SWS OBSERVATIONS OF IR EMISSION FEATURES TOWARDS COMPACT HII-REGIONS

We present ISO Short Wavelength Spectrometer (SWS*) grating spectra o f six compact HII regions. In addition to strong emission lines from a tomic species these spectra display infrared bands attributed to Polyc yclic Aromatic Hydrocarbons (PAHs). The continuous spectral coverage o f the present observations and the high spectral resolution allow to d escribe the detailed structure of the emission bands: the 7.7 mu m ban d is composed of two bands at 7.6 and 7.8 mu m, the 6.2 mu m band has a long wavelength extension, there is a plateau of emission between 6 and 7 mu m and a new feature is reported at 11.0 mu m in addition to t he well-known 11.2 mu m band. These observations also reveal large var iations in the relative intensities of the dust bands, in particular b etween the 7.7 and 8.6 mu m bands. In one extreme case, the 8.6 mu m b and is stronger than the 7.7 mu m band. These observations are compare d to a mixed population of ionized PAHs, using new laboratory measurem ents.