Projectile charge dependence of electron-capture cross sections (cosmic rays-interstellar gas interactions)

The cross section for electron capture by highly charged ions from atomic hydrogen is computed in the framework of the continuum distorted-wave (CDW) approximation. The theoretical results are in remarkably good agreement with experiments on K X-ray emission in F 9+ -H collisions and on electron capture of K-shell electrons in Ar by high-energy protons. The results are also used to determine the limits of usefulness of the Brinkman-Kramers (BK) approximation. A brief discussion is given of the consequences on the production of X-ray lines in the interstellar medium by low-energy cosmic rays.