New theoretical study of potassium perturbed by He and a comparison to laboratory spectra

The visible and near-infrared spectra of late L- and T-type dwarf stars are dominated in large part by the resonance lines of neutral Na and K. It is the collision broadening of these atomic lines by H-2 and He in the stellar atmosphere that determines the continuum from below 0.5 mu m to above 0.9 mu m in the spectrum. Their line profiles can be detected as far as 3000 cm(-1) from the line center in T dwarfs and consequently an accurate and detailed determination of the complete profile, including the extreme far wing, is required to model the contribution of these strong alkali resonance lines to brown dwarf spectra. We report on our new calculations of unified line profiles of K perturbed by He using ab initio potential data for the conditions prevailing in cool substellar brown dwarfs and hot dense planetary atmospheres with temperatures from T-eff = 500 to 3000 K. For such objects with atmospheres of H-2 and He, conventional laboratory absorption spectroscopy can be used to examine the line wings and test the line shape theories and molecular potentials. We find that an analytical Lorentzian profile is useful for a few cm(-1) from the line center, but not in the line wings, where the radiative transfer is a consequence of the K-He radiative collisions that are sensitive to the interaction potentials. Tables of the K-He absorption coefficients of the resonance lines allow accurate model atmospheres and synthetic spectra. For this purpose, we present new opacities from comprehensive line shape theory incorporating accurate ab initio potentials. Use of these new tables for the modeling of emergent spectra will be an improvement over previous line shape approximations based on incomplete or inaccurate potentials. We also present Lorentzian impact parameters obtained in the semi-classical and quantum theory for the K 4s - 4p resonance line centered at 0.77 mu m specifically for the line core regime.