Optical Line Emission from Photodissociation Regions

We present a parameter study for the optical, low-ionization [O I] λ6300, λ6363, and λ5577 and the [S II] λ6731 and λ6716 lines emitted by photodissociation regions (PDRs). We assume in these models that the PDR has a plane-parallel, semi-infinite geometry and is illuminated from one side. The PDR models are characterized by the total hydrogen density 10⁴ cm^-3 ≤ n ≤ 10⁷ cm^-3 and the strength of the FUV field G₀ in terms of the Habing field 10³ < G₀ < 10⁷. Oxygen and singly ionized sulfur are excited thermally in the outer hot PDR layers due to collisions with electrons and atomic hydrogen. [O I] photons are also produced in the photodissociation of OH. We apply the results of the parameter study to the protoplanetary disks in Orion, layers between H II regions and molecular clouds, low-ionization nuclear emission-line regions, T Tauri stars, and planetary nebulae. We discuss the extent to which PDRs dominate or contribute to the observed [O I] and [S II] emission in these different objects.