ISO observations of the reflection nebula Ced 201 : evolution of carbonaceous dust ⋆

We present spectrophotometric imaging mid–IR observations of the reflection nebula Ced 201. Ced 201 is a part of a molecular cloud illuminated by a B9.5V star moving through it at more than 12 km s−1. The spectra of Ced 201 give evidence for transformation of very small carbonaceous grains into the carriers of the Aromatic Infrared Bands (AIBs), due to the radiation field of the illuminating star and/or to shock waves created by its motion. These very small grains emit mainly very broad bands and a continuum. We suggest that they are present everywhere in the interstellar medium but can only be detected in the mid–IR under special circumstances such as those prevailing in this reflection nebula. The efficiency of energy conversion of stellar light into mid–infrared emission is 7.5% for both the very small grains and the AIB carriers, and the fraction of interstellar carbon locked in these emitters is approximately 15%.