R = 100,000 Spectroscopy of Photodissociation Regions: H₂ Rotational Lines in the Orion Bar

Abstract

Photodissociation regions (PDRs) form on the surfaces of molecular clouds whenever these clouds are struck by far-ultraviolet radiation from hot young stars. These regions are characterized by the transition from hot, ionized gas to cold, molecular gas as the far-ultraviolet field is attenuated farther from the ionization front. The temperature profile of the PDR varies depending on the density and strength of the FUV field. Derivation of this profile must take into account the local heating and cooling, the chemical equilibrium, and radiative coupling to other layers within the structure. The Orion Bar is a dense molecular structure at the southeast boundary of the Orion Nebula. Early theoretical models of the Orion Bar by Tielens and Hollenbach [1] (for a density of 2.3 × 10⁴ and UV field strength (G₀) of 10⁵) predict temperatures of ~ 1000 K at A_V = 0–2, dropping to less than 100 K by A_V = 4.