MOLECULAR CLOUD HII REGION INTERFACES IN THE STAR-FORMING REGION NGC-6357/

We have performed (CO)-C-12(1-0), (CO)-C-12(2-1), (CO)-C-13(1-0), (CO) -C-13(2-1), (CO)-O-18(1-0), (CO)-O-18(2-1), HCO+(1-0) and (HCO+)-C-13( 1-0) observations towards two selected areas in the star forming compl ex NGC 6357 with angular resolutions from 21 '' to 55 ''. In particula r, we have mapped the molecular gas around the two I-III regions G353. 1+0.6 and G353.2+0.9 in the (CO)-C-12(1-0) and (CO)-C-13(1-0) transiti ons with a resolution of similar to 43 ''. This improves on the coarse r (CO)-C-12(1-0) observations previously carried out by others. We hav e also studied the physical properties of gas along strips through the molecular cloud/HII region interfaces. For G353.1+0.6, the observatio ns confirm that an ionization front is eroding a warm, dense molecular cloud located to the north of the optical nebula. The molecular gas a ppears fragmented on a scale size > 0.5 pc and not all the components an related to the HII region. There is evidence of a density increase near the front and indications of temperature gradients toward the exc iting stars. This is further suggested by the presence of (CO)-C-12(1- 0) self-absorption produced by a cooler external layer. The isotopic r atio X((CO)-C-13)/X((CO)-O-18) decreases from the inner part of the cl ouds towards the front, contrary to what is theoretically predicted an d observed in many other regions. This may be due to excitation or opt ical depths effects. An evolutionary scenario is proposed when the exc iting stars form at the edge of a molecular cloud. The morphology of G 353.2+0.9 is rather different from what previously believed. Only a we ak ''bar'' of molecular material was found to the south of the sharp i onization front observed optically and in the radio-continuum, and mos t 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 > 0.5 pc, and a warm, dense and compact molecular fragment coinci des with the elephant trunk visible in H alpha images. Other clouds wi th high (similar to 40 It) (CO)-C-12(1-0) brightness temperatures surr ound the nebula to the north. Around their peaks, the (CO)-C-12(2-1)/( CO)-C-12(1-0) main beam temperatures ratio is significantly < 1, contr ary 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 ionizat ion fronts, and the molecular structures interacting with the ionized gas have virial masses greater than masses calculated under the assump tion of LTE. These observations have allowed us to better understand t he morphology of the two regions and to sketch the physical properties of molecular clouds exposed to UV radiation. Dynamical interactions b etween ionized and molecular gas are used in order to estimate the age (less than or similar to a few 10(5) yrs) of both nebulae. Also the e ffects of UV radiation in determining the morphology of molecular gas are considered. The heating sources of the molecular clouds are the ea rly type stars of the HII regions, at the edges of the clouds. The (CO )-C-12(1-0) opacity, tau, seems to affect (CO)-C-12(1-0) main beam tem peratures and an empirical relation between tau and the visual extinct ion A(V) has been determined.