The interaction of young massive stars with their environment ^*,**

A millimeter and submillimeter line study of NGC 6334 FIR II

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: dennyzhang998@hotmail.com
² Purple Mountain Observatory, Chinese Academy of Sciences, NanJing 210008, PR China
³ Center for Astrophysics, GuangZhou University, GuangZhou 510006, PR China
⁴ Instituto de Radioastronomía Millimétrica (IRAM), Avenida Divina Pastora 7, Local 20, 18012 Granada, Spain
⁵ Department of Physics, Tamkang University, 251-37 Tamsui, Taipeh County, Taiwan
⁶ Im Acker 21b, 56072 Koblenz, Germany

Received: 29 June 2006
Accepted: 7 January 2007

Abstract

Using the 15-m Swedish ESO Sub-millimeter Telescope (SEST), CO, HCN, and HCO⁺ observations of the galactic star-forming region NGC 6334 FIR II are presented, complemented by [C i] ³PP₀ and ³PP₁ data from the Atacama Pathfinder Experiment (APEX 12-m telescope). Embedded in the extended molecular cloud and associated with the H ii region NGC 6334–D, there is a molecular “void”. [C i] correlates well with ¹³CO 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 ~40-50 K in the molecular cloud and 200 K 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 2.2m H₂ emission indicates that the well-evolved H ii region expands into a medium that is homogeneous on pc-scales. If the H₂ 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.