The "lithium test" and the spectra of late M dwarfs and brown dwarfs: Condensation effects

A system of ionization-dissociation equilibrium equations for model atmosph eres for late M dwarfs are solved, and spectra of late M dwarfs and brown d warfs are calculated in the region of lithium resonance lines, taking into account the decrease of the TiO concentration due to condensation. In the c ontext of the approach adopted, the TiO partial pressure could not exceed t he saturation pressure. It was found that (1) in the outer atmospheres of l ate M dwarfs and brown dwarfs with T-eff approximate to 1000 K, TiO molecul es are oversaturated; (2) in these regions, the TiO density is substantiall y reduced due to condensation (for T-eff approximate to 1000 K and log g = 5.0), which reduces the intensity of the TiO bands in the spectra of M dwar fs; (3) in the atmospheres of late M dwarfs, lithium-containing molecules a re obviously undersaturated; (4) the "lithium test" is quite applicable for T-eff less than or similar to 1000 K, and even for lower temperatures.