Grain populations in the M 17 south - west star forming complex ?

The different spectral components of the 3–12 μm dust emission are imaged in the M 17 south-west molecular cloud / HII region interface. The various images result from a combination of space borne (ISO) and ground based (CFHT) observations. A spatial correlation analysis shows that there are two main components, which correspond to the continuum (including broad features) and to the emission bands (3.3, 6.2, 7.7 and 11.3 μm) respectively. The former peaks towards the HII region whereas the later dominates at the molecular cloud interface. This is consistent with a model which attributes the bands to PAH molecules and the continuum to very small grains. In the frame of the coal dust model, these observations put constraints on the properties of the grains, implying a band-to-continuum ratio which varies strongly with the excitation conditions. This analysis, which points out the signature of very small particles, does not favor one model relative to the other.