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

The different spectral components of the 3-12 mu m dust emission are imaged in the M 17 south-west molecular cloud/HII region interface. The various i mages result from a combination of space borne (ISO) and ground based (CFHT ) observations. A spatial correlation analysis shows that there are two mai n components, which correspond to the continuum (including broad features) and to the emission bands (3.3, 6.2, 7.7 and 11.3 mu m) respectively. The f ormer peaks towards the HII region whereas the later dominates at the molec ular 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 propel-t ies of the grains, implying a band-to-continuum ratio which varies strongly with the excitation conditions. This analysis, which points out the signat ure of very small particles, does not favor one model relative to the other .