Optical line emission from photodissociation regions

We present a parameter study for the optical, low-ionization [O I] lambda 6300, lambda 6363, and lambda 5577 and the [S II] lambda 6731 and lambda 6716 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 10(4) cm(-3) less than or equal to n less than or equal to 10(7) cm(-3) and the strength of the FUV held G(0) in terms of the Habing held 10(3) < G(0) < 10(7). 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 n 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.