Ion runaway instability in low-density, line-driven stellar winds

We examine the linear instability of low-density, line-driven stellar winds to runaway of the heavy minor ions when the drift speed of these ions relative to the bulk, passive plasma of hydrogen and helium approaches or exceeds the plasma thermal speed. We first focus on the surprising results of recent steady state, two-component models, which indicate that the limited Coulomb coupling associated with suprathermal ion drift leads not to an ion runaway, but instead to a relatively sharp shift of both the ion and passive fluids to a much lower outward acceleration. Drawing on analogies with subsonic out flow in the solar wind, we provide a physical discussion of how this lower acceleration is the natural consequence of the weaker frictional coupling, allowing the ion line driving to maintain its steady state balance against collisional drag with a comparatively shallow ion velocity gradient. However, we then carry out a time-dependent, linearized stability analysis of these two-component steady solutions and thereby find that, as the ion drift increases from sub- to suprathermal speeds, a wave mode characterized by separation between the ion versus passive plasma goes from being strongly damped to being strongly amplified. Unlike the usual line-driven flow instability of high-density, strongly coupled flows, this ion separation instability occurs even in the long-wavelength Sobolev limit, although with only a modest spatial growth rate. At shorter wavelengths, the onset of instability occurs for ion drift speeds that are still somewhat below the plasma thermal speed and, moreover, generally has a very large spatial growth. For all wavelengths, however, the temporal growth rate exceeds the already rapid growth of line-driven instability by a typical factor of similar to100, corresponding to the mass-density ratio between the bulk plasma and the driven minor ions. We further show that this ion separation mode has an inward propagation speed that is strongly enhanced ( at its maximum by a similar factor of similar to100) over the usual Abbott wave speed of a fully coupled, line-driven flow, implying that in the context of this separation mode, the entire domain of any steady state solution can be considered as subcritical. Finally, we note that, despite the extremely rapid linear growth rate, further analyses and/or simulations will be needed to determine whether the nonlinear evolution of this instability should lead to true ion runaway or instead might perhaps be limited by damping from two-stream plasma instabilities.