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Astron. Astrophys. 336, 662-666 (1998)

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5. Conclusions

From our broad-band ISO mapping of the region of N 4 in the LMC at 6.75 and 15 µm and comparison with H[FORMULA] and CO(1-0) images, we derive the following conclusions.

i) The mid-IR emission far from the H II   regions comes from UIBs excited by the general radiation field at the surface of the molecular cloud. The ratio of the 15 µm emission to that at 6.75 µm ([FORMULA]0.6-0.7) is comparable to that found in quiet areas of our galaxy (0.55-1.0), suggesting similar properties for the UIB carriers. This however has to be checked by mid-IR spectroscopy which is presently programmed with ISO.

ii) The (15 µm)/(6.75 µm) flux ratio is considerably enhanced in regions of strong UV flux near the H II   regions, as soon as the far-UV radiation density reaches approximately [FORMULA] times that in the vicinity of the Sun. This is due to the contribution of very small grains at 15 µm. These very small grains seem to survive inside H II   regions as shown by the mid-IR emission from the centers of these H II   regions.

iii) However the emissions at both wavelengths peak on an interface between the H II   region N 4A and the molecular cloud. The situation in this region is very similar to that in the classical interface of M 17 in our Galaxy (Cesarsky et al.  1996b), including the presence of an ultra-compact H II   region.

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© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998