2011A&A...525A...8R

We aim at deriving the excitation conditions of the interstellar gas as well as the local FUV intensities in the molecular cloud surrounding NGC 3603 to get a coherent picture of how the gas is energized by the central stars. The NANTEN2-4m submillimeter antenna is used to map the [CI] 1-0, 2-1 and CO 4-3, 7-6 lines in a 2'x2' region around the young OB cluster NGC 3603 YC. These data are combined with C¹⁸O 2-1 data, HIRES-processed IRAS 60µm and 100µm maps of the FIR continuum, and Spitzer/IRAC maps. The NANTEN2 observations show the presence of two molecular clumps located south-east and south-west of the cluster and confirm the overall structure already found by previous CS and C¹⁸O observations. We find a slight position offset of the peak intensity of CO and [CI], and the atomic carbon appears to be further extended compared to the molecular material. We used the HIRES far-infrared dust data to derive a map of the FUV field heating the dust. We constrain the FUV field to values of χ=3-6x10³ in units of the Draine field across the clouds. Approximately 0.2 to 0.3% of the total FUV energy is re-emitted in the [CII] 158µm cooling line observed by ISO. Applying LTE and escape probability calculations, we derive temperatures (T_MM1=43K, T_MM2=47K), column densities (N_MM1=0.9x10²²cm^–2, N_MM2=2.5x10²²cm^–2) and densities (n_MM1=3x10³cm^–3, n_MM2=10³-10⁴cm^–3) for the two observed molecular clumps MM1 and MM2. The cluster is strongly interacting with the ambient molecular cloud, governing its structure and physical conditions. A stability analysis shows the existence of gravitationally collapsing gas clumps which should lead to star formation. Embedded IR sources have already been observed in the outskirts of the molecular cloud and seem to support our conclusions.