  Ultracool stellar atmospheres show absorption by alkali resonance lines
severely broadened by collisions with neutral perturbers. In the coolest and
densest atmospheres, such as those of T dwarfs, Na I and K I broadened by
molecular hydrogen and helium can come to dominate the entire optical spectrum.
The effects of NaHe collision broadening are also central to understanding the
opacity of cool DZ white dwarf stars. In order to be able to construct
synthetic spectra of brown dwarfs and cool DZ white dwarfs, where helium
density can reach several 10^21~cm-3 NaHe line profiles of the resonance lines
have been computed over a wide range of densities and temperatures. Unified
line profiles that are valid from the core to the far wings at high densities
are calculated in the semiclassical approach using up-to-date molecular data
including in particular electronic spin-orbit coupling from the sodium atom. We
present a comprehensive study of Na-He collisional profiles at high density,
and temperatures from 5000~K, the temperature prevailing in the atmosphere of
ultra-cool DZ white dwarf stars, down to 1~K, the temperature in liquid helium
clusters. Collision broadening and shift parameters within the impact
approximation obtained in the semiclassical and quantum theory using our new
accurate molecular data are presented.
