2004A&A...424..887K

We used the KOSMA 3m telescope to map the core 7'x5' of the Galactic massive star forming region W3Main 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 IRS5 for which we obtained ISO/LWS data comprising four high-J CO transitions, [CII], and [OI] at 63 and 145µm. In combination with a KAO map of integrated line intensities of [CII] (Howe et al., 1991ApJ...373..158H), 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 W3Main 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 0.84pc beam, filling between 3 and 9% of the volume, with a typical clump radius of 0.025pc (2.2''), and typical mass of 0.44M_☉. The high-excitation lines of CO stem from a 100-200K layer, as do the [CI] lines. The bulk of the gas mass is however at lower temperatures.