Astron. Astrophys. 320, 972-992 (1997)

5. Conclusions

We have presented and analyzed line observations of molecular clouds associated with the two HII regions G353.1+0.6 and G353.2+0.9, which are part of the star forming complex NGC 6357. The main results can be summarized as follows:

1. The molecular emission towards G353.1+0.6 arises north of the optical nebula, where an ionization front is eroding a molecular cloud of pc of diameter which we have called component A. An elongated structure, component E, runs along the ionization front and is blue-shifted with respect to A. This suggests that this is placed between the observer and the ionization front and causes its obscuration. Towards the ionization front there is also red-shifted molecular material ( ), which could be behind it. We have interpreted these features as an expanding bubble powered by the OB stars in the optical nebula.
2. A cooler, clumpy molecular layer external to component A may account for a ¹² CO(1-0) self-absorption feature detected at .
3. Gaussian fits to the line profiles show that component A is composed of at least two main subcomponents at and . Typical H₂ densities, derived from and an LVG model, are - cm^-3, and there is evidence of a density increase near the ionization front. A mass estimate for component A is , and its maximum is cm^-2.
4. A kinetic temperature of -50 K is obtained from our observations, although there are clear indications of the existence of temperature gradients in component A. This cloud is not only colder in the inner parts, but the O excitation temperature peaks near the ionization front, confirming that the heating sources (the cluster of OB stars) are external and located south of the cloud.
5. We estimated that the values for CO abundances are typical (although ¹² CO seems slightly underabundant) in G353.1+0.6. A decrease of the ¹³ CO/ O isotopic ratio towards the ionization front was detected across component A.
6. The morphology of G353.2+0.9 is rather different from what was expected from the previously available indications (Fea90). In fact, only a weak "bar" of molecular gas was found to the south of the sharp ionization front, whereas the majority of the molecular emission comes from a region behind or to the north of the HII region. This is composed of two main features with and , whose origins should be investigated by observations on a larger scale. The newly-born stars within the nebula, which are also its exciting sources, are more probably related to the cloud at -2 ; if so, the (H109 ), , suggests that the ionized gas is slowly expanding towards the observer. Thus, in this case we are clearly viewing an ionization front face-on.
7. The molecular fragments directly associated to G353.2+0.9 are component C, which coincides with the elephant trunk visible in the H image (Fea90), and components E, G and H, small clouds that border the southern edge of the nebula. These have very different 's, in the range from -5 to 2 , suggesting they are located on the surface of an expanding ionized bubble powered by IRS 4, which is probably the main source of energy in the nebula (Fea90). Since component C is visible in all observed lines, including (1-0) and CO (1-0), it must be very dense ( cm^-3). It houses two compact radio sources and IRS 4 is located to the south, at the apex of the elephant trunk. On the other hand, components E, G and H are relatively weak, with [¹² CO(1-0)] in the range 5-13 K, and their sizes are pc. Density estimates vary between and cm^-3, depending on the adopted model, and masses are .
8. The other components surrounding G353.2+0.9, i.e. A, B and F, have densities - cm^-3, [¹² CO(1-0)] K and LTE masses between 300 and 1500 . In this case, too, a -50 K seems appropriate, and there are some indications of colder inner regions. Maximum 's are cm^-2 and we found roughly the same abundances for optically thin CO isotopes as in G353.1+0.6, while ¹² CO abundances range from to .
9. As shown by Fea90, the Pis24 cluster appears unrelated to G353.2+0.9, and it is located inside a large cavity with relatively little molecular gas. We can speculate that the interaction between the cluster and the gas originated this cavity much before the formation of the HII region and has not left other traces beside the large molecular hole.
10. G353.1+0.6 is an evolved HII region with an age of yrs, powered by a cluster of OB stars within the optical nebula. The progenitor molecular cloud which originated these stars was photoevaporated by the UV radiation and expanded to form the low density sphere of ionized gas around them. G353.2+0.9 appears instead as a younger, more compact HII region.
11. The main difference between the two regions is in the ratios, which are in G353.1+0.6 and in G353.2+0.9. In the latter case, we have proposed the existence of a diffuse warmer and low density interclump gas which absorbs the ¹² CO(2-1) emission more than the ¹² CO(1-0) emission.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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