We used the KOSMA 3m telescope to map the core 7 ^ { \prime } \times 5 ^ { \prime } 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 ii ] , and [ O i ] at 63 and 145 \mu m. In combination with a KAO map of integrated line intensities of [ C ii ] ( Howe et al .
1991 ) , this data set allows 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 ^ { \prime \prime } ) , and typical mass of 0.44 M _ { \odot } .
The high-excitation lines of CO stem from a 100 - 200 K layer , as also the [ C i ] lines .
The bulk of the gas mass is however at lower temperatures .