Where Light Meets Darkness
Photodissociation regions mark the transition between the harsh radiation environment near hot stars and the shielded interiors of molecular clouds. In these boundary layers, far-ultraviolet photons with energies of 6-13.6 eV penetrate the gas, dissociating molecules and ionizing atoms with low ionization potentials. The resulting chemical stratification — ionized → atomic → molecular — produces a rich spectrum of emission lines that dominates the infrared output of galaxies.
The Layered Structure
A classic PDR exhibits a characteristic onion-skin structure as UV photons are progressively absorbed with depth. At the surface, hydrogen is atomic and carbon is ionized (C⁺), producing bright [CII] 158 μm emission. At A_V ≈ 1-2 magnitudes of dust extinction, H₂ self-shielding triggers an abrupt transition to molecular hydrogen. Deeper still, at A_V ≈ 2-4, CO forms as carbon transitions from C⁺ through neutral C to CO. The exact locations depend on the ratio G₀/n_H.
Heating and Cooling
PDR gas is heated primarily by the photoelectric effect on dust grains and polycyclic aromatic hydrocarbons (PAHs): UV photons eject energetic electrons from grain surfaces, which thermalize with the gas. This heating is balanced by cooling through fine-structure line emission — [CII] 158 μm and [OI] 63 μm dominate, with CO rotational lines becoming important deeper in. The balance sets gas temperatures ranging from ~5000 K at the ionization front to ~10-50 K in the shielded interior.
Diagnostic Power
PDR emission lines are powerful diagnostics of physical conditions in distant objects. The [CII]/[OI] ratio constrains gas density, while [CII]/FIR traces the UV field strength. CO ladder excitation reveals temperature and density profiles. These diagnostics, calibrated against detailed PDR models, allow astronomers to characterize the interstellar medium in galaxies billions of light-years away using facilities like ALMA and JWST.