FUV irradiated molecular clumps: Spherical geometry and density gradients

We examine the effects of spherical geometry and density gradients on the thermal and chemical structure of photon dominated regions in individual molecular clumps inside molecular clouds. We investigate the effects of spherical geometry and density gradients on the [CII] fine-structure and ¹²CO and ¹³CO low-J rotational line intensities produced in clouds illuminated by external far-ultraviolet radiation fields. We compare plane-parallel and spherical models for a range of cloud parameters. The models are characterized by an UV intensity of χ = 1000, a mean hydrogen particle density of n(H)= 10⁵cm^-3 and clump sizes ranging from R = 0.05 pc to R = 0.005 pc. We argue that spherical models including density gradients are necessary to realistically account for the range of absolute and relative CII and CO line strengths observed in star forming cloud cores. We compare our model results in detail with observations of the Orion molecular cloud complex.