The Extended Infrared Emission of the Southern Parts Of Orion B

Abergel, A., Bernard, J. P., Boulanger, F., Miville-Deschênes, M.-A., Puget, J.-L., Nordh, L., Olofsson, G., Huldtgren, M., Kaas, A. A., Falgarone, E., Pérault, M., André, P., Montmerle, T., Copet, E., Persi, P., and Sibille, F.
ISO Beyond Point Sources: Studies of Extended Infrared Emission, September 14-17, 1999, ISO Data Centre, Villafranca del Castillo, Madrid, Spain. Edited by R. J. Laureijs, K. Leech and M. F. Kessler, ESA-SP 455, 2000. p. 119.


ISOCAM observations reveal the extended emission from dense molecular clouds to low brightness diffuse regions with an unprecedented sensitivity and angular resolution. We present observations of the molecular cloud L1630, located at the southern part of Orion B. The striking variations of the infrared color deduced from the broad-band maps of ISOCAM (from ~5 to ~18 microns) are combined with spectroscopic observations. We show that the ratio of the intensity of the aromatic features to the continuum like emission systematically detected in interstellar spectra around ~15 microns strongly varies from place to place. We suggest it is the signature of abundance variations of two different families of emitting particles (for instance aromatic and non aromatic particles), or of different size distributions of one family (aromatic particles). The illuminated edges of dense structures have a specific infrared color due to a strong continuum emission around 15 microns. It may be due to a specific state of evolution of the emitting particles, since the emission comes from a thin layer where the emitting particles could be detached from grain mantles, and destroyed by incident UV radiation. The large scale emission coming from the outer parts of dense structures presents a very different infrared color (compatible with no continuum emission around 15 microns). The emitting properties of the small particles appear to evolve strongly in the interstellar medium.