PAHs in reflection nebulae

Reflection nebulae are particularly interesting objects for the study of the interstellar matter. The reasons are the following: (i) the matter has not been too much processed, (ii) the geometry of the IR nebula is simple, because the exciting source is usually a single star. This kind of objects is thus well suited for assessing the questions of the variations of IR properties (features, continuum, ratios...) with the incident flux.