Charge Transfer and Ionization Equilibrium in the Interstellar Medium

It is shown that charge transfer strongly affects the ionization equilibrium of oxygen in the H i regions of interstellar space. Ion-atom charge at thermal energies (E < 1 eV) are evaluated by using the orbiting approximation of Bahcall and Wolf. The rate coefficients for the reactions 0+ + H 0 + H are evaluated for temperatures between 10 and 10 K. The ionization level of oxygen is determined for a range of temperatures and densities. Over the entire range considered, n(O+)/n(O) < -89n(H+)/n(H), instead of the value 10 n(H+)/n(H) which would obtain if charge exchange did not occur.