2007A&A...465..887Z

Using the 15-m Swedish ESO Sub-millimeter Telescope (SEST), CO, HCN, and HCO⁺ observations of the galactic star-forming region NGC6334FIRII are presented, complemented by [CI] ³P₁-³P₀ and ³P₂-³P₁ data from the Atacama Pathfinder Experiment (APEX 12-m telescope). Embedded in the extended molecular cloud and associated with the Hii region NGC6334-D, there is a molecular ``void''. [CI] correlates well with <sup>13</sup>CO and other molecular lines and shows no rim brightening relative to molecular cloud regions farther off the void. While an interpretation in terms of a highly clumped cloud morphology is possible, with photon dominated regions (PDRs) reaching deep into the cloud, the data do not provide any direct evidence for a close association of [CI] with PDRs. Kinetic temperatures are ∼40-50K in the molecular cloud and >200K toward the void. CO and [CI] excitation temperatures are similar. A comparison of molecular and atomic fine structure line emission with the far infrared and radio continuum as well as the distribution of 2.2µm H<sub>2</sub> emission indicates that the well-evolved Hii region expands into a medium that is homogeneous on pc-scales. If the H<sub>2</sub> emission is predominantly shock excited, both the expanding ionization front (classified as subsonic, ``D-type'') and the associated shock front farther out (traced by H₂) can be identified, observationally confirming for the first time a classical scenario that is predicted by evolutionary models of Hii regions. Integrated line intensity ratios of the observed molecules are determined, implying a mean C¹⁸O/C¹⁷O abundance ratio of 4.13±0.13 that reflects the ¹⁸O/¹⁷O isotope ratio. This ratio is consistent with values determined in nearby clouds. Right at the edge of the void, however, the oxygen isotope ratio might be smaller.