H II region

An H II region is a volume of ionized hydrogen surrounding one or more sources of ultraviolet photons. Photoionization heating, recombination, gas dynamics, and dust determine its structure and emission spectrum.

An ionization front is the transition between predominantly ionized and predominantly neutral gas. Its motion and thickness depend on photon supply, recombination, gas density, and hydrodynamic response.

Photoionization equilibrium holds when the local or integrated photoionization rate equals the radiative recombination rate, so the ionized fraction is stationary.

The recombination time is the characteristic time for an ionized plasma to recombine after its ionizing source is removed. Uniform hydrogen with electron density $n_e$ and recombination coefficient $\alpha$ has $t_{\mathrm{rec}}=1/(\alpha n_e)$.

Case B recombination excludes direct recombinations to the ground state because their ionizing photons are assumed to be absorbed locally. The coefficient ${\alpha }_B$ therefore counts recombinations to excited states.

The hydrogen-ionizing photon production rate is the number of photons emitted per unit time above the hydrogen ionization threshold:

A Strömgren sphere is the idealized ionized region around a steady source embedded in uniform hydrogen gas, bounded where integrated recombinations consume the source's ionizing-photon supply.

For ionizing-photon rate $Q_{\mathrm{H}}$, uniform pure-hydrogen density $n$, and Case B recombination coefficient ${\alpha }_B$, the Strömgren radius is

Before hydrodynamic expansion matters, photon counting in a uniform initially neutral medium givesThe ionization front approaches the Strömgren radius on the recombination time.

A dusty Strömgren sphere includes ionizing photons absorbed by dust before they can ionize hydrogen. Dust therefore decreases the ionized radius below the dust-free Strömgren radius at fixed source luminosity and gas density.