1998A&A...331..742L

The unidentified infrared (UIR) emission bands at 7.7 and 11.3 µm have been detected in the emission of an isolated cirrus cloud externally heated by the typical interstellar radiation field (ISRF) of the solar neighbourhood. For the observations we used ISOPHOT, the photometer aboard ISO. The detected UIR bands have absolute intensities ∼1/1000th of the values typically observed in planetary and reflection nebulae. The intensity ratio 11.3 to 7.7µm is at the higher end of the range observed for reflection nebulae, planetaries, HII regions, or for the diffuse emission of the inner Galaxy. An enhanced emission level is observed also in the bands 6-9µm and 12-14µm, suggesting the presence of the 6.2, 8.6 and 12.7µm UIR bands. At 10 and 16µm continuum emission has been detected which must be due to very small grains. The observations presented give further evidence that the UIR features seem to be ubiquitous throughout the galaxy. This is in support of the PAHs as carriers of the features and, moreover, as key agents in transforming the stellar UV into infrared radiation.