Photodissociation regions. II. A model for the Orion photodissociation region.

We have constructed a model for the photodissociation region in Orion. This model matches the observed emission in the fine-structure lines of O I 63 μm and 145 μm, C I 609 μm, and C II 158 μ m and the low-lying rotational lines of CO. It also explains the extended H₂ 1-0 S(t) emission, the carbon recombination lines, and the C I 9850 Å line. The emission in the C I 370 μm, Si II 35 μm, and Fe II 26 μm, 35 μm lines is predicted. The model fit implies that a fairly uniform density (∼ 10 ⁵ cm ^-3) photodissociation region lies behind the Trapezium stars. The peak temperature in this region is about 500 K, and the intensity of the incident farultraviolet (FUV) field is approximately 10⁵ times as intense as the ambient interstellar field. This model shows that the observed high brightness temperature of the C I 609 μ m line can be explained by emission from the C⁺/C/CO transition region. This difference with previous chemical models is due to a higher gas phase elemental abundance of carbon, to the charge exchange reactions of C⁺ with S and SiO, and to carbon self-shielding.