1997A&A...320..972M

We have performed ¹²CO(1-0), ¹²CO(2-1), ¹³CO(1-0), ¹³CO(2-1), C¹⁸O(1-0), C¹⁸O(2-1), HCO⁺(1-0) and H¹³CO⁺(1-0) observations towards two selected areas in the star forming complex NGC 6357 with angular resolutions from 21" to 55". In particular, we have mapped the molecular gas around the two HII regions G353.1+0.6 and G353.2+0.9 in the ¹²CO(1-0) and ¹³CO(1-0) transitions with a resolution of ∼43". This improves on the coarser ¹²CO(1-0) observations previously carried out by others. We have also studied the physical properties of gas along strips through the molecular cloud/HII region interfaces. For G353.1+0.6, the observations confirm that an ionization front is eroding a warm, dense molecular cloud located to the north of the optical nebula. The molecular gas appears fragmented on a scale size >0.5pc and not all the components are related to the HII region. There is evidence of a density increase near the front and indications of temperature gradients toward the exciting stars. This is further suggested by the presence of ¹²CO(1-0) self-absorption produced by a cooler external layer. The isotopic ratio X(¹³CO)/X(C¹⁸O) decreases from the inner part of the clouds towards the front, contrary to what is theoretically predicted and observed in many other regions. This may be due to excitation or optical depths effects. An evolutionary scenario is proposed where the exciting stars form at the edge of a molecular cloud. The morphology of G353.2+0.9 is rather different from what previously believed. Only a weak "bar" of molecular material was found to the south of the sharp ionization front observed optically and in the radio-continuum, and most of the molecular emission arises from regions behind or to the north of the HII region. This indicates that we are viewing a late stage "blister" configuration face-on. This region is fragmented on a scale size >0.5pc, and a warm, dense and compact molecular fragment coincides with the elephant trunk visible in Hα images. Other clouds with high (∼40K) ¹²CO(1-0) brightness temperatures surround the nebula to the north. Around their peaks, the ¹²CO(2-1)/¹²CO(1-0) main beam temperatures ratio is significantly <1, contrary to what is observed in G353.1+0.6. In both G353.1+0.6 and G353.2+0.9 a large range of radial velocities is observed close to the ionization fronts, and the molecular structures interacting with the ionized gas have virial masses greater than masses calculated under the assumption of LTE. These observations have allowed us to better understand the morphology of the two regions and to sketch the physical properties of molecular clouds exposed to UV radiation. Dynamical interactions between ionized and molecular gas are used in order to estimate the age (≲ a few 10⁵yrs) of both nebulae. Also the effects of UV radiation in determining the morphology of molecular gas are considered. The heating sources of the molecular clouds are the early type stars of the HII regions, at the edges of the clouds. The ¹²CO(1-0) opacity, τ, seems to affect ¹²CO(1-0) main beam temperatures and an empirical relation between τ and the visual extinction A_V has been determined.