The interaction of young massive stars with their environment: A millimeter and submillimeter line study of NGC6334 FIR II

Abstract

Using the 15-m Swedish ESO Sub-millimeter Telescope (SEST), CO, HCN, and HCO+ observations of the galactic star-forming region NGC6334 FIR II are presented, complemented by [C I] 3^P_1--3^P_0 and 3^P_2--3^P_1 data from the Atacama Pathfinder Experiment (APEX 12-m telescope). Embedded in the extended molecular cloud and associated with the H II region NGC6334--D, there is a molecular "void". [C I] correlates well with 13^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 [C I] with PDRs. Kinetic temperatures are ~40--50K in the molecular cloud and >=200K toward the void. CO and [C I] 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 2.2um H_2 emission indicates that the well-evolved H II region expands into a medium that is homogeneous on pc-scales. If the H_2 emission is predominantly shock excited, both the expanding ionization front (classified as subsonic, "D-type") and the associated shock front farther out (traced by H_2) can be identified, observationally confirming for the first time a classical scenario that is predicted by evolutionary models of H II regions. Integrated line intensity ratios of the observed molecules are determined, implying a mean C18^O/C17^O abundance ratio of 4.13+-0.13 that reflects the 18^O/17^O 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.