Kelvin‐Helmholtz Instability in a Weakly Ionized Medium

Ambient interstellar material may become entrained in outflows from massivestars as a result of shear flow instabilities. We study the linear theory ofthe Kelvin - Helmholtz instability, the simplest example of shear flowinstability, in a partially ionized medium. We model the interaction as a twofluid system (charged and neutral) in a planar geometry. Our principal resultis that for much of the relevant parameter space, neutrals and ions aresufficiently decoupled that the neutrals are unstable while the ions are heldin place by the magnetic field. Thus, we predict that there should be adetectably narrower line profile in ionized species tracing the outflowcompared with neutral species since ionized species are not participating inthe turbulent interface with the ambient ISM. Since the magnetic field isfrozen to the plasma, it is not tangled by the turbulence in the boundarylayer.Comment: 21 pages, 4 figure