Emission of CO, C i, and C ii in W3 Main

We used the KOSMA 3m telescope to map the core 7 � × 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 13 CO. 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 ), and (O ) at 63 and 145 µm. In combination with a KAO map of integrated line intensities of (C ) (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� . The high-excitation lines of CO stem from a 100−200 K layer, as do the (C ) lines. The bulk of the gas mass is however at lower temperatures.