The photoevaporation of interstellar clouds. I - Radiation-driven implosion

An approximate analytical solution is developed for the evolution of an interstellar neutral cloud exposed to the ionizing radiation of a newly formed star. The structure of a steady photoevaporation gas flow off a spherical ionization front is derived even for cases where the ionization front cannot be considered thin, and the downstream flow is not in ionization and thermal equilibrium. Under a wide range of conditions, the ionization front is approximately D-critical, and the ionized gas expands supersonically into the intercloud medium. The ionization front typically drives a strong shock into the initial cloud.