We review the current theory of very low mass stars model atmospheres including the coolest known M dwarfs, M subdwarfs, and brown dwarfs, i.e. Teff ≤ 5,000K and −2.0 ≤ [M/H] ≤ +0.0. We discuss ongoing efforts to incorporate molecular and grain opacities in cool stellar spectra, as well as the latest progress in deriving the effective temperature scale of M dwarfs. We especially present the latest results of the models related to the search for brown dwarfs.

Very low mass star models and the Teff scale

Very Low Mass stars (VLMs) with masses from about 0.3 M⊙ to the hydrogen burning minimum mass (0.075 M⊙, Baraffe et al. 1995) and young substellar brown dwarfs share similar atmospheric properties. Most of their photospheric hydrogen is locked in H2 and most of the carbon in CO, with the excess oxygen forming important molecular absorbers such as TiO, VO, and H2O. They are subject to an efficient convective mixing often reaching the uppermost layers of their photosphere. Their energy distribution is governed by the millions of absorption lines of TiO, VO, CaH, and FeH in the optical to near-infrared, and H2O and CO in the infrared, which leave no window of true continuum. But as brown dwarfs cool with age, they begin to differentiate themselves with the formation of methane (CH4) in the infrared (Tsuji et al. 1995; Allard et al. 1996).