Ionization mechanisms of the intercloud medium.

A generalized treatment of the ionization equilibrium of the intercloud medium is developed, which is modelindependent in the steady-state approximation, and which is suited to an interpretation of ultraviolet absorption data. It is found that the most satisfactory models require the presence of density inhomogeneities along the line of sight. In all models considered for A Sco, 2-MeV cosmic rays and X-rays are found to lead to large disagreements with the obseryations, and data on v Sco, a Leo, and other stars seem to support this conclusion. Therefore, it would seem that these cannot be the physical agents responsible for the bulk of the ionization of the gas; and we develop an alternative model, where the ionization below 24.58 eV is provided by ultraviolet photons. This model appears to satisfy theoretical requirements, and leads to good agreement with the data. Subject headings: cosmic rays - interstellar matter - X-rays