CHAMP+ observations of warm gas in M17 SW

Abstract

Context: Sub-millimeter and Far-IR observations have shown the presence of a significant amount of warm (few hundred K) and dense (n(H2) ≥ 104 cm-3) gas in sources ranging from active star-forming regions to the vicinity of the Galactic center. Since the main cooling lines of the gas phase are important tracers of the interstellar medium in Galactic and extragalactic sources, proper and detailed understanding of their emission and the ambient conditions of the emitting gas, is necessary for a robust interpretation of the observations. Aims: With high resolution (7″-9″) maps (∼3 × 3 pc2) ofmid-J molecular lines we aim to probe the physical conditions and spatial distribution of the warm (50 to several hundred K) and dense gas (n(H2) > 10 5 cm-3) across the interface region of the nearly edge-on M17 SWnebula. Methods: We have used the dual color multiple pixel receiver CHAMP+ on the APEX telescope to obtain a 5.′3 × 4.′7 map of the J = 6 → 5 and J = 7 → 6 transitions of 12CO, the 13CO J = 6 → 5 line, and the 3P2 → 3P1 370 μm fine-structure transition of [C I] in M17 SW. LTE and non-LTE radiative transfer models are used to constrain the ambient conditions. Results: The warm gas extends up to a distance of ∼2.2 pc from theM17 SWridge. The 13CO J = 6 → 5 and [CI] 370 μm lines have a narrower spatial extent of about 1.3 pc along a strip line at PA = 63°. The structure and distribution of the [C I] 3P2 → 3P1 370 μm map indicate that its emission arises from the interclump medium with densities on the order of 103 cm-3. Conclusions. The warmest gas is located along the ridge of the cloud, close to the ionization front. An LTE approximation indicates that the excitation temperature of the embedded clumps reaches ∼120 K. The non-LTE model suggests that the kinetic temperature at four selected positions cannot exceed 230 K in clumps of a density of n(H 2) ∼ 5 × 105 cm-3 and that the warm (Tk > 100 K) and dense (n(H2) ≥ 104 cm-3) gas traced by the mid-J 12CO lines represents just about 2% of the bulk of the molecular gas. The clump volume-filling factor ranges between 0.04 and 0.11 at these positions. © 2010 ESO.