Photon‐dominated Region Models for Global Properties of the Galaxy

Abstract

New photon-dominated region (PDR) models were constructed for investigation of the [C II] 158 μm fine-structure line emission, particularly of the Galactic [C II] emission observed with the Balloon-borne Infrared Carbon Explorer (BICE). The new models have two noteworthy features: (1) a spherical model cloud is immersed in isotropic stellar radiation, and (2) the incident stellar radiation has a wavelength range from the ultraviolet (UV) to the near-infrared. These features are in contrast to those of previous plane-parallel models in which the incident stellar radiation has been restricted to the UV. This makes the new model more suitable for general Galactic PDRs. Equations for chemical equilibrium and thermal balance are solved simultaneously with radiative transfer in the model cloud. Then the luminosities of the cloud are derived for the [C II] line, the 12CO (J = 1-0) line, and the continuum at a wavelength of λ = 100 μm. In order to explain the intensity ratios of [C II]/CO (I/ICO = 1.3 × 103) and [C II]/100 μm [I/λIλ(100 μm) = 5 × 10-3] observed toward the inner Galactic plane, the new models require a moderate (only several times as large as the solar neighborhood value) UV flux of the incident radiation. This flux is an order of magnitude smaller than those required by the previous models. Since the previous models required large incident UV fluxes, PDRs were sometimes considered not to be the primary origin of the diffuse [C II] emission in the Galactic plane. However, the new models show that the observed [C II] emission can originate in molecular clouds immersed in the general Galactic radiation field. This indicates that general Galactic PDRs are a promising candidate for the primary origin of the Galactic [C II] emission.