Bands of NaH lines in Spectra of Late Type Stars

Abstract: The absorption of radiation by systems of NaH molecule bands in the atmospheres of late-type stars is modeled. Calculations of synthetic spectra with model atmosphere parameters, which corresponds to M stars, show that the lines of this molecule form notable spectral details approximately the same intensity at wavelengths from λ ≈ 380 nm to almost ~1100 nm. The recently calculated parameters of the NaH molecule from the Exomol database, as well as a new value of the dissociation potential of this molecule D₀ = 1.975, were used in the calculations. The dependences of the calculated spectral energy distributions on the dissociation potential of the NaH molecule and on the parameters of the stellar atmospheres (T_eff, log g, [Fe/H]) were considered. Analysis of synthetic spectra shows that the lines of the NaH molecule become weak with temperature increasing and gravity decreasing in the stellar atmosphere. Thus, sodium hydride lines cannot be observed either in stars with effective temperatures corresponding to early M stars nor in M giants. NaH lines should appear only in the spectra of cold dwarfs, although the strong absorption of other molecules (TiO, CrH, and FeH) in visible and near-infrared region of the spectrum and absorption by atoms in the blue region make the NaH lines' detection a very complicated task. The energy distribution in the spectrum of the red dwarf VB 10 (M8V) in the blue region of the spectrum is modeled. The results of the analysis show that, under normal conditions and close to the solar chemical composition, NaH molecules provide only an additional component in the opacity of the spectra of cold dwarfs and substellar objects.