Ion-ion charge exchange and X-rays from the winds of hot stars

Charge exchange occurs between charged ions with enough energy to overcome Coulomb repulsion, a condition satisfied for collisions at velocities like those of the winds driven from hot stars by radiation pressure. X-ray line ratios in some hot stars are inconsistent with those expected from thermal plasmas excited by electron impact. Ion-ion interactions including charge exchange might be responsible instead if high-velocity collisions between ions are enabled by the presence of a magnetic field in the wind, suggesting a possible alternative mechanism to the widely accepted instability-driven shock model. The nature of a plasma in charge-exchange equilibrium is yet to be determined.