ionizationbounded

Ionization-bounded is a term used in astrophysics to describe an H II region or nebula in which the extent of the ionized gas is set by the supply of hydrogen-ionizing photons from embedded stars, rather than by the amount or distribution of gas. In an ionization-bounded nebula, the gas is optically thick to Lyman-continuum photons, and the rate of ionizations balances the rate of recombinations within the nebula. The condition can be written as Q(H0) = ∫ α_B n_e n_p dV, where Q(H0) is the production rate of ionizing photons and α_B is the case-B recombination coefficient. The outer boundary is an ionization front separating the fully ionized interior from surrounding neutral gas. This structure is often approximated by a Strömgren sphere, whose radius depends on the ionizing luminosity and the ambient gas density.

In contrast, a density-bounded (or leakage) nebula is truncated by the gas distribution before all ionizing photons are absorbed, allowing some ionizing photons to escape. Observationally, ionization-bounded regions tend to show a well-defined outer edge in emission lines such as Hα and [O III], with a neutral or low-ionization layer beyond the boundary. The exact edge can be influenced by clumping, geometry, and external radiation fields.

Ionization-bounded conditions are relevant in studies of H II regions around young star clusters and in photoionization models of galaxies, where they affect line ratios, inferred abundances, and the escape fraction of ionizing photons.