D. Tran 1, D. Cesarsky 2, M. Sauvage 3, & al.
1 Max Planck Institut fur Extraterrestrische Physik - Garching, Germany
2 Institut d'Astrophysique Spatiale, Orsay, France
3 DSM/DAPNIA/SAp, CEA-Saclay, 91191 Gif sur Yvette Cedex FRANCE
We present here a study of two differents regions : NGC7023 and M17 in order to show the impact of the physical environment on the mid infrared spectrum. These two regions allow us two characterize the variations of the Unidentified Infrared Bands in the case of NGC 7023 and the behaviour of the ``hot dust'' continuum that is important in the HII region of M17. A special emphasis will be placed on the physical conditions of the dust in the M17 region compared to what is expected from a new theoretical model.