1996A&A...305L...1T

If dust condenses in any stellar photosphere, the most likely place is that of late M dwarfs, where the thermo-chemical condition of condensation is easily fulfilled because of the low temperature and high density. Assuming that the dust formation follows the thermodynamics of the phase transition, dust opacities are included in the construction of non-grey model atmospheres in radiative and convective equilibria. Once dust is formed in the stellar surface, the photosphere is heated significantly because it is enveloped by an efficient heat absorber. The dust formation is suppressed by this heating, but an equilibrium is reached with a small amount of dust in the atmospheres of M dwarfs with T_eff<2,800K. The higher temperature of the dusty atmospheres and the extinction by the dust weaken the molecular line absorption, which was too strong in dust-free models of cool M dwarfs as compared with observations. Thus, the dusty models provide a consistent interpretation of observed spectra for the first time and hence the physical basis for determining the effective temperature scale of M dwarfs. The introduction of dust formation in stellar photospheres, however, offers new challenges: for example, for theoretical modeling we must consider the phase transition with associated physical processes as in the planetary atmospheres, and we need sensitive infrared observations such as by the ISO for detailed observational tests of the dusty models.