Emission of CO, CI, and CII in W3 Main

We used the KOSMA 3m telescope to map the core 7' x 5' 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 (CO)-C-13. The maps are centered on the luminous infrared source IRS 5 for which we obtained ISO/LWS data comprising four high-J CO transitions, [C II], and [O I] at 63 and 145 mum. In combination with a KAO map of integrated line intensities of [C II] (Howe et al. 1991), 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 C-0, 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 (2.2), and typical mass of 0.44 M-circle dot. The high-excitation lines of CO stem from a 100-200 K layer, as do the [C I] lines. The bulk of the gas mass is however at lower temperatures.