Evolution of very small particles in the southern part of Orion B observed by ISOCAM

We present ISOCAM observations (5-18 mum) of the southern part of Orion B, including the reflection nebula NGC 2023 and the Horsehead nebula illuminated by the B star HD 37903 and the O star sigma Orionis, respectively. Due to the limited radiation field, the emission in these wavelengths is due to very small particles which are heated each time they absorb a UV photon. A filamentary structure is detected at small angular scales (down to the angular resolution of 6) on top of a smoother background. The particular case of the Horsehead nebula suggests that the filaments in general result from the illuminated surfaces of dense structures, while the smoother background comes from lower density matters probably ionised. Striking spatial variations of the infrared colour (5-8.5 mum/12-18 mum) are also detected. Spectroscopic observations show that they are due to variations of the intensity of the aromatic features (especially at 7.7 mum) relative to a continuum emission increasing in intensity towards longer wavelengths. The contribution of the continuum relative to the aromatic features appears significantly higher at the illuminated surfaces of dense structures than in lower density matter. This effect could be the signature of the evolution of the very small particles from shielded molecular material to photo-dissociated and photo-ionised matter. We also show that size segregation due to grain dynamics in uni-directional radiation fields may play a major role.