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 (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)