A MODEL FOR CLUMPY GIANT MOLECULAR CLOUDS WITH EXTERNAL ULTRAVIOLET HEATING

We have developed a model of clumpy molecular clouds based on that of Kwan and Sanders, in which a clump filling factor which is very low at the cloud surface and increases to ≃ 1 at the cloud center allows the UV radiation from an H II region located at the surface to permeate the volume of the cloud and to create a temperature gradient and varying abundance of carbon monoxide within each clump. The model is in general agreement with the enhanced intensity of the 12CO J = 3 → 2 transition relative to the J = 1 → 0 line which we have observed throughout the central ≃10′ region of the Orion molecular cloud. It also produces centrally peaked spectral lines whose intensity is maximum in a shell-like distribution centered on the H II region as is observed.