K-H2 line shapes for the spectra of cool brown dwarfs

Observations of cooler and cooler brown dwarfs show that the contribution from broadening at many bars pressure is becoming important. The opacity in the red optical to near-IR region under these conditions is dominated by the extremely pressure-broadened wings of the alkali resonance lines, in particular, the K I resonance doublet at 0.77 mu m. Collisions with H-2 are preponderant in brown dwarf atmospheres at an effective temperature of about 1000 K; the H-2 perturber densities reach several 10(19) even in Jupiter-mass planets and exceed 10(20) for super-Jupiters and older Y dwarfs. As a consequence, it appears that when the far wing absorption due to alkali atoms in a dense H-2 atmosphere is significant, accurate pressure broadened profiles that are valid at high densities of H-2 should be incorporated into spectral models.