2010A&A...510A..87P

Sub-millimeter and Far-IR observations have shown the presence of a significant amount of warm (few hundred K) and dense (n(H₂)≥10⁴cm) 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. With high resolution (7''-9'') maps (∼3x3 pc²) of mid-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)>10⁵cm) across the interface region of the nearly edge-on M17 SW nebula. We have used the dual color multiple pixel receiver CHAMP⁺ on the APEX telescope to obtain a 5.3'x4.7' map of the J=6->5 and J=7->6 transitions of ¹²CO, the ¹³CO J=6->5 line, and the ³P₂->³P₁ 370µm fine-structure transition of [CI] in M17 SW. LTE and non-LTE radiative transfer models are used to constrain the ambient conditions. The warm gas extends up to a distance of ∼2.2pc from the M17 SW ridge. The ¹³CO J=6->5 and [CI] 370µm lines have a narrower spatial extent of about 1.3 pc along a strip line at PA=63°. The structure and distribution of the [CI] ³P₂->³P₁ 370µm map indicate that its emission arises from the interclump medium with densities on the order of 10³cm. 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 ∼120K. The non-LTE model suggests that the kinetic temperature at four selected positions cannot exceed 230K in clumps of a density of n(H₂)∼5x10⁵cm and that the warm (T_k>100K) and dense (n(H₂)≥10⁴cm) 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.