2013A&A...550A..10L

Most studies of high-mass star formation focus on massive and/or luminous clumps, but the physical properties of their larger scale environment are poorly known. In this work, we aim at characterising the effects of clustered star formation and feedback of massive stars on the surrounding medium by studying the distribution of warm gas through mid-J ¹²CO and ¹³CO observations. We present APEX ¹²CO(6-5), (7-6), ¹³CO(6-5), (8-7) and HIFI ¹³CO(10-9) maps of the star forming region G327.36-0.6 with a linear size of ∼3pcx4pc. We infer the physical properties of the emitting gas on large scales through a local thermodynamic equilibrium analysis, while we apply a more sophisticated large velocity gradient approach on selected positions. Maps of all lines are dominated in intensity by the photon dominated region around the HII region G327.3-0.5. Mid-J ¹²CO emission is detected over the whole extent of the maps with excitation temperatures ranging from 20K up to 80K in the gas around the HII region, and H₂ column densities from few 10²¹cm^–2 in the inter-clump gas to 3x10²²/cm2 towards the hot core G327.3-0.6. The warm gas (traced by ¹² and ¹³CO(6-5) emission) is only a small percentage (∼10%) of the total gas in the infrared dark cloud, while it reaches values up to ∼35% of the total gas in the ring surrounding the HII region. The ¹²CO ladders are qualitatively compatible with photon dominated region models for high density gas, but the much weaker than predicted ¹³CO emission suggests that it comes from a large number of clumps along the line of sight. All lines are detected in the inter-clump gas when averaged over a large region with an equivalent radius of 50"(∼0.8pc), implying that the mid-J ¹²CO and ¹³CO inter-clump emission is due to high density components with low filling factor. Finally, the detection of the ¹³CO(10-9) line allows to disentangle the effects of gas temperature and gas density on the CO emission, which are degenerate in the APEX observations alone.