Study of the K-H${_2}$ quasi-molecular line satellite in the potassium resonance line

Abstract

Context. The optical spectra of L and T-type dwarfs exhibit a continuum dominated by the far wings of the absorption profiles of the Na 3s-3p and K 4s-4p doublet perturbed by molecular hydrogen and helium. The presence of a broad absorption feature in the blue wing of the K 4s-4p doublet may be attributed to the K-H 2 quasi-molecular line satellite as we predicted in a previous work. Aims. The position of the predicted line satellite was not in good agreement with the observed feature. This was a motivation to compute new interaction potentials and corresponding profiles. Methods. New molecular potentials for K-H 2 were computed using a valence pseudopotential with Gaussian orbitals on potassium, for an H 2 molecule assumed to be in its ground state. A quasi-full three-electron multireference calculation agrees very well with available spectroscopic data. The potentials and radiative dipole transition moments were input data for a unified spectral line shape evaluation of the complete K resonance line profile. Results. We show that these improved potentials now predict a line satellite at the position that is observed in late brown dwarf spectra. The existence, shape and strength of this feature is found to depend strongly on temperature, as well on the abundance ratio of potassium and sodium. Conclusions. The structure of the continuum in brown dwarf spectra from 500 to 1000 nm is determined by radiative collisions of alkali atoms with H 2 molecules. We conclude that the sensitivity of the spectrum to temperature, pressure and abundances is a tool for determining basic parameters of brown dwarf atmospheres, provided that the physics underlying its formation is described well.