  We present recent observations of small hydrocarbons (C3H2, C2H, C4H) with
high abundances in the Photo-Dissociation Region of the Horsehead nebula. Our
results show for the first time observational indications that the small
hydrocarbon distribution follows the Aromatic Infrared Bands (AIBs) emission
traced by ISO-LW2 (5-8.5 microns), whereas it does not coincide with the CO and
isotopes large-scale distribution. The derived abundances are significantly
higher than in local clouds. This enhancement might be explained by an in situ
formation assisted by the release of carbonaceous molecules from UV-irradiated
aromatic particles.
