ISOCAM and molecular observations of the edge of the Horsehead nebula

We present ISOCAM observations (5-18 mum) of the Horsehead nebula, together with observations of the (J = 1-0) and (J = 2-1) transitions of (CO)-C-12, (CO)-C-13 and (CO)-O-18 taken at the IRAM 30-m telescope. The Horsehead nebula presents a typical photodissociation region illuminated by the O9.5 V system sigma Ori. The ISOCAM emission is due to very small particles transiently heated to high temperature each time they absorb a UV photon. A very sharp filament (width: similar to10" or similar to0.02 pc) is detected by ISOCAM at the illuminated edge of the nebula. This filament is due to a combined effect of steep increase of the column density and extinction of incident radiation, on typical sizes below similar to0.01 pc. Both the three-dimensional shape and the local density of the illuminated interface are strongly constrained. The dense material forming the edge of the Horsehead nebula appears illuminated edge-on by sigma Ori, and the particles located beyond the border should not be affected by the incident radiation field. This structure may be due to dense filaments in the parental cloud which have shielded the material located in their shadow from the photo-dissociating radiations. The measurement of the penetration depth of the incident radiation from the infrared data (similar to0.01 pc) gives a density of a few 10(4) cm(-3) just behind the bright filament. This value is comparable to the estimate of the density beyond the edge and deduced from our molecular observations, and also to the density behind the ionization front calculated in the stationary case. The material behind the illuminated edge could also be non-homogeneous, with clump sizes significantly smaller than the observed penetration depth of similar to0.01 pc. In that case no upper limit on the average density just behind the illuminated edge can be given.