  At the low temperatures achieved in cool brown dwarf and hot giant planet
atmospheres, the less refractory neutral alkali metals assume an
uncharacteristically prominent role in spectrum formation. In particular, the
wings of the Na-D (5890 \AA) and K I (7700 \AA) resonance lines come to define
the continuum and dominate the spectrum of T dwarfs from 0.4 to 1.0 \mic.
Whereas in standard stellar atmospheres the strengths and shapes of the wings
of atomic spectral lines are rarely needed beyond 25 \AA of a line center, in
brown dwarfs the far wings of the Na and K resonance lines out to 1000's of \AA
detunings are important. Using standard quantum chemical codes and the Unified
Franck-Condon model for line profiles in the quasi-static limit, we calculate
the interaction potentials and the wing line shapes for the dominant Na and K
resonance lines in H$_2$- and helium-rich atmospheres. Our theory has natural
absorption profile cutoffs, has no free parameters, and is readily adapted to
spectral synthesis calculations for stars, brown dwarfs, and planets with
effective temperatures below 2000 Kelvin.

Astrophysics