Introduction

Photodissociation regions (PDRs) are regions where FUV photons dominate the energy balance or chemistry of the gas. In this chapter, we will examine the physical characteristics of dense and luminous PDRs near bright O and B stars, starting with the ionization balance (Section 9.2) and the energy balance (Section 9.3) of the gas in PDRs. We follow that with a discussion of the dust temperature in PDRs. The chemistry of PDRs (Section 9.5) is very similar to that of diffuse clouds (cf. Section 8.7), except for possible time-dependent effects. We have, then, all the ingredients to understand the structure of PDRs (Section 9.6). The remainder of this chapter focusses on the analysis and interpretation of observations of PDRs. We will start this off with back-of-the-envelope estimates of the incident FUV field, density, temperature, and mass based on a few key observations (Section 9.7). More thorough analysis tools are discussed in Section 9.8. These different techniques for estimating the physical conditions in PDRs are then compared, based on a case study of the Orion Bar (Section 9.9). Section 9.10 contrasts the physical conditions derived for various well-known PDRs. Finally, we examine the H2 IR fluorescence spectrum of PDRs in Section 9.11.

Figure 9.1 illustrates the structure of a PDR. FUV photons penetrate a molecular cloud, ionizing, dissociating, and heating the gas.