DETECTION OF UIR BANDS IN AN ISOLATED LOCAL INTERSTELLAR CIRRUS CLOUD

The unidentified infrared (UIR) emission bands at 7.7 and 11.3 mu m ha ve been detected in the emission of an isolated cirrus cloud externall y heated by the typical interstellar radiation field (ISRF) of the sol ar neighbourhood. For the observations we used ISOPHOT, the photometer aboard ISO. The detected ULR bands have absolute intensities similar to 1/1000th of the values typically observed in planetary and reflecti on nebulae. The intensity ratio 11.3 to 7.7 mu m 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 emissio n level is observed also in the bands 6-9 mu m and 12 -14 mu m, sugges ting the presence of the 6.2, 8.6 and 12.7 mu m UIR bands. At 10 and 1 6 mu m continuum emission has been detected which must be due to very small grains. The observations presented give further evidence that th e 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.