteristics by setting the surface conditions of model interiors and providing transformations to the various obser vational planes. They can reveal the spectroscopic properties of brown dwarfs and help establish their detectability. In this paper, we revi ew the current state-of-the-art theory and modeling of the atmospheres of very low mass stars, including the coolest known M dwarfs, M subdw arfs, and brown dwarfs, i.e. T-eff less than or equal to 4,000 K and - 4.0 less than or equal to [M/H] less than or equal to +0.0. We discuss ongoing efforts to incorporate molecular and grain opacities in cool stellar spectra, as well as the latest progress in (a) deriving the ef fective temperature scale of M dwarfs, (b) reproducing the lower main sequences of metal-poor subdwarfs in the halo and globular clusters, a nd (c) results of the models related to the search for brown dwarfs.