  Ambient interstellar material may become entrained in outflows from massive
stars as a result of shear flow instabilities. We study the linear theory of
the Kelvin - Helmholtz instability, the simplest example of shear flow
instability, in a partially ionized medium. We model the interaction as a two
fluid system (charged and neutral) in a planar geometry. Our principal result
is that for much of the relevant parameter space, neutrals and ions are
sufficiently decoupled that the neutrals are unstable while the ions are held
in place by the magnetic field. Thus, we predict that there should be a
detectably narrower line profile in ionized species tracing the outflow
compared with neutral species since ionized species are not participating in
the turbulent interface with the ambient ISM. Since the magnetic field is
frozen to the plasma, it is not tangled by the turbulence in the boundary
layer.
