ISOCAM observations of the southern part of Orion B:
Constrains on small scale structure and geometry

A. Abergel ¹, J.P. Bernard ¹, F. Boulanger ¹, J.-L. Puget ¹, M. Gérin ², E. Falgarone ², L. Nordh ³, & G. Olofsson ³

¹ Institut d'Astrophysique Spatiale, Université Paris-Sud, Bât. 121, 91405 Orsay Cedex, France

² Radioastronomie Millimétrique, Ecole Normale Supérieure, Paris, France

³ Stockholm Observatory, S-133 36 Saltsjöbaden, Sweden

The large scale ISOCAM survey of the southern part of Orion B conducted in the broad-band filters LW2 (5-8.5 m) and LW3 (12-18 m) reveals the small scale structure (6'') of the emission in a wide range of physical conditions and geometry:
- The reflection nebula NGC2023 and NGC2024 (southern part)
- The molecular cloud L1630
- The North-South photodissociation region at the western edge of the molecular cloud
- The HorseHead Nebula which is a dense globule emerging from its parental cloud due to UV erroding radiations.

Small scale observations of the edge of a cloud illuminated by a few bright stars allow to strongly constrain the density structure and the geometry of the cloud. This is spectacularly illustrated in the HorseHead nebula whose illuminated edge presents one of the sharpest IR filaments detected by ISOCAM in the interstellar medium. It gives unprecedented constrains on the geometry and the penetration of the UV radiations.