Emission of CO, C I, and C II in W3 Main

We used the KOSMA 3m telescope to map the core of the Galactic massive star forming region W3 Main in the two fine structure lines of atomic carbon and four mid-J transitions of CO and 13CO. The maps are centered on the luminous infrared source IRS 5 for which we obtained ISO/LWS data comprising four high-J CO transitions, [], and [] at 63 and m. In combination with a KAO map of integrated line intensities of [] [CITE], this data set allows us to study the physical structure of the molecular cloud interface regions where the occurence of carbon is believed to change from C+ to C0, and to CO. The molecular gas in W3 Main is warmed by the far ultraviolet (FUV) field created by more than a dozen OB stars. Detailed modelling shows that most of the observed line intensity ratios and absolute intensities are consistent with a clumpy photon dominated region (PDR) of a few hundred unresolved clumps per 0.84 pc beam, filling between 3 and 9% of the volume, with a typical clump radius of 0.025 pc (), and typical mass of 0.44 . The high-excitation lines of CO stem from a K layer, as do the [] lines. The bulk of the gas mass is however at lower temperatures.