Alkali Line Profiles in Ultracool White Dwarfs

We present PHOENIX atmosphere models for metal-rich cool white dwarfs using improved line shapes for the Na I and K I resonance doublets. Profiles for collisional broadening due to H2 and He based on the adiabatic representation show strong deviations from Van der Waals interaction at short distances. Comparison with observed spectra that show extremely broadened Na I lines indicates that a He-rich atmospheric composition is required to explain the line strengths and spectral energy distributions. Our current synthetic spectra, using an expansion in powers of density to the third order optimised for brown dwarf atmosphere conditions, significantly underestimate the observed absorption in the far wings, even predicting smaller total line strength than a Lorentzian profile. This is due to the handling of multiple perturber interactions becoming inadequate for the extreme densities of the coolest white dwarfs. The density expansion would have to be extended at least to the 7th order for an accurate treatment of such conditions and might break down altogether in the densest objects. The results of a direct calculation of the unified profile should therefore be used for model atmospheres of cool metal-rich white dwarfs. Qualitative comparison of the full adiabatic profile to the spectrum of WD2356-209 indicates good agreement with the observed line shape. Observations of the coolest white dwarfs may therefore serve as a laboratory for testing the physics of the deeper atmospheres and interiors of brown dwarfs and giant planets.