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.

Theoretical profiles of the Mg+resonance lines perturbed by collisions with He

“…1), mainly in the slope of the repulsive walls, leads to this close agreement in the line wings and the prediction of the quasi-molecular line satellite at 0.23 µm. In Allard et al (2007c), we have shown that ab initio K-H 2 potentials systematically less repulsive than the pseudo-potentials of Rossi & Pascale (1985) allowed a K-H 2 quasi-molecular line satellite to be predicted, which closely matches the position and shape of an observed feature in the spectrum of the T1 dwarf ε Indi Ba (Allard et al 2007a). More recently, in Allard et al (2016b), the position of the predicted line satellite was confirmed by a laboratory measurement.…”

K–H2line shapes for the spectra of cool brown dwarfs

“…In this continuation of previous work (Allard et al 2003(Allard et al , 2007c, we illustrate the evolution of the absorption spectra of K-H 2 collisional profiles for the densities and temperatures A&A 589, A21 (2016) prevailing in atmosphere of cool brown dwarf stars (Sect. 4).…”

“…Alkali-H 2 collisional profiles have been the subject of previous studies by Burrows & Volobuyev (2003) and Allard et al (2003Allard et al ( , 2007c. Absorption profiles of sodium and potassium perturbed by helium and molecular hydrogen were presented in Burrows & Volobuyev (2003) using multiconfiguration selfconsistent field Hartree-Fock potentials in the Szudy theory (Szudy & Baylis 1975.…”

“…This approach to calculating the spectral line profile requires the knowledge of molecular potentials with high accuracy because the shape and strength of the line profile are very sensitive to the details of the molecular potential curves describing the K-H 2 collisions. Ab initio calculations of the potentials of K-H 2 for different orientations of the molecule have been reported in Santra & Kirby (2005) and Allard et al (2007c), neglecting the spin-orbit coupling (SO). In a previous paper (Allard et al 1999), we derived a classical path expression for a pressure-broadened atomic spectral line shape that allows for an electric dipole moment that is dependent on the position of perturbers, which is not included in the more usual approximation of Anderson & Talman (1956) and Baranger (1958a,b).…”

Water vapour in the atmosphere of a transiting extrasolar planet

“…The line shapes of these alkali metals were calculated at different temperatures and interpolated for intermediate values 21,22 . Their spectral contribution becomes important in the visible wavelength range (Fig.…”