ar X iv : a st ro - p h / 97 05 07 1 v 1 9 M ay 1 99 7 MODEL ATMOSPHERES : Brown Dwarfs from the Stellar Perspective

In this paper, 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. T eff ≤ 5, 000 K 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 also present the latest results of the models related to the search for brown dwarfs. 1. Very Low Mass Star models and the T eff 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 H 2 and most of the carbon in CO, with the excess oxygen forming important molecular absorbers such as TiO, VO, FeH and H 2 O. 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 and VO in the optical, and H 2 O 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 (CH 4) in the infrared (Tsuji et al. 1995) at the expense of CO which bands begin to weaken in their spectra (Allard et al. 1996). Across the stellar-to-substellar boundary, clouds of e.g. corundum (Al 2 O 3), perovskite (CaTiO 3), iron, enstatite (MgSiO 3), and forsterite (Mg 2 SiO 4) may form, depleting the oxygen compounds and heavy elements and profoundly modifying the thermal structure and opacity of their photosphere Because these processes also occur in the stellar regime where a greater census of cool dwarfs is currently available for study, a proper quantitative understanding of VLM stars near the hydrogen burning limit is a prerequisite to an understanding of the spectroscopic properties and parameters of brown dwarfs and jovian-type planets. Model atmospheres have been constructed by several investigators over recent years with the primary goals of: 1