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 rst focus on the surprising results of recent steady-state, two-component models, which indicate that the limited Coulomb coupling asso- ciated with suprathermal ion drift leads not to an ion runaway, but instead to a relatively sharp shift of both the ion and passive uids to a much lower outward acceleration. Drawing upon analogies with subsonic outo w in the solar wind, we provide a physical discussion of how this lower acceleration is the natural conse- quence of the weaker frictional coupling allowing the ion line-driving to maintain its steady-state balance against collisional drag with a comparitively shallow ion velocity gradient. However, we then carry out a time-dependent, linearized sta- bility analysis of these two-component steady solutions, and thereby nd that, as the ion drift increases from sub- to suprathermal speeds, a wave mode character- ized by separation between the ion vs. passive-plasma goes from being strongly damped to being strongly amplied. Unlike the usual line-driven-o w instabil- ity of high-density, strongly-coupled o ws, this ion separation instability occurs even in the long-wavelength, Sobolev limit, although with only 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 tem- poral growth rate exceeds the already rapid growth of line-driven instability by a