Optical Line Emission from Photodissociation Regions

Abstract

We present a parameter study for the optical, low-ionization [O I] j6300, j6363, and j5577 and the [S II] j6731 and j6716 lines emitted by photodissociation regions (PDRs). We assume in these models that the PDR has a plane-parallel, semi-inÐnite geometry and is illuminated from one side. The PDR models are characterized by the total hydrogen density 104 cm~3 ¹ n ¹ 107 cm~3 and the strength of the FUV Ðeld in terms of the Habing Ðeld Oxygen and singly ionized sulfur are G 0 103 \ G 0 \ 107. 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 di †erent objects.