J.Ph Bernard 1, A. Abergel 1, F. Boulanger 1, J.L. Puget 1, G. Helou 2, & W. Langern 3
1 IAS, Orsay
2 IPAC
3 JPL
Many regions of the sky have shown to have important color variations in the IRAS data (in particular in the 12/100 color ratio), which are believed to trace, not only excitation conditions, but also physical properties and abundances of the smallest dust particles of the ISM. Some nearby molecular clouds even exhibit strong 12 micron haloes where the abundance of small particles, in particular PAH, is strongly increased (Bernard et al. 1993, A&A 277, 609).
We will present recent ISO (ISOCAM, PHOT, LWS) observations toward such molecular cloud edges. The ISOCAM data were obtained in a set of filters (LW2, LW3, LW4, LW6, LW7, LW8) well adapted to the study of the PAH features, as well as in the CVF mode for selected positions. PHOT observations in the 3.3 micron PAH feature and nearby continuum were also obtained. These observations complement the large scale surveys with ISOCAM large filters and CVF spectra which can only be obtained toward a limited number of regions. They allow a detailed understanding of the spatial distribution of the PAH physical conditions at the interface of molecular clouds. These data will be compared to PHOT data at 90 and 180 micron and LWS data in the C+[156 mic] line, in order to test the gas eating properties of PAH through photoelectric effect. We will also present a search for the long predicted fluorescence of PAH, using UBVRI deep imaging of these regions.
Altogether, these new results confirm that desorption from the surface of larger dust grains, or even PAH formation in the gas phase can strongly enhance the PAH abundance at the edges of some molecular clouds. The ISO data will allow for the first time a detailed analysis of the physical processes involved.