Environs of HII regions.

The time evolution of conditions in the neutral gas ahead of the ionization front of the expanding H II region of an early-type star is explored in a review of recent observations and analytical investigations. The initial conditions in the molecular cloud (containing H2, He, CO, and dust grains) are defined, and four stages are characterized: formation of an extended C(+) region, rapid dissociation of H2 in a narrow region, formation of a broad H I zone, and narrowing of the H I zone. Diagrams, graphs, and maps of sample data are provided, and unresolved problems are discussed.