Simulation of energy distributions in optical and IR spectra of late-type M4–M6 dwarfs

Abstract

Spectral types (M4–M6), effective temperatures Tef (2700–2900 K), and free fall accelerations logg (4.0–4.5) are determined for five M dwarfs using their energy distributions in the spectral range λλ = 680…840 nm. Stellar spectra with resolutions R = 4000 were obtained using the IMACS spectrograph mounted on the ESO Walter Baade 6.5-m telescope. The spectral types are derived from spectral indices and the effective temperatures of the stars are estimated based on their spectral types. Values of Tef and logg are also derived from the comparison between the observed and theoretical energy distributions, calculated both for dust-free, standard NextGen model atmospheres of red dwarfs, and for semiempirical models considering the presence of dust in stellar atmospheres according to the technique developed by Pavlenko et al. We determine dust parameters for stellar atmospheres of these stars, and establish that it is necessary to account for the decrease in concentration of TiO molecules due to their condensation on dust grains, when Tef < 3000 K. We conclude that the radiation scattering by dust grains does not have an appreciable effect on energy distributions in the spectra of the considered stars.